STRUCTURE magazine | May 2022

STRUCTURE MAY 2022

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MASONRY

INSIDE: New University Fieldhouse Masonry Weight Loss Plan Masonry Society Guidelines Community Bridge Restoration

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M AY 2 0 2 2

Contents M AY 2022

STRUCTURAL AND NON-STRUCTURAL MASONRY WORKOUT FOR A NEW FIELDHOUSE By Pat Conway, AIA

Cover Feature

The University of Wisconsin – La Crosse is constructing a new 144,000 gross-square-foot Student Fieldhouse & Soccer Support Facility.

Masonry was the choice for interior infill walls within the structural steel frame and key loadbearing walls. What was not so apparent was how the mason contractor and design team would develop creative uses of prefabricated masonry lintels, lightweight CMU, and a seldomused alternative engineered method to eliminate or minimize the number of control joints.

F E A T U R E S IT TAKES A VILLAGE TO SAVE A BRIDGE

By Mark Kanonik, P.E

A covered pedestrian bridge, constructed in the early 1800s, was built to protect employees traveling to work and, over its life, became a community treasure. The bridge was moved from its site (2017) due to safety issues resulting from age and deterioration. The project's success was truly a community effort – strong support for saving the bridge, non-profits efforts to apply for a grant, volunteer labor, and a contractor’s reduced fees due to the importance of the project to the Community.

TWICE REPURPOSED

By Thomas Leech, P.E., S.E.

Bypassed by the National Road Program, Brownsville, Indiana, pushed to construct their own bridge. They approached Stephen Long, looking to promote his newly patented truss design. The design was state-of-theart for the 1830s. Chord member sizes were based on mathematics and diagonals were “wedged,” introducing an elementary form of prestressing. Successfully completed in 1840, the bridge was moved to storage in the 1970s. Twenty years later, the Long Covered Bridge made its final stop in Columbus, Indiana.

STRUCTUREmagazine

C O L U M N S a n d D E PA RT M E N TS Editorial

The Time Has Come

By David Horos, P.E., S.E.

Building Blocks

6-inch CMU: Masonry’s Weight Loss Plan

By Philippe Ledent, P.E., S.E.

Construction Issues The Masonry Society

New Dry-Stack Guidelines from

By David Biggs, P.E., S.E.

Structural Integrity

Structural Safety Evaluation and Selection of Adhesives for Concrete Bonding By Dr. Martin Brandtner-Hafner

Structural Components

Specifying Masonry Component Strength

By John Chrysler, P.E.

Guest Column

Anchor Bolt Provisions in the Masonry Code

By Richard M. Bennett, Ph.D., P.E., and Luke A. Scoggins, S.E., P.E.

Structural Practices

Glass Railings

By Kevin Perttu, P.E.

Structural Design

Special Steel Reinforced Concrete

Structural Walls – Part 2

By David A. Fanella, Ph.D., S.E., P.E.

Engineer’s Notebook

Identifying Cold-Formed Steel Material

Thickness on the Job Site

By Tony Coviello, M.S., P.E., S.E.

Historic Structures

Tampa Bay (Sunshine Skyway) Bridge

Disaster

By Frank Griggs, Jr., D.Eng, P.E.

InSights

Peer Review in SE Practice

By James O. Malley, S.E., P.E., SECB

Legal Perspectives

Which Building Materials Are Responsible for Most Construction Accidents? By Neil Flynn, Esq.

Structural Influencers

Advice from a Career – Robert Silman

By Eytan Solomon, P.E.

Spotlight

New Life for the Historic Savannah Power Plant

In Every Issue

Advertiser Index Resource Guide – Steel/CFS NCSEA News SEI Update CASE in Point

Spotlight

Unique Structural Gems on China’s Landscape

Structural Forum

Ethical Perspectives and Decisions

By Scott Civjan, Ph.D., P.E.

Publication of any article, image, or advertisement in STRUCTURE® magazine does not constitute endorsement by NCSEA, CASE, SEI, the Publisher, or the Editorial Board. Authors, contributors, and advertisers retain sole responsibility for the content of their submissions.

STRUCTURE magazine is not a peer-reviewed publication. Readers are encouraged to do their due diligence through personal research on topics.

M AY 2 0 2 2

EDITORIAL The Time Has Come

By David Horos, P.E., S.E., LEED AP

The time has come for all good men to come to the aid of their country.

T

his famous quote resides somewhere in my brain, and as I start my tenure as NCSEA President, it recently resurfaced. Not that it is applicable word for word. And in the current geopolitical environment, this phrase has much more serious connotations. But maybe substitute “profession” or “environment” for “country” and exchange “people” for “men.” With those edits, this phrase seems to suit my mindset well.

I have spent the last ten years on the Boards of the Structural Engineers Association of Illinois (SEAOI) and NCSEA. When I think back to the fateful phone call asking if I would consider joining the SEAOI Board, one thought went through my head: “Maybe now is the time that I give something back to our profession.” I was not overly active in professional organizations before that time but decided it was time for me to contribute.

Fast forward, and now I am helping to lead one of our profession’s three major organizations (maybe I overdid it?). I only hope to live up to the example put forth by my predecessors.

I am a military brat who has lived in 9 states and two foreign countries. I have worked in partnerships, corporations, and small businesses, have owned a business, and have practiced in consulting engineering, AE, and EA firms. My personal and professional experience leads me to view our profession from many different perspectives. From those perspectives, I see NCSEA as an association of associations (the state SEAs), each of which has its own unique perspective, strengths, needs, and place in its professional community. It is what provides NCSEA with the richness that we appreciate and creates some of our challenges. How to represent and serve the wide range of our member SEAs. NCSEA also operates in conjunction with our two national allied organizations, CASE and SEI. Many of us are involved with two or even all three of these organizations. Beyond that, our profession operates in the intertwined AEC industry, including many related professions that work together to create the built environment. At one level, it can make your head spin. On another, however, the organizations create multitudes of ways to get involved.

As for this coming year, NCSEA continues to support and move forward our strategic plan through the work of staff, committees, and SEAs, including but not limited to: • our external marketing/messaging efforts, currently manifested as the We SEE Above and Beyond campaign, • public outreach/media interaction, the importance of which we were made acutely aware of in the aftermath of the event at Surfside, • our diversity, equity, and inclusion initiatives, • the birth and planned growth of the NCSEA Foundation.

I also would like to challenge us to find our voice and increase our impact on sustainable design and how it relates to resilient STRUCTURE magazine

communities. Climate Change has been politicized, and discussions around sustainability have gone on for some time. While some of us may be tired of hearing about it or do not believe it deserves much attention, Climate Change is a complex subject where knowledge evolves rapidly. With an increasing focus on embodied carbon in structures and the growing SE2050 Commitment, I believe that now is the time for structural engineers to increase our engagement in sustainable design.

The time of relying solely on experts has passed. The topics of sustainability and resilient communities must now be fundamental subjects familiar to all structural engineers. We have the knowledge and wherewithal to lead practicing engineers to include sustainability in our workflow and integrate it into our designs. Efficiency is critical, but we must extend beyond efficient design.

I also believe that the relationship between sustainability and resilient communities is not well understood. There are opportunities to lead in explaining and communicating the interconnectivity between the two.

While I am not an expert, I believe that the topics of sustainable design and resilient communities will continue to increase in importance in the coming years in ways we can’t imagine. However, if structural engineers do not prepare for the conversation, engage in it properly, and lead it, and if we don’t adapt our education and practices accordingly, we will lose stature and relevance to professions that do.

More optimistically, there is an opportunity for individual structural engineers, SE firms, and our profession to do more when it comes to efficiency, the strategic use of new and established materials, and partnering with contractors and other designers within the AEC community to create a more sustainable built environment and more resilient communities.

The time has come for us all to contribute to the profession of structural engineering. The time has come for us all to contribute to sustainability and resilient communities.

Maybe these quotes are not quite as memorable as the original version. My challenge is for each of us to determine the right way to work to better the profession and the built environment. However best, each of you can contribute as many opportunities exist to get involved. And for those who have done and continue to do so, thank you. Please continue your excellent work.

By the way, I now recall the where and why of the quote I used at the beginning of this column. Do you know?■ David Horos is a Principal in the Structural Engineering Studio at SOM and President of the NCSEA Board of Directors.

M AY 2 0 2 2

7

building BLOCKS 6-inch CMU: Masonry’s Weight Loss Plan

By Philippe Ledent, P.E., S.E.

I

n terms of structural concrete masonry unit (CMU) construction, the predominant unit sizes are nominally 8-inch and 12-inch-wide units. However, with rising material costs, 6-inch-wide units are ideal for interior and exterior wall construction depending on wall heights and design loads.

In terms of interior partition walls, ungrouted and unreinforced CMU wall construction is often the most cost-effective solution compared to other building systems. In addition, CMU construction provides superior fire resistance and sound transmission performance. Unreinforced masonry partition walls are typically designed for lateral forces prescribed under the International Building Code (IBC) and the American Society of Civil Engineers’ Minimum Design Loads and Associated Criteria for Buildings and Other Structures (ASCE 7).

These lateral loads typically include a minimum horizontal load specific to interior walls and partitions included in IBC Section 1607.15.

IBC Section 1607.15 Interior walls and partitions. Interior walls and partitions that exceed 6 feet (1829 mm) in height, including their finish materials, shall have adequate strength and stiffness to resist the loads to which they are subjected by not less than a horizontal load of 5 psf (0.240 kN/m2).

Relative to the partition lateral load, recommendations are provided in The Masonry Society’s (TMS) Strength Design of Masonry publication. “There is not a direct strength load combination that addresses the partition load. Use 0.9D for flexural tension to be consistent with other strength load combinations where the dead load is acting as a ‘resistance’ and use 1.6 as a load factor for out-of-plane partition load (live load).”

Based on this minimum interior wall and partition horizontal load, a maximum wall height is calculated based on the mortar type specified and the density of the masonry units. The Masonry Society standard TMS 402-16, Building Code Requirements for Masonry Structures, includes values for the modulus of rupture ( fr ) Table 1. Maximum wall height 5 psf interior horizontal load.

Unit Density NW (135 pcf)

MW (125 pcf)

LW (115 pcf)

8 STRUCTURE magazine

Mortar Type

Maximum Height

Type N Masonry Cement

9' - 4"

Type S Masonry Cement

12' - 0"

Type N PCL/Mortar Cement

12' - 8"

Type S PCL/Mortar Cement

14' - 8"

Type N Masonry Cement

9' - 4"

Type S Masonry Cement

11' - 4"

Type N PCL/Mortar Cement

12' - 8"

Type S PCL/Mortar Cement

14' - 8"

Type N Masonry Cement

9' - 4"

Type S Masonry Cement

11' - 4"

Type N PCL/Mortar Cement

12' - 8"

Type S PCL/Mortar Cement

14' - 8"

Figure 1. Maximum wall height (normal weight).

of masonry based on mortar type, the direction of flexural tensile stress, and the masonry type in Table 2.5-3. For interior walls and partitions, the maximum wall height is typically controlled by flexural tension, which can be calculated as the difference between the axial compressive stress and the bending stress, and then compared to the factored design flexural tension stress: P M – u + u ≤φf r An Sn where: • Pu = factored axial load at the critical section • An = net area (24.0 in2/ft for ungrouted 6-inch CMU) • Mu = factored moment at the critical section • Sn = net section modulus (43.3 in3/ft for ungrouted 6-inch CMU) • φ = strength-reduction factor (0.6 for unreinforced masonry subject to combinations of flexure and axial load) • fr = modulus of rupture (Table 2.5-3) For walls spanning vertically and having the net flexural tension normal to bed joints, 6-inch ungrouted interior CMU walls and partitions in low seismic areas, which are controlled by the minimum 5 psf (0.240 kN/m2) horizontal load, have maximum wall heights presented in Table 1. These wall heights are based on the wall having pinned support conditions at both the base and top of wall conditions.

For interior walls and partitions, out-of-plane seismic forces must also be considered. Out-of-plane seismic forces are determined following Chapter 13 of ASCE 7.

ASCE 7 Section 13.3.1.1 Horizontal force. The horizontal seismic design force (Fp) shall be applied at the component’s center of gravity and distributed relative to the component’s mass distribution and shall be determined in accordance with Eq. (13.3-1): z 0.4apSDSWP (1+2 h ) Fp = Rp Ip where: • Fp = seismic design force

( )

Figure 2. Maximum wall height (medium weight).

Figure 3. Maximum wall height (lightweight).

• SDS = spectral acceleration, short period, as determined from ASCE 7 Section 11.4.5 • ap = component amplification factor (1.0 for unreinforced masonry partitions in ASCE 7 Table 13.5-1 • Wp = wall weight • Rp = component response modification factor (1.5 for unreinforced masonry partitions in ASCE 7 Table 13.5-1) • Ip = component importance factor • z = height in the structure of the partition wall • h = average roof height Conservatively, a masonry wall can be assumed to have z equal to h. As stated in Strength Design of Masonry, “The value for Ip is permitted to be taken as 1.0 unless the interior wall or partition contains highly toxic, explosive, or hazardous substances, or is part of an egress stairway, or is in a Risk Category IV structure and its failure could impair continued operation of the facility, in which case Ip = 1.5.” Based on these assumptions, the maximum wall height can be calculated. The results are presented in Figure 1 through Figure 3.

These figures are based on Ip = 1.0 and would need to be modified if a higher importance factor were required. Additionally, the maximum wall height in each figure has been limited to the maximum wall height based on the minimum horizontal load prescribed by Section 1607.15 of the IBC. For interior walls and partitions designed to span horizontally between supports, such as intersecting walls or pilasters, maximum horizontal spans can be calculated similarly and are based on an fr corresponding to tensile stress parallel to bed joints provided in TMS 402-16 Table 2.5-3. For interior walls and partitions where the 5 psf (0.240 kN/m2) controls, maximum horizontal spans are presented in Table 2.

A maximum horizontal span can similarly be calculated based on mortar type and a factored out-of-plane load (Figure 4).

For exterior walls, the capacity of a 6-inch CMU wall can be calculated based on the strength design provisions in Chapter 9 of TMS 402. Typically, a complete interaction diagram is developed for a wall to ensure all factored loads are within the capacity envelope.

However, for nonbearing, reinforced CMU walls, designers can conservatively neglect the masonry self-weight and solve for the pure flexural capacity of the wall. To solve for the flexural capacity, first, use the strength design provisions to solve for the depth of the equivalent compressive stress block (a). If a is less than the face shell thickness (1-inch for a 6-inch CMU), it can be analyzed as a solid section. If a is less than the face shell thickness, then the required area of reinforcement can be solved directly. Equations for a and the required reinforcement area are included in the Strength Design of Masonry and presented below. a = d − d 2− 2Mu √ 0.8φf ḿb and 0.8φf ḿba As,reqd = fy For comparison, the required reinforcement areas were calculated for both 6-inch and 8-inch CMU walls based on a 30 psf (1.44 kN/m2) factored out-of-plane load, and the results are presented in Figure 5 (page 10).

As shown, the required reinforcement is greater for 6-inch CMU walls, which is expected, and the values diverge as wall height increases. continued on next page

Table 2. Maximum horizontal span 5 psf interior horizontal load.

Mortar Type

Maximum Length

Type N Masonry Cement

12' - 0"

Type S Masonry Cement

14' - 8"

Type N PCL/Mortar Cement

16' - 8"

Type S PCL/Mortar Cement

19' - 4"

Figure 4. Maximum horizontal span (6-inch CMU).

M AY 2 0 2 2

9

average of 23%. Representative CMU wall weights are presented in National Concrete Masonry Association’s (NCMA) publication TEK 14-13B. Beyond the reduction in the design lateral force, the lower weight can impact foundation design. For interior walls specifically, thickened slab sections can potentially be used in place of wall footings, allowing for more efficient project scheduling.

Fire Resistance Ratings Methods for determining fire resistance ratings are provided in Section 703.3 of the IBC. Specific to masonry construction, fire resistance ratings are typically calculated following the procedure presented in Section 722 of the IBC. As stated in IBC Section 722.3.2 and specific to concrete masonry walls, the fire-resistance Figure 5. Required reinforcement. rating of walls and partitions shall be determined from Table In terms of reinforcement spacing, the required spacing of a #5 722.3.2. The rating shall be based on the equivalent thickness of vertical bar is provided in Table 3. The reinforcement spacing was the masonry and type of aggregate used, and minimum equivalimited to a maximum of 120 inches (304.8 cm), assuming the wall lent thicknesses are provided in IBC Table 722.3.2. For brevity, is an ordinary reinforced masonry shear wall and subject to seismic the minimum equivalent thickness values are provided based on detailing requirements. ½-hour increments in Table 4 but can be found based on ¼-hour increments in the IBC. For partially grouted walls where the unfilled cells are left empty, Wall Weights the equivalent thickness for calculation of the fire-resistance rating Specifically, in higher seismic design categories, dead load impacts is equal to that of an ungrouted unit. Values for the equivalent the lateral force on the building structure and the design of the lateral thickness of typical hollow units are presented in NCMA TEK force-resisting system. Using 6-inch CMU in lieu of an 8-inch CMU 07-01D. For a typical 6-inch wide hollow unit, the equivalent for interior wall construction decreases the weight of the wall by an thickness provided by NCMA is equal to 3.1 inches (7.9 cm).

Thus, a 6-inch partially grouted or ungrouted wall achieves a 1-hour fireTable 3. Required reinforcement spacing #5 vertical reinforcement, 30 psf out-of-plane pressure. resistance rating depending on the type As,req (in2/ft) Required Spacing (in) Wall Height of aggregate used. Depending on the Delta (ft) 6-inch 8-inch 6-inch 8-inch mix design and geometry of the unit, higher fire-resistance ratings may be 8 0.0192 0.0141 36% 120 120 possible.

9 0.0243 0.0178 36% 120 120 The equivalent thickness of a 100% 10 0.0301 0.0221 37% 112 120 solid unit or solid grouted unit is equal 11 0.0366 0.0267 37% 96 120 to the actual thickness. Additionally, IBC Section 722.3.1.4 allows for the 12 0.0437 0.0319 37% 80 112 equivalent thickness to be equal to 13 0.0516 0.0375 37% 64 96 the actual thickness of the unit if the 14 0.0601 0.0436 38% 56 80 units are filled with loose-fill materi15 0.0693 0.0502 38% 48 72 als specified in that section. Thus, for a 6-inch unit, either grouted solid or 16 0.0793 0.0573 38% 40 56 filled with an approved material, the 17 0.0901 0.0649 39% 40 56 equivalent thickness would be equal to 18 0.1018 0.0730 39% 32 48 the specified thickness of 5.625-inches 19 0.1142 0.0816 40% 32 40 (14.3 cm). Therefore, depending on the 20 0.1276 0.0908 40% 24 40 aggregate used in the manufacture of the CMU, a 6-inch CMU wall, either grouted solid or filled with an approved Table 4. Minimum equivalent thickness (inches) of bearing or nonbearing concrete masonry walls. material, could achieve a 3-hour fireresistance rating.

Fire-Resistance Rating (hours) Type of Aggregate

½

1

1½

2

2½

3

3½

4

Pumice or expanded slag

1.5

2.1

2.7

3.2

3.6

4.0

4.4

4.7

Expanded shale, clay, or slate

1.8

2.6

3.3

3.6

4.0

4.4

4.8

5.1

Limestone, cinders, or unexpanded slag

1.9

2.7

3.4

4.0

4.5

5.0

5.5

5.9

Calcareous or siliceous gravel

2.0

2.8

3.6

4.2

4.8

5.3

5.8

6.2

10 STRUCTURE magazine

Sound Transmission Class Masonry construction exhibits superior performance in terms of sound transmission, which is especially useful in environments such as mixed-use and

residential buildings. Depending on owner Table 5. Comparison of STC ratings. requirements, a sound transmission class STC Rating Density (STC) rating of 50 is required specifically Unit Width 3 ) (lb/ft Hollow Unit Grout-Filled Unit Sand-Filled Unit Solid Unit for dwelling units and sleeping units separated from adjacent occupancies. Specifically, the 115 45 52 50 51 IBC has sound transmission requirements 125 45 53 51 52 6-inch presented in Section 1206.

1206.2 Airborne Sound. Walls, 135 46 53 51 53 partitions, and floor-ceiling assemblies 115 47 56 54 55 separating dwelling units and sleeping 125 48 57 54 56 8-inch units from other or from public or service areas shall have a sound trans135 49 57 55 57 mission class of not less than 50, or not less than 45 if field tested, for airborne noise where tested in • Based on contractors polled for this article, an ungrouted accordance with ASTM E90.

6-inch CMU partition wall provides approximately 3% to In masonry construction, STC ratings can be estimated based on 5% cost savings per square foot compared to an ungrouted testing performed by NCMA and calculated values presented in TEK 8-inch CMU partition wall. Compared to a partially grouted 13-01C. Sample results are presented in Table 5, showing that, typi8-inch CMU partition wall, an ungrouted 6-inch CMU parcally, 6-inch CMU walls will have an STC rating of 3 to 4 less than tition wall provides approximately 10% to 20% cost savings the corresponding 8-inch CMU wall. For a range of densities, STC per square foot. ratings for 6-inch walls comprising hollow units vary from 45 to 46.

6-inch CMU provides an excellent solution for mixed-use, residential, If 6-inch CMU walls are used to separate dwelling units and sleeping and school projects, specifically for partition wall construction. When units from the areas noted in the IBC, there are two common options specifying CMUs on your next project, verify availability for increasing the STC rating to exceed 50: with local contractors and suppliers and discuss options for 1) Add drywall to one or both sides of the CMU wall, corner details when using 6-inch CMU.■ potentially including a furring space with soundabsorbing material.

References are included in the PDF version of the 2) Fill the units with grout or sand or use solid units. online article at STRUCTUREmag.org.

As shown, the difference between the performance of 6-inch and 8-inch CMU walls in terms of STC ratings is minimal. Additional recomPhilippe Ledent is the Executive Director of the Masonry Institute of mendations and considerations are presented in NCMA TEK 13-01C.

Summary

Michigan and an adjunct faculty member in the Construction Engineering Technology department at the University of Toledo. In addition, Philippe serves on the Board of Directors for The Masonry Society and as an at-large representative for the Masonry Alliance for Codes and Standards.

ADVERTISEMENT–For Advertiser Information, visit STRUCTUREmag.org

Especially for interior walls and partitions, 6-inch CMU wall construction can be economical and schedule-friendly. In addition, 6-inch CMU wall construction provides the following advantages: Structural design prowess • It reduces the wall thickness by 2 inches (5.0 cm), adding to meets architectural vision. the usable floor square footage, especially on larger projects such as school construction.

• It can achieve a minimum 1-hour fire-resistance rating, either partially grouted or ungrouted, and a minimum 3-hour fire-resistance rating if grouted solid or filled with an approved material.

• It has a similar performance in terms of STC ratings when compared to an 8-inch CMU wall.

• It reduces wall dead load by approximately 23%, reducing the design lateral force and allowing Seattle | Tacoma | Portland | pcs-structural.com for smaller foundations, including thickened slab construction.

© Benjamin Benschneider

M AY 2 0 2 2

11

construction ISSUES New Dry-Stack Guidelines from The Masonry Society

By David Biggs, P.E., S.E., Dist.M.ASCE, FSEI, HonTMS

T

he use of masonry began with unreinforced dry-stack (mortarless) construction. For millennia, stone has been laid dry without mortar. The Great Pyramid of Giza (c. 2600 BC) is an example of limestone and granite ground smooth and fits tight without mortar (Figure 1). There are numerous examples of residences, walls, fortresses, and towers constructed using dry-stack masonry throughout the world. Unreinforced, these structures rely on gravity and friction to maintain their stability.

The fascination with dry-stack masonry continues today. While stone retaining walls are popular, the emphasis in North America is now primarily on manufactured masonry units. In addition to building applications, dry-stack masonry for structures has been applied to segmental retaining walls using gravity or mechanically stabilized earth techniques.

Internationally, dry-stack construction is being used in modern residential and commercial construction. There are several features common to many contemporary dry-stack concrete masonry systems, including: • Most use a form of grouted reinforcement with proprietary unit configurations that interlock and have aligning cores to accommodate the grout.

• Prestressing may or may not be grouted in place.

• Their use is often justified based on successful performance, experience, and available research.

In the United States, dry-stack masonry has been in our building codes for decades. However, it utilizes a surface bonding coating on both sides of the wall to provide structural flexural capacity.

The current International Building Code (IBC), Section 2114 DryStack Masonry, is based upon Allowable Stress Design procedures in Chapter 8 of TMS 402, Building Code Requirements of Masonry Structures. The materials and construction are based upon ASTM

Figure 1. Great Pyramid, Egypt.

C946 – 18 Standard Practice for Construction of Dry-Stacked, Surface-Bonded Walls.

Based on new research, The Masonry Society (TMS) has developed a guide to update the design and construction methodology for dry-stack concrete masonry unit (CMU) walls.

TMS 1430-21, Design and Construction Guidelines for Dry-Stack Concrete Masonry, is a non-mandatory aid for designers, building officials, contractors, educators, and others interested in drystack masonry construction. The guide is expected to be a precursor for inclusion in a future edition of TMS 402.

Systems

Figure 2. Reinforced dry-stack CMU.

12 STRUCTURE magazine

The guide addresses two specific systems of dry-stack (Figures 2 through 5). The units are always laid in a running bond (when units are placed with head joints

Figure 3a. Industrial facility. Courtesy of The Constructor.

horizontally offset at least one-quarter of the unit length in each course, as seen in Figure 2). The guide does not address other types of dry-stack masonry.

The differences between those in the guide compared to the methodology in the IBC includes: • Unreinforced, surface-bonded masonry is not addressed.

• A surface-bonded coating can be used with reinforced CMU as an exterior finish (Figure 2), but it cannot be considered as providing any structural capacity. Only the grouted reinforcement provides flexural strength.

• A surface-bonded coating can also be used as an exterior finish with prestressed CMU (Figures 4 and 5, page 15).

However, if the coating is applied to both faces of the CMU and the wall prestressing is designed to keep the wall in compression, the coating can be designed to provide increased in-plane shear capacity.

Figure 3b. Comfort Block System produced by Genest.

that limits the assembly strength to 1,500 psi. However, if the bedded surfaces of the units are ground smooth, f d́m increases with the increased unit strength. (The guide also uses a lower strength reduction factor, ϕ, for designs using unground units.) Before using the ground interface values, it is best to check with local manufacturers to verify that they can produce ground units.

The guide is based upon Strength Design provisions (Chapter 9) and Prestressed Masonry provisions (Chapter 10) from TMS 402.

However, engineers can use ASD provisions from Chapter 8. continued on next page

Table 2. Compressive strength of masonry based on the compressive strength of concrete masonry units and type of mortar used in construction.

Net area compressive strength of concrete masonry, psi (MPa)1

Design Most engineers are familiar with reinforced masonry design using TMS 402. The procedures of the guide for dry-stack are similar, as shown below.

For mortared masonry, the design is based upon f ḿ , the net area compressive strength of the masonry system. It can be obtained by testing masonry prisms or conservatively determined by the Unit Strength Method shown in Table 2 taken from TMS 602, Specifications for Masonry Structures (courtesy of The Masonry Society). For the Unit Strength method, f ḿ is derived from the net area compressive strength of the units and the mortar type.

For dry-stack masonry, the design is based upon f d́m , the net area compressive strength of the concrete masonry assembly, as shown in Table 2.6-1 from the guide.

Notice the distinct differences in this table. First, there is no mortar since it is a dry-stack. Second, the f d́m is based upon the interface between units, either unground or ground. Research has shown that the bedding surface of a manufactured CMU that meets ASTM C90 standards is irregular such that when the dry-laid units are stacked and loaded in compression, the interface undergoes some micro-crushing

Net area compressive strength of ASTM C90 concrete masonry units, psi (MPa) Type M or S mortar

Type N mortar

1,750 (12.07)

-----

2,000 ( 13.79)

2,000 (13.79)

2,000 (13.79)

2,650 (18.27)

2,250 (15.51)

2,600 (17.93)

3,400 (23.44)

2,500 (17.24)

3,250 (22.41)

4,350 (28.96)

2,750 (18.96)

3,900 (26.89)

-----

3,000 (20.69)

4,500 (31.03)

-----

For units of less than 4 in. (102 mm) nominal height, use 85 percent of the values listed.

1

Table 2.6-1. Compressive strength of dry-stack masonry assemblies (f'dm) based on compressive strength of concrete masonry units and interface condition.

Net Area Compressive Strength of Concrete Masonry Units, psi (MPa)

Net Area Compressive Strength of Concrete Masonry Assembly f'dm, psi (MPa) Unground Interface

Ground Interface

2,000 (13.8)

1,500 (10.3)

1,500 (10.3)

2,800 (19.3)

1,500 (10.3)

1,500 (10.3)

3,150 (21.7)

1,500 (10.3)

1,600 ( 11.0)

3,500 (24.1)

1,500 (10.3)

1,800 (12.4)

3,850 (26.5)

1,500 (10.3)

2,000 (13.8)

M AY 2 0 2 2

13

Commonly Asked Questions 1) How do you level the bottom of the wall if the concrete foundation is not perfectly level? a. The first course is set in mortar for leveling.

2) Will the courses remain level, and what are the tolerances? a. If bedding courses become unequal, shims or mortar are used for correction.

3) Do dry-stack walls require movement joints? a. Rarely, since the units can shift slightly. However, joints should be used for fully-grouted walls and to protect coatings or finishes from random cracking.

4) Can joint reinforcement or bond beams be used in the design? a. No for joint reinforcement since the courses are not mortared. However, reinforced bond beams can be used since they are grouted.

5) Can dry-stack walls be designed using stack bonded construction? a. No, the guide only recommends running bond construction.

6) Won’t the grout leak out the joints without any mortar? a. Yes, while this is not detrimental to the system’s performance, it can be an aesthetic concern if the wall is exposed in service.

In addition, extra care may be warranted if the walls are partially grouted, particularly if the ungrouted cells are to be filled with another material, such as insulation.

7) Can an adhesive be used in the joints to level and provide an air seal? a. The guide does not address this specifically, but the adhesive could be part of a proprietary system.

8) Are cleanouts required? a. No, since no mortar was used.

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9) Won’t the prestressing forces crack the CMU if there is no mortar in the joints? a. Not if the units are bedded without “hard spots.” The prestressed method is particularly appropriate for using ground units to assure smooth bearing.

10) Why can’t a surface bonded coating increase the wall strength for reinforced dry-stack walls, yet it can for prestressed walls? a. The surface-bonded coating is only effective if it does not crack. Unfortunately, cracking is inherent in the design of reinforced walls. However, prestressed walls can be designed to avoid flexural tension.

11) Can dry-stack masonry systems be used anywhere? a. Since dry-stack design and construction using the guide is not yet code approved, users need to consider one of the following alternatives: i. Considering dry-stack CMU as a special system under IBC Section 104.10 – Modifications and IBC Section 104.11 – Alternative materials, design, and methods of construction and equipment. ii. Having the system reviewed for code compliance using an established evaluation service, such as the International Code Council Evaluation Services (ICC-ES).

12) Since dry-stack CMU is not a listed system in ASCE 7, what seismic design coefficients and factors (e.g., R, Cd, and Ώ0) are recommended? a. For reinforced walls constructed with ground units, the recommendation is to use the comparable system values for mortared systems in ASCE 7. Otherwise, downgrade the system by one level and use those values. b. For prestressed walls with a surface-bonded coating designed structurally, use the comparable system values for mortared systems in ASCE 7. Otherwise, use the values for unreinforced masonry. c. These recommendations are expected to change as additional research into dry-stack is completed.

13) What research is behind dry-stack masonry? a. Research has been done internationally, but more recent U.S. research was performed at Clemson University and NCMA. 14) Does the guide apply to the use of proprietary units as well as generic ones? a. Yes, provided the proprietary units meet ASTM C90 standards.

15) When used with prestressed dry-stack CMU, does the surface-bonded coating require a proprietary mix? a. No, but it must meet the requirements of ASTM C887, Standard Specification for KPFF is an Equal Packaged, Dry, Combined Opportunity Materials for Surface Bonding Employer Mortar, and be installed www.kpff.com per ASTM C946.

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14 STRUCTURE magazine

S an Die go, California Seattle Tacoma Lacey Spokane Portland Eugene Sacramento San Francisco

Los Angeles Long Beach Orange County San Diego Boise Salt Lake City Des Moines St. Louis

Chicago Nashville Birmingham Lousiville Cincinnati Washington, DC New York

Figure 4. Prestressed dry-stack CMU.

Figure 5. Residence in Texas (wall construction and before stucco application). Courtesy of Cercorp Initiatives Inc. (www.cercorp.com).

16) Is dry-stack masonry expected to replace mortared systems? a. No, but there are situations where the system offers some economic savings without sacrificing performance.

17) Will these systems require masons to build? a. Yes, mason craftsmanship and understanding techniques are still essential.

18) Can dry-stack walls be deconstructed and rebuilt elsewhere? a. That is an option.

19) Does dry-stack CMU provide fire resistance? a. The guide outlines possible tests that could be performed to determine if a rating is possible.

20) Can dry-stack CMU be used with a cavity wall and anchored veneer? a. Yes; however, without using grouting, the anchors must be selected to attach to the face shells of the CMU.

21) How do you attach anchor bolts or expansion bolts to dry-stack CMU? a. Either grout the cores at anchor locations or check the capacity of the face shells.

22) Does the guide offer design examples? a. Yes.■ TMS 1430-21, Design and Construction Guidelines for Dry-Stack Concrete Masonry is available through The Masonry Society (https://masonrysociety.org).

David Biggs is a Principal of Biggs Consulting Engineering, Saratoga Springs, NY. He is the Chair of the Dry-Stack Task Group for TMS and a former member of the STRUCTURE Editorial Board.

M AY 2 0 2 2

15

structural INTEGRITY Structural Safety Evaluation and Selection of Adhesives for Concrete Bonding

By Dr. Martin Brandtner-Hafner

T

oday, price and strength may not be a decisive selection criteria and super-strong adhesives are not a gamechanger for concrete bonding.

Adhesives are increasingly being used in the construction and building industries. On the one hand, this concerns dowel reinforcements that use chemical anchors. On the other hand, the sealing and repair of cracks in concrete structural components, such as bridges, columns, or facades, are still on the rise. In adhesive bonding, the interface between the joined materials is the most critical area.

Therefore, it is of immense importance to characterize and investigate this section sufficiently. Literature reviews regarding concrete bonding reveal that only a few publications using adhesive systems are found. Most publications Figure 1. Overview of three basic types of adhesion bonding qualities. deal with pure concrete-to-concrete bonding, where new concrete layers are applied on top of old ones. However, since these are to close the economic knowledge gaps on technical product sheets not traditional polymeric adhesive bonds, those concrete-to-concrete provided by manufacturers. Furthermore, a mathematical concept bonding situations are not focused on in this article. was applied to create an adhesive bonding performance index that Another major field of research concerns the bonding of fiber-com- allows for independent empirical peer ratings of the adhesives under posite components for external reinforcement of concrete buildings. investigation. Finally, the study completed experimental tests on ten This often takes the form of mats joined by epoxy resin bonding. different polymeric adhesive systems on concrete bonds. The results Traditional mechanical test methods, such as the pull-off test or the show that only a fraction of the adhesive systems tested are suitable shear test, are used to characterize the adhesion properties of the for the structural bonding of concrete components. This enables the bonded bulk. Also, fracture analysis already plays a major role, where creation of objective evaluation parameters on a techno-economic basis substantial information about a joint’s delamination behavior by that can lead to a significant knowledge gain compared to information means of cracking can be obtained. Unfortunately, to the best of the provided by manufacturers’ technical datasheets. author’s knowledge, no comprehensive evaluation concept of adhesives for concrete bonding can be found in the literature.

Materials and Methods Thus, this article highlights the above gaps and alternatively provides decision-makers with helpful information for objective adhesive evalu- The Table shows a compilation of evaluated polymeric adhesive ation and selection. For this purpose, a specially developed structural systems from this study used to bond concrete joints. In total, ten adhesive safety factor and an adhesive safety premium were developed adhesives of seven chemical systems were selected. All information

Overview of evaluated adhesive systems for bonding concrete.

Number

Adhesive System

Notation

Application

Substrate

1

Acrylic

URF

Plastering

Concrete

2

Acrylic

MAC

Plastering

Concrete

3

Silane-Modifed Polymer (MS)

FAF

Construction Adhesive

Concrete

4

Silane-Modifed Polymer (MS)

FAX

Construction Adhesive

Concrete

5

Styrene-Acrylic Copolymer

KSB

Construction Adhesive

Concrete

6

Silicone

SIL

Liquid Sealant

Concrete

7

Silane-Modifed Polymer (MS)

MSE

Elastic Adhesive

Concrete

8

Cyanoacrylate/Acrylate Hybrid

HYS

Structural Adhesive

Concrete

9

Polyurethane

PUV

Vehicle Body Adhesive

Concrete

Epoxy

EPV

Vehicle Body Adhesive

Concrete

10

16 STRUCTURE magazine

was taken from the manufacturer’s datasheets. The producers classified the candidates as suitable for joining concrete components or at least not declared unsuitable.

σc strength

Adhesion Bonding Quality The first evaluation methodology relates to the adhesion bonding quality (ABQ) of the interface. It is measured employing fracture analysis and describes the ex-post wetting of the adhesive surface after complete separation of the test specimens. The adhesion bonding quality can be determined via fractography based on the percentage distribution of the adhesive wetting in [%] of the fractured surface.

Standards and technical guides generally distinguish between cohesive failure, adhesive failure, and mixed failure. The author has made his classifications and marked these with type A [cohesive failure], type B [mixed failure], and type C [adhesive failure], and coined the term adhesion bonding quality.

Figure 1 shows the above divisions by way of illustration: 1) excellent adhesion bonding quality due to pure cohesive failure (type A), 2) moderate adhesion bonding quality due to mixed failure (type B), 3) poor adhesion bonding quality due to pure adhesion failure (type C).

Structural Safety Factor There are well-established and proven approaches widely used in fracture analysis, such as the non-linear plastic fracture mechanics approach called the “GF principle.” from Hillerborg (1985). For the interested reader, further applications of his method are reported for concrete, wood, adhesives, and bio-composites. Unfortunately, since the GF value is a single fracture analysis criterion, other material-specific factors, such as strength and toughness, are not fully considered. Therefore, a multi-parameter approach was created, and a single evaluation index was formed using three fracture characteristic values. Such an approach benefits from an effective and holistic characterization of empirical material properties into one metric: a simpler interpretation of their meaning and a more straightforward presentation of a complex issue for decision-makers. The alternative is offered by forming a so-called structural safety factor, SF, a multiparameter hybrid figure incorporating several fundamental fracture analytical material properties based on fracture analysis. Equation 1 describes the relationship: SF = f (GF ∙ σc ∙ l ch)

(Eqn.1)

Figure 3. Schematic illustration of characterization of peer bonding performance (Brandtner-Hafner, 2017).

Load

Evaluation Methods

SF = f(σc,lch,GF)

lch toughness

GF damage tolerance Displacement Figure 2. Schematic illustration of the structural safety factor principle (Brandtner-Hafner, 2021).

with GF as the specific fracture energy in foot-pounds per square inch [foot-pounds/square inch], σc the interfacial cohesive strength in pound-force per square inch [psi], and lch as the characteristic length in inches [inch]. Figure 2 depicts these single metrics used to create the safety factor. They represent the size, shape, and course of the stable load-displacement diagram of the adhesive under investigation.

Peer Bonding Performance Finally, as evaluation metrics have been created, a mathematical value analysis in the form of a peer group evaluation was conducted. This was accomplished by creating an adhesive bonding performance (ABP) parameter, which measures the adhesive’s relative peer performance, including both safety costs and bonding safety. This ABP enables an empirically valid performance rating. Figure 3 illustrates the basic concept of this mathematical approach.

Results and Discussion The results of this study are presented based on ten different adhesives for concrete bonding. Figure 4 (page 18) shows the peer safety portfolio of the tested adhesive systems for concrete bonding. It considers the adhesive bonding efficiency formed by the adhesion bonding quality described above and the safety factor. The different colors represent the structural safety according to the traffic light system highlighting the risk of unstable failure.

It is noticeable that three clusters have been formed. The first one focuses on a two-component structural adhesive of types epoxy (EPV), cyanoacrylate/acrylate hybrid (HYS), and polyurethane (PUV), with basically high strength and low elasticity, especially for metal bonding. However, they exhibit very low adhesion properties at concrete interfaces, which leads to a massive deterioration of the structural bonding safety. This fact is expressed by low safety factors ranging from 6.4% for the epoxy-based EPV adhesive up to 16.1% for the SAC-based candidate named KSB. Consequently, those candidates are marked with small red balloons indicating low bonding safety (high failure risk). Styrene-acrylic and pure acrylicbased adhesives (KSB, URF, MAC) form the second cluster, likewise revealing very low structural safety and bonding performance values. continued on next page M AY 2 0 2 2

17

Figure 4. Peer safety portfolio of adhesives systems used for concrete bonding.

Consequently, they were also marked with small red balloons demonstrating a high risk of unstable failure.

The final cluster is formed by silicone (SIL) and silane-modified polymer adhesives (MSE, FAF, FAX). They are marked in yellow and demonstrate adhesive safety factors ranging from 18.3% to 32.9%. Interestingly, the silane-modified polymer-based adhesive FAX highlights by far the highest bonding performance with a measured bonding efficiency of 70% at a safety factor level of 66.8%.

This could be explained by the high energy storage capacity of the adhesive during the fracture process. In a final step, the bonding performance of the entire group was calculated, comprising bonding safety, bonding quality, and adhesive safety premium. Figure 5 summarizes the total results in a rating and ranking compilation. Again, the colored distinctions emphasize the risk of unstable failure.

parameters could be generated by combining the GF test method and an innovative evaluation method from Brandtner-Hafner (2017, 2019, 2020, 2021). These were then used to create mathematical evaluation models for generating adhesive bonding performance incorporating structural safety and structural premium. Ten different adhesive candidates used for bonding concrete joints adhesively were tested experimentally. This holistic analysis demonstrated that only adhesive systems with high energy storage capacity during the fracture process could fail safely. Specifically, this means that epoxy-, polyurethane-, and cyanoacrylate-based adhesives do not improve the structural integrity of bonded concrete joints, such as anchors or crack repair fillers, as is the standard used for refurbishment in the construction and building industry. Therefore, more suitable alternatives are necessary to overcome such shortcomings. Further future research will show how this new finding can and will affect the joining technology of concrete.■ References are included in the PDF version of the online article at STRUCTUREmag.org.

Dr. Martin Brandtner-Hafner is the Founder of FRACTURE ANALYTICS, specializing in the structural safety evaluation of adhesives, composites, and high-tech materials.

Conclusion In this study, different polymeric adhesive bonding systems – some of which are commercially used by practitioners and operators in the construction industry – were evaluated for their safety, performance, and efficiency at concrete bonds. This became necessary as technical data sheets and manufacturer’s specifications often lack valid data and decision support. Furthermore, the literature has shown that standardized test methods are technologically incapable of providing such information.

Therefore, fracture analysis was used as an evaluation tool to create empirical analysis data. Multifactorial assessment

18 STRUCTURE magazine

Figure 5. Peer bonding performance of adhesives systems used for concrete bonding.

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structural COMPONENTS Specifying Masonry Component Strength How Much is Enough?

By John Chrysler, P.E., FTMS

S

tructural engineers often get a greater level of comfort by specifying masonry component strengths higher than are needed. However, this can be counterproductive since some masonry components, such as mortar, may sacrifice bond in lieu of strength, leading to cracking and water intrusion into structural masonry walls. Masonry grout may also be considered a form of concrete, which it is not. A better approach is understanding how the individual materials work together based on code requirements supported by extensive research.

The ultimate goal should be to specify masonry components that are economical with maximum structural integrity.

The Masonry System Concrete block masonry is designed as a homogeneous system that assumes the compression components of masonry units, mortar, and grout are equal. Code design parameters are conservative, which takes into account that there will be some variation in material properties.

Joint reinforcement or structural deformed reinforcement, if present, are sole tension components and not subject to multiple material variations.

Based on the Unit Strength Table 2 of The Masonry Society’s TMS 602, Specification for Masonry Structures, and minimum strength requirement of 2,000 psi for concrete masonry units as required by ASTM C90, Standard Specification for Loadbearing Concrete Masonry Units, the natural design threshold for concrete block masonry is 2,000 psi. When one looks at TMS 602, Table 2, the 2,000 psi strength verification is based on Type M or S mortar, but the strength verification is reduced to 1,750 psi when using Type N mortar. The natural instinct is that more strength yields better performance, but this is not always true. Mortar with less cement content provides a better bond to the masonry unit, mitigating or eliminating moisture intrusion into the masonry system.

Moisture intrusion can lead to walls that leak which is never desirable.

Table 2 in TMS 602 lists Unit Strength verifications in increments of 250 psi which provides thresholds for design compressive strengths above 2,000 psi; however, the availability of unit strength from local concrete block producers must be confirmed.

Grout must be high slump to allow flow in masonry cells.

Masonry Units Masonry unit strength is the primary component when considering compressive strength design of masonry. Mortar and masonry grout naturally vary in strength properties, and the goal should be to specify strengths compatible with the masonry units.

Even though ASTM C90 requires the minimum compressive strength of concrete masonry units to be 2,000 psi, units are typically manufactured at strengths 20 to 30% greater than the minimum.

This provides unit strengths that are comfortably compliant without far greater than desirable strength.

Mortar

Mortar is unquestionably the most misunderstood component of masonry. The consensus seems to be that mortar strength less than the masonry design strength means that the actual system strength is inadequate, but that is not true. When tested as an individual component, there are two primary reasons that mortar Table 2 — Compressive strength of masonry based on the compressive strength of concrete does not replicate the actual mortar strength in the masonry units and type of mortar used in construction. masonry wall.

• The aspect ratio (height divided by width) of mortar Net area compressive Net area compressive strength of ASTM C90 in a masonry wall is not reflected in the test specistrength of concrete concrete masonry units, psi (MPa) men (3⁄8 inch to 1¼ inches for mortar joint in a wall masonry, psi (MPa) Type M or S mortar Type N mortar vs. 1:1 or 2:1 in a test specimen).

1,750 (12.07) --2,000 (13.79) • The moisture curing content in a test specimen 2,000 (13.79) 2,000 (13.79) 2,650 (18.27) is not similar to that in a masonry wall (masonry units absorb excess moisture whereas specimen 2,250 (15.51) 2,600 (17.93) 3,400 (23.44) form does not).

2,500 (17.24) 3,250 (22.41) 4,350 (28.96) There are 13 statements within 3 ASTM Standards 2,750 (18.96) 3,900 (26.89) --(ASTM C270, C780, C1586) addressing masonry mortar 3,000 (20.69) 4,500 (31.03) --field testing and explaining why mortar strengths do not directly correlate to the compressive strength values Reference: TMS 602

20 STRUCTURE magazine

ASTM C270, Table 1 Property Specification RequirementsA

Mortar

Cement-Lime

Mortar Cement

Masonry Cement

A

Type

Average Compressive Strength at 28 days min. psi (MPa)

Water Retention, min. %

Air Content, max. %

M

2500 (17.2)

75

12

Aggregate Ratio (Measured in Damp, Loose Conditions)

S

1800 (12.4)

75

12

N

750 (5.2)

75

14

O

350 (2.4)

75

14

Not less than

M

2500 (17.2)

75

18

2¼ and not more

S

1800 (12.4)

75

18

than 3½ times the sum

N

750 (5.2)

75

20

of the separate volumes

O

350 (2.4)

75

20

of cementitious

M

2500 (17.2)

75

18

materials

N

1800 (12.4)

75

18

S

750 (5.2)

75

20

O

350 (2.4)

75

20

Laboratory prepared mortar only

Ref: ASTM C270

contained in ASTM C270 Table 1. Additionally, the table notes that the application is for laboratory-prepared mortar only.

Other published information on masonry mortar explains the strength disparity between mortar in a masonry wall and mortar test specimens. The National Concrete Masonry Association and the Brick Industry Association publish information on mortar testing and performance and consistently explain why field-tested mortar compressive strength is not representative of mortar strength in the wall and how bond strength is essential to take into consideration.

The Code (TMS 602) and Material Standards should be used as guidance to specify the mortar most compatible with the masonry unit and wall system. The Code contains certain seismic limitations, and the Material Standard, ASTM C270, provides valuable recommendations. However, neither source goes in the direction of recommending compressive strength, and this absence of recommendations acknowledges that mortar prepared by proportion without testing for compressive strength safely provides a performing masonry wall system. Type M or S mortar is required by

TMS 402, Building Code Requirements for Masonry Structures, for participating elements (shear walls) in Seismic Design Category D and above.

One often overlooked but very useful resource is contained in the Appendix of ASTM C270. Table X1.1 is a guide for the Selection of Masonry Mortars.

There are 4 mortar classifications by Type, M, S, N, and O. Type M has the highest Portland cement content and Type O the lowest.

Mortar bond is enhanced by lower Portland cement content which results in the industry recommendation that the most appropriate mortar selection is the one providing the best bond while complying with Code requirements.

Type M mortar should be used sparingly. Applications would include the need for durability, such as retaining walls, or when very high design strengths are specified. For structural walls in high Seismic Design Categories (D and above), Type S mortar is most appropriate, and Type N mortar is typical for structural walls in lower Seismic Design Categories.

continued on next page

ASTM C270 Table X1.1 Guide for the Selection of Masonry MortarsA

Location

Exterior, above grade

Exterior, at or below grade

Interior Interior or Exterior

Building Segment

Mortar Type Recommended

Alternative

load-bearing wall

N

S or M

non-load bearing wall

O

N or S

parapet wall

N

S

foundation wall, retaining wall, manholes, sewers, pavements, walks and patios

SC

M or NC

load-bearing wall

N

S or M

non-bearing partitions

O

N

see Appendix X3

see Appendix X3

tuck pointing

B

This table does not provide for many specialized mortar uses, such as chimney, reinforced masonry, and acid-resistant mortars.

Type O mortar is recommended for use where the masonry is unlikely to be frozen when saturated, or unlikely to be subjected to high winds or other significant lateral loads.

Type N or S mortar should be used in other cases.

C Masonry exposed to weather in a nominally horizontal surface is extremely vulnerable to weathering. Mortar for such masonry should be selected with due caution.

A B

Ref. ASTM C270

M AY 2 0 2 2

21

absorbed by the masonry units, placing the grout in a moisture state similar to concrete.

Masonry grout has many similarities with concrete, so it is natural Also important is the aggregate size. With the small cell spaces to think of grout as a different type of concrete. Unfortunately, this accommodating grout, the limitation of coarse aggregate is generally can lead to specifying or providing grout that may not be appropriate 100% of the aggregate passing the ½-inch sieve, with a maximum of for masonry walls.

15% of the aggregate retained on a ⁄-inch sieve (reference ASTM The first thing to consider is that grout, unlike concrete, must C404, Standard Specification for Aggregates for Masonry Grout). be a very high slump mixture to flow in the restricted and conThese differences are rarely an issue for pre-blended grout or grout gested spaces in the cells of masonry units. The Code (TMS 602) mixed in the field, but when larger volumes of grout are supplied and Material Standard (ASTM C476) require a slump of 8 to 11 by ready-mixed trucks, the grout is proportioned by a design mix. inches, unthinkable in a concrete world. The next process in the Unfortunately, the design mix is often generated by software sensiplacement of masonry grout is that the excess water is immediately tive to the properties of concrete. This can lead to grout design mixes that are enhanced with plasticizers that replace water and may ultimately lead to an insufficient amount of water to hydrate the masonry grout adequately.

Alternately, the design may increase the content of Portland cement to compensate for the perceived high water content in grout during the curing process. This can easily lead to grout strengths of 3,000 to 4,000 psi or more which is wasteful and environmentally unfriendly by using more Portland cement than needed.

The masonry grout should be specified to the minimum of 2,000 psi or the design strength, whichever is greater. Then submittals should be closely reviewed so that the proposed grout is not a modified form of concrete but truly a grout submittal. Using grout that is reasonably close to the masonry unit strength provides a structural masonry system that follows the intent of the Code.

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22 STRUCTURE magazine

Summary Structural masonry walls are designed as a homogeneous system, and specifying and providing components with reasonably close properties is economical and structurally sound. The designer should consider that neither masonry units nor masonry grout should be excessively above the design strength. Additionally, mortar is not the weak link in the compressive strength chain and should not be specified by compressive strength. Mortar bond strength, obtained by using a softer mortar with less Portland cement, is a key property that is rarely given the appropriate recognition.■ John Chrysler is the current committee chair of TMS 402/602, the Material Standard referenced in the International Building Code. After 26 years as Executive Director of the Masonry Institute of America, Mr. Chrysler has recently retired but still can be reached at jc@masonryinstitute.org.

guest COLUMN Anchor Bolt Provisions in the Masonry Code

By Richard M. Bennett, Ph.D., P.E., and Luke A. Scoggins, S.E., P.E.

T

he phrase “the devil is in the details” applies to anchor bolt design in masonry structures. The anchorage of masonry members is essential both for load transfer into the member and for stability and support of the member. There are typically two types of anchors used in masonry. Cast-in-place anchors, or anchor bolts, are generally designed using TMS 402 Building Code Requirements for Masonry Structures.

Post-installed anchors are generally designed based on manufacturers’ data, with the design capacities of the anchors determined through International Code Council (ICC) Evaluation Service reports. This article focuses on anchor bolt design and, particularly, several recent revisions in the 2016 version of TMS 402 that will help with the design of anchor bolts.

Brief History of Anchor Bolt Design Before the 2008 TMS 402 code, there was a vast difference between strength design and allowable stress design in how the strength of an anchor bolt was determined. The provisions were harmonized in 2008, so that allowable stress design and strength design give reasonably the same design. There are now three failure modes considered for tension: steel yielding, masonry breakout, and anchor bolt pullout (bent-bar anchors only). Often, the anchor bolt pullout equation will control for bent-bar anchors; thus, headed anchors are often preferred for large-tension loads. Four failure modes are considered for shear: steel yielding, masonry breakout, masonry crushing, and anchor bolt pryout.

Masonry breakout will only control if the anchor is near an edge. Anchor bolt pryout will only control for very small embedment lengths.

Masonry crushing is usually the failure mode that controls the design capacity.

The masonry code has always required at least a ¼-inch clearance between the anchor bolt and masonry unit for fine grout and a ½-inch clearance between the anchor bolt and masonry unit for coarse grout. In the 2011 code edition, this provision was clarified for anchor bolts placed in drilled holes in the face shells of hollow masonry units.

The anchor bolt is permitAnchor bolts at the top of wall. Courtesy ted to contact the masonry of the Southeast Concrete Masonry unit where the bolt passes Association

Hot-forged headed anchor bolts. Courtesy of Portland Bolt Co.

through the face shell, but the portion of the bolt within the grouted cell still has to maintain the minimum space between the bolt and the unit.

Anchor Bolts Provisions in TMS 402-16 There were two major changes to anchor bolt provisions in the 2016 version of TMS 402. One change was to increase the shear capacity due to masonry crushing by 67 percent. The second change modified the interaction equation for combined tension and shear loading from a linear interaction to an elliptical interaction, increasing the calculated capacity of anchor bolts.

Designers have expressed concerns about the masonry crushing equation. The existing equation results in very closely spaced anchor bolts, to the point that designs seem unreasonably conservative. When the masonry crushing equation was added to the TMS 402 code in 2008, there was no comparison to test data. The nominal masonry crushing strength was 1050√f ḿ Ab , where fḿ is the specified masonry compressive strength and Ab is the bolt area.

Test data from 345 anchor bolt tests were recently examined. Shear crushing controlled in 188 of the tests. Shear crushing was not necessarily the actual failure mode but the equation that controlled the design capacity. The average ratio of the experimental strength to the nominal strength was determined to be 2.33. In other words, anchor bolts were failing at over twice the predicted nominal strength.

Several modifications to the current crushing strength were examined, and the equation recommended by FEMA 368 (2000 NEHRP, Recommended Provisions for Seismic Regulations for New Buildings and Other Structures) was chosen. This equation is 17504√f ḿ Ab . A similar increase was made for allowable stress design. Re-examining the 345 tests showed that shear crushing was controlled in 131 of the tests.

The average ratio of the experimental strength to the nominal strength was 1.49, or the anchor bolts were failing at approximately 1.5 times the predicted nominal strength. continued on next page

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There are now three failure modes considered for tension: steel yielding, masonry breakout, and anchor bolt pullout (bent-bar anchors only).

Numerous anchor bolts are required in this structure to attach the roof to the masonry walls.

The second change in anchor bolt provisions in TMS 402-16 was related to anchor bolts under combined tension and shear loading.

The interaction equation for combined axial tension and shear in anchor bolts in masonry has historically been a linear interaction equation in the TMS 402 code. This is quite conservative. The TMS 402-16 code changed this to an elliptical interaction equation with an exponent of 5⁄3, as shown below. A similar change was also made to the allowable stress design interaction equation: bau 5⁄3 bvu 5⁄3 + ≤1 φBan φBvn where bau is the strength level axial load on an anchor bolt, bvu is the strength level shear load on an anchor bolt, ban is the nominal axial strength of an anchor bolt, and bvn is the nominal shear strength of an anchor bolt.

The basis for this change was again the examination of test data of anchor bolts under combined tension and shear loading. The change is significant. For example, for a value of bau /φBan = 0.5, a linear interaction would limit bvu/φBvn to 0.5. With the new elliptical interaction, bvu/φBvn could be as high as 0.8 for bau /φBan = 0.5.

( ) ( )

Anchor bolts are used to attach the top plate to the masonry wall.

24 STRUCTURE magazine

Changes to ASCE 7 In addition to the changes to the anchor bolt requirements in TMS 402, there was a change in anchor provisions in ASCE 7, Minimum Design Loads and Associated Criteria for Buildings and Other Structures.

In ASCE 7-10, the seismic requirements in Chapter 13 (nonstructural components), Chapter 14 (material-specific requirements), and Chapter 15 (nonbuilding structures) required that the anchor loads be increased by a factor of 2.5 if the failure was controlled by other than tensile or shear yielding. This factor was reduced from 2.5 to 2.0 in Chapter 14 of ASCE 7-16. The increase in Chapters 13 and 15 is based on an overstrength factor that ranges from 1.5 to 2.5 but is 2.0 for almost all cases.

Conclusions The two major changes in the anchor bolt provisions of TMS 402-16, increasing the shear crushing strength and changing from a linear to an elliptical interaction equation, will significantly affect anchor bolt design. The changes will result in more efficient designs and reduce

Anchor bolts are used to attach the wood ledger to the side of the masonry wall.

the excessive conservatism in previous codes. Based on several trial designs, these two changes, combined with the change in the ASCE 7-16 seismic provisions, result in an approximate doubling of the calculated capacity of anchor bolts.■ We are excited to see a lessening of the pandemics’s impact on our authors and advertisers and expect to have more normal page counts in the future.

As a result, we have taken the opportunity to showcase the work that other organizations do in supporting SEs by reinvigorating our Guest Column program. It is a pleasure to have these organizations add to STRUCTURE’s knowledge base. If your organization would like to submit an article proposal, please contact Chair@STRUCTUREmag.org.

This article, all or in part, has been previously published in the Masonry Design, May 2018 issue. It is reprinted with permission.

Richard M. Bennett is a Professor of Civil and Environmental Engineering at the University of Tennessee. He is a Fellow of The Masonry Society. Dr. Bennett has been very active on the TMS 402/602 Code Committee, chairing the Flexural, Axial Loads, and Shear subcommittee and serving as the vice-chair of the 2013 MSJC Committee. Dr. Bennett was the chair of the main committee that developed the 2016 code and is currently 2nd vice-chair of the TMS 402/602 Code Committee.

Anchor bolts are used to attach the ledger angle to the side of the masonry wall.

Additional anchor bolts are used in the beam pocket for the base plate. Note that the ledger angle was installed over the face of the beam pocket, which had to be amended in the field.

Luke A. Scoggins is a Consulting Engineer at Jacobs. As a subcommittee corresponding member, he currently participates on the TMS 402/602 Code Committee.

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structural PRACTICES Glass Railings To Top Rail or not to Top Rail

By Kevin Perttu, P.E.

T

he engineering of guardrails has generally been straightforward ever since they were first addressed in building codes. Even the earliest building codes, like the Uniform Building Code (UBC) and the Building Officials and Code Administrators’ (BOCA) National Building Code, had live load requirements for typical handrail and guardrail scenarios.

Historically, the building codes addressed guardrails under the assumption that they were fabricated metal pipe or metal tube assemblies that provided fall protection for any walking surfaces greater than 30 inches above the surface below. Standard two-line metal pipe railings have been used for decades in industrial settings, and ornamental metal railings are used to this day as architectural features for buildings around the world. It is common for these typical metal railings to see a minimum code-required 200-pound concentrated live load and a separate 50-pound-per-foot uniform load case applied at the top of the guard. Often, an additional 50-pound load Figure 1. Obstructed view for fans (Google Images Creative Commons Use). applied over one square foot of the infill of the guardrail was required if the guard was functioning as a decorative railing in have increased in popularity exponentially since the 1970s and a residential or commercial setting.

1980s. Changing times also means changing codes, and since glass In the mid-1900s, the architectural world started to move toward behaves so differently than metal, this new structural glass balustrade glass as a desirable construction material, and the development needed to be addressed. The 1988 edition of the UBC was the first of the glass balustrade guardrail was introduced. The first patent mainstream code to address glass railings, and the requirements for a structural glass balustrade shoe was issued in 1967, and they introduced in this code laid the groundwork for the requirements found in the current editions of the International Building Code (IBC).

The 1988 UBC addressed glass strength requirements, safety glazing testing, and precautions to include if a glass panel breaks. In addition, this code noted an interesting requirement for a glass balustrade guardrail: “Glass balusters shall not be installed without a handrail or guardrail attached.”

This one requirement has provided much debate in the glass railing industry and certainly has caused a few headaches for engineers, glass railing manufacturers, building owners, and architects over the past 10 years. This article focuses on this code requirement and how changes in the code over the last decade have resulted in some significant structural issues and a new glass testing procedure required in the 2018 edition of the IBC. The terms top cap and top rail are used interchangeably throughout the article.

Glass Railing Code Requirements Figure 2. Base shoe glass railing (Bing Image Free Use Search).

26 STRUCTURE magazine

As mentioned above, handrail and guardrail live load requirements in the most recent governing

building codes (OSHA, ASCE, IBC, etc.) have all essentially remained the same as summarized in IBC Chapter 16, Section 1607: 200-pound concentrated load and a separate load case of 50-pound-per-foot uniform load applied perpendicular to the top of the guard. Additional code requirements for glass railings can be found in IBC Chapter 24, Section 2407, Glass in Handrails and Guards, including (emphasis added): 2407.1.2 Support. Each handrail or guard section shall be supported by a minimum of three glass balusters or shall be otherwise supported to remain in place should one baluster panel fail.

Glass balusters shall not be installed without an attached handrail or guard.

These requirements in IBC Chapter 24 are essentially the same as the first glass railing requirements introduced in the 1988 edition of the Uniform Building Code. The code is unclear as to what constitutes an “attached handrail or guard,” but it is generally assumed to refer to a rail section on top of the glass baluster, otherwise referred to as a top cap. Figure 3. Point-Supported glass railing (Bing Image Free Use Search).

Starting with the 2009 edition of the IBC, an exception to the glass baluster top cap requirement was introduced amenity terraces, and other typical building guardrails, including and is noted in IBC 2009 Section 2407 as (emphasis added): lobby/monumental stairways, are being planned without a top rail Exception: A top rail shall not be required where the glass balor top cap. The code relies on the building official to decide whether usters are laminated glass with two or more glass plies of equal or not the guardrail application can use this exception, but that thickness and the same glass type when approved by the building approval procedure does not seem to be happening. Otherwise, in official. The panels shall be designed to withstand the loads speci- the author’s practical opinion, a monumental stairway in a hotel fied in Section 1607.7. lobby would not be considered an event-viewing venue, and the The International Code Council (ICC) further explained this “excep- glass guardrail would require a top cap. tion” in their published commentary by stating the following: The exception allows an option where a top rail is an undesirable Base Shoe Railings and visual barrier. An example is a guard at the front of the spectator Point-Supported Railings levels of sports arenas and theaters. The balusters must be laminated glass complying with the live load requirements for guards The IBC is also unclear on what type of glass railings the top rail and handrails. exception should apply. Structural glass balustrades typically have Based on the ICC’s explanation two configurations for attaching to of the exception introduced in the the host structure. Figure 2 shows an 2009 IBC, the intent was to provide example of a base shoe style railing, for unobstructed viewing of a speand Figure 3 shows an example of a cial live event, such as being able to standoff style or point supported glass watch your favorite baseball pitcher railing. It is important to differenthrow a perfect game (Figure 1) tiate the two attachment methods or to see the play “Hamilton” withbecause glass behaves differently out a top rail running through when point-supported than when Alexander’s head. Two essential secured in a base shoe. Base shoe qualifications must be met to install supported glass allows structural a glass railing without a top cap: 1) design loads to distribute evenly as the glass must be laminated, and 2) bearing stress along the bottom face the railing without a top cap must of the glass that is captured within be approved by the building official. the shoe. Figure 4 shows how glass However, from 2010 onward, stresses are distributed along the manufacturers and architects seem bottom edge of a base shoe railto take advantage of this top cap ing. That is not the case, however, exception and apply it to all types for point-supported glass railings. of glass guardrails, regardless of Figures 5 (page 28) and 6 (page 29) the application or if there really is show how the stress concentrates a viewing event. Currently, high- Figure 4. FEA showing stress distribution along the bottom of a base shoe around the standoff support that is rise balconies, rooftop windscreens, glass railing. nearest the applied design load. This M AY 2 0 2 2

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The most impactful issue when the top rail is allowed to be removed per the exception in the code is that it requires the glass panels to be laminated. causes increased stress in the glass surrounding the standoff, primarily at the edge of the hole in the glass where the standoff fastener passes through. In both FEA examples, a 200-pound concentrated load has been applied to the top corner of the glass panel.

member since it allows the adjacent glass panels to help share the load, reducing stress in the glass and minimizing deflection of the guard. However, the 2009 IBC top rail exception provision introduced a structural issue that is two-fold for laminated glass standoff systems: 1) significantly increased deflections and 2) higher stress concentrations in a single pane (or ply) of the laminated glass What Happens Without a Top Rail? panel at the standoff hole locations; in fact, much higher stresses Top rails may seem insignificant because they are often just small than a monolithic glass panel would be subject to. stainless-steel channels, but they serve a purpose when engineering For example, 9⁄16-inch-thick laminated glass is comprised of (2) structural glass balustrades. This is especially true when the top rail ¼-inch-thick tempered glass panels bonded together by a polymeric is continuous over the entire glass balustrade because it helps share interlayer. Laminated glass panels have different structural behaviors the design loads over multiple panels than monolithic glass panels, particularly of glass, significantly reducing peak Table of FEA stress comparison. bearing stresses occurring on the edge of stresses and overall deflection of the glass holes in the glass. Figure 7 shows a typical FEA Stress at Glass Hole Edge panels. The most impactful issue when 9⁄16-inch-thick laminated glass panel for STRESS the top rail is allowed to be removed a point supported glass railing tested to per the exception in the code is that it failure. The glass breakage occurred at Laminated Glass requires the glass panels to be laminated. the left side of the standoff attachment ¼” Compression Side 7,910 psi Intuitively, people think laminated glass hole, and, in this case, the compression is better or safer, but it presents strucside ¼-inch glass panel layer failed. It ¼” Tension Side 5,460 psi tural design challenges for both stress is consistent with the high stress con5,860 psi Monolithic Glass and deflection. For standard base shoe centrations shown near the hole edge glass railings using laminated glass within Figure 6. Not surprisingly, ½-inch out a top rail, the designer needs to be monolithic glass panels resist the concencautious to resolve the design stresses trated stresses much better, and the same within a single glass panel (e.g., lack of testing process resulted in much higher load sharing to adjacent panels). Also, test loads being applied before breakage. the designer must be aware that the The Table shows an FEA stress comlaminated glass deflects more, particuparison analysis for a point supported larly with PVB interlayers which are not 9⁄16-inch-thick laminated glass and a recommended for any structural glass ½-inch-thick monolithic glass with an balustrades. Designers should also conidentical layout and design load applied. sider differential deflection of the glass The stress in a ¼-inch single ply in the panels when top caps are not used. The laminated glass is significantly higher challenges of laminated glass panels are than the stress that the FEA model more significant for point-supported shows for the full ½-inch-thick panel. railings.

In summary, this data shows that laminated glass panels do not work as well as monolithic glass panels. The glass railing Structural Behavior of engineer should carefully analyze the Point-Supported Glass hole edge stresses for point-supported Before the top rail exception was added glass railings without top caps. to the code in the 2009 IBC, pointFor this example, there are generally supported railings were always installed two engineering solutions to accomwith a top rail. They used ½-inch-thick modate laminated glass in standoff monolithic (solid single panel) glass guards without a top cap: 1) use thicker without any significant history of problaminated glass and/or 2) increase the lems. For standoff glass guard systems, Figure 5. FEA model showing stress concentration at number of standoff supports. Thicker the top cap can be an essential structural standoff support. laminated glass is much more costly to

28 STRUCTURE magazine

the building owner, and added standoffs change the look, which can be unappealing to the architect. Both options typically make the structural engineer the bearer of bad news.

has been included in IBC 2018, Chapter 24, Section 2407.1.2: 2407.1.2 Structural glass baluster support. Guards with structural glass baluster panels shall be installed with an attached top rail or handrail. The top rail or handrail shall be supported Guidance for the Glass by not fewer than three glass baluster Railing Structural Engineer panels or shall be otherwise supSince base shoe style balustrade railported to remain in place should one ings are more commonly used in sports baluster panel fail. arenas, theaters, and other public gathException: An attached top rail or ering areas common for live viewing handrail is not required where the events, designers could infer that the glass baluster panels are laminated Code Council intended the top rail glass with two or more glass plies of exception in the code to apply to base equal thickness and of the same glass shoe glass balustrades only. It is questiontype. The panels shall be tested to able whether standoff style guardrails remain in place as a barrier following should be included in the top rail excepimpact or glass breakage in accortion rule, as the Code Council does not dance with ASTM E2353. define what exactly constitutes a strucThe critical point with the re-worded tural baluster. Based on the engineering exception is that it replaces the previous data above, the top rail exception rule disclaimer “when approved by the building inadvertently causes structural design official” with a provision that all laminated issues in the glass for point-supported Figure 6. Close up of FEA stresses around the hole in the glass. panels used without top caps need to be glass railings due to the laminated glass tested per ASTM E2353: Standard Test requirement. Regardless, whenever the architect exercises the right Methods for Performance of Glazing in Permanent Railing Systems, to use the top rail exception for point-supported glass railings, the Guards and Balustrades. The testing requirement applies to all glass delegated design engineer should exercise caution to ensure the baluster railing configurations, including base shoe and point-suplaminated glass is thick enough to resolve the concentrated flexural ported glass balustrades. and edge stresses at the standoff support holes.

The updated exception in the 2018 IBC could affect architects and glass guardrail manufacturers that plan to continue using laminated glass panels without a top rail because the ASTM E2353 method is To Top Rail or Not to Top Rail? an extensive testing procedure that includes impact testing and post In the 2018 edition of the International Building Code, the Code breakage requirements. However, this added testing requirement will Council has provided an answer to whether or not a glass guard can help alleviate structural design concerns with laminated glass used in exclude the top rail. An updated version of the top rail exception rule glass balustrades that do not have top rails.

Where Do We Go from Here?

Although the International Code Council can be commended for addressing the lack of oversight regarding the top rail exception rule, it is unclear how ASTM E2353 testing is supposed to be incorporated into the design process. From an engineering perspective, the ASTM E2353 testing qualification is similar to the safety glazing requirements already referenced in IBC Chapter 24, Section 2407.1. Historically, safety glazing qualification testing is the responsibility of the glass manufacturer. As the IBC 2018 and IBC 2021 become the governing codes for construction projects throughout the U.S., glass railing manufacturers are encouraged to communicate with their glass suppliers to be prepared for this latest update of the top rail exception rule for glass balustrades. Currently, twenty-five U.S. states have adopted the IBC 2018, while South Dakota, Colorado, and Wyoming have already adopted the IBC 2021, including the same top rail exception discussed in this article.■

Figure 7. Testing failure at standoff hole location in a laminated glass panel.

Kevin Perttu is the Engineering Manager of the Architectural Products Group at Rice Engineering (kevinperttu@rice-inc.com).

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It Takes a Village to

Save a Bridge

By Mark Kanonik, P.E., FASCE

H

ome to less than 2,000 residents, the quaint Village of Cambridge, NY, lies between the foothills of the Adirondack Mountains of upstate New York and the Green Mountains of Vermont. About 150 years ago, Cambridge was home to the Jerome B. Rice Seed Company, the second-largest seed company in America at that time, with clients throughout New England and the Eastern Seaboard.

Pictured above: Completed bridge; note the failed stone lining wall on the left and the leaning stone wall on the right.

30 STRUCTURE magazine

The Rice Company started in the early 1800s when R. Niles Rice wall that supported the south end of the bridge collapsed. Thankfully, traveled throughout the country and sold his seeds from the back of the bridge did not fall into the stream or sustain any damage, and a horse-drawn wagon. His business was very successful, and his son it remained open to the public. In 2012, the Cambridge Valley Jerome B. Rice began a construction campaign between 1879 and Community Development and Preservation Partnership, a nonprofit 1895 to erect several buildings for warehousing and distribution of the community development corporation that facilitates downtown company’s seeds. The Rice Seed Company chose the least expensive real revitalization projects, asked the author to explore the feasibility of estate in Cambridge – a swamp, which was drained to become the site rehabilitating the bridge. Unfortunately, the property owner at that of its buildings. The access road time was not at all interested in to the property was unpaved and the bridge, and the feasibility crossed Blair’s Brook (now called study was never completed. The Owl Kill). Needless to say, the property was later sold to 17 road was often a muddy mess, Mile VARAK Park, LLC, and especially in the spring when the the new owners wholeheartedly snow melted, and Jerome did not embraced the idea of saving the want his nearly 200 employees bridge. Fearing for the public’s getting themselves (and his safety and the bridge itself, buildings) dirty as they walked the owners donated the bridge to work. to the Village of Cambridge, Rice hired local architect hoping it could be restored. In Florans Hoxie to design and con2017, the bridge was removed struct a covered pedestrian bridge and transported to the grounds across the stream so his workers of the Village Department of could stay dry and clean. Hoxie Public Works, where it sat in built the bridge in his front yard, obscurity for the next couple of then disassembled it and hauled years. Village residents thought it by horse-drawn wagon to the the bridge was gone for good, site, where he reassembled it. and the National Society for The bridge is a relatively modest the Preservation of Covered structure, about 23-feet long by Bridges website simply stated about 6-feet wide, with three that the bridge was “GONE.” stringers and four heavy-timber Interesting Fact: The New York portals framed with traditional State Covered Bridge Society heavy-timber joinery. Still, Hoxie does not consider this bridge an incorporated several Victorian authentic covered bridge because details into the bridge, including it does not have trusses, even two sitting benches at midspan, though it is thought to be the where workers could stop and oldest surviving covered pedesenjoy the views and sounds of trian bridge in America. the stream below.

After the bridge was moved offBridge being set on new abutments with the help of donated crane services.

Although the Rice Seed site, the Community Partnership Company no longer exists, the former Rice Seed Company buildings collaborated with the Homefront Development Corporation, another are home to dozens of small companies, including professional offices, nonprofit organization that assists communities with revitalizaartists’ studios, and small-scale manufacturing. Many employees of tion projects, to apply to the New York State Office of Homes and these companies lived in the Village and walked across the bridge Community Renewal for a Main Street Grant to rehabilitate the daily as they went to and from work, and it was a common sight bridge. Unfortunately, the grant was denied, but the members of the to see residents relaxing on the sitting benches. Since its construc- Community Partnership never lost hope; they tried again in 2019 tion, the Green Bridge (as it is known locally) has been the defining and obtained a grant for $15,000. streetscape feature for the Village’s Main Street. It is even pictured The bridge remains a beloved piece of local history. A purposeful on postcards from the 1930s. effort was made to involve as many people as possible in its rehaThe bridge, however, was nearly lost about 10 years ago. The stone bilitation, and regular social media posts were critical to engaging walls lining the stream were in poor condition, and a portion of the the community. In short order, a team of volunteers was assembled,

Hoxie built the bridge in his front yard, then disassembled it and hauled it by horse-drawn wagon to the site, where he reassembled it.

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The bridge remains a beloved piece of local history. A purposeful effort was made to involve as many people as possible in its rehabilitation, and regular social media posts were critical to engaging the community.

Completed bridge decorated with Christmas lights.

including the Architect of Record (Jeff Goldstone of Goldstone Architecture) and the author, both of whom live in the Village.

Construction documents were prepared as three separate contracts – one for the new substructures, one for the rehabilitation of the bridge, and one for its return across the stream. A condition of the grant was that the bridge had to be open to the public by the end of 2021. It seems that $15,000 does not buy much in the construction world today, but it was hoped that at least the bridge could be relocated onto new abutments for the dollar value of the grant, even if the bridge was not rehabilitated due to lack of funds.

While the bridge itself is public property owned by the Village, the land, including the stone walls lining the stream, is privately owned by 17 Mile VARAK Park. Eventually, the owners of the Park want to donate the grounds adjacent to the bridge to the Village, but the current condition of the stone walls precludes that. Repairing the stone walls is not a realistic option, as this would be prohibitively expensive. Instead, new concrete abutments were designed to fit within the confines of the existing stone walls that line the stream, even though one of those walls is now partially collapsed. Plans are being prepared to return the Owl Kill to a natural meandering stream, and the siting of the new abutments relative to the existing walls was carefully considered to accommodate that.

A contract was awarded to Dave Clark Excavation in early 2021.

Clark said he wanted to “do something for the community” and agreed to complete all contracts for significantly less than the dollar value of the grant. Since the bridge was stored off-site, it could be rehabilitated without concern for impact to the stream, significantly reducing the time, effort, and cost to complete the work. Thankfully, the basic structure of the bridge was sound. The three floor beams were replaced about 25 years ago, but everything else was original.

Deteriorated wood framing, generally limited to the railings and roof

32 STRUCTURE magazine

boards, was replaced in kind, and the bridge was repainted. The most extensive single effort in the rehabilitation of the bridge was replacing the cedar shingles. The Community Partnership purchased new cedar shingles for $4,500, and they were finish-cut by members who then handed them over to the Contractor for installation.

While completing the repairs to the bridge itself, the Contractor prepared the site for the new abutments. Work within the stream was regulated by the New York State Department of Environmental Conservation, with stipulations on maintaining water clarity and protecting the fish in the stream. The Contractor precast both abutments off-site to limit the time and extent of work within the stream and install them with minimal effort. The bridge was raised by about one foot to increase the freeboard in anticipation of the stream channel being naturalized.

After four years in storage, the bridge was finally set on its new abutments in September, but something did not seem quite right – the bridge sagged by a few inches and it vibrated whenever anyone walked across it. The distance between the stone walls lining the stream was much shorter than the bridge’s overall length, meaning that the bridge was cantilevered over the stone walls by a couple of feet on each side. Probably for the first time in its 130-year history, the bridge was spanning its full length, and the stringers, while adequate to carry the dead and live loads, deflected noticeably. The Contractor temporarily jacked the bridge to as close to level as possible and installed engineered lumber beams alongside the existing stringers.

The bridge remained relatively level when the jacks were removed, and the vibrational characteristics were significantly improved.

In late 2021, the bridge was opened to the public, and it quickly became the place to be. A local history buff (who also directed the social media campaign) and the local chapter of the Knights of Columbus decorated the bridge with Christmas lights powered by an electrical

cord run through a window of a tenant of the Park. The history buff even brought students from the 4th-grade class to light the bridge in a touching ceremony. Almost overnight, the refurbished bridge became a media sensation – many were spotted photographing the bridge, families used the bridge as a background for Christmas photos, and at least one marriage proposal took place on the bridge. (She said yes.)

The bridge’s rightful repair and return is an example of what can be accomplished when a community rallies together for a good cause.

A word of caution, however. Community projects like these can take several years from conception to completion, especially when design and construction services are donated pro-bono and the project is small. In fact, talk of refurbishing the bridge began almost 10 years ago, and it took about eight years of start-and-stop before a grant was awarded. At times it seemed the Green Bridge would be lost forever. Instead, the little bridge was saved through perseverance, collaboration, and deliberate community engagement. Architectural, structural engineering, and landscape design services were donated.

The truck used to haul the bridge from the Village DPW yard and the crane used to set the bridge were also donated. The Contractor donated significant labor, clearly spending several times the value of the grant to complete the work. Even the Christmas lights and the electricity to power them were donated. The adage many hands make for light work certainly was true for this project.

It is the author’s conviction that all engineers should give back to their communities in some way. Indeed, the Code of Ethics of the American Society of Civil Engineers states that “engineers (shall) enhance the quality of life for humanity” (Ethical Responsibility, Section I.b) and (shall) “endeavor to be of service in civic affairs” (Ethical Responsibility, Section I.e). As this project demonstrates, just a little bit of engineering effort significantly enhanced the quality of life for many of the Village’s residents.■ Mark Kanonik is the National Director of Structural Engineering for EYP and is an adjunct at Rensselaer Polytechnic Institute (mkanonik@eypae.com).

Project Team Owner: Village of Cambridge, NY Structural Engineer: EYP, Inc., Albany, NY Architect: Goldstone Architecture, Bennington, VT Contractor: Dave Clark Excavation, Cambridge, NY Bridge in storage off-site at the Village DPW yard; note that many diagonal braces were added to stabilize the bridge during lifting and transportation.

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Structural and Non-Structural Masonry Workout for a

New Fieldhouse

By Pat Conway, AIA

Student Fieldhouse & Soccer Support Facility at the University of Wisconsin – La Crosse, under construction.

T

he University of Wisconsin – La Crosse (UWL) is constructing a new $49 million, 144,000 gross-square-foot Student Fieldhouse & Soccer Support Facility for their campus. Masonry was the obvious choice for interior infill walls within the structural steel frame and key loadbearing walls. But what was not so apparent was how the mason contractor and design team would work together to develop creative uses of prefabricated masonry lintels, lightweight CMU, and a seldom-used alternative engineered method to eliminate or minimize the number of control joints (Figure 1).

to support during installation, or they can also be prefabricated to eliminate the need for any temporary shoring (Figure 2).

At the UWL Fieldhouse, project engineer Chad Allen with Oneida Total Integrated Enterprises (OTIE) states, “From the start of the project, we wanted to use masonry lintels wherever possible, and we were able to do so for most of the main portion of the fieldhouse.

Masonry lintels offer a pure solution to spanning openings without introducing dissimilar materials. There were four larger arched openings in the CMU walls that needed to be precast. The precast concrete arches work well with a masonry wall for an easy-to-install and robust solution that we knew Masonry Lintels would not crack” (Figure 3, page 36 ).

It is rare that a masonry lintel cannot be used Regarding masonry lintels, project archito span openings in a CMU wall – even tect Kyle Schauf, with HSR Associates substantial openings. Products to conout of La Crosse, gave this insight, Perhaps the most struct masonry lintels are typically local, “Over the years, we have learned from innovative masonry on this available, and less costly than structhe International Masonry Institute’s tural steel lintels. In many scenarios, (IMI) local Structural Masonry project was the use of the steel lintels may have to be delivered Coalition design services that conlong distances and be a long-lead item tractors prefer masonry lintels from alternative engineered method that can negatively impact project a constructability perspective. We like sequencing and schedules. For added masonry lintels because it allows us to for controlling masonry advantages, masonry lintels move at develop architectural details to minithe same rate as surrounding masonry mize thermal transfer at window heads movement and walls, eliminating potential cracking instead of having a structural steel beam due to differential movement that occurs eliminating cracking. bottom plate spanning from the inside with steel lintels. More than ever, modern of the building to the edge of the veneer” masonry lintel installation techniques are easy (Figure 4, page 36 ).

34 STRUCTURE magazine

Lightweight CMU Including the brick and cast stone veneer, the project’s masonry contract was nearly $4 million. There were over 144,000 CMU on this project; 33,000 were 16-inch CMU units. After being awarded the job, the contractor, Market & Johnson, worked with the design team to approve lightweight CMU for the entire project.

Kevin Fabry, Masonry Project Manager for the Market & Johnson La Crosse office, says, “We prefer installing lightweight CMU because ultimately it is good for our masonry crew’s longevity and reduces chances for injuries and fatigue. For example, using a lightweight 16-inch CMU, one Figure 1. Model image from masonry contractor’s Tradesman 5.0N4 3D estimating software. mason lifts approximately 1,000 pounds less per day!” Kevin continues, “Even though lightweight units cost a little more per unit, we see approximately a 10% installa- off-the-shelf masonry units were used on this project without a cost tion increase and much better attitudes on the job site. When masons premium, other than the upcharge for lightweight aggregate specific are happy, you always get a better product. Additionally, lightweight to this block supplier.

CMU is easier to transport on-site. They chip less than normal-weight units, and using lightweight allows us to have lower mason-to-tender Alternative Engineered Method ratios, less rubbing, and less saw time and blades.”

Chad Allen (OTIE) states, “Obviously, during the design phase, we Perhaps the most innovative masonry on this project was the use of do not know what system/preferences the contractor prefers, so we the alternative engineered method for controlling masonry movedefault to normal weight products. Structurally, my concern was the ment and eliminating cracking. On one wall that is 201.25 feet block compressive strength and its impact on the masonry compres- long by 61 feet tall at its gabled peak, the contractor, structural sive strength. If the contractor could find a block that produced the engineer, and IMI worked together to implement this movement required specified compressive strength of masonry (f´m), we would control strategy to eliminate control joints. Because the wall had be good to go with the lightweight block.” so many misaligned openings, accommodating CMU movement For efficient use of reinforcement bars and grout, the engineer for with standard control joint placement rules-of-thumb would have this project used the Unit Strength method to verify f´m compliance. been challenging – both structurally and from a constructability The design value for masonry on this project was set at an f´m of 2,250 perspective (Figure 5, page 37). psi. Block strengths had to have a net compressive strength of at least The engineer referenced the National Concrete Masonry 2,600 psi and Type S mortar to achieve the f´m value. Locally available Association’s (NCMA) TEK 10-03, Control Joints for Concrete

Figure 2. Robust shoring for continuous 7-course masonry lintel at large openings. This wall supports the roof (and drift load) on the opposite side of the wall.

Other openings shored with a simple Z-clip and lumber technique.

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Figure 3. Precast concrete arch being installed on 16-inch CMU wall.

Masonry Walls – Alternative Engineered Method, to better understand this unique movement control strategy. With the installation of mid-wall bond beams at 48 inches on-center and the extension of masonry lintels the entire length of the wall, the structural engineer had enough horizontal steel in the wall to restrain masonry movement. This allowed the contractor to eliminate control joints!

The contractor used self-consolidating grout to ensure proper grout flow with all the added grout and rebar in the wall. The contractor applauded this decision because it increased CMU installation production and decreased complicated and abundant temporary wall bracing, which aided site movement logistics.

Structural engineer Chad Allen (OTIE) agreed, “For this particular wall, the engineering approach was actually much easier for everyone than figuring out where to locate vertical joints, how to communicate them on a plan, and then hope they were installed correctly. Because this wall functions as the transition between two building geometries/ masses, a lot was happening here, and the engineering approach simplified everything.”

Sixteen-inch CMU infill walls were used between the large steel columns that support steel roof trusses over 200 feet long. Masonry lintels spanning 13.25-foot-wide windows centered in each structural bay were extended to the full length of the bay. These lintels were prefabricated on-site at two-course height. Then another two courses were added to them after being placed on the wall. The block is not tied to the steel columns at the edges of the masonry infill walls but is connected at the roof diaphragm. This proved to be a critical design decision because the steel frame moved a remarkable 1½ to 2 inches during construction, which would have potentially cracked the masonry wall.

The engineer explains, “We had to look at the outward deflection of the steel trusses and compare that to the allowable drift of the CMU wall and find a balancing point where everything worked together. Internal masonry piers adjacent to the steel columns were designed as the boundary elements for the in-plane and outof-plane lateral loads. Additional piers at the window jambs were designed to carry the out-of-plane Figure 4. Thermally-efficient window head detail showing long masonry lintel and loose lintel in veneer. lateral loads.”

36 STRUCTURE magazine

Summary The use of masonry for interior walls and backup walls for a masonry exterior veneer proved to be an excellent choice for durability, speed, and cost benefits.

Masonry offered the designers a robust solution and allowed the contractor to complete a building during a challenging construction climate with material delivery Figure 5. Diagram of long wall showing mid-wall bond beams and extended masonry lintels to restrain CMU movement in and price issues for non-masonry lieu of accommodating movement with control joints. Courtesy of ForSE Consulting. products. On your next CMU project, challenge yourself to use higher f´m design values reflecting the typical strength of locally available masonry units for more economical and environmentally-friendly use of masonry. Of course, you can always special-order higher strength blocks for special conditions.

Then consider using lightweight concrete masonry units and masonry lintels to keep masonry projects quick to install, mason-friendly, and low-cost. Lastly, take the time to listen to your mason contractor’s suggestions for creative installation techniques that may benefit the whole project.■ Pat Conway is a registered architect in Wisconsin, a member of the American Institute of Architects (AIA) and Construction Specification Institute (CSI), Director of Industry Development and Technical Services for the International Masonry Institute (IMI) and distinguished masonry speaker and author.

Project Team Owner: State of Wisconsin Engineer: Oneida Total Integrated Enterprises (OTIE), Milwaukee, WI Architect: HSR Associates, La Crosse, WI General Contractor and Masons: Market & Johnson, Inc., Eau Claire, WI

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37

Twice Repurposed R The (Stephen) Long Covered Bridge, from Brownsville to Columbus (Indiana) B Y T HOMAS L EECH , P.E., S.E.

The Truss of the 1840 Long “Wagon” Covered Bridge over the East Fork of the Whitewater River, Brownsville, Indiana. The only remaining Long Truss Bridge in Indiana. Dismantled and relocated to Indianapolis, Indiana, in 1974. Courtesy of the Library of Congress.

T

he year is 1840, and the townsfolk of Brownsville, Indiana,

were dismayed that the newly constructed National Road was crossing the East Fork of the Whitewater River, just a few miles north of town. They engaged Stephen Long to construct a bridge for the town, and his patented Long Truss soon spanned the river…

The year is 1993, and the same bridge now lies in Columbus, Indiana – with many miles in between at the end of an interesting journey.

The Journey – from Brownsville to Indianapolis to Columbus.

38 STRUCTURE magazine

Long’s system of trusses was similar to that of Palladio [Renaissance scientist and engineer of early truss bridge designs], but he [Long] evidently knew a valid method of calculating stresses in truss members and, in his work, gave very reasonable proportions for all the members of structures of various spans.”

Stephen Timoshenko

First Stop In 1830, the National Road construction was alive and active in Indiana, with construction marching onward to the Mississippi River (although the road never made it that far). Brownsville, Indiana, townsfolk were “agitated” that the National Road had just constructed a bridge across the East Fork of the Whitewater River, two miles upstream. Fearing a loss of commerce, the town approached agents of Stephen Long, who was looking to promote his newly patented truss design. The design was “state of the art” for the 1830s. Chord member sizes were based on mathematical calculations, and diagonals were “wedged,” introducing an elementary form of pre-stressing. Both design insights were landmark innovations for the day. The bridge was successfully completed in 1840 and became locally known as the Wagon Bridge, and provided an immediate economic boom to the townsfolk of Brownsville.

Second Stop – 75 Miles West After careful dismantling, the (Stephen) Long Covered (Wagon) Bridge members were transported to Indianapolis, Indiana, and

placed in storage for its intended destination, Eagle Creek Park – the largest recreational park in Indianapolis, featuring more than 1,400 acres of water and 3,900 acres of forest, and offering a wide variety of activities such as fishing, boating, hiking, and, at one time, a (planned) covered bridge enjoyment. An iconic painting of the Wagon Bridge was displayed in the park office for many years. Originally intended as a signature element in the park from the earliest planning stages in the 1970s, the Wagon Bridge was moved from Brownsville to Indianapolis. Ultimately, it was never rebuilt and laid in storage for many years before being relocated to Columbus, Indiana.

Third and Final Stop – 40 Miles South After approximately twenty years in storage, the (Stephen) Long Covered (Wagon) Bridge made its final stop of a long, two-legged journey as it arrived in Columbus, Indiana.

An area once known as “Death Valley,” a flood-prone spit of land adjacent to the oxbow of the Driftwood River, underwent a slow transformation starting in the 1960s. In over 30 years, the former industrial area along the western edge of Columbus, Indiana, was

The (Stephen) Long Covered (Wagon) Bridge. Relocated in 1993 to Mill Race Park, Columbus, Indiana.

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This small country town [Columbus, Indiana] of 30,000 in southeastern Indiana calls itself “The Athens of the Prairie” – and with good reason. Seldom, if ever, has so small a community contained so many examples of innovative architectural achievements… with accomplishments of men such as I. M. Pei, designer of the John Fitzgerald Kennedy Library in Cambridge, Massachusetts; Robert Trent Jones, known the world over for his golf courses; Eliel Saarinen, co-designer of the national museum in Helsinki and many others…”

New York Times, March 22, 1970 transformed into an 85-acre riverfront park, appropriately named Mill Race Park. The transformed setting is now recognized as one of the top 100 parks in the nation for design, reputation, and accessibility. As a testament to the collaborative spirit of design with many contributions from the community, the park has evolved into a community destination Longs Peak is a prominent mountain featuring an 84-foot observation in the northern Front Range, located tower, people [walking] trails, in Rocky Mountain National Park picnic shelters, playground Wilderness, near Estes Park, Colorado. equipment, horseshoe pits, basThe mountain was named in honor ketball courts, an amphitheater, of (once topographer) Stephen Long the creation of two lakes and… and is featured on the Colorado state the relocation of the now [2022] quarter. Courtesy of Wiki Commons.

182-year-old Long Covered (Wagon) Bridge… completing its journey from Brownsville, Indiana, temporarily to Indianapolis, and then to its present location.

Biography – Stephen (Harriman) Long (1784-1864) A remarkable engineer with a remarkable career. A snapshot of his wide range of engineering practice and accomplishments follows.

• Graduated from Dartmouth College in 1809.

Courtesy of the Library of Congress

40 STRUCTURE magazine

• Upon graduation, taught mathematics in New Hampshire and Pennsylvania.

• Joined the Corps of Engineers as a second lieutenant in 1814 and served as assistant professor of mathematics at West Point.

• Became a Major (in 1816) and was assigned the responsibility of Topographical Engineer, where he led five expeditions (1817-1823) through the Upper Mississippi Valley and the borderlands with Canada. His explorations included the upper Mississippi River and its tributaries, Missouri, Platte, and South Platte Rivers, the eastern range of the Rocky Mountains in Colorado, and a considerable part of the Arkansas River basin.

• Assigned by the Department of War (in the late 1820s) to serve as consulting engineer for the Baltimore and Ohio Railroad Company, where he came to be regarded as an expert in railroad engineering.

• Became the chief engineer for the Atlantic and Great Western Railroad.

• Served as a consulting engineer for a number of other railroad companies.

• Received (in 1829) his first (of many) patents for his work on railroad steam locomotives.

• Authored a publication entitled the Rail Road Manual (1829), which presented tables that eliminated the need for field computations.

• Published (in 1830) a booklet entitled Description of the Jackson Bridge [near Baltimore, Maryland] Together with Directions to Builders of Wood or Frame Bridges (which was the basis for his 1830 patent).

Courtesy of the Library of Congress

• Received patents in 1830 and 1839 for pre-stressing the wooden diagonal trussed members in covered bridges, leading to many covered bridge designs in New England and the mid-west.

• Unsuccessfully, claimed patent infringement when Howe patented his truss concept. (The Howe Truss patent was based on post-tensioning vertical rods, alternatively, accomplishing and improving on the goals of Long’s patent.)

The 1830 Patent of the Long Truss The 1830 patent of Stephen Long was remarkable in two features.

1) The patent provided a series of tables that rationally proportioned the upper and lower chords based on engineering mechanics principles, much as one would compute the stresses in the upper and lower flanges of a wide flange beam. This is one of the earliest examples of a rational approach in an era where handed-down craftsmanship principles were the norm.

2) The diagonals were of simple fabrication with joggle joints for ease of installation. However, the most important feature was the use of wedges, which were driven along the joggle lines of the lower end of the diagonals. During driving, the wedges induced compressive stresses in the diagonals – an early form of pre-stressing. Without a clear understanding of force flow in an internally indeterminate system, the wedge-induced pre-stressing, in a broad sense, recognized that live loading conditions would introduce tensile stresses in certain diagonals that would otherwise loosen unstressed diagonal members at their joints unless a conscious strategy was employed to render the “...truss frames … stiff and unyielding.”■ We are excited to see a lessening of the pandemics’ s impact on our authors and advertisers and expect to have more normal page counts in the future. As a result, we have taken the opportunity to showcase the work that other organizations do in supporting S.E.s. It is a pleasure to have these organizations add to STRUCTURE’s knowledge base. If your organization would like to submit an article proposal, please contact Chair@STRUCTUREmag.org.

This article is courtesy of the Engineers’ Society of Western Pennsylvania, initially published in the Pittsburgh Engineer in June 2021. It is reprinted with permission.

Thomas Leech is the retired Chief Engineer of Bridges and Structures for Gannett Fleming, Inc. Currently, he is an instructor of Civil Engineering and Environmental Engineering at Carnegie Mellon University, Pittsburgh, Pennsylvania. He is also an author of several books. His book, co-authored with Linda Kaplan, entitled Bridges … Pittsburgh at the Point … A Journey Through History, is available from Word Association Publishers, Inc., Tarentum, Pennsylvania.

“…The objects aimed at in this invention are, greater simplicity of and economy in the construction of bridges… a system of counter bracing, by means of which the truss frames are rendered stiff and unyielding…” Stephen Long Basis for the 1830 Patent, University of Michigan Files https://bit.ly/37nPD62 M AY 2 0 2 2

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structural DESIGN Special Steel Reinforced Concrete Structural Walls Part 2: Significant Changes to Design and Detailing Requirements

By David A. Fanella, Ph.D., S.E., P.E., F.ACI, F.ASCE, F.SEI

S

ignificant changes were made to the design and detailing requirements for special steel-reinforced concrete structural walls in the 2019 edition of Building Code Requirements for Structural Concrete (ACI 318-19) (hereafter referred to as ACI 318).

Part 1 of this article (STRUCTURE, April 2022) covered the following changes: • The introduction of Grade 80 and Grade 100 deformed bars to resist the effects of flexure, axial force, a combination of axial force and flexure, and shear.

• New requirements for longitudinal bar termination and splice locations.

• New requirements for minimum area of boundary longitudinal reinforcement for slender walls.

Part 2 provides information on the following revisions: • Increase of the design shear force for slender special structural walls.

• New requirements for expected wall deformation capacity and expected wall drift demand.

• Revised details for transverse reinforcement within boundary elements and the wall web.

• New maximum vertical spacing requirements of transverse reinforcement at wall boundaries.

Design Shear Force Prior to the 2019 edition of ACI 318, shear strength requirements for special structural walls were based on the maximum factored shear force, Vu, obtained from analysis of the structure using the prescribed earthquake loads in ASCE/SEI 7 Minimum Design Loads and Associated

Criteria for Buildings and Other Structures. These shear requirements are inconsistent with those for beams, columns, and joints in special moment frames, which are based on capacity design. Also, due to actual earthquake ground motion, shear forces can be considerably larger than those obtained using ASCE/SEI 7 loads. Thus, the shear design requirements for special structural walls in ACI 318 before 2019 do not ensure ductile flexural yielding occurs prior to shear failure at the critical section.

New shear design requirements were introduced in the 2019 edition of ACI 318 to address these issues. The design shear force, Ve, for special structural walls must be calculated by ACI 318 Eq. (18.10.3.1): Ve = ΩvωvVu ≤ 3Vu In this equation, Ωv is the overstrength factor, which is equal to the greater of Mpr /Mu and 1.5 for walls with a height-to-length ratio hwcs /lw > 1.5. The term hwcs is the height of the entire structural wall above the critical section. For walls with hwcs /lw ≤ 1.5, Ωv = 1.0. The minimum Ωv = 1.5 is based on Mpr = 1.25Mn = 1.25Mu /0.9 = 1.4Mu. A value of 1.5 was selected because the amount of longitudinal reinforcement provided in a structural wall, which has a direct impact on Mn, is typically greater than the required amount.

The probable flexural strength, Mpr, is determined using the properties of the wall at the critical section, assuming the stress in the longitudinal bars is equal to 1.25fy and the strength reduction factor, φ, is equal to 1.0. The factored bending moment at the critical section, Mu, is obtained from analysis of the structure using the ASCE/SEI 7 prescribed loads. Because Mpr and Mn depend on the factored axial force at the critical section, the condition producing the largest value of Ωv must be used to determine Ve. continued on page 44

Also, due to actual earthquake ground motion, shear forces can be considerably larger than those obtained using ASCE/SEI 7 loads.

42 STRUCTURE magazine

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Includes full digital interactive access to the ACI Detailing Manual and the ACI Reinforced Concrete Design Handbook, subscribers can make digital notes alongside ACI CODE-318-19 provisions and commentary, and navigate content by section, by chapter, and/or by provision.

For access and to subscribe, visit www.concrete.org/ACI318.

Figure 3. Detailing requirements for special structural walls with rectilinear hoops and crossties (See table on page 45 for details).

The dynamic amplification factor, ωv, is determined by ACI 318 Eq. (18.10.3.1.3) for walls with hwcs/lw ≥ 2.0: ns 0.9 + 10 for ns ≤ 6 ωv = n 1.3 + 30s ≤ 1.8 for ns > 6

{

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In this equation, ns is the number of stories above the critical section, which must not be taken less than 0.007hwcs (this limit is imposed on ns to account for buildings with large story heights). For walls with hwcs/lw < 2.0), ωv = 1.0.

The dynamic amplification factor accounts for the response of a multistory building with changing patterns of lateral inertial forces (mode shapes). Accounting for higher modes can shift the centroid of the lateral forces downward, thereby increasing the shear force at the critical section.

Where Ve is calculated by ACI 318 Eq. (18.10.3.1), it is permitted to take φ = 0.75 when determining the design shear strength, φVn, of a slender special structural wall. In addition to the overstrength and dynamic amplification factors, the redundancy factor, ρ, must be included in the determination of Ve where required by ASCE/SEI 7 Section 12.3.4.2.

It is evident that Ve can be significantly greater than Vu (up to a factor of 3 times), which will most likely require thicker walls and/or more transverse reinforcement. The SEAOC Seismology Committee’s SEAOC Blue Book: Seismic Design Recommendations has contained a similar method for determining Ve for many years.

Drift Capacity Check New drift capacity requirements must be satisfied for slender special structural walls (hwcs /lw ≥ 2.0) where special boundary elements are required in accordance with ACI 318 Sect.

18.10.6.2. Either of the following drift capacity checks must be satisfied in such cases:

[

( )( )

]

δc 1 4 − 1 lw c − Ve = ≥ 1.5δu hwcs 100 50 b b 8√f´c Acv hwcs

(Eqn. 1)

b ≥ √0.025clw (Eqn. 2) In these equations, δc = wall displacement capacity at the top of the wall; b = width of the compression face of the wall; c = largest neutral axis depth calculated for the factored axial force and bending moment consistent with the direction of the design displacement, δu, at the top of the wall; and Acv = gross area of the wall web minus the area of any openings.

Figure 3. Detailing requirements. Note: Horizontal web reinforcement consisting of deformed bars with standard hooks or headed deformed bars must be anchored to develop fy within the confined core of the boundary element.

Length of boundary element, lbe

lbe ≥ greater of

{

hu/16 where hu = unsupported height of wall compression fiber b ≥ greater of √0.025clw where ACI 18.10.6.2 is used 12” where hw/lw ≥ 2.0 and c/lw ≥ 3/8

Width of flexural compression zone, b Horizontal spacing, hx

14” hx ≤ lesser of 2b/3┤

Vertical spacing, s

Transverse

s ≥ lesser of

reinforcement –

lesser lesser lesser lesser

of of of of

b/3 and lbe/3 6db of the smallest long.bar and 6” for Grade 60 bars 5db of the smallest long.bar and 6” for Grade 80 bars 4db of the smallest long.bar and 6” for Grade 100 bars 14 − hx ≤ 6” 4”┤≤ so = 4 + 3

(

boundary element

c − 0.1lw c/2

)

lbeb f’ −1 c fyt bc1bc2 0.09sbc2 f ’c fyt

0.3sbc2

Area, Ash

Ash = greater of

Hoop overlap, sover Crossties

Transverse reinforcement – web

sover ≥ lesser of

6” 2b/3

At least #3 bars enclosing #10 and smaller longitudinal bars At least #4 bars enclosing #11 and larger longitudinal bars Vertical spacing s ≤ 12”┤ Bar size: At least #3 bars enclosing #10 and smaller longitudinal bars At least #4 bars enclosing #11 and larger longitudinal bars

The term δc /hwcs represents the drift capacity, which need not be taken less than 0.015, and (1.5δu /hwcs ) represents the drift demand. Satisfying this drift capacity check results in a low probability of strength loss for the design earthquake. For walls without flanges, b is equal to the thickness of the wall. If b varies over c, like for walls with flanges, an average value of b should be used considering the effective flange width defined in ACI 318 Section 18.10.5.2.

The second equation is derived from the first equation, assuming Ve /8√f´c Acv = 1.0 and δu/hwcs = 0.015.

Detailing Requirements Many new and revised detailing requirements have been introduced in the 2019 edition of ACI 318 for special structural walls. These requirements, found in ACI 318 Sections 18.10.6.4 and 18.10.6.5, are based primarily on observations from recent earthquakes and laboratory tests. The main revisions are: • Transverse reinforcement spacing limits were revised in special boundary elements, including new maximum vertical spacing limits for Grade 80 and Grade 100 transverse reinforcement within and outside of the region of anticipated yielding.

• A new limit is placed on the length of a hoop leg relative to the thickness of the confined core within a special boundary element. Where this limit cannot be satisfied, adjacent overlapping hoops must be used with an overlap length of at least the lesser of 6 inches and two-thirds of the boundary element thickness.

• Web vertical reinforcement must have lateral support provided by the corner of a hoop or by a crosstie with seismic hooks at each end for a distance above and below the critical section of at least the greater of lw and Mu/4Vu (that is, within the region of anticipated yielding).

These revisions, along with the other detailing requirements for special structural walls, are illustrated in Figure 3. Hooked and headed transverse reinforcing bars can be utilized in any special structural wall and not just in the configurations indicated in the figure.

More in-depth information on the changes outlined in this article, including design aids and worked-out examples, can be found in the Concrete Reinforcing Steel Institute’s (CRSI) Design Guide on the ACI 318 Building Code Requirements for Structural Concrete. Also available is the CRSI Design Checklist for Special Steel Reinforced Concrete Structural Walls, which contains an easy-to-use list of essential items that must be completed when designing and detailing special structural walls. Visit www.crsi.org for more information on these and other CRSI resources.■ References are included in the PDF version of the online article at STRUCTUREmag.org.

David A. Fanella is Senior Director of Engineering at the Concrete Reinforcing Steel Institute (dfanella@crsi.org).

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engineer's NOTEBOOK Identifying Cold-Formed Steel Material Thickness on the Job Site

By Tony Coviello, M.S., P.E., S.E.

C

an you identify cold-formed steel thickness just by the

feel of the stud? If you cannot, you are not alone. There are many ways to easily identify CFS thicknesses, including using materials you just might have in your pocket.

Learn How to Read the Printed Code It is required for metal stud manufacturers to stamp the stud size and gauge on each product. If you are not sure how to read the printed code, here is an example from Clark Dietrich, a cold-formed steel manufacturer: 6-inch, 16-gauge studs are labeled CD600S162-54 50 KSI (Figure 1).

This stamp example means the stud has the following dimensions: • 6 inches wide • 1.62-inch flange • 16-gauge (54 mils) thick • Steel strength is 50 KSI

Note: A mil is equal to one-thousandth of an inch or 0.001 inch.

This measurement is a typical manufacturing dimension used to specify the thickness of a product.

Consider the Colors The ends of steel studs and tracks are painted with color markings to indicate their thickness, which means you can keep a color guide handy for a quick reference (Figure 2). Here is the rundown: • Blue – 118 mils (10-gauge) • Red – 97 mils (12-gauge) • Orange – 68 mils (14-gauge) • Green – 54 mils (16-gauge) • Yellow – 43 mils (18-gauge) • White – 33 mils (20-gauge) • Pink – 30 mils (20-gauge; interior only) • Black – 27 mils (22-gauge; interior only) You may be wondering why two studs would be different for the same thickness, as demonstrated by the white and pink color markings. This is because interior framing often has less galvanizing than exterior members. Therefore, interior-grade members should not be used in exterior walls.

Use a Micrometer to Measure Material Thickness

Figure 1. Example. Courtesy of Clark Dietrich.

46 STRUCTURE magazine

If you cannot see any identifying marks, use a micrometer, a tool designed to make exact measurements within one-thousandth of an inch. Even though there are inside and outside micrometers available, it is recommended that you choose a C-shaped outside micrometer, which is easier to use with the rounded, sheared edges of cold-formed steel. Also, try measuring the knockout in the stud web rather than at the rounded edges at the lip.

Color

Part Number

Mil

Blue

10-ga

118

Red

12-ga

97

Orange

14-ga

68

Green

16-ga

54

Yellow

18-ga

43

White

20-ga

33

Pink

20-ga (interior only)

30

Black

22-ga (interior only)

27

Figure 2. Color guide for metal studs.

Figure 3. Common thickness reference materials.

Consult Online Gauge Guides

Table of thickness references by shape.

There are several metal thickness instrument gauge guides available online. It is important to point out that most are designed to measure sheet metal thicknesses and not cold-formed structural steel, but they can still be helpful. According to BuildSteel, “since a wide variety of CFS member profiles, depths, widths, and material thicknesses exist, the steel framing manufacturing industry developed a standard designator system that all CFS manufacturers and suppliers use. It is important to note that this universal designator system is used in identifying cold-formed steel framing in building codes as well.”

You can probably obtain enough information from these guides to make a relatively accurate estimate of cold-formed steel thickness.

Reference Material

Use Materials You Have on Hand – Or in Your Pocket!

If you are stuck, just reach into your pocket for some handy commonthickness reference materials (Figure 3).

• A quarter = 0.069” thick (approx. 14-gauge) • A dime = 0.053” thick (approx. 16-gauge) • Dollar bill = 0.0043” thick. Fold it to get 10 plies, and you have 0.043” (approx. 18-gauge) It is also helpful to know the most common cold-formed metal framing sizes and shapes. You do not have to memorize this list, but the Table shows a quick overview.

At the end of the day, learning how to read the printed code is one of your best options. But the good news is that there is more than one way to identify cold-formed steel thicknesses while you are in the field, especially if there are no visible marks on the studs.■ Tony Coviello is the owner and chief structural engineer of Iron Engineering, a structural engineering firm servicing the East Coast of the United States.

Tony’s expertise is with load-bearing and non-load bearing framing, interior framing, blast design, and multi-story panels (tony@iron-eng.com).

Studs

Track

Common Sizes 14”, 12”, 10”, 8”, 6” and 35 ⁄8” wide

14”, 12”, 10”, 8”, 6” and 35 ⁄8” wide

U-channel

¾”, 1½”, and 2”

Furring channel

7

⁄8” and 1½”

Available in a wide variety of

L-header

dimensions, typically custom bent.

Straps

2” to 12” wide

M AY 2 0 2 2

47

historic STRUCTURES Tampa Bay (Sunshine Skyway) Bridge Disaster

By Frank Griggs, Jr., Dist. M.ASCE, D.Eng, P.E., P.L.S.

T

he first bridge across Tampa Bay connecting Rubonia, FL, with St. Petersburg, FL, opened on September 6, 1954. The main structure was a steel cantilever span with a length of 1,584 feet built by the Virginia Bridge & Iron Company. It replaced a ferry from Point Pinellas on the southerly side of St. Petersburg to Piney Point just north of Rubonia. There were two flanking deck truss spans of 250 feet 3 inches on each side, followed by low-level deck spans. A second parallel span of the same dimensions was built just to the west in 1969 but did not open until 1971 due to foundation problems. This provided two lanes in each direction, and the road became I-275, with the old bridge handling northbound traffic and the newer bridge handling southbound traffic. The total length of the central bridge was approximately 4.4 miles, and the total length of the entire project was 14 miles.

The main shipping lane from the Gulf of Mexico to the bay was called the Mullet Key Channel. It was dredged to a depth of about 35 feet and passed under the main span of the cantilever section of the Tampa Bay bridge. The central span consisting of two cantilever arms and a suspended span, had a center-to-center length of 864 feet, and provided a horizontal clearance of 800 feet and a vertical clearance of 155 feet. The two anchor arms were 360 feet long, and the cantilever arms 252 feet. The suspended span was 360 feet in length.

The two main shipping lane piers had fender protection. The two anchor arm piers had no protection, as they were outside the shipping channel. The anchor piers, consisting of two concrete columns mounted on a wide concrete base (plinth), were smaller than the two main towers as they carried a compressive load from the flanking span and a tension load from the cantilever. They were also shorter as they only came up to the lower chord of the flanking trusses.

Weather in the area was frequently volatile and, during the construction of the bridges, there were many delays. Vessels were normally not moved in dense fog and/or during strong northwest winds. Pilots were usually boarded at Egmont Key, a small island to the west of the bridge, to guide the ships into the bay. In January 1972, August 1973, May 1977, and February 1980, there were collisions of ships with the fenders of the main piers. In addition, two ships collided in the channel on January 28, 1980, near the bridge.

After these collisions, repairs were made to the fender system, but no significant upgrades were made to the structure.

On May 9, 1980, this was the situation when the Summit Venture, a Liberian registered phosphate carrier ship, running empty, was steaming towards Tampa Bay. The ship, built in 1976 in

Twin cantilever spans of the Sunshine Skyway.

Nagasaki, Japan, was 580 feet long and 86 feet wide with a displacement of 19,734 tons. Captain John Lerro boarded the ship at Egmont Key around 6:30 A.M., then checked the weather and channel traffic.

At the time, there was only a slight mist in the air. He had been told another ship would be leaving the port at about the time he would be entering, so he had to be careful to avoid a collision. Lerro, who had guided over 800 ships into the bay, proceeded to guide it to the entrance to Tampa Bay. The trip’s difficulty was compounded by the shallow depth of the bay outside the shipping channel and frequently unpredictable weather. He soon had to deal with poor visibility due to fog and rain. It turned out that the other ship anchored short of the bridge to ride out the storm. What happened next can be seen in the illustration of the ship’s path.

First, a severe thunderstorm blew in from the northwest, pushing the ship, which was riding high in its unloaded condition, outside the shipping channel. Lerro sent two lookouts to the bow of the bridge when the weather turned worse. He was to make an 18° turn to the left between buoys 1A and 2A, shown in the graphic. An unpredicted strong wind came up out of the north-northwest and blew the ship farther outside the channel. In addition, the ship’s radar failed.

Lerro later testified, “It was heavy, heavy, heavy rain. The radar screen was a sheet of water. It turned yellow. You could see nothing.” He could not see the bridge until a small break in the weather

It was heavy, heavy,

heavy rain. The radar screen was a sheet of water.

48 STRUCTURE magazine

caused it to come into view. He tried to stop the ship by ordering a dropping of the anchor and the reversing of its engines, but everything he attempted could not slow the ship fast enough. Yet he was moving slow enough that he could not steer the ship away from the pier. The ship hit the main pier with a glancing blow causing no damage to the bridge, but then hit the anchor span pier head-on, causing its collapse at 7:33 in the morning. When that pier failed, the weight of the cantilever caused the anchor span to lift, overloading it until it separated from the far cantilever arm, and the anchor span, cantilever arm, and suspended span dropped into the bay with a portion of the bridge landing on the bow of the ship.

On the bridge, drivers were also driving through a heavy rain shower, cutting down visibility. At the time, six cars, one truck, and a Greyhound Bus carrying 23 people fell into the bay, killing 35 people. One man in a pickup truck was the only survivor as his truck fell on the bow of the ship before falling into the bay, where the ship’s crew picked him up.

As was usual, the National Transportation Board made a study of the collapse. In addition, the Pilot Commissioners found that Lerro had “acted reasonably…and his only choice was navigating blindly through the existing weather.” Lerro was called to testify by many groups and lawyers. At one of these sessions, he was quoted as follows, “…between buoys 14 and 16, the rain was intermittently heavier and lighter with the wind blowing from the southwest. Radar was still clear, and, at buoy 16, I saw buoys 1A and 2A, signaling the turn to go through the two main spans of the Skyway. When I got two-tenths of a mile in front of 1A and 2A, a storm hit me, hit the ship, hit us.

I couldn’t see the bow of the ship…the rain was swirling overhead, and the wind was gusting. When I first saw the bridge, it was on my starboard bow at a 45-degree angle dead ahead…I knew immediately you’re not supposed to see that at a 45-degree angle. I ordered full stern, the rudder hard left and dropped anchor, but it was too late.

I looked up to see the right side of the ship glance off the pier. It crumbled. It looked like a bunch of cornflakes crumbling down.”

The NTSB wrote in part, “Theoretically, a cantilever bridge structure remains stable by a system of balanced weights. The weight of the anchor arm spans balance the weight of the cantilever arm span and the suspended span, with the

main channel piers acting as fulcrums and main supports. The anchor piers perform the dual functions of providing support for the anchor arm span and the steel deck truss span and of maintaining the stability of the structure’s balance. Because of these major functions of support and balance, the anchor piers are critical elements of the structure.

The mass and design of bridge piers and pier protection systems and the configuration, weight, and speed of vessels have a direct effect on the damage which may result from a collision. The bulwark and the forecastle of the SUMMIT VENTURE struck the pier column before the lower bow struck the pier crash wall. If the pier crash wall had been larger or a pier protection system had been installed at that location, the initial impact would have occurred near the waterline. Because the pier crash wall is anchored through the pier footer directly into the bay bottom and is larger and stronger than the columns, it is possible that sufficient energy might have been absorbed to reduce the vessel’s forward motion and perhaps redirect the vessel before the bulwark and forecastle struck the column.

While the pier still could have been damaged, only the vessel’s mast would have struck the bridge span if the vessel had been redirected to starboard. The vessel could have passed under the bridge span if it had been redirected to port, and the damage to the bridge span might have been minimized…

Bridge owners should consider protecting existing vulnerable bridges and take particular care in pier placement in future bridge construction. The FHWA should examine this issue carefully in its review process for bridges built with Federal-aid funds.

Final resting positions, vehicle damage patterns, and witnesses’ statements indicated that the Courier pickup truck was the southernmost involved highway vehicle, and all traffic ahead of that vehicle crossed the bridge safely. The Courier pickup truck, the El Camino, and the Scirocco were definitely on the collapsed section of the bridge. The remaining five vehicles were driven off the downward-sloped bridge section and fell into the water after the bridge section had collapsed.

Those five vehicles carried 32 persons…Since the bus was resting over the Fairmont and the Nova, it must have followed them off the bridge…

The bus and four sedans ran off the bridge substantially after the collapse. The time available was more than sufficient to allow the

Path of the ship. Courtesy St. Petersburg Times.

M AY 2 0 2 2

49

The mass and design of bridge piers and pier protection systems and the configuration, weight, and speed of vessels have a direct effect on the damage which may result from a collision.

Ship and bridge after the collapse; rammed pier center-right.

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drivers to stop safely, but they were not aware of the bridge condition ahead. If a bridge span failure detection and warning system had been installed and activated, it might have alerted the drivers of those vehicles of the danger ahead, and many lives might have been saved.

Therefore, the National Transportation Safety Board recommends that the Federal Highway Administration: • Develop standards for the design, performance, and installation of bridge span failure detection and warning systems.

• Establish criteria to evaluate the need for installing bridge span failure detection and warning systems on existing and proposed bridges.

• In cooperation with the U.S. Coast Guard, develop standards for the design, performance, and location of structural bridge pier protection systems, which consider that the impact from an off-course vessel can occur significantly above as well as below the water surface.

• In cooperation with the U.S. Coast Guard, conduct a study to determine which existing bridges over the navigable waterways of United States ports and harbors are not equipped with adequate structural pier protection.

• Use the results of the study conducted under recommendation M-81-21 to advise appropriate bridge authorities of the benefits of installing additional pier protection systems.

(Class U, Priority Action) (M-81-22)”

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50 STRUCTURE magazine

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L O V E L A N D

|

C A R B O N D A L E

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B U F FA L O

The National Transportation Safety Board voted 3 to 2 that Lerro had been partly responsible but said that other factors, including the severe storm, had contributed to the accident. Only three members approved the recommendations, with two members not participating.

Lawsuits went on for years against the owners of the Summit Venture.

The FHWA issued Technical Advisory 5140.19, Pier Protection and Warning Systems for Bridges Subject to Ship Collisions, on February 11, 1983. In the Background section, it stated, “The increase in the occurrence of ship/bridge collisions during the past 10 years warrants additional emphasis on the need to consider protection for bridge piers as well as the installation of warning systems to alert motorists in the event of a span collapse.

The purpose of this directive is to provide guidance on these subjects to the Federal Highway Administration’s (FHWA) field offices and to State and local agencies involved with Federal-aid highway projects which cross navigable waters. This material is not regulatory but has been developed to provide additional support and emphasis for developing appropriate protective and warning systems.”

It went on to make recommendations on Motorists Warning Systems and Pier Protections. Under Pier Protections, it wrote, “Because of the tremendous momentum achieved by modern oceangoing vessels even while traveling at low speeds in inland channels, it may be extremely difficult to retrofit some existing bridge piers with protective systems which can successfully withstand the anticipated impact loadings. For this reason, it becomes particularly important to recognize the potential hazards from ship collisions and to locate and design piers on new bridges in such a way that the risks of collision are reduced to an acceptable level.”

When the new Sunshine Skyway Bridge, a cable-stayed bridge, opened in February 1987, it was located east of the twin cantilevers with a span of 1,200 feet and a vertical clearance of 180 feet. The pier foundations had large concrete islands, called dolphins, built around each of the bridge’s six piers to absorb ship impacts. Now all bridge piers in navigable waterways must be designed to resist ship impacts. Another lesson had been learned the hard way.■ Dr. Frank Griggs, Jr. specializes in the restoration of historic bridges, having restored many 19 th Century cast and wrought iron bridges. He is now an Independent Consulting Engineer (fgriggsjr@twc.com).

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51

INSIGHTS Peer Review in SE Practice Assisting in Innovation, Development, and Progress

By James O. Malley, S.E., P.E.

O

ver the last three decades, structural design standards have clearly grown more prescriptive and complex. Some engineers argue that this has stifled structural engineering innovation. While this may be true to some extent, our codes and standards have always left the door open for engineers to design structures that do not fully meet the letter of the prescriptive codes and standards via demonstrating equivalent performance. In fact, ASCE 7, Minimum Design Loads and Associated Criteria for Buildings and Other Structures, and the International Building Code (IBC) both now have specifically recognized performance-based design procedures (see Section 1.3.1.3 of ASCE 7-16, e.g.).

Commonly referred to as Peer Review (ASCE 7 uses the term Independent Design Review), it is a key element of implementing performance-based designs and projects that incorporate high-performance elements or are more sophisticated analysis procedures. This article summarizes some of the key aspects of this review process via the “Who, What, When, Where, Why, and How” of structural peer review, hoping to encourage our profession toward more widespread application. In addition, a series of references that include more detailed information on implementing peer reviews are included in the online version of the article.

Who Does the Review?

Peer reviews are typically done by a panel that includes a practitioner and an expert in the establishment of the hazard(s) being designed to resist, generally either earthquake or extreme wind. Many panels also include an academic with expertise in the structural system proposed for the design. Various documents identify the required qualifications to act as a peer reviewer, with language such as “…having the necessary expertise and knowledge to evaluate performance, the structural and component behavior, the particular load considered… to determine structural resistance and component behavior…” (ASCE 7-16 Section 1.3.1.3.4). Peer reviewers are independent engineers without conflicts of interest for the project under review who have previously designed similar structures and/or have participated in developing design standards and performance-based design guidelines documents.

What Projects are Reviewed?

Independent Design Review has been required for many cycles of ASCE 7 for the implementation of base isolation and viscous damping on seismic designs and for projects designed using nonlinear response history analysis. The other prevalent project type that implements peer review is where the design proposes to demonstrate equivalent performance to take exceptions to some elements of the prescriptive requirements of ASCE 7 or the material design standards. On the West Coast, this is often done to exceed structural system height limits specified in Table 12.2-1 of ASCE 7 for high-rise residential, office, or mixed construction, or where the structure’s architectural expression results in a hybrid or undefined structural system. Peer 52 STRUCTURE magazine

reviews are also performed outside of building code compliance for government agencies, corporations, and other entities for numerous reasons, including implementing designs intended for higher than code-level performance.

When are the Projects Reviewed?

A critical element is to start the review as early as possible, preferably during the conceptual design phase when major decisions (such as structural system, configuration, etc.) are made. Reviewer input at this early stage can significantly impact the ultimate structural design and performance. Starting the review too late in the process can result in disagreements on major design issues that could cause project redesign and subsequent delays. The reviews continue throughout the design process at milestone submittals, with the focus changing from more global/general issues and topics to more detailed and specific.

At the completion of the review, it is customary for the review team to issue a letter(s) summarizing the results of the review and confirming that the design intent has been met, with multiple letters being issued for fast-tracked designs.

Where Do Peer Reviews Occur?

Peer reviews occur all over the country, though a preponderance is for tall buildings on the West Coast to allow structural systems such as special reinforced concrete shear walls and buckling restrained braced frames to exceed the ASCE 7 height limits. For over thirty years, peer reviews have been required for many projects in the State of Connecticut for code-level prescriptive designs depending on parameters such as building height, area, occupancy, etc. In addition, federal agencies such as the Department of Affairs, the General Services Administration, and the State Department require peer review for major projects since they are not bound to local jurisdiction plan review and permitting. It is expected that this practice will increase across the country as performance-based design for wind becomes more commonplace.

Peer reviews provide a mechanism for structural engineers to innovate and extend boundaries of construction through the application of advancements in materials, modeling and analysis capabilities, research results, etc. With peer review as a means of ensuring that these extended boundaries are within reason, this type of innovation pushes the profession forward, allowing us to better serve our clients and communities.

for review and acceptance prior to the analyses being performed.

Review of analysis and design submittals, and construction documents, occurs throughout the process. Typically, a comment log tracks all review comments and helps manage the resolution process. By getting the design team and peer review team on the same page at the outset and throughout the execution of the project development, analysis/evaluation, and final design stages, the review process can be accomplished providing the most value to the project design and without undue impact to design schedules.

How are Peer Reviews Performed?

Summary

The reviews start with the establishment of the project design criteria.

This document becomes the de facto “code” for the project, setting the rules for demonstrating that the design intent has been met. It also describes the project and intended performance objectives, and defines the design loading parameters, proposed code exceptions and design assumptions for key structural elements, identifies which elements of the lateral force-resisting system will be permitted to yield in a controlled fashion and which will be protected from yielding, discusses the design approach for any unique elements in the structural system, and lists all the reference codes, standards, and guidelines to be used in the project design. At the same time, since virtually all these projects include some form of response history analyses, a parallel document is prepared to establish the loading criteria, such as seismic response spectra and ground motion time histories or a wind hazard assessment and wind loading time histories. For many projects, a more detailed supplementary document on the nonlinear modeling analysis input parameters is developed

The profession widely uses peer review as a mechanism to allow unique and innovative projects within the context of demonstrating performance at least equivalent to that of prescriptive designs. When properly implemented, for a small additional cost to the project (and hopefully no schedule impact), significant benefits can be realized by extending the boundaries of engineering practice and construction applications via a collaborative process of independent assessment and review. Engineers are encouraged to embrace this process when they face unique challenges or desire to push forward an innovative approach.■

Why is Peer Review Important?

References are included in the PDF version of the online article at STRUCTUREmag.org.

James O. Malley is a Senior Principal at Degenkolb Engineers and NCSEA Past President (malley@degenkolb.com).

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M AY 2 0 2 2

53

legal PERSPECTIVES Which Building Materials Are Responsible for Most Construction Accidents?

By Neil Flynn, Esq.

E

ach year, American construction workers sustain approximately two hundred thousand injuries that are serious enough to cost them at least one day of work. This number leads all fields of endeavor, representing almost ten percent of all workplace injuries. In examining these accidents and their frequency, most studies look to the mechanism of the occurrence, most prominently the fatal four: falls from heights, strikes from objects, being crushed between objects, and electrical shocks. Combined, these types of accidents cause six of every ten fatal construction workplace accidents. Less frequently considered are the materials involved in construction accidents.

Perhaps most prominent among this category of objects is the most ubiquitous on any construction site: tools. Although not always considered in the category of “materials,” tools are involved in many construction accidents, especially when the term is used to encompass devices such as forklifts and man lifts. However, even setting those aside, as well as elevation devices such as ladders and scaffolds, construction equipment – ranging from hand tools, such as pipe wrenches, nail guns, and welding torches, to large, stationary equipment, such as table saws and pipe threaders – cause thousands of injuries each year.

These injuries include unsecured tools falling from atop ladders (where they were left for “just a minute”), trips and falls over improperly stored tools in walkways, burns, lacerations, and even amputations resulting from the removal of safety devices or other improper use of inherently dangerous equipment. Devices intended to cut iron pipes make short work of flesh and bone when safety practices are not strictly followed.

In addition, the materials that these tools are used to shape, alter, and mold are also involved in a significant number of construction workplace accidents.

While many of these events remain outside the power of the structural engineer to affect, this is not universally true. For instance, the seemingly common-sense requirement to bolt new steel instead of welding it in pre-existing wood structures can significantly reduce the risk of costly and dangerous fires.

STRUCTURE magazine

Whether made of steel, lead, cast iron, or copper, pipes are invariably denser, heavier, and capable of making short work of the human body.

Required safety precautions, most notably hard hats, have significantly reduced the number of injuries caused by construction workers coming into unintended contact with metal pipes.

Unfortunately, however, these materials are still the source of a significant percentage of workplace accidents.

Due to their weight and unyielding nature, pipes are inherently dangerous whether they fall onto an unsuspecting worker or, when improperly stored, cause an impact injury or a trip and fall. Improper storage is also the cause of many injuries from cladding materials, drywall, roofing (shingles, tar paper, etc.), or flooring. Something as simple as an unevenly stacked rebar or improperly stored floor tiles can easily cut a worker or cause one to trip and fall. Given the nature of construction sites, where almost everything is sharp, hard, and/or heavy, what might otherwise be a benign occurrence can quickly become a career or an even life-ending event. Other dangerous materials invariably found on commercial and residential construction sites are electrical wiring, conduit, and fixtures.

Of course, the most obvious source of injury from such materials is the risk of electrical shock.

While most closely associated with injuries to electricians, a significant number of electrification injuries are sustained by construction workers in every trade (electrical injuries). In addition, wiring by its very nature creates a high risk of trip and fall injuries stemming from the ease of becoming entangled in loosely or otherwise improperly stored materials.

Another example of inherently dangerous materials that pose an everyday risk of catastrophic injuries is those that are inherently flammable. Solvents, liquid petroleum gas, and adhesives pose multiple risks ranging from flammability and combustion to

inhalation dangers. As the general public has become inured over the last two years to wear masks due to the COVID pandemic, construction workers and especially safety officers have long been aware of proper breathing protection procedures.

Such materials must be stored and used in adequately ventilated spaces to prevent the accrual of toxins’ harmful and often lethal levels. Fire or even explosions are the most obvious risk associated with combustible and/ or flammable substances. However, they pose additional risks that are often overlooked.

These materials can frequently cause injury simply from dermal contact but are often more dangerous when inhaled due to the lack of or failure to use proper respiratory protection. Damage to the lungs and respiratory system overall can range from immediate to long-term and can, in many cases, be fatal.

Of course, this type of injury is also associated with perhaps the most well-known worksite hazard: asbestos.

Asbestos, mesothelioma, and other respiratory diseases have claimed the lives of thousands of construction workers and resulted in some of the most high-profile workplace-related litigations (asbestos litigations). Entire law firms exist solely to represent workers who have been sickened and/or killed by asbestos.

Gypsum, vermiculite, and other dangerous inhalable substances are used in everything from floor tiling, to wallboards, electrical casing, pipe insulation, and roof shingles.

Unfortunately, these materials are silent killers without proper safety training and

equipment, causing their damage over time without a single notable occurrence as the source of their harm. The risks associated with these materials have been well known for decades, but they remain in use for various reasons ranging from financial to efficacy.

Thankfully, safety equipment and practices have improved exponentially over time, but the number of construction and other workers who fall victim to chronic inhalation risks is still staggering.

As we can see, the risks associated with construction work are not limited to the most obvious culprits. Whether environmental or situational, the risk of exposure to construction equipment, tools, and materials bears its own risk level ranging from minor injury to death. While many jurisdictions provide legislative frameworks designed to compensate

the victims of such risks, the goal is, of course, to make such compensation unnecessary.

Strictly enforced site safety practices, OSHA regulations, and, quite often, simple common sense can all play significant roles in reducing injuries on construction sites and should all be pursued as aggressively as possible. By collecting and monitoring information about the project, increasingly in real-time, structural engineers can anticipate problems before they reach the critical stage and can be managed before someone gets hurt.

Frequent site inspections, regular safety meetings, and rigorously enforced site safety standards are now being supplemented with real-time data collection systems. Comprised of wireless on-site sensors coupled with data collection nodes, such systems streamline the structural engineer’s ability to monitor the

entire building site and anticipate problems before they arise. These systems can be customized to monitor material storage and the location and movement of workers throughout the site, thereby giving the engineer the ability to anticipate and prevent dangerous or even deadly accidents before they happen.■ For more information on construction accident lawyers, visit https://plattalaw.com/new-yorkconstruction-accident-lawyer Neil Flynn is an attorney with The Platta Law Firm, PLLC in New York, NY, and litigating construction accident cases throughout the Courts of New York.

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M AY 2 0 2 2

structural INFLUENCERS Advice from a Career – Robert Silman

By Eytan Solomon, P.E., LEED AP

R

obert (Bob) Silman, founder and President Emeritus of the structural engineering firm Silman, formerly Robert Silman Associates, passed away in 2018. Prior to the firm’s founding in 1966, Bob worked at Severud Associates, Ove Arup & Partners, and Amman & Whitney.

Bob taught at Cornell, Columbia, Yale, and Harvard and was a fellow of the American Society of Civil Engineers. He was also an honorary member of the AIA New York Chapter, the Structural Engineers Association of New York, and the International Association for Bridge and Structural Engineering.

What always inspired me about having Bob as a boss was how deeply and admirably human he was at all times. He was an incredible schmoozer but never exuded sleazy salesmanship. He could talk about classical music and art, but in a way that drew you in if you were less learned, without him coming off as snooty. He was sharp with engineering insight but never pretentious about knowing more than someone else, even an entry engineer or layperson. He was a patient listener and a patient explainer.

Below are selections from his “notes to the firm” posts on the company intranet, which serve as excellent advice to any structural engineer:

Be Safe On-Site During my first year of practice, when this was a one-person firm, I received a call on a very cold wintry morning. A piece of cast iron cornice had fallen from the 10th floor of the Ethical Culture School on Central Park West [and 63rd street]. Could I come out immediately?

They had a scaffold on the premises because they were doing façade work at the time.

I was dressed in a business suit, an overcoat, leather-soled shoes, and gloves. So that is how I appeared on site.

The scaffold was a three-person maximum, 16-foot-long wooden plank scaffold, supported by hemp lines at each end that were manually hauled to raise and lower the scaffold. There was a “safety” arrangement consisting of a web belt tied off to the scaffold rail with a spare piece of light rope. No independent safety lines, no toe board, a thin film of ice on the planks of the scaffold! I climbed through the top floor window to access the scaffold, and as my leather-soled shoe hit the ice, I started to fly out into Central Park as I had not yet attached the safety belt. One of the mechanics grabbed me and laughed.

Upon returning to the office, I went out and immediately bought a pair of stout, Vibram-soled shoes at a significant cost (Vibram soles at

that time could be had only on costly imported Italian hiking and walking shoes). I still have these Fabiano shoes, and they work well. I then kept a set of appropriate clothing for site visits stored in the office that I could change into on short notice.

What a far cry from today – motorized scaffold on steel wire suspension, full harness arrest system clipped to an independent safety line, toe board, grating floors that do not ice up, railing with intermediate horizontals, and most of all – training and certification required before you can even go out on a scaffold.

Not such fond memories of the old days.

Use Technology with Caution We are a firm of technologists. We call ourselves structural engineers, and, by definition, that makes us proponents of technology. Most of us take it for granted that if we are at the leading edge of the uses of technology, we are doing the best thing possible. That is what progress is all about, and who can argue that “progress is our most important product” (that was General Electric Company’s slogan for many years).

Perhaps we should distance ourselves for a moment from what we are embroiled in every day. Perhaps we should consider whether modern technology is different from the erstwhile technology that has allowed us to arrive at our present stage of development. Many of you know that I teach the Philosophy of Technology course at the Graduate School of Design at Harvard. One of the philosophers that we read, Hans Jonas, points out that there are many dramatic differences in contemporary technology compared to what has gone before. Most importantly, the results of earlier technologies were proximate. We could see all of the results almost immediately.

Today’s technologies sometimes result in consequences that we could never have dreamed of, occurring far down the road, maybe long after we are gone. Many of these result in environmental degradation or serious health consequences. The push for sustainability and green design in our buildings addresses some of these issues. But the suggestions of sustainability advocates address the known. What about the unknown? I would like to suggest that each of us seriously consider what we are designing and drawing. Will it possibly result in unknown future consequences? Will we be guilty of the next DDT debacle?

Maybe it is OK to use caution, think twice, or raise a yellow “go slow” flag before endorsing the next new thing. I am not advocating that we become Luddites, only that we consider the future.

Today’s technologies sometimes result in consequences that we could never have dreamed of...

56 STRUCTURE magazine

Remember that part of our culture is to function with joy.

Be Proactive We talk about “being proactive” all the time, but what do we mean?

• Do not wait to be contacted, but initiate the contact yourself.

If you need information to proceed with your work, ask for it immediately or, better still, anticipate what you will need and be sure you have it before you need it.

• Make lists and send them on to the client. If you know what you will require by when, talk to the client in advance about upcoming deadlines and when you can expect the information.

Then once it has been decided upon, memorialize the conversation by writing it down in a friendly way and sending it on in the form of a list or a memo. That way, there is no misunderstanding. Clients easily forget what they have promised to send us when under pressures of their own.

• Do not allow long periods of silence to go by when working on a project. Even if you are in the midst of a long series of calculations that will have no meaning to the client until you have completed them, call them frequently to tell them what you are doing so that they do not think that you have forgotten them or are not working on the job. Put yourself in their place. Even if you are between phases or the project has been put on hold temporarily, contact the client regularly to check on the status; this shows our interest.

• Even if you are not working directly on a project with a client now, but you have had a good past relationship, call the former client every once in a while just to keep in touch. You would be amazed how far this goes.

• If you see or read something that may not pertain to the project directly but would be of interest to the client, send it along.

Make sure that it is appropriate, however – nothing politically incorrect!

Whatever you do to be proactive, treat it gently. Do not come down like a hammer and make demands. Remember that part of our culture is to function with joy. Pass some of that along.

Own Up to Mistakes Who of us never makes an error? It is said that the great Turkish rug makers always intentionally wove a small mistake into their pattern, saying, “Only God is Perfect.”

What should be our response as an individual in a firm when we find that we have made a mistake? The natural impulse is to cover it up if possible. Or blame it on circumstances beyond our control or on others. But most of us have learned that such actions come back to haunt us.

The only acceptable course of action is to own up to it as soon as we realize it. We must go to our immediate

supervisor and discuss the whole thing in total candor. That does not mean that it must be broadcast through the PA system so that everyone knows! Then a decision has to be made as to how to handle the situation. Most rational people understand that others make mistakes, and they allow for it. That is generally thought to be why jobs carry a contingency during the construction phase, to allow for such mistakes or omissions. And it is said that the best way to learn is from our mistakes.

Personally, I once made a glaring design error during my first year of practice when I used the wrong method to design a 30-foot-high braced wall of steel sheeting…. The job was out to bid, and I had to go to the [client] and tell him that we had to recall the job, fix the error, and rebid the job. His response was, “Good that you found the error now. It’s no big deal to fix the drawings and rebid the job.

Go to it.” And we got lots more work from that client. Had I tried to make excuses, I think that this particular person would have thrown me out. But he appreciated my honesty.

Go to your supervisor if you make a mistake, and together you can work out a strategy for fixing it. Don’t be afraid to be honest about it. We all make mistakes. Only God is perfect.

Broaden Your Horizons As I think back over the years to the many mentoring sessions that I have participated in, there is a recurring theme. People ask me, “What should I do so that I can improve my performance or even ‘stand out’?” One area that is generally lacking is intellectualism.

Without trying to be elitist or pedantic, or preachy, I would say that all of us can stretch our minds a little further every day in all kinds of ways that may not seem directly related to our job performance.

Read a book that challenges you, that makes you think about what you just read, or perhaps requires you to go back and reread what you just read simply to understand it. Go to a museum, look at something you have never seen before, and then do some research on what you just encountered. Go to a performance where the milieu is a different one from what you usually encounter – classical music if you usually listen to rock, a play if you usually go to a movie, an opera if you’ve never been before, a lecture, a reading. These don’t have to cost a lot (or anything) if you go on a free night to the museum or go to a student performance.

We all know the benefits of exercising our bodies and stretching our muscles.

How about our minds? It is amazing how much better you will feel. And without possibly noticing it, how much better you will perform your role at work.■

And it is said that the best way to learn is from

our mistakes.

Eytan Solomon is a Senior Associate at Silman and a member of STRUCTURE’s Editorial Board (solomon@silman.com).

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SPOTLIGHT New Life for the Historic Savannah Power Plant

T

his historic adaptive reuse project is a part of the Plant Riverside District, a 670,000-square-foot complex located just feet away from the Savannah River in Georgia.

Initially built in 1912 as the Savannah Electric & Power Company’s power station, the building sat vacant from 2005 until 2016.

Transforming the skeleton of the Savannah Power Plant building into a new JW Marriott Hotel from the inside out, the open space of the original boiler room was replaced with five levels of new framing. The engineers used composite steel joists topped with four inches of lightweight concrete on composite metal deck. The use of this system eliminated the need for foundation retrofit at all gravity columns while satisfying the tight floor-to-floor clearance requirements.

For the structural design firm on the project, Browder + LeGuizamon and Associates, a major structural challenge was the analysis and re-design of the building’s lateral system, triggered by replacing and adding entire floor levels as well as installing large new openings in the exterior masonry walls. Most of the original building was not designed for lateral resistance; however, thanks to the availability of archived structural drawings, the engineers pieced together a 3-D analysis model, which included details of built-up beams and columns to study the global behavior of the building pre- and post-renovation. The structural retrofit necessitated the installation of new steel braces through both new and existing beams and the complete replacement of existing braces to accommodate stairs and corridors. The engineer’s design and sequencing eliminated the need for temporary bracing while supporting two 115-ton existing steel

58 STRUCTURE magazine

stacks. Additionally, brace lines had to be located to work with the usage of the public areas on the lower level. The industrial feel of the power plant lent itself to the incorporation of primary structural elements into the interiors. It was a key in successfully combining structural functionality with design aesthetics.

Outside of design and constructability considerations, Browder + LeGuizamon engineers collaborated closely with the project’s architect, Campo Architects, to maintain the historical character of the renovated building. This was most apparent in the roof framing of the powerplant’s turbine hall with its existing 55-foot-long roof trusses spanning across the three-story atrium, which became the hotel’s main lobby. The original trusses had to be maintained despite the need to install two levels of a suspended walkway around three sides of the atrium to provide access to guest rooms. Suspending the walkway from the trusses required strengthening of all chords and web members, including their connections, therefore altering most of the historically authentic features of these members. Engineers solved this issue by installing new back-span beams behind the existing columns together with cantilevered outriggers on the lobby side that were moment-connected through the column. Walkway beams then spanned between the outriggers, and faux hanger rods and clevis were added to give the appearance of a suspended walkway.

However, creating the moment connection posed a challenge of its own due to the joint’s inaccessibility and space constraints for welding. This had to be resolved through intensive coordination with the steel fabricator.

Beyond usage as a luxury hotel and entertainment center in Savannah, the building had also to perform an essential function of being the central link to the

other buildings in the complex. This called for designing connecting tunnels through the basement level to the adjacent buildings on the west and east side. The tunnel to the West Hotel was particularly challenging due to an existing five-foot-deep grade beam supporting the entire west façade and new entrance canopy, which meant that the grade beam had to remain in place during construction.

The challenge was to maneuver the position and slope of the 6-foot by 10-foot tunnel in such a way as to pass under the grade beam, a particularly difficult task due to limited space and adjacent foundation walls that had to be shored during construction. The engineers further utilized the tunnel’s concrete walls as support for the canopy column to simplify construction.

Browder + LeGuizamon developed multiple design options for each renovation component and examined these for constructability and level of impact on the existing structure before selecting optimal solutions. This complex renovation was successfully completed thanks to the well-thought-out design, quick reactions to unforeseen conditions in the field, and close collaboration with Campo Architects and AECOM Hunt, the general contractor for the project.■ Browder + LeGuizamon and Associates, Inc. was an Outstanding Award Winner for the Savannah Plant Riverside Project in the 2021 Annual NCSEA Excellence in Structural Engineering Awards Program in the Category – Forensic/Renovation/Retrofit/Rehabilitation Structures over $20M. M AY 2 0 2 2

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SPOTLIGHT Unique Structural Gems on China’s Landscape

T

he new Taiyuan Botanical Garden complex in Taiyuan, China, features three paraboloid domes ranging from 140 to 290 feet in diameter and from 40 to 100 feet in height. The gridshells comprise light doubly curved glulam beams arranged in two or three crossing layers. The project pushes the boundaries of structural engineering and construction technique in a country with little experience using timber for long-span applications, creating three beautiful gems for this growing city.

Timber was chosen for this project due to its adaptability to the geometric demands, inherent fire resistance, structural flexibility, natural aesthetic, and environmental sustainability. Working with Austrian architect DMAA, StructureCraft engineers developed an optimized geometry while looking at constraints like daylighting, structural performance, shipping, and fabrication and pre-assembly, all meticulously described, not only with digital files for the fabrication but with kit-of-parts erection and sequencing drawings for site crews.

Domes are usually constructed for efficiency using triangulation in the dome surface.

However, for architectural and sun-shading reasons, the architect and client wished to create a more tightly spaced grid on the southern side, more open on the northern side.

This led to the development of a less efficient gridshell-like and irregular rectangular grid.

Further, domes are usually spherical, leading to more repetitive surface patterns. The

STRUCTURE magazine

client and architect were insistent on a unique paraboloid and challenged StructureCraft: how could all this be built economically? And how could structural efficiency be enhanced, given the less stable non-triangulated surface?

The solution for the first question lay in exploiting the full potential of and writing new scripts for the latest in computational geometry software, seeking to optimize the precise shape of the paraboloid to minimize waste in the doubly curved glulam pieces while seeking structural efficiency at the same time.

To solve the second, more global structural issue, inherent local buckling instability resulting from the non-triangulated surface: a diagrid of almost invisible cables was inserted below the gridshell surface, which stabilized and organized the buckling modes.

The complex structure also needed to be buildable. Building piece-by-piece up to nearly 90.5 feet (30m) in the air, expecting all to fit with structural forces properly transferred, would be impossible.

The solution lay in prefabricating precisely the individual pieces and a pattern of roughly 33- x 40-foot (10m x 12m) modules that could be placed on shoring towers and stitched together using simple, custom-designed scarf joints.

Gridshells are not entirely new.

Traditionally, a non- triangulated gridshell was constructed by fitting wooden laths together onsite – labor-intensive and not very precise. But it was the only practical way to deal with the geometrical complexity.

The highly complex geometry was solved in advance with this project, allowing each unique piece to be prefabricated with confidence that

it would fit on site. In addition, the geometry software was pushed so that the amount of two-way curvature necessary in each piece was rationalized. Thus, the volume of waste and CNC cutting time was minimized.

Secondly, a non-triangulated gridshell dome is especially vulnerable to snap-through buckling in numerous complex modes. This vulnerability was efficiently dealt with using a novel, relatively light grid of cable diagonal bracing installed just below the gridshell surface, almost invisible, yet allowing the timber members to be much lighter and saving tremendously in material costs and erection labor.

A significant challenge lay in how to tension the cables, given that they had to connect numerous nodes in a two-way pattern. Single adjustable pieces between nodes would have been prohibitively expensive and laborintensive. Long lengths of cable continuous through the nodes also created a challenge in that tensioning at the extreme ends would be impossible due to friction at each node. The problem was neatly solved by devising for each node a unique casting with separate pieces joined with an adjusting bolt which could essentially “pull” tension into the cable. It proved to be a highly effective, constructible method onsite.

The result of these efforts is a unique series of long-span timber structures created through the cooperation of team members on three different continents in their desire for a world-class attraction.■ StructureCraft was an Outstanding Award Winner for the Taiyuan Botanical Garden Domes Project in the 2021 Annual NCSEA Excellence in Structural Engineering Awards Program in the Category – New Buildings $30M to $80M

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NCSEA

NCSEA News

National Council of Structural Engineers Associations

Call for Submissions for the 2022 Structural Engineering Excellence (SEE) Awards Would you like to see your firm’s prized project on the list of winners for the 2022 Structural Engineering Excellence (SEE) Awards program? Visit bit.ly/2022SEEAwards to submit your project before the July 12th deadline. The SEE Awards annually highlight some of the best examples of structural engineering ingenuity throughout the world. Projects are judged on innovative design, engineering achievement, and creativity.

Winners will be announced at the Awards Celebration at the Structural Engineering Summit, November 3, 2022, in downtown Chicago, IL. Winning projects will also receive significant promotional benefits including a poster board presentation at the Awards Celebration, coverage in STRUCTURE magazine and NCSEA media outlets, the opportunity to present in the annual awards webinar series, and more.

Call for Nominations for the 2022 Special Awards

Do you know a fellow engineer who has contributed exemplary service and commitment to the betterment of the structural engineering profession? NCSEA is now accepting nominations for the 2022 Special Awards including the Service Award, Robert Cornforth Award, Susan M. Frey Educator Award, and Susan Ann “Susie” Jorgensen Presidential Leadership Award.

Submit your nomination at bit.ly/2022SpecialAwardsNominate before the July 12th deadline. Special Award honorees will be announced in advance of the Summit via NCSEA and STRUCTURE media outlets and recognized at the Awards Celebration at the Structural Engineering Summit November 3, 2022, in downtown Chicago, IL.

Diversity in Structural Engineering Scholarship Program The NCSEA Diversity in Structural Engineering Scholarship was established by the NCSEA Foundation to award students who have been traditionally underrepresented in structural engineering (including but not limited to Black/African Americans, Native/Indigenous Americans, Hispanics/Latinos, and other people of color).

Two state engineers associations (SEAs) have recently partnered with the NCSEA Foundation on the diversity scholarship program – the Structural Engineers Association of Northern California (SEAONC) for an endowed scholarship, and the Structural Engineers Association of Metro Washington (SEA-MW) for a named scholarship.

Kevin S. Moore, P.E., S.E. remarked, “SEAONC is excited to support NCSEA’s efforts at reaching across the nation to find a deserving, underrepresented student to help the profession diversify. In almost 30 years of practicing structural engineering, I cannot recall an effort that has the potential to change the profession in a more meaningful way.”

Jennifer Greenawalt, P.E., S.E., LEED Green Associate shared, “SEA-MW has been pursuing starting a scholarship for several years and, with the founding of our SEA-MW SE3 Committee last year, we are excited to be able to sponsor a DEI scholarship through NCSEA to support students in the DC Metro region and surrounding areas.”

If your SEA or firm is interested in supporting the NCSEA Diversity in Structural Engineering Scholarship Program, please visit bit.ly/EndowNameDiversityScholarship to learn more.

follow @NCSEA on social media for the latest news & events!

60 STRUCTURE magazine

News from the National Council of Structural Engineers Associations

FREE Webinar Series Featuring the 2021 Excellence in Structural Engineering Outstanding Projects

Join us for a special, free NCSEA webinar series in May and June featuring the Outstanding Project winners of NCSEA’s 2021 Excellence in Structural Engineering Awards. Thank you to Atlas Tube for sponsoring this series, making it freely available to all!

This series will be held on six Thursdays – May 26 and June 2, 9, 16, 23, and 30; starting at 12 p.m. CST each day – and includes up to 8.5 hours of education. The award-winning structural engineers will present on their projects – highlights and successes, challenges and innovations, all from the structural engineer perspective. Learn more and register at bit.ly/2021SEEAwardsWebinar.

Save the Date to Join Us in Chicago, November 1-4, for the 2022 Structural Engineering Summit Mark your calendars for November 1-4 in Chicago, and meet us in the “windy city” to network and learn with those who know “wind loads” best! The NCSEA Structural Engineering Summit offers unrivaled educational opportunities with leading experts, unique networking opportunities at the trade show, inspirational keynote speakers, and a celebration of structural engineering ingenuity and service like no other.

If you would like to join our growing exhibitor list, please visit https://bit.ly/Summit2022Prospectus to learn more.

NCSEA Webinars

Visit www.ncsea.com/education for the latest news on upcoming webinars and other virtual events.

May 19, 2022

Pedestrian Bridge Collapse Forensic Investigation: What Went Wrong and Lessons Learned

May 31, 2022

Loving your Local Mason

June 14, 2022

Fire-Resistance Design of Mass Timber and Connections per 2021 IBC

June 21, 2022

An Adaptation of the Uniform Force Method

Purchase an NCSEA webinar subscription and get access to all the educational content you’ll ever need! Subscribers receive access to a full year’s worth of live NCSEA education webinars (25+) and a recorded library of past webinars (170+) – all developed by leading experts; available whenever, wherever you need them!

Courses award 1.0-1.5 hours of Diamond Review-approved continuing education after the completion a quiz.

M AY 2 0 2 2

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SEI Update Learning / Networking

NEW Peer-to-Peer Standards Exchange Forum

Join the discussion about ASCE Standards! Launching in May 2022, you can discuss technical issues about ASCE standards in the Peerto-Peer Standards Exchange. The Peer-to-Peer Standards Exchange is a new ASCE Collaborate forum. Dive into your technical area by discussing questions and issues with your community. Members can ask and answer questions. Nonmembers will have view-only capability. https://collaborate.asce.org/forums

Access a Selection of Papers from 2022 Structures Congress Proceedings and Journal Content Freely open to registered ASCE Library users through May 31 at https://ascelibrary.org/structures22collection.

The 2022 SEI Standards Series

Preview ASCE 7-22, changes from ASCE 7-16, the Digital Products/ Hazard Tool, and join the discussion with the expert standard developers. 1.5 PDHs per session.

• May 12, 2022: ASCE 7-22 Seismic Join host Emily Guglielmo, P.E., F.SEI, M.ASCE, with guests John Hooper, P.E., F.SEI, Dist.M.ASCE, and Sanaz Rezaeian, Ph.D. • June 9, 2022: ASCE 7-22 Wind & Tornado • July 14, 2022: ASCE 7-22 Snow/Rain • September 8, 2022: How & Why to Use ASCE 7-22 in Your Practice Learn more and register at https://collaborate.asce.org/integratedstructures/sei-standards

LIVING WELL with DESIGN-BUILD A workshop where firms share their experiences with the design-build project delivery system.

Join colleagues from HNTB, KPFF, Kleinfelder, HDR, and Black & Veatch to share your successes and lessons learned, and policies and procedures to reduce the chance of claims.

MAY 20 AND 21, 2022 ASCE Headquarters 1801 Alexander Bell Drive, Reston, VA 20191-4382

For agenda, workshop details and registration visit https://asceinsurance.com/risk-management-hub.

ASCE Continuing Education: Structural Training Guided Online Course – May 9 - July 29, 2022

Seismic Design and Detailing of Steel and Concrete Buildings GOCSDD22 Finley A. Charney, Ph.D., P.E., F.ASCE, F.SEI Justin D. Marshall, Ph.D., P.E., M.ASCE Upon completion of this course, you will be able to: • Identify the elements of the seismic force-resisting system for a steel or reinforced concrete structure and classify them as force-controlled or displacement-controlled elements.

• Design and detail braces, gusset connections, and columns for ordinary and special steel concentrically braced frames.

Wednesday, June 15, 2022 | 11:30 am – 1:00 pm ET Case Study: A Fresh Look at the Citicorp Engineering Ethics Dilemma – NEW Dave K. Adams, P.E., S.E., M.ASCE 2 PDHs | 7047IW2022

Errata 62 STRUCTURE magazine

Tuesday, July 19, 2022 11:30 am – 1:00 pm ET Performance-Based Seismic Design of Structural Buildings – NEW Praveen K. Malhotra, P.E., M.ASCE 2 PDHs | 7039IW2022 go.asce.org/StructuralTraining

ON-DEMAND COURSES Choose your topic and learn on your own time.

• Industry’s largest catalog of on-demand topics • Convenient 24/7 access • Individual and organization rates go.asce.org/OnDemand

SEI Standards Supplements and Errata including ASCE 7. See www.asce.org/SEI. To submit errata, contact sei@asce.org.

News of the Structural Engineering Institute of ASCE Membership

Congratulations New Graduates

• Make sure to take your SEI/ASCE member benefits with you – update your contact info and member profile at www.asce.org • Check out and get involved with your local SEI Chapter or Grad Student Chapter www.asce.org/SEILocal

Advancing the Profession

Thank You, OneClick, LTD

SEI SE2050 was the only American entity to receive a donation in recognition of work as an organization whose mission includes Powering the Makers of a Zero-Carbon Future from the international firm OneClick LCA, LTD, with offices in Finland and the UK. As part of celebrating OneClick’s 20th birthday, €5,000 (over $6,500) was donated to each of 11 organizations that have grass-roots movements to support a zero-carbon future. Learn more about OneClick and this recognition at www.oneclicklca.com/20th-birthday. Join SEI SE 2050 and learn how you can support our mission at www.se2050.org. Thank you, OneClick, LTD, for this recognition.

Open Public Comment for ASCE 76

Comment through May 16 on the new ASCE/SEI 76-XX Standard for Mitigation of Disproportionate Collapse Potential in Buildings and Other Structures. Use your ASCE user account to access the ASCE Public Comment System. https://bit.ly/38F045f For additional questions, contact James Neckel at jneckel@asce.org.

Reach Structural/Civil Engineers

Opportunities to Sponsor, Exhibit, and Advertise.

Contact Sean Scully at sscully@asce.org.

Be a mentor.

Find a mentor.

Virtually. collaborate.asce.org/mentoring

GET PAID WHAT YOU’RE

WORTH www.asce.org/salaries

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CASE in Point Structural Engineering Events ACEC Annual Convention and Legislative Summit May 22 – 25, 2022

The ACEC Annual Convention is back in-person! This 3-day convention features high-profile speakers, education sessions on AEC industry topics, roundtables, networking, and Hill visits.

Join the Structural Engineering Roundtable on May 23rd at 3 pm.

To register to attend, go to: www.acec.org/conferences/2022-annual-convention. Early Bird Registration ends Thursday, April 21, 2022.

Joint Town Hall On Tuesday, March 15, 2022, leadership from CASE, NCSEA, and SEI hosted a virtual Joint Town Hall event For the Betterment of the Structural Engineering Profession to discuss how the three organizations are progressing to fulfill the Vision for the Future of Structural Engineering (adopted in April 2019). Discussions specifically highlighted current and future initiatives to advance the profession and enhance member engagement. If you missed the Town Hall, watch it at www.ncsea.com/events/past. This recording is provided free of charge.

Engagement Join One of the CASE Committees* National Guidelines | Contracts | Toolkits Programs and Communications *Committee members must be a member of ACEC and the CASE coalition. Not sure if you’re a member? Contact coalitions@acec.org.

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News of the Coalition of American Structural Engineers CASE Tools and Resources Looking for Guidance on Recruiting and Retaining Staff?

Recruitment and retention of new engineering and technical staff is always a challenge for structural engineering firms, and this challenge seems to be ever-increasing.

A firm’s success now and in the future will be closely tied to how well it can recruit, develop, and retain younger employees. By 2025, 75 percent of the workforce will be Millennials and Generation Z – generations who often place a higher value on benefits related to company culture, work environment, flexible working hours, and work-life balance compared to previous generations.

TOOL 1-4: Creating a Culture of Recruitment and Retention provides a brief background discussion related to the challenges and opportunities in recruitment and retention of employees and includes brainstorming and planning tools and a sample Employee Handbook to get you started.

If you are a member of CASE, this tool and all publications are FREE to you. NCSEA and SEI members receive a discount on publications. Use discount code NCSEASEI2022 when you check out.

Coming Soon…

Tool No. 2-5: Insurance Management: Minimize Your Professional Liability Premium CASE surveyed several underwriters serving the A/E industry for professional liability, asking what information they would like to see in addition to that on their own application and why. This tool compiles their input into ten items. Tool 2-5 is designed as a guide to help you provide critical additional information to the underwriter to differentiate your firm from the pack.

CASE has tools and practice guidelines to help firms deal with a wide variety of business scenarios that structural engineering firms face daily. So whether your firm needs to establish a new Quality Assurance Program, update its risk management program, keep track of the skills their young engineers are learning at each level of experience, or need a sample contract document – CASE has the tools you need!

You can purchase these and the other Tools at www.acec.org/bookstore

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65

structural FORUM Ethical Perspectives and Decisions

By Scott Civjan, Ph.D., P.E.

S

ome of us think of ethical conflicts as having an ideal resolution, where we can discuss the scenario and assign blame to someone who would dare to arrive at a different decision. This is often reinforced by ethics discussions, where we assess a scenario and scoff at how the transgressor should know better and be subjected to punishment. “How can those people be so unethical!” we think to ourselves. However, an ethical conflict is not always a situation where a clear answer is apparent. Therefore, before deciding what the “right” answer is, let’s step back and reassess. What could lead us to make another decision and, by extension, lead another person to make a choice different from ours? Could they still be behaving ethically, and if so, can we define an ethical person solely through their actions, or do we need to understand their decision process? I propose that giving some thought to these issues can be very important in understanding ethics and providing sound mentoring.

To be clear, there can be a complete ethical breakdown in how a person conducts their business, but that is not what this article focuses on. Instead, the author discusses situations where viewing the dilemma from another perspective can make us reassess our gut reaction that the other person has made a terrible mistake. With different knowledge and/or experience, we may come up with opposing yet equally ethical decisions to resolve an ethical conflict. If we can do it, why can’t we believe that someone else with a different background and perspective could as well? Acknowledging this possibility opens us up to some important steps in mentoring.

Once we promote good decision-making, we can focus on the problems of ethical fading (when ethical aspects move to the background and are replaced with aspects like profitability, etc.) and confronting truly unethical behavior.

Consider an example where you have inspection responsibilities on a project.

A concrete batch is delivered to the site.

Slump is significantly high and out of spec for the project. Some water was added to the truck but was not recorded. The sample was taken from the middle of the load, so much of the concrete had already been placed. The driver and contractor say the next trucks will be corrected, this is typical, 66 STRUCTURE magazine

and this concrete should be placed rather than rejecting the truck or removing any concrete. As an inspector, you need to make a decision.

Assume that you are an inexperienced engineer or intern and that the contractor and workers at the site have significantly more experience. From your perspective, you might have very limited information, knowing that the specification is meant to ensure “safety to the public” and that the higher slump concrete will be weaker but might be a way of making it easier to place or less expensive to produce. Therefore, it would make sense to reject the truck based on your understanding of the situation. Alternatively, you might contact your supervisor and be told that the delivered concrete is acceptable, changing your decision through more information or advice. However, without further guidance, you might assume that the specifications are generally too conservative or arbitrary and extrapolate that any future truck with this deviation is acceptable. If this becomes standard practice on a job, a senior engineer in the company may be surprised to be told that the inspection protocols are not being followed with the rigor they expected.

Does the supervisor’s experience add to the decision? Some of you may already be asking for more information: was a high range water reducer added, where is the concrete being placed, how critical are the members being cast, and did previous strengths exceed requirements? A senior engineer may know the answers to many of these questions or quickly get this information before deciding.

Based on this information, they may accept or reject the truck or divert the concrete to a less critical member. Imagining yourself as the supervisor, you see there is a lot more information at your disposal to make the decision and evaluate the risks. However, the amount of additional information you can collect is dependent on the time available to arrive at a decision. So, even this experienced engineer may decide to accept or reject the truck.

Would this have relevance in a design office?

Consider an inexperienced engineer who understands that the life safety provisions of AISC and ACI specifications rigidly represent the “safety of the public.” Without a full understanding of load paths, load redistribution, and assumptions in approximate

analysis, this engineer may not understand why a supervisor decides that a slightly overstressed member (per simplified design) is acceptable. The inexperienced engineer may feel that they are being asked to risk public safety or extrapolate this statement to erroneously think that overstressed members are generally acceptable since “we use all of those load factors to be conservative and account for this.” On the other hand, the senior engineer may have spent many years investigating the conservative aspects of typical designs and feel comfortable that a more in-depth analysis would result in excess capacity for this specific design. To be clear, the senior engineer’s decision is not more ethical but is based on a different perspective. Without this knowledge, it would be problematic to blindly allow a variance from specifications, and calculations should be provided to justify the variation.

These examples of a new engineer versus senior personnel are fairly common based on the author’s experience and conversations.

Perhaps senior engineers can remember how uncertain they were about similar decisions earlier in their careers. Perhaps engineers early in their career can see why being over-ruled on a decision could have sound reasoning rather than seeming arbitrary. Most importantly, whether a decision is ethical or unethical relies less on the final decision but more on making the best decision based on the available time and information.

Once we realize that individuals have different perspectives, experiences, and information, we can apply this as a core part of mentoring. As a mentor, spell out scenarios that the new employee might face and discuss how to make decisions and who/when to call for more information. As a mentee, feel comfortable asking for information and advice.

Develop relationships within companies and project teams that rely on clear and open communication, acknowledge different perspectives, and focus on making informed and ethical decisions.■ Scott Civjan is a Professor at the University of Massachusetts Amherst Department of Civil and Environmental Engineering. He teaches classes in structural engineering, including design classes, where he has been introducing and modifying ethics content.

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