STRUCTURE magazine - May 2021

STRUCTURE MAY 2021

NCSEA | CASE | SEI

MASONRY

INSIDE: Rebuilding the Sperry Chalet 22 Historic Masonry Façades Loving Your Local Mason Salt Lake City Earthquake Aftermath

11 16 28

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Contents M AY 2021

Columns and Departments 7

Editorial Drinking the Kool-Aid®

By Ed Quesenberry, S.E.

8

Codes and Standards Upcoming Change to TMS 402/602

By Richard M. Bennett, Ph.D., P.E.

11

Structural Rehabilitation Engineering Judgment for Historic Masonry Façades

By Edward Gerns, RA, and Rachel Will, P.E.

14

Structural Connections

Cover Feature

Modern Wood Fasteners – Part 3

22 UP FROM THE ASHES: REBUILDING THE SPERRY CHALET – PART 1

and David Moses, P.E., Ph.D.

By Alex Salenikovich, Eng, Ph.D.,

16

By Ian Glaser, P.E., Jeffrey Schalk, P.E., S.E., and Laine McLaughlin, AIA

In 2017, the Sprague Fire roared through Glacier National Park.

A spark from the nearby fire caught in one of the eaves of the Sperry Chalet, and the log-framed interior structure burned completely, leaving only four stone masonry walls and two interior stone chimneys still standing. National Park Service committed resources to stabilize and rebuild the Sperry Chalet.

Structural Practices Loving Your Local Mason

By Donald Harvey, P.E., Gary Ogden, and Kelsey Stithem

19

Structural Design Beyond the Shelf Angle

By Cortney Fried, P.E.

24 THE CITY CREEK’S CENTER BRICK MASONRY FAÇADE

28 EARTHQUAKE AFTERMATH

By John G. Tawresey, S.E.

and Mason T. Walters, S.E.

The City Creek Center project

In 2020, the Salt Lake City & County

demonstrates the value of performancebased design and developing the curtainwall system in coordination with the rest of the design. The inevitable conflicts between parties involved in a complex curtainwall construction project were significantly reduced.

32

2019 NCSEA Structural Engineering

By Jerod G. Johnson, S.E., Ph.D.,

Building suffered only cosmetic damage from an M5.7 event. Damage

Curriculum Survey Results

By Scott M. Francis, P.E., SECB

34

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

unreinforced masonry structure was of the original stakeholders and the

42

Additional Content Available Only at – STRUCTUREmag.org

Construction Issues Damage Control for Deep Excavations

By Hee Yang Ng, MIStructE, C.Eng, P.E.

Spotlight Virginia State Capitol Building Dome

Engineer’s Notebook Error Checking and the Black Box – Part 2

designers of its 1980s retrofit.

May 2021 Bonus Content

Historic Structures West Hartford (Woodstock) Bridge Disaster

experienced by this base-isolated, hardly perceptible …a credit to the vision

Education Issues

By Scott N. Jones, S.E.

In Every Issue 4 36 38 40

Advertiser Index NCSEA News SEI Update CASE in Point

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.

M A Y 2 0 21

5

EDITORIAL Drinking the Kool-Aid®

By Ed Quesenberry, S.E.

W

hen I reflect on my experience as an association volunteer, I am honestly surprised that I now serve as NCSEA President.

Ten years ago, I was on the Board of Directors for the Structural Engineers Association of Oregon (SEAO). At that time, I was relatively naive about NCSEA. Despite the great work that NCSEA was doing on behalf of the profession, I only knew them as being part of the group that published STRUCTURE magazine. Luckily for me, an amazing structural engineer, the late Sue Frey, intervened a couple of years into my term on the SEAO Board and drastically changed my perception and understanding of NCSEA. I realize now that this simple shift in perception was made possible by my willingness to drink some Kool-Aid. Sue convinced me to attend an NCSEA Summit as the Alternate Delegate from Oregon. Those three days I spent in Oklahoma City opened my eyes to the role NCSEA plays in the lives of practicing engineers and to all the behind-the-scenes work they do to support, promote, and advance the profession. Returning to Oregon, I shared my discoveries and excitement with my fellow engineers, to which many responded, “Ah, so you are drinking the Kool-Aid, eh?” I could not deny it – I was hooked.

I see one of my most important sworn duties as NCSEA President as encouraging each of you readers to try the Kool-Aid made by the national organizations that support our profession (NCSEA, SEI, CASE). To clarify, I am suggesting that by “drinking the Kool-Aid” these organizations offer, you would adopt the “extreme dedication to a cause or purpose” connotation of the phrase (credit Wikipedia) rather than the tragic Jonestown, Guyana, connotation. National level involvement in support of our profession can have significant upsides. So, as I hand you a nice, cold glass of refreshment, sit back and consider some of these upsides.

network has grown to include leading experts in many different facets of structural engineering practice. They have all been more than willing to offer their advice and assistance whenever I have asked. Involvement at the national level can broaden your view of the profession and help foster your growth and effectiveness as a practitioner.

Share Your Voice Are you tired of people thinking you are an architect when you try to explain to them what you do? Is there a code provision that really drives you crazy? Do you want more diversity and equity in structural engineering practice? If you answered yes to any of these, the national platform could be the place for you. There are ongoing initiatives on each of these topics at the national level right now and, to be fully realized, we need your unique voice. Whether it is acting on these nationallevel initiatives at the local level or joining the national effort, the opportunity for you to make a difference is real and there for the taking. All you have to do is speak up, and you can help create change.

STRUCTURAL ENGINEERING IS ARGUABLY ONE OF THE NOBLEST PROFESSIONS OUT THERE.

Expand Your Network From a business perspective, it is important to focus on the most immediate marketplace to where you live, as it is likely the source of most of the work you have on your desk. Because of this, it is very easy to develop a regional view of what is going on in our industry. The first NCSEA Summit I attended expanded that view for me. I met engineers from all over the country and was captivated by how similar yet different structural engineering practice is in states other than those I had worked in. I was also able to meet legends of our industry that I had only read about, including Les Robertson, Gene Corley, and Charlie Thornton, and to hear them share their perspectives on structural engineering practice. Through my involvement in NCSEA, my STRUCTURE magazine

Help Secure the Future Structural Engineering is arguably one of the noblest professions out there. Think about it; without us, the built environment would not be as safe and cool looking as it is today, yet we are somewhat of a hidden profession in the public eye. We do amazing things using math and science yet have generally accepted relative unanimity as the status quo.

While this might be due in part to our nature as engineers, it is my opinion that this puts our profession at risk of being commoditized and outmoded. Volunteer organizations such as NCSEA, SEI, and CASE all have the support and advancement of the practice of structural engineering as part of their core missions. Due to the enormity of this undertaking, these organizations all require a robust pipeline of volunteers like you to make significant strides toward securing a resilient and prominent future for the structural engineering profession.

Enjoy Your Refreshments Well, there are your three cups of Kool-Aid; I hope that at least one will be appealing enough for you to take a sip. To completely exhaust the metaphor, I will close by predicting that you will find the Kool-Aid highly refreshing and something you will want to share with all your friends.■ Ed Quesenberry is the Founding Principal of Equilibrium Engineers LLC and is President of the NCSEA Board of Directors. (edq@equilibriumllc.com)

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CODES and STANDARDS Upcoming Change to TMS 402/602

By Richard M. Bennett, Ph.D., P.E.

T

he next edition of the Masonry Society’s TMS 402/602, Building result in a tributary area of 2.67 square feet and meet the prescriptive Code Requirements and Specifications for Masonry Structures, is requirements. Use the engineered design method with slotted ties and due to be published in 2022. Some of the anticipated changes are try a tributary area of 4 square feet or 24- x 24-inch tie spacing. The reviewed in this article, including some stiffness of a slotted tie is 3000 pound/inch, things that designers can use now. One Table 1. Tie forces. so the design force is 2.5pu At = 2.5(32.7psf ) Tie Stiffness Tie Force of the most significant changes is not a (4ft2) = 327 pounds. This is less than the technical change but a change in the length design strength of 330 pounds, so a 24- x ktie ≤ 2000 lb/in.

2.0pu At of the code cycle. The Masonry Society 24-inch tie spacing could be used. Since 2000 lb/in. < ktie ≤ 5000 lb/in. 2.5pu At board approved a trial six-year cycle for this method meets the alternative design 5000 lb/in. < ktie ≤ 8000 lb/in. 3.0pu At updating the code in response to feedback provisions of TMS 402-16 Section 12.2.1, from practicing engineers who are being this design could be used now. ktie > 8000 lb/in.

4.0pu At overwhelmed by the constantly changing Another change to veneer design that can ktie = tie stiffness; pu = strength level out-of-plane load; codes. The six-year cycle also enabled the be used now is related to the out-of-plane At = tributary area of veneer tie committee to tackle larger issues. stability of a backing. TMS 402-16 has the requirement, in the alternative design provisions, that the out-ofplane deflection of the backing shall be limited to maintain veneer Veneer Design stability. A maximum deflection for different backing height-toOne of the most significant changes is a complete rewrite of the thickness ratios (hb /tsp) provides an easy means to verify out-of-plane veneer chapter. This included extensive reorganization, simplify- stability. The method was developed assuming a rigid veneer and ing the prescriptive design provisions, developing an engineered a single mid-height crack, both conservative assumptions. The design method, and designing tables for fasteners for adhered veneer limiting ratios are given in Table 3. For the above example, with a systems. This article covers two of the changes, and these changes height of 20 feet and a brick thickness of 35⁄8 inches, hb /tsp = 66.2. can be used now.

Thus, a backing with a deflection under service level wind loads of A simplified engineered design method was developed to determine anything less than hb /240 = 1.0 inch is acceptable for stability. The the load on a veneer tie. The load is a factor multiplied by the tributary backing stiffness would also need to be sufficient to limit a crack area of the veneer tie, with the factor based only on the veneer tie width, and typically a stiffer backing than hb /240 would be used stiffness. The requirements are given in Table 1. Until standardized to control crack width. It is important to note that hb is the height test data becomes available for ties, the Chapter also includes deemed- of the backing and not the height of the veneer. If there were two to-comply values, as shown in Table 2.

10-foot stories in the example, the value of hb would be 10 feet.

Consider a 20-foot-high building in an area with a basic wind speed of 105 mph, Exposure Category C, Risk Category II, and an elevation Reinforcement Harmonization of 1000 feet. The components and cladding design wind pressure would be 32.7 psf. Based on prescriptive design, the maximum tie TMS 402-16 had different provisions for the maximum size and perspacing would be 24 inches, and the maximum tributary area would centage of reinforcement depending on whether the design method be 2.67 square feet. A tie spacing of 16 inches x 24 inches would was allowable stress or strength design. The requirements have been Table 2. Deemed-to-comply tie strength and stiffness values.

Tie Type Corrugated

Adjustable: Slotted

Diagram

Table 3. Maximum deflection of the backing to provide out-of-plane stability.

Design Allowable Strength Load Stiffness 125 lb

330 lb

75 lb

200 lb

500 lb/in.

3000 lb/in.

hb/tsp

Wind1, δser

Seismic2, δu

67

hb / 240

hb / 100

100

hb / 360

hb / 150

133

hb / 480

hb / 200

167

hb / 600

hb / 250

Under application of 0.42 times the strength level wind load and applicable to backing whose stiffness is the same for service level and strength level wind loads. If the stiffness is not the same, evaluate stability using strength level wind loads and using the deflection limits for seismic loads.

2 Under application of the strength level seismic load.

1

Adjustable: Two-leg pintle

8 STRUCTURE magazine

210 lb

125 lb

2500 lb/in.

Maximum Deflection of the Backing for Stability

harmonized and simplified. The proposed provisions are: • Maximum bar size is #11 • Nominal bar diameter size cannot exceed one-eighth the least nominal wall thickness dimension (inches) • Bar diameter cannot exceed one-third the least dimension of the gross grout space • Maximum reinforcement percentage is 4% of the gross grout space, with 8% Figure 1. Gross grout space. allowed at laps The gross grout space is defined as the area or dimensions available With the adoption of compression-controlled sections, the within the continuous grouted cell, core, bond beam course, or collar maximum reinforcement restrictions were removed, except for joint, considering the effect of unit offset in adjacent courses but intermediate and special reinforced masonry shear walls under inneglecting possible mortar protrusions and the presence of perpendicu- plane loads and for beams. No change was made in the maximum lar reinforcement, if any. The gross grout space is shown in Figure 1 reinforcement requirements for intermediate and special reinforced for flanged units laid in a one-half running bond. shear walls, as the maximum reinforcement provisions are needed to For designer convenience, tables are provided for maximum rein- ensure there is adequate ductility in the walls. Maximum reinforceforcement for common configurations. Table 4 is a partial example ment provisions were kept for beams to ensure a ductile failure mode. of the tables.

Figure 2 shows the interaction diagram for a 12-foot-high 8-inch concrete masonry wall with Grade 60 #5 @ 8 inches under outof-plane loads. As mentioned previously, with TMS 402-16, the maximum reinforcement requirements can only be met if there is tension in the wall. Although there is a small region of flexure and axial load combinations allowed with TMS 402-16 that would not be allowed with TMS 402-22, the compression-controlled provision in TMS 402-22 allows significantly higher axial loads.

Net Shear Area

Figure 2. Comparison of TMS 402-16 and TMS 402-22.

Tension- and Compression-Controlled Sections

The net shear area, Anv, which is used to determine the shear strength of masonry members, has not been well defined and has caused confusion for designers. Figure 3 (page 10) has been added to clarify the net shear area for reinforced masonry members. As shown, partially grouted beams are now allowed for masonry beams that do not require shear reinforcement. If shear reinforcement is required, the code mandates full grouting of the beam. Feedback from designers on the figure has been overwhelmingly positive.

Other Changes

A major change was made in strength design provisions, with the introduction of compression-controlled sections. In TMS 402-16, Other revisions include changing the anchor bolt tension and shear there were only tension-controlled sections (ϕ = 0.9 for all cases of strength provisions to be based on the ultimate strength of the anchor moment and axial load) and rather stringent limits on the maximum reinforcement. There were several Table 4. Maximum vertical reinforcement for one-half running bond per 8-inch length (per cell). issues with this approach, including that it was posMaximum reinforcement sible to have values on the interaction diagram above Nominal Unit Thickness the balance point, even with the maximum reinforceFlanged Units Open-end Units ment provisions. On the other hand, the maximum 8 in.

1-#7 or 2-#5 1-#8 or 2-#6 reinforcement provisions could be quite stringent in 12 in.

1-#9 or 2-#6 1-#11 or 2-#8 other cases, with #5 bars spaced at 8 inches in an 8-inch concrete masonry wall exceeding the maximum reinforcement limits under out-of-plane load, even Table 5. Strength reduction factor ϕ for moment, axial load, or combined moment and axial load. with no axial load. The strength-reduction factor is ϕ Net tensile strain, εt Classification determined from Table 5. The value of εty is deterεt ≤εεty Compression-controlled 0.65 mined as fy /Es, with fy being the yield strength of the reinforcement and Es being the modulus of elasticity εt – εty 0.65 + 0.25 Transition εty < εt < 0.003 + εty of steel. This approach is similar to that used in the 0.003 American Concrete Institute’s ACI 318, Building Code ε ≥0.003 +εε Tension-controlled 0.90 t ty Requirements for Structural Concrete and Commentary.

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Figure 3. Net shear area.

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and not the yield strength. This provides consistency with anchor strength between TMS 402, ACI 318, and the American Institute of Steel Construction’s AISC 360, Specification for Structural Steel Buildings. Provisions were added for the use of deformed wire reinforcement. The smaller size of deformed wire reinforcement can be advantageous in some situations, such as when shear reinforcement is required in masonry beams. Prestressed masonry provisions had been limited to just walls. Provisions were added for prestressed masonry

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Summary

In summary, there are both technical changes and changes made to help designers in the 2022 edition of TMS 402. At the time of this article, the TMS 402/602 Code Committee is responding to comments from the Technical Activities Committee. The draft document will soon be available for public comment, with the anticipated public comment period being June 1 to July 15, 2021. Visit the Masonry Society Verified best-in-class thermal performance website, www.masonrysociety.org, for Ideal engineered solution for all cavity sizes additional information, to review the Cost-effective, easy installation and on-site adjustability public comment version, and to submit Delegated design and engineering comments. The committee has scheduled about 9 months to review and respond THERMAL TIES™ & CONNECTORS Industry-leading structural and thermal performance without to public comments. The anticipated unnecessary and expensive composite materials publication date of the next version of High-strength ties allow for increased spacing, larger cavity widths, TMS 402/602 is October 2022, which reduced penetrations and lower costs will be in time for the hearings for the 2024 edition of the FAST™ THERMAL BRACKETS International Building Code.■ Offset shelf angle from back-up wall with manufactured structural thermal breaks providing near continuous insulation and improving thermal performance by 84% compared to conventional shelf angle Significantly reduce shelf angle costs and make installation easy with smaller and lighter shelf angle

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10 STRUCTURE magazine

beams. Appendix A on empirical design was deleted. Appendix B on infills was moved to the main code and retitled as Chapter 12.

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Richard Bennett is a Professor of Civil and Environmental Engineering at the University of Tennessee, Knoxville. He was chair of the 2016 TMS 402/602 Committee and is currently 2nd Vice-Chair of the 2022 TMS 402/602 Committee.

structural REHABILITATION Engineering Judgment for Historic Masonry Façades

By Edward Gerns, RA, and Rachel Will, P.E.

B

uilding codes and standards are typically enacted in response to hazardous situations that threaten public health, safety, and welfare, or to natural disasters such as floods, fires, and earthquakes.

Fire protection was the first major issue leading to the establishment of early building codes in the United States. In 1896, the National Fire Protection Agency (NFPA) was founded to establish uniform sprinkler standards for the mills and warehouses in the Northeast. First published in 1897, NFPA standards were used until the 1950s. Modern structural related building codes were first introduced in the United States around 1900 in cities such as New York and Chicago.

Most building codes are updated on three- to five-year cycles to incorporate new knowledge from research as well as in-service occurrences.

Keeping codes current to existing knowledge for new construction can create confusion about their applicability to existing structures, particularly historic structures. An unintended consequence is that when codes are taken at face value, they often discourage rehabilitation projects due to inconsistency in how the codes apply to rehabilitation compared to new construction.

The Distress of In-Plane Façade Elements In-plane elements refer to units or assemblies within the plane of the main façade and can be grouped into three categories: intermedi- Figure 1. Lack of continuous intermediate ate support, hung elements, support at a corner. and combined support assemblies. Each of these support conditions presents potentially unique distress scenarios related to the structural support of the units and, therefore, will potentially require different repair approaches.

Intermediate Support Elements

These assemblies or units are supported at floor lines, window/door heads, or corners of the building. Distress at intermediate supports is often related to the configuration of shelf angles and the lack of accommodation of general building movements. Distress is often the Masonry FaÇade Rehabilitation result of discontinuous gravity support of the masonry. Examples Applying modern structural evaluation criteria and contemporary include corners of the building where the shelf angles from the two standards/codes to historic masonry façades without rational, different façades do not align or are held short of the corner (Figure 1), practical engineering judgment can result in overly conservative or discrete lintel elements that are limited to openings rather than conanalysis and unnecessary repairs. Often, distress in historic masonry tinuous shelf angles extending around the perimeter of the building. façades is caused by corrosion of the underlying steel support. These conditions can result in vertical cracking due to the change in Still, there are also instances when the relative stiffness of the support. Additionally, cause of the distress is not specifically cordistress can be the result of unit geometries, rosion-related. Understanding how these unanticipated load paths, inadequate bearhistoric masonry systems behave is critical ing, construction tolerances, and inadequate to understanding appropriate repairs. In or inappropriate field modifications to the addition to corrosion, masonry distress masonry units themselves. can result from unaccommodated moveIn the case of continuous piers and corment, unit geometry, variable support, ners, the effect of differential movement modified load paths, poor installation, between the masonry and the underlying field modifications, and previous inapstructure can result in significant internal propriate repairs. Historic masonry stresses within the cladding system leadfaçades were typically constructed with ing to localized crushing, in-plane shear corrodible metals and without flashings. cracking, vertical cracking, and outward As a result, the support components are displacement of units. The manifestation highly susceptible to corrosion. It should of this distress typically occurs where the be understood that all of the conditions masonry is restrained, such as at supports listed above, and resulting distress, will within piers or at intersecting walls at the be exacerbated by the accumulation of corners of the building. corrosion scale. The following examples Masonry cladding with distress related to illustrate some of the common distress discontinuous or lack of adequate intermediconditions that are often encountered and Figure 2. Hung lintel elements. Original construction ate support generally requires repairs. Often, sometimes misinterpreted. these repairs address the structural support circa the 1890s (top); repair detail circa 2015 (bottom).

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by removing masonry units and repairing, supplementing, or replacing the steel to establish adequate bearing.

conditions. In glazed masonry products such as terra cotta, the stress occurring within these units is often manifested by crazing in the glaze, which indirectly illusHung Elements trates the alternate load path (Figure 3).

Hung elements typically exist at openings Masonry with distress related to elements in the façades, most commonly windows. with combined support often does not The individual units are hung with j-hooks require structural repairs. The configuraand rods or other anchors from the suption of the existing units and structural porting structural steel above (Figure 2, support is challenging to change, and, as page 11). Distress in hung components is long as significant out of plane movement often related to corrosion of the anchoror displacement is not observed, the associages rather than other structural issues. ated cracking of the units is essentially the Corrosion related distress at hung ele- Figure 3. Units with combined support at the pier. Note masonry system’s way of providing stress ments needs to be addressed to maintain diagonal stress relief cracks and crazing. relief. Minor repairs, including treating adequate structural support due to the cracks and adding supplemental in-situ lack of redundancy of support with hung elements. pins, are generally the methods necessary to limit water infiltration Non-corrosion related distress is typically associated with deflection and secure the individual pieces. Repairs that are often performed of the units due to the flexibility of the support. When displacement include installing supplemental steel support at every floor line and of the supporting lintel is observed, particularly at larger openings, other approaches that change the unit geometries and potentially it does not necessarily warrant repairs. If the condition of the steel require extensive rebuilding and unanticipated future issues. Often, can be verified and it is determined to be serviceable, deflections these repairs are not necessary due to the slip plane detailing that was may be a result of installation tolerances, catenary behavior, inelastic often used historically. bending, or building movements. Often repairs are not necessary to “correct” the deflections. Generally, the masonry at these locations Distress of Projecting Façade Elements will behave as an arch in addition to the support provided by the hanger elements. Thus, while the hung assemblies may not work Projecting elements generally refer to façade assemblies that are “on paper,” they remain functional and typically can be “repaired in exposed on three surfaces, including 1) horizontal projections such kind,” as shown in Figure 2. as cornices and watertables, and 2) vertical projections such as parapets and balustrades. These elements present specific distress scenarios Combined Support Elements particularly related to the structural support of the assemblies and Elements with combined support are units that are located at the often require extensive repairs, which makes meeting current structural interfaces of piers and spandrels or two different support conditions. regulations challenging.

Distress at masonry units with combined support is often a result Projecting masonry elements that experience cracking and displaceof the load sharing between support elements, i.e., units that extend ments are likely the result of accumulated stresses along the length of between the lintel and the piers, sills and piers, etc. Differential load- the façade. These stresses are mostly due to unaccommodated thermal ing of individual units can result in cracking. Examples include the and moisture movements of the cladding materials, as well as differcorners of the windows where lintel and units extend between the ential movements between the masonry and the underlying structure. piers and spandrel area, causing differential stress within the unit.

Fired clay products expand during the first few years after fabricaThe portion of each unit within the pier is subjected to significantly tion due to the absorption of moisture from the atmosphere. The rate higher compression stresses and restraint than the portion within the of expansion slows as the moisture content of the masonry material spandrel. This differential stress within the same unit, as well as general equalizes with the atmosphere. Over the service life of a building, the building movements, often causes cracking at the transition of support façade materials also expand and contract due to thermal cycles. If the moisture expansion and thermal expansion are not accommodated, internal stresses accumulate within the cladding system. These internal stresses often result in cracking and (a) (b) displacement in long continuous bands of masonry, such as parapets, cornices, watertables, and continuous spandrel areas.

These conditions are most pronounced at corners where the accumulated movements occur in two directions.

Cornices And Watertables

Figure 4. Terra cotta cornice and parapet section. Original condition (left); repaired section (right).

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Cornices and watertables are continuous projecting horizontal bands around part or all of the perimeter of the building. Distress at these locations is often related to corrosion of the anchorages (similar to hung elements), and cyclical movements.

The non-corrosion related distress is typically associated with displacement of the units due to the flexibility of the support, unaccommodated expansion, original construction techniques (including filling units), and lack of maintenance

geometric stability and loading eccentricity could and should be considered. There are also instances where only one wythe of the masonry roof side or opposite side may be distressed. Thus, replacing only the outer wythe, whether it is the roof side and/or exterior wythes, may be a viable option. Understanding the behavior of both the roof and parapet wall, along with the observed distress, is essential in evaluating the potential repair options.

Balustrades These are ornamental rails and copings supporting a series of balusters that can be single or multiple units.

Balustrades were intended to act as monolithic elements with the function of the tensile bond capacity of the mortar, with mortar keys between units as the primary load resistance mechanisms (Figure 6). In some systems, these mechanisms are combined with metal bars extendFigure 5. Parapet failure due to lack of Figure 6. Terra cotta balustrade detailing, 1927. ing through the units and across the top rail. As the anchors at the slip plane of flashing. mortar deteriorates and the embedded metal corrodes, or improper maintenance over the years. While the flexibility of the balustrades experience distress, including displacements, crackthe system generally does not work “on paper” with current code- ing, and bowing. For most balustrades, as long as vertical bars are prescribed loadings, in some instances – such as seismic loading – the installed in the balusters and the bars stitching the top and bottom flexibility may improve the performance of the system. Historically, rails are generally in good condition, the “unreinforced” balustrades cornice systems were designed empirically, often with convoluted may experience deflections and displacements but may not require load paths, as shown in Figure 4a, and it is, therefore, challenging repair or reconstruction. Current codes typically treat balustrades as to “upgrade” the support to current standards. Often, the most handrails or guardrails, and it is often assumed that meeting these appropriate and rational approach for cornice repairs is “in-kind” loading criteria is required, resulting in significant repairs to “reinreplacement using non-corrodible elements for the anchors, hangers, force” the system. Current “code compliant” balustrade repairs have etc., as shown in Figure 4b. The failure mechanisms in this instance consisted of dismantling and reconstructing according to the principles were due to the units 1) having been backfilled with concrete, and 2) for reinforced masonry, which is often inappropriate for the existing lack of maintenance, which contributed to moisture infiltration and application and may result in unanticipated distress. corrosion of the support elements. Many drastic “repairs” were made Instead, in-kind repairs can consist of dismantling the balustrade and to cornices and watertables as they became maintenance concerns. reconstructing it with stainless steel threaded rods and re-establishing In extreme instances, repairs included removal or partial removal of the tensile capacity of the mortar by installing new mortar at the the elements to meet current regulations and protect public safety. full bed joint of the unit. This repair is analogous to a net that keeps elements from dislodging from the building, instead of significant reinParapets forcing of the balustrade elements to meet the code prescribed loads.

Historic masonry parapets typically are multi-wythe and are rarely reinforced. Non-corrosion related distress at parapets is generally Conclusion related to bowing, bulging, and displacements, which are often a result of the stresses from unaccommodated expansion and cyclical move- The application of modern structural evaluation criteria and contemporary ment. General deterioration due to being exposed on three sides, and standards/codes to historic façade systems can often result in excessively subsequent increased freeze-thaw exposure, can also contribute to these conservative or unnecessary repairs. These examples illustrate many of the conditions. In some instances, non-corrosion distress is exacerbated issues faced when completing rehabilitation work and highlight ways of by previous repairs such as installed flashings that introduced a slip successfully working within the structural provisions in the code. plane into the system without accommodation of anchorage of the Without further development of the regulatory system, the question portions of the wall above and below the slip plane, as illustrated in remaining is how to bring an existing building up to “modern” codes the example from a 1960s parapet wall (Figure 5). While all of the and standards. The most significant issue stems from the question: conditions above may warrant some level of repair, it is engineering “Does a building necessarily have to meet modern structural provisions judgment for each specific real-world instance and weighing factors to provide for life safety?” Moreover, what does it mean if a building such as serviceability, past performance, and current regulations that is not up-to-code? The need for sound engineering judgment, with need to be evaluated to determine the level of repair required for an understanding of historic façade systems and basic structural each condition. behavior, is essential in the successful utilization, application, Displacements, bowing, and bulging of parapets do not always development, regulation, and modification of the structural require significant structural repairs. Significant displacements and rehabilitation codes.■ bowing may justify reconstruction of the parapet to account for appropriate lateral anchorages and accommodation of thermal and Ed Gerns is a Senior Principal with the Chicago office of Wiss, Janney, moisture expansion and other movements, but may not require Elstner Associates, Inc. redesign for full reinforcement to meet all of the current additional Rachel Will is an Associate Principal and Associate Director of Knowledge code regulations such as seismic and increased wind load. While Sharing with Wiss, Janney, Elstner Associates, Inc. out-of-plane displacement may not be ideal, judgment relative to M A Y 2 0 21

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structural CONNECTIONS Modern Wood Fasteners The Key to Mass Timber Construction Part 3: Design Guidelines for Glued-in Rods

By Alex Salenikovich, Eng, Ph.D., and David Moses, P.E., Ph.D.

T

his is the third part of the series of articles on modern wood fasteners. Part 1 (STRUCTURE, August 2020) focused on self-tapping screws (STS). Part 2 (STRUCTURE, Figure 1. Potential failure modes: a) pull-out due to bondline or wood shear; b) net-tension; February 2021) introduced the reader to glued-in rods (GIR) c) group tear-out; d) splitting; e) rod tensile failure. From Tlustochowicz et al. (2011) and the components making up these joints. This concluding part summarizes design guidelines for the GIR connections. Despite most often cited in the literature, again, still being researched and the interest among designers of mass timber construction, there is not adopted. The volume of timber around the glued-in rod may no official recognition in U.S. and Canadian design codes for GIR be increased by countersinking the bonded portion and leaving an connections. This article sheds light on the state of the art of this unbonded length of approximately 5d at the end of the member to emerging technology. We caution the reader that this an area of reduce the risk of splitting (Figure 3). Such countersinking proved development without code approvals in the U.S. and Canada – the to be effective and allowed further reductions of rod spacing to 3.5d. content is provided as informational and is not to be used for design. Also, the ductility and displacement capacity of the joint is enhanced when the countersinking is employed.

Transverse reinforcement near the ends of timber members has proved Design Considerations to be effective in protecting the timber members from splitting, and it is mandatory in the Russian design standard. In New Zealand, it Connection Configuration is recommended to place not more than three closely spaced rods in When designing GIR connections, the load path and failure modes one row and to offset the ends of the rods by at least 3 inches (75 of adjoining members should be carefully considered. Apart from mm) to avoid stress concentrations and minimize the risk of rupture the tensile failure of the rods or pull-out due to bondline failure, at the ends of the rods. It is important that the rods are evenly spaced wood failure modes in the vicinity of the rods due to shear, splitting, to achieve the optimum force flow in high-capacity joints. net-tension, or group tear-out are possible (Figure 1). Rods may also Group Effects experience buckling if loaded in compression, although it has rarely been observed. Sizing a GIR joint is a compromise between efficiency Suppose the yielding capacity of rods is higher than their pull-out capac(equal strength and stiffness with the adjoining members) and unde- ity. In that case, premature failure of an individual rod may occur due to sirable wood failures. Assuming the ratio of Young’s modulus of steel irregular force distribution in a joint with multiple rods. The stiffer the to timber along the grain equals 20, it can be shown that spacing the joint, the less stress redistribution is possible. To account for the uneven rods at four times the diamforce distribution in a joint, a eter of the rod would provide 10% reduction is applied to the equal axial stiffness of the rods resistance of GIR joints with and the timber member. But two rods and a 25% reduction smaller spacing between rods for all other configurations, in and closer edge distances to accordance with the Russian the face of the timber member design standard on glulam increases the probability of timber structures with glued-in brittle shear, splitting, or tenrods (SP 382.1325800.2017). sile failures.

In New Zealand, no reduction The following design paris applied on groups of two ticulars can be found in rods, 10% reduction on 3 and Figure 2. Stresses in timber around the glued-in rod loaded in withdrawal.

European literature. To cal- From Fabris (2001) 4 rods, and 20% reduction is culate the tensile strength applied for groups of 5 and 6 of timber in the joint, it is assumed that each rod of diameter d rods – larger groups are not recommended. In tests, the force distribuglued parallel to grain can activate a maximum cross-section tion between rods in a joint depends on the joint configuration, the of 6d × 6d = 36d 2 of surrounding wood (note this has not been orientation of rods relative to wood grain, and load direction. adopted and is still being researched). Also, sufficient shear area and Inclined Rods volume of the surrounding wood are needed to resist the longitudinal shear and transverse tensile stresses around the rod (Figure 2); According to the Russian standard, rods glued at angles between hence, joints with closely spaced rods would be prone to splitting 20° and 90° to the grain are considered inclined. Often, inclined and shear failures. The spacing and edge distance allowing an area of rods are used for reinforcement of notched supports (Figure 4). The 5d × 5d = 25d 2 around each rod inserted parallel to grain has been length and positioning of rods should prevent transverse splitting

14 STRUCTURE magazine

Figure 3. Countersinking of glued-in rod. Notation: L b = bonded length, L u = unbonded length.

of timber at the embedded end. The minimum spacing along the grain depends on the angle of inclination and varies between 7.5d (for α > 60°) and 14d (for α < 30°), otherwise, 10d.

Service Conditions and Corrosion GIR connections are suitable for structures designed for dry service conditions. A noticeable reduction of pull-out strength at high moisture content has been reported in the literature. Furthermore, wood shrinkage and swelling are not compatible with the linear expansion of steel and may lead to excessive splitting of timber or bond failure. For similar reasons, joints should be fabricated in conditions as near as possible to the in-service environment. Although the adhesive and surrounding wood protects the bonded part of the rods, the rods usually are exposed at their ends. In severely corrosive environments, it is recommended to use stainless or zinc-coated steel rods. In accordance with the Russian standard, the structures with GIR joints are permitted in environments with sustained elevated temperatures above 95°F (35°C) but not higher than 122°F (50°C) and the relative humidity not less than 50%. In Europe, wood adhesives are rated for use up to 140°F (60°C) via stringent tests.

Fire and High-Temperature Applications

GIR in their codes, may raise a legitimate question for the reader: “Is this even a realistic option for my project?” It is, although it will take more work than more common connections because prescriptive building code provisions lag behind new technologies. Building codes like the International Building Code (IBC) in the U.S. or the National Building Code (NBC) of Canada allow for innovation through provisions for the approval of alternative, non-codified means and methods. In the U.S., this is addressed in IBC section 104.11, the American Society of Civil Engineers’ ASCE 7-16, Minimum Design Loads for Buildings and Other Structures (section 1.3.1.3), and the National Design Specification (NDS®) for Wood Construction (sections 1.1.1.5 and 11.1.1.3). In Canada, see the CSA Group’s standard CSA O86, Engineering Design in Wood (section 4.3.2). These provisions involve demonstrating that something not addressed by the code nevertheless meets the code’s intent by performing equivalently to code-recognized products or procedures.

Much like the International Code Council’s ICC-ES reports that many U.S. engineers are already familiar with, other nations have product evaluation reports, such as the European Technical Assessment (ETA) or the National Technical Assessment (NTA) reports that may be used to demonstrate equivalence. The potential use must be within the range of applicability and restrictions

Figure 4. Reinforcement of notched beams at support. Reproduced from SP 382.1325800.2017.

Guidelines for fire resistance need to be developed. In principle, the timber surrounding the steel has an insulating effect on the rods. The wood will char and provide protection to the steel rods, as is now required in certain applications noted in timber design standards in the U.S. and Canada. Since steel is known to have low heat resistance, all steel parts of the GIR joints should be protected against fire. Research on the heat resistance of adhesives and GIR joints is underway.

Other Current Considerations Hardwood (beech) glulam and LVL are being studied right now in Europe and have great potential due to their high density leading to high bondline strength and efficient GIR connections. Currently, a new draft of design rules for joints with bonded-in rods, considering all findings accumulated over the last fifty years, has been circulated in Europe, and similar efforts are being undertaken in Canada and at the ISO level. Time will tell if consensus can be reached on the general rules that will lead to new U.S. and Canadian standards.

Application That lack of recognition in North American building codes, as well as lack of consensus among other nations that have recognized

defined in those documents for them to be valid support. The engineer should understand the basis of published strengths and any differences in design factors from foreign reports and translate those as needed to values compatible with the project jurisdiction’s design methods. The Authority Having Jurisdiction (AHJ) may require testing if the supporting documents are not accepted, so the engineer should be prepared to make a thorough, well-reasoned case to the building official initially, explaining foreign terminology and derivations of values. An independent peer review may also be necessary. However, with good planning, glued-in rods can open up new avenues to safe, strong, concealed wood connections in North America as they have elsewhere.■ References are included in the PDF version of the article at STRUCTUREmag.org.

Alex Salenikovich is a Professor of Timber Engineering at Laval University in Quebec City, QC, Canada. (alsal10@ulaval.ca) David Moses is a Structural Engineer and owner of Moses Structural Engineers Inc. in Toronto, ON, Canada. (dmoses@mosesstructures.com)

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structural PRACTICES Loving Your Local Mason

By Donald Harvey, P.E., Gary Ogden, and Kelsey Stithem

S

tructural engineering education includes fundamental principles of mechanics of materials and structural analysis that help engineers to understand and design structural members and systems. Occasionally, it is necessary to wander into design considerations outside of the structural realm, such as corrosion protection and serviceability. Still, rarely are structural engineers taught to prioritize constructability as a primary focus of structural design. This consideration often comes with experience as it is learned that the most efficient or elegant structural solution is not always the easiest to construct or (arguably more important) the least expensive. Structural engineers’ limited education on masonry materials and limited exposure to masonry construction processes can magnify installation issues. This article highlights how engineers can avoid some of the most common Figure 1. Typical elevation drawing from NCMA TEK 14-18B showing minimum reinforcement requirements constructability issues with modern structural for walls that resist shear (Ordinary Reinforced shear walls). masonry.

What is the best way to understand masonry constructability conRead Appendix X1 of ASTM C270, where Type N is the cerns? Ask masons! The authors surveyed masons and masonry experts recommended mortar for load-bearing walls! across the country to identify common field issues that can help • Local Materials – Ask your local block supplier about local engineers develop more constructible masonry design practices. Three block strengths since they often depend on local aggregate of the most common themes from the survey are: availability; avoid specifying excessive (expensive) f´m. For • Detailing – Provide sufficient detail in structural drawings. example, Colorado has lightweight aggregate readily available • Bar Congestion – Avoid the most common bar crowding with typical block strengths around 2,200 psi. When paired conditions. with type N mortar, this block yields an f´m value of about • Different Materials and Trades – Limit conflicts, differing 1,830 psi using the unit strength method. By comparison, tolerances, and trade coordination challenges by limiting the typical block strengths of CMU produced with normal weight integration of other structural materials into masonry walls. aggregate common to areas in the Midwest are often around Other common concerns identified by the survey that should be 3,000 psi, providing an f´m of approximately 2,120 psi when kept in mind during design are: type N mortar is specified.

• Control Joints – The Building Code places design responsibility • Designing for Modularity – Rule of thumb: dimensions should for control joints on the designer; contractors should not be be divisible by 8 inches, including rough openings. Minimizing responsible for layout. the number of cuts a mason is required to make can provide • Mason-Preferred Bar Size – Based on the survey, 78% of savings in both cost and construction schedule. NCMA Tek masons prefer #5 bars for vertical reinforcement in partially Note 05-12 explains that masonry openings are typically 4 grouted walls, with #6 bars being the second choice for most. inches wider and 2 to 8 inches taller than doors and windows When larger bars are specified, lap lengths and bar weights to allow for framing and window sills. This means that a become concerns for masons. modular opening size of 40 inches can usually accommodate • Bar Size Selection – Consider using only odd bar sizes (#3, #5, the installation of a 36-inch standard-sized door, for example. #7) for ease of distinguishing size on site with a quick glance.

• Means and Methods – The masonry Code and Specifications Also, if #5 bars are selected for vertical reinforcement, using #5 allow the mason to choose low-lift vs. high-lift grout placement bars for lintel and bond beam reinforcement limits the need and the means of locating reinforcement (e.g., bar positioners). for ordering and organizing different bar sizes on site.

• Mortar Type – Consider Type N mortar unless seismically Detailing restricted (yes, even in structural walls) to increase water resistance, improve workmanship, and decrease cleaning effort. f´m When thinking of how much detail is adequate for masonry drawis actually not usually very important in structural design since ings, consider how a mason uses the construction documents to lay tensile and shear reinforcement generally dictate capacity. out a single wall with control joints, openings, and bond beams. Do

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the drawings require a mason to assemble information from numerous details, tables, and (worst of all) specifications to configure a simple wall? A typical wall elevation that shows various wall conditions (doors, windows, control joints, etc.) can greatly improve a mason’s understanding of how various reinforcing requirements interact. An excellent example of this type of drawing can be found in the National Concrete Masonry Association (NCMA) Tek Note TEK 14-18B (Figure 1). Even better, incorporating elevation drawings for the entire structure provides specific reinforcement information and virtually eliminates conflicts (Figure 2). Ensure that your details include all dimensions, including Figure 2. Example of a well-detailed structural masonry elevation drawing. Courtesy of Larsen Structural Design dimensions (or acceptable ranges of dimen- of Fort Collins, Colorado. sions) for reinforcement locations in plan, elevation, and section. lengths are reduced substantially for single bars centered in cells One of the most common omissions from structural drawings is since the masonry code increases the required lap length when sufficient lintel detailing. Lintel details are often omitted from the cover is reduced. Using a #5 bar centered, versus at the face shell, drawing set altogether. When they are present, they are often limited reduces the lap length by more than 50%, from 49 to 19 inches. in terms of reinforcement location, bearing length, and lintel height. Increased lap lengths add costs due to materials and labor and can One tip to simplify your masonry lintel design is to increase lintel even lead to laps that are longer than grout lift heights, which credepths to ensure that all lintel span-to-reinforcement-depth ratios are ates a nasty constructability problem. To illustrate the advantages less than or equal to 8. Increasing lintel of using a single bar centered in a cell, depth is a relatively easy and inexpensive consider a typical wall using an 8-inch modification that ensures that you do CMU with an f´m of 1,750 psi (2,000 psi not need to perform complicated deflecblock with Type N mortar). Which wall tion checks on the lintels. would have the larger capacity: one #5 bar centered in the cell or two #5 bars, one at each face of the cell? While it may Bar Congestion be evident that the wall with two bars Another common constructability has a greater capacity, is this an efficient concern with reinforced masonry is bar and constructible design? The two-bar congestion in the cells. Congestion is design has a capacity 69% higher than not always obvious in the design prothe single-bar design. However, due to cess because reinforcement is drawn as increased lap lengths required for the single dots or lines. However, especially two-bar design, the weight of the reinin low-lift masonry construction, wall forcement used is increased by 162%. reinforcement is lapped at relatively tight Figure 3. Close-up view of reinforced masonry corner condition While the two-bar design has a higher and regular spacing. It is common for the with single vertical bars and single bond-beam reinforcement capacity, the increase in strength is not length of reinforcement that is lapped with realistic laps showing moderate congestion. proportional to the amount of reinforceto be much greater than the length ment or labor required to build this wall. where only a single bar is present. For And, the additional congestion makes this reason, a single bar on the drawconstruction issues more likely. ings should generally be considered to require the installation of two bars in the Different Materials field. One of the most common areas and Trades of bar congestion occurs at wall corners since these areas include vertical bar laps It is common to encounter designs where and corner bar laps at bond beams. In various structural materials are integrated addition to reinforcement, mortar prointo masonry walls. Perhaps the most trusions can even further restrict cell frequent examples are steel lintels and area. Designing bond beams with only steel columns embedded in masonry. one bar is critical to avoid congestion Embedding other structural materiissues and associated grout consolidation als into the field of a masonry wall can concerns at corners (Figures 3 and 4 ). create several constructability concerns.

Using a single bar centered in a cell Figure 4. Close-up view of reinforced masonry corner condition First, there is often inefficiency because has a variety of significant benefits. with single vertical bars and double bond-beam reinforcement of coordinating trades. If a mason must Besides avoiding congestion issues, lap showing significant congestion. stop work to allow an ironworker to erect M A Y 2 0 21

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steel, this interruption breaks the typical workflow. It can lead to delays, discontinuities in grout lifts, and conflict between trades. This can be further exacerbated by the need for weld inspections or field treatments of embedded elements.

Additionally, embedding non-masonry materials in masonry walls often significantly increases time and labor for masons in placing units around the embedded objects. For example, if a 13-inch-wide steel column is placed in line with a masonry wall, it is likely that every single masonry unit on both sides of the column will need to be cut to fit the masonry around the column. Similarly, a 10-inchdeep beam or lintel element will require cutting down unit heights to maintain coursing, and masons frequently must place faceshells (soaps) around embedded elements for fire protection or to conceal these embedded elements, which is labor-intensive and inefficient.

Finally, differing construction tolerances for various materials can lead to conflicts between masonry and embedded elements.

Although masonry industry codes and guidelines generally permit out-of-plumb variations of ± ¼ inch in 10 feet, it is very common for project specifications to limit visible corners and edges to a variation of ± 1⁄8 inch in 10 feet. This is an incredibly small permissible variation compared with other construction materials. The thicknesses of connection plates or even bolt heads in steel elements generally exceed 1⁄8 inch, and wood member dimensions may vary by more than 1⁄8 inch from piece to piece (not to mention bowing and warpage). Often, masons are put in an untenable position where they must maintain tight tolerances on the masonry despite variation in elements that their work is resting on or adjacent to. These awkward

interfaces can also be stressed when differential volumetric behavior, such as shrinkage of block or thermal expansion of steel, push and pull on the different materials.

As a structural engineer, consider whether these constructability concerns can be avoided by using all-masonry construction. Modern reinforced masonry has excellent compression capacity, so using a pilaster instead of a steel column is generally more efficient and economical. Similarly, using masonry lintels rather than steel tends to expedite projects and reduce trade conflicts.

Conclusions In conclusion, there are some relatively simple changes to structural drawings, such as adding typical elevation views, detailing lintels, using single bars for vertical reinforcement and bond beams, and limiting non-masonry embedded structural materials, that can help transform your relationship with the masons building your projects from a conflict to collaboration. So, love your local mason today!■ Donald Harvey is an Associate Vice President and Engineer with AtkinsonNoland & Associates in the Boulder office. (dharvey@ana-usa.com) Gary Ogden is an Engineer Intern in Atkinson-Noland’s New York Office. (gogden@ana-usa.com) Kelsey Stithem is an Engineer Intern in Atkinson-Noland’s Boulder office. (kstithem@ana-usa.com)

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structural DESIGN Beyond the Shelf Angle Brick Masonry Façades and the Structural Engineer

By Cortney Fried, P.E.

S

tructural engineers typically have had little involvement with the design of brick masonry veneers other than the selection of lintels, shelf angles, and the attachment of these supports to the structure where warranted. In most cases, this is because brick masonry veneers are generally detailed prescriptively, which does not require engineering design. However, modern designs demanding high-performance enclosures and unique façade profiles increasingly require a structural engineer’s involvement for the design to conform to code requirements while achieving the intended effect.

Prescriptive vs. Engineered

Figure 1. Anchored brick veneer on concrete masonry unit and steel studs.

Many engineers tend to be most familiar with engineered or performance-based methods. Project-specific loads are determined under a substrate. The substrate can consist of either lath with a scratch coat the procedures in model building codes and referenced resources such of Portland cement mortar (lath and scratch coat), cement board, or a as the International Building Code (IBC) or the American Society of proprietary modular panel. The substrate is mechanically fastened to Civil Engineers’ ASCE 7, Minimum Design a backing. Where the backing is concrete or Loads and Associated Criteria for Buildings masonry, and the surface is stable and suitably and Other Structures. Material codes, such prepared, the units can be directly applied to as the American Concrete Institute’s ACI the backing. The attachment to the backing 318 (concrete), Building Code Requirements supports the veneer both laterally and vertifor Structural Concrete and Commentary, cally. Figure 2 depicts an example of adhered or ANSI/AISC 360 (hot-rolled steel), veneer on lath and scratch over a light gauge Specification for Structural Steel Buildings, frame (stud) backing. This type of adhered are used to determine or select appropriately veneer is often referred to as “thickset.” sized structural members.

Brick cast into architectural precast concrete In contrast, the prescriptive method is or tilt-up concrete panels does not fall into the meant only for the most common and category of veneer. It is considered a finish to straightforward applications. Prescriptive the concrete panels and therefore governed provisions can only be applied when the by the architectural precast concrete or tiltproject meets the criteria defining its appliup concrete manufacturers’ requirements, as cation. If all these criteria cannot be met, the Figure 2. Thin brick on studs using thick set method. applicable. However, brick installed after such prescriptive requirements cannot be used, panels have fully cured is considered a veneer and an alternate compliance path is required. Brick masonry has and would be designed and constructed as described herein. historically been designed prescriptively, with architects selecting compliant components (brick units, veneer ties) and specifying the Code Requirements – General required veneer tie spacing.

The current requirements for masonry veneer design and construction are included in the 2016 version of TMS 402 – Building Code Brick Veneer Construction Requirements for Masonry Structures and TMS 602 – Specification In the case of masonry, the term “veneer” means that the outer wythe serves for Masonry Structures. These requirements include provisions for as the exterior finish and transfers out-of-plane load directly to a backing general design, prescriptive code, and alternative design. The 2018 but is not considered to add strength or stiffness to the wall assembly. IBC references the TMS 402/602 requirements for masonry veneer.

Brick veneer is classified as either anchored veneer or adhered veneer.

Select code provisions for anchored and adhered brick veneer are As depicted in Figure 1, anchored brick veneer consists of clay masonry summarized in Table 1 and 2 (page 21). units, nominally 3 inches or 4 inches wide, separated from a backing As shown in Tables 1 and 2, there are fewer prescriptive code requireor back-up wall construction, yet secured to it and supported laterally ments for adhered veneers compared to anchored veneers. The code with regularly spaced veneer ties. The masonry units must be supported has not kept pace with the growing popularity of adhered veneer by the foundation and, as required, intermediate supports such as shelf systems and the development of new proprietary technologies such angles. In this case, the masonry only supports its self-weight. as improved modified mortars and modular panel systems. Industry An adhered brick veneer consists of clay masonry units, typically organizations such as the Brick Industry Association (thin clay brick) ranging from ½ inch to 1 inch in thickness, adhered using mortar to and the National Concrete Masonry Association (manufactured stone M A Y 2 0 21

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veneer) offer design guides and recommendations for these systems that can be used to supplement the code requirements. These organizations also offer technical assistance to architects, engineers, and contractors.

Anchored Brick Veneer – Corbelling Corbelling is a technique in which masonry is projected outward or inward incrementally over several adjacent courses. TMS 402 establishes prescriptive limits on the extent of corbelling for the stability of the veneer. Each course in the corbel cannot project more than one-half the nominal unit height or one-third the nominal unit thickness. The overall horizontal projection of the corbel cannot exceed one-half the nominal unit thickness. There are no provisions in TMS 402 for corbelling adhered veneers.

Alternate Compliance Path

there are two primary options: use custom or semi-custom veneer ties or modify the backing to reduce the air space. The use of custom/semicustom veneer ties is discussed in Case 3.

To modify the backing and reduce the air space to prescriptive dimensions while not excessively restricting the extent of insulation, consider vertical battens aligned with the studs that project into the air space/drainage cavity. The structural engineer would size the battens and design their connection to the studs or other framing to ensure continuity of the load path from the veneer to the structure.

Because the battens would be located within the typical wet zone of the wall assembly, the batten material should be appropriate for that exposure, or instead, installed behind the water-resistive barrier and detailed accordingly to resist moisture.

Adhered Veneer. Placing continuous insulation behind adhered veneer has only recently become more common. Including continuous insulation within the wall assembly requires placing the weight of the veneer further from the backing where typical anchorage details for adhered veneer systems cannot handle the additional eccentricity. The IBC includes tables in Chapter 26 for fastening requirements over foam plastic sheathing. Still, many cases require the design of these connections by a structural engineer due to the cladding weight or the desired thickness of insulation.

Due to the lack of restraint from the insulation, the fasteners are subjected to bending because of the stand-off between the load and the anchorage of the fastener into the backing. This addition of bending to the expected tension and shear forces will likely require larger fasteners

As noted previously, when the project in question does not or cannot conform to the prescriptive code requirements, other compliance paths must be used to meet the code. The veneer chapter in TMS 402 includes anchored veneer alternative design provisions that serve as a substitute compliance path in lieu of the prescriptive provisions. These alternative design provisions consist of an engineered rational design method that applies the following four requirements or guidelines: • Loads shall be distributed through the veneer to anchors (as applicable) and the backing through the principles of engineering mechanics • Veneer stability must be maintained Table1. Prescriptive code requirements – anchored brick veneer. through limits to out-of-plane deflection Clay Brick Unit Dimensions • The veneer need not be subject to the Typical thickness 23⁄4 to 35⁄8 inches flexural tensile strength provisions of the Maximum thickness of veneer may be restricted based Allowable thickness 25⁄8 to 5 inches Allowable Stress Design and Strength on Seismic Design Category (SDC) of project Design chapters in TMS 402.

• The general requirements for veneer must Air Space/Drainage Cavity Dimensions be met, along with the requirements for Minimum Specified 1 inch veneer not laid in running bond in the Maximum 45⁄8 or 65⁄8 inches Air spaces exceeding 45⁄8 inches must use veneer ties that case of anchored veneer and the seismic meet additional requirements. Air spaces exceeding 65⁄8 prescriptive provisions. inches must use the alternate compliance path.

The use of the alternative design provisions does Boundaries Measured from the face of the backing to the back of the brick. For a light not need to apply to the entire façade, only the frame (stud) backing, the measurement begins at the face of the stud, not at portions that exceed the prescriptive code limitathe exterior face of the sheathing. tions. For instance, if a building has a prominent Contents May include insulation, drainage mats, and mortar dropping collection cornice, but the remaining brickwork meets the devices. When insulation is present, maintain a minimum of 1 inch between prescriptive requirements, only the cornice must the insulation and the back of the brick comply with the alternative design requirements.

Veneer Ties

Tie Components

Examples The following examples describe common situations in which a structural engineer will need to become involved in brick veneer design and provide conceptual solutions to these situations.

Case 1 – Continuous Insulation Anchored Veneer. The maximum air space dimension in the prescriptive code requirements results in a maximum 5-inch thickness of insulation that can be installed. Architects wishing to follow high-performance enclosure design principles that recommend superinsulation can find the limit of 5 inches restrictive. Where the air space/drainage cavity exceeds the prescriptive limit of 6⅝ inches, 20 STRUCTURE magazine

Air Spaces ≤ 4 ⁄8 inches

Air Spaces over 45⁄8 and ≤ 65⁄8 inches

Plates

Min. 0.60 inch thick

Min. 0.74 inch thick

Wires

Min. 0.148 inch (9 gauge, W1.7) diameter

Min. 3⁄16 inch (W2.8) diameter

5

Tie Installation Spacing Tributary Area

25 inches vertically max.

32 inches horizontally max.

2.67 square feet max.

Reduce to 75% when SDC is D or higher.

Reduce to 70% when wind pressure velocity (qz) exceeds 40 psf but is less than or equal to 55 psf, and the mean roof height does not exceed 60 feet. Projects with wind pressure velocities and mean roof heights exceeding these values must use the alternate compliance path.

Embedment Minimum of 1½ inches from the back of the brick.

Minimum cover of 5⁄8 inch from the exterior face.

Table 2. Prescriptive code requirements – adhered brick veneer.

Unit Sizes Thickness Face dimensions Area Weight

2-5⁄8 inches max.

36 inches max. in any direction 5 square feet max.

15 psf max.

Backing Materials

Masonry, concrete, lath, and scratch coat applied over wood framing, steel framing.

Lath with scratch coat can also be applied over masonry and concrete.

Permitted Installation Direct applied (to concrete or masonry backings) Methods Thick-set, lath, and scratch coat (to all backings) Applications where the veneer is adhered to cement board or modular panels must meet the code through the alternate compliance path.

Bond Shear Strength Min. 50 pounds-per-square-inch per ASTM C482

than expected for the weight of the veneer. Since these same fasteners carry the gravity load of the system in addition to the lateral load, it is recommended to limit the vertical deflection of the fasteners to ⅛ inch.

Case 2 – Corbelling The prescriptive limits for corbelling may not be satisfactory for designs that feature projecting brickwork. Further, the prescriptive corbelling limits are often misunderstood. In projects with sloped or slanted walls, designers sometimes assume that an angled backing following the slope of the corbel absolves them from complying with the limits because the air space/drainage cavity is consistent in dimension. In other cases, the designer only implements the individual projection limits and omits the overall horizontal projection limit, not realizing that both conditions must be met.

The overall horizontal projection limit is critical because, when corbelled brickwork projects beyond the footprint of the brick course at the base, the weight of the brickwork creates an overturning moment that either pulls the veneer off the wall or pushes it inward toward the backing. While veneer ties can restrain this overturning moment, it is critical to understand that the veneer ties are only intended to transfer the prescriptive lateral load to the backing and are manufactured accordingly. By using the veneer ties to restrain brickwork corbelled beyond the prescriptive limits, the additional tension or compression load from the overturning moment can potentially exceed the capacity of the ties. The structural engineer should coordinate with the veneer tie manufacturer to determine whether the specified veneer ties will be adequate for the intended corbelling effects. In many cases, more frequently spaced ties or veneer ties with heavier gauge components are sufficient to accommodate corbelling beyond the prescriptive limits.

Another consideration in corbelling that requires evaluation is the support of the brickwork until mortar cures when the overturning moment caused by corbelling will be restrained by the veneer ties. The load capacity of typical veneer ties relies on their bond and engagement with hardened mortar. Many projects with extreme corbelling incorporate custom veneer ties that mechanically engage the brick units instead of relying solely on embedment in mortar.

Adhered Veneer. As noted previously, TMS 402 does not include provisions for corbelling in adhered veneers. However, thin units intended for adhered veneer applications are available in various thicknesses and designers have combined units of different thicknesses

such that the appearance of corbelled brickwork or a pattern of projections is achieved. These cases must be addressed through the alternative design provisions in TMS 402. Similar to the case of installing adhered veneer over continuous insulation, the engineer should consider the variation in unit thicknesses by accounting for the additional overturning forces and non-uniform distribution of forces within the anchorage design.

Case 3 – Dimensional Errors The author received an inquiry about a project where the construction team found that the completed cold-formed steel stud backing did not correspond with the intended location of the exterior face of the anchored brick veneer. The dimensional bust resulted in an actual air space dimension of 8½ inches compared to the originally specified 4 inches. In order to restore an air space less than 4⅝ inches in width, the construction team proposed adding a second wythe of masonry on the interior side of the brick, attached to the backing using veneer ties and connecting the wythes together with joint reinforcing, but not compositely. Expansion or relocation of the backing was not feasible.

The proposed solution represented a misunderstanding of the prescriptive code limitations. The addition of the second wythe of masonry would reduce the air space/drainage cavity within the prescriptive code dimension, but the creation of a “double veneer” violates the limits on veneer thickness in addition to creating an unusual assembly with complex behavior. While the alternative design provisions would allow for an engineered double wythe veneer, the recommended solution involved eliminating the inner wythe of masonry and using customized veneer ties. This solution still required using the alternative design provisions because the dimension of the air space exceeded 6⅝ inches, but upgraded ties were a simpler and faster option that better suited the construction schedule.

Veneer ties are proprietary products designed primarily for prescriptive code provisions. However, veneer tie manufacturers perform their own development and testing of the ties. As a result, they can upgrade wires and plates beyond the sizes used in the typical product line when higher capacities and longer ties are needed to create a semi-custom veneer tie. For this scenario, the structural engineer should coordinate directly with the veneer tie manufacturer to discuss available options and obtain test data for review.

Conclusion Increasingly, structural engineers may find themselves facing complicated brick veneer façades. Still, it need not be a cause for concern due to flexibility in the alternative design provisions and known approaches for common applications. To date, the current provisions have been used successfully on a variety of projects; however, the industry recognizes that the veneer code requirements could be improved. The code cycle for the 2022 version of TMS 402/602 is nearing completion and will include an extensive reorganization and expansion of content for masonry veneers to provide more resources for engineers. Examples include a new engineered method developed for anchored veneer to serve as an intermediate step between the prescriptive requirements and full rational design, as well as tables that more concisely present the prescriptive anchored veneer requirements and present prescriptive solutions for adhered veneer applied over drainage cavities or continuous insulation of various dimensions.■ Cortney Fried is a Managing Senior Engineer at the Brick Industry Association (BIA). (cfried@bia.org)

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drawings without the benefit of having been to the site. The team created a Revit model based on the handful of drawings and photographs made available. Roof pitches, wall thicknesses, floor-to-floor heights, log diameters, and log spacing were estimated based on the limited documentation.

Undocumented detailing was assumed based on experience with similarly constructed buildings from the era.

With a short timeline to complete the design, structural system selection was a critical early decision. Vital considerations were code compliance and constructability. The preferred alternative, derived from the public outreach process, favored rebuilding the chalet in a way that honored, if not matched, the original character. Structural system selection had to incorporate all of these objectives. In the winter months, the chalet is almost entirely blanketed in snow and previous studies indicated a ground snow load of 350 psf. The new roof design was based on a roof snow load of 275 psf. Reframing the

By Ian Glaser, P.E., Jeffrey Schalk, P.E., S.E., and Laine McLaughlin, AIA, LEED AP building with logs that were strategically designed with concealed steel flitches satisfied all the design imperatives. The The Sperry Chalet proudly rebuilt in 2020. Courtesy of Mark Bryant Photographics. log joists and rafters were manageable to erect and were field adjustable by the timber framers.

The other early pivotal design decision was to phase construction by framing the building in the first summer and n August 31, 2017, the Sprague Fire, roaring through Glacier repairing the masonry walls in the next summer. The drawings specified National Park, reached the remote backcountry site of Sperry a new exterior raker bracing scheme that supplemented the interior Chalet. It was one of two remaining chalets out of nine built by timber bracing. The design stipulated progressive removal of the the Great Northern Railway Company in the 1910s within Glacier interior bracing as the floor levels were installed from the bottom National Park’s high country. Despite preemptive mitigation efforts up. Temporary stud sister walls, located just inboard of the exterior by park staff to protect the building, a spark from the nearby fire’s masonry walls, were designed to support the new structure’s weight ember storm caught in one of the chalet’s eaves. The log-framed interior structure burned completely, leaving only the four stone masonry walls and the two interior stone chimneys still standing.

Two accessory structures on the site were spared.

In October, National Park Service (NPS) crews raced to temporarily stabilize the free-standing walls with interior timber bracing before the snows began. The character-defining exterior masonry walls were at risk of collapse from snowdrifts and wind gusts over the upcoming winter season (Figure 1). This emergency stabilization effort was spearheaded by the NPS and the Glacier National Park Conservancy, the park’s non-profit funding partner.

Backed by a groundswell of public support, the NPS committed resources to rebuild the Sperry Chalet. In March 2018, Anderson Hallas Architects was engaged to lead the design team, which included JVA, Inc. as the structural Engineer of Record and Atkinson-Noland Associates, who evaluated the existing masonry and provided expertise regarding its condition, characteristics, and repair.

With the goal of starting construction immediately following snowmelt (July) and enclosing the building before the onset of winter Figure 1. The chalet survives the first winter after the fire, thanks to emergency (October), the design team was tasked with producing construction bracing. Courtesy of GravityShots.com.

Up Ashes FROM THE

Rebuilding the Sperry Chalet: Part 1

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22 STRUCTURE magazine

beam pockets revealed discrepancies in the estimated floor joist spacing. The profile of the rock below the crawlspace varied more than anticipated. In some places, crawlspace depth was too shallow to allow for dropped framing. In other places, the crawlspace needed to be excavated so that new footings could bear directly on the rock. JVA returned to the office to revise the foundation and first-floor plan and was back on-site five days later to help the contractor understand and implement the changes (Figure 4). This process of visiting the site to document existing conditions and construction progress followed by structural drawing revisions continued throughout the summer, level by level, to avoid any delays in the short construction window.

The contractor had initially assumed the ability to use helicopters with a 3000-pound payload capacity to transport materials to the site. But fires in the summer of 2018 reassigned these larger helicopters to firefighting. An available helicopter with a 1000-pound payload capacity was ultimately used. Consequently, more flights were required, and they were scheduled with sensitivity to the natural habitat and busy visitor season.

Figure 2. Designers assess the structure in July 2018 Figure 3. Engineers core-drill the With two alternating crews working seven days a week, amongst the forest of braces. masonry wall. the Chalet was enclosed (Figure 5) and the damaged as it was being built since the existing masonry was too fragile to masonry walls re-supported laterally at the end of the first construcbear upon. The sister walls were also detailed to provide out-of-plane tion phase. The crew demobilized in October as the cold and snow stability to what was, in the interim, essentially just a masonry façade. prevented further work. In the summer of 2019, the crews completed The design team started and completed the accelerated design, the second phase of work, which included repairing the exterior concurrent with NPS reviews, within six weeks. The approved masonry walls and tying them permanently into the structure. The Construction Documents were then issued for competitive bidding. exterior balconies were rebuilt, and the doors, windows, and interior

By June, Dick Anderson Construction was awarded the contract, and features were installed. Although dampened by COVID-19, the Sperry they mobilized in early July.

Chalet welcomed its first guests since the fire in July 2020.

Two days after the general contractor arrived on site, the design Notwithstanding the aggressive schedule and the logistics of the team trekked the 6.7-mile trail to reach the structure perched on the backcountry site, the ambitious design required creative thinking to cliffside 3,300 feet above the trailhead at Lake MacDonald. Provisions overcome several structural challenges. The designers sought to baland hand tools were carried by mule-train, and testing equipment ance code compliance with original framing proportionality while was delivered by helicopter. The purpose of the visit was to validate integrating the remaining masonry walls into the structural the construction drawings and begin assessing the fire-damaged stone system. Details of the structural design will be highlighted masonry (Figure 2). While the contractor mobilized and the design in the second part of this article.■ team investigated, they collaborated.

Ian Glaser is JVA, Inc.’s Historic Preservation Director.

Bedrock in the crawlspace was cored for laboratory analysis that ultimately confirmed that the geotechnical criteria assumed for the Jeffrey Schalk is a Senior Project Manager at JVA, Inc. design were appropriate. Surface penetrating radar was implemented to Laine McLaughlin is a Project Manager at Anderson Hallas Architects. profile the historic masonry walls’ layup, and scans were substantiated in strategic locations with video-scoped cores (Figure 3).

Several anchors were installed into the masonry walls on the first reconnaissance day. They were load-tested at the end of the trip to verify anchor shear and tension capacities used for the seismic calculations.

During this first trip to the site, several discoveries were made that illuminated inaccuracies in the geometry assumed a few months prior. The assumed grid-togrid dimensions were off by as much as twelve inches. Figure 4. The first floor is framed as interior bracing Figure 5. Timber framers set the log rafters via helicopter assist.

Measurement of existing is progressively removed.

Courtesy of A Boring Photo.

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(structural brick veneer), the following describes the innovations on two of the six buildings. For these two buildings, a brick veneer on steel stud system (BV/SS) was selected (Figure 1).

Performance-Based-Design The advantages of a performance-based-design include the ability to more precisely define the expected performance, increase the chance of achieving that performance, and allow for cost-reducing innovations that deviate from standard practice and/ or prescriptive code compliance.

The first challenge was to convince the stakeholders to expand the structural engineering involvement to include performance-based-design.

Eventually, all agreed that there were opportunities to customize the BV/SS system to reduce cost and to better meet the owner’s needs. The complexities of the walls helped drive the decision.

Figure 1. Two residential buildings at City Creek Center. Courtesy of ZGF Architects.

By John G. Tawresey, S.E.

The City Creek Center's

Performance Criteria/Expectations

Although BV/SS walls are common, structural engineers typically are not involved in the design except for a limited analysis of the backup wall to determine the expected wall deflection (the codes and standards do not provide consistent criteria for the deflection limit). The applicable building codes for this project were the 2006 International Building Code (IBC), ASCE 7-05, Minimum Design Loads for Buildings and Other Structures, and TMS 402-05, Building Code Requirements and Specification for Masonry Structures. These codes contain limited brick veneer performance requirements, leaving considerable freedom to customize performance criteria.

Early in the process, the Owner, General Contractor, Construction Manager, and consultants were involved in discussions about wall performance. The wind and seismic loads were well defined, but the required performance was not. Since the project is located in a high seismic risk category, the seismic performance was a primary issue.

Seismic loads were divided into three intensity levels; 1) frequent event, 2) 500-year return period, and 3) 2⁄3 maximum considered.

The engineer of record provided seismic displacements for each floor at each seismic level. Building wall elements were differentiated by location and geometry – flat or linear walls at the base and typical floors, corners at the base and typical floors, and parapets. Four levels of performance were defined: operational, immediate occupancy, life safety, and collapse.

1) Operational (No damage) – Hairline cracking of masonry bed joints may exist, with or without a seismic event.

Brick Masonry Façade

T

he City Creek Center's masonry façade is an example of a structural engineering project that used performance-based-design to improve upon conventional, mundane brick façade systems.

Located in Salt Lake City, Utah, the project site encompasses 23 acres of land representing two large city blocks. It contains four residential buildings with 535 units, three office buildings, and 700,000 square feet of retail space. A creek runs through the site, thus the name. The exterior walls of the project are brick, precast concrete, and glass.

Eight architectural firms were involved in the project, but only one structural engineer of record, Magnusson Klemencic and Associates (MKA), Seattle. MKA approached KPFF Structural Engineers early in the project to assist in the design of the brick exterior walls.

As a structural engineer specializing in the design of facades or curtainwalls, including walls constructed of brick masonry, opportunities to design a project of this size rarely occurred during the author’s career.

It has been a challenge to convince owners and architects to design brick walls before bidding and show those designs on a set of drawings, instead of specifying performance and requiring the contractor to design the wall. The City Creek project opened the door to this alternative delivery system.

Because of the complexity of the wall, all parties agreed, after much discussion, to include the wall Element Frequent Event design as part of the project design Flat wall – base Operational documents, allowing for a perforCorners – base Operational mance-based-design (PBD) method Flat wall – typical floor Operational to be used.

Although several different masonry Corners – typical floor Operational wall systems were used on the projParapets Operational ect, including reinforced veneer 24 STRUCTURE magazine

Earthquake 500 Year Return

⁄3 Maximum Considered

2

Operational

Immediate Occupancy

Immediate Occupancy

Life Safety

Operational

Immediate Occupancy

Immediate Occupancy

Life Safety

Life Safety

Collapse

2) Immediate Occupancy (Minor damage, repairable) – Failure of caulked joints and separation of window seals is expected and can be repaired. Cracking of masonry bed joints is expected. Some cracking of brick at corners. Some vertical cracking through brick units is likely but limited. Some separation of face shells from the wall and units from parapets and other appendages.

3) Life Safety (Major damage, repairable) – Severe damage to portions of the wall and minor separation from the building, with no panel falling hazard.

4) Collapse (Major damage, not repairable) – Large portions of the wall have substantial damage and create falling hazards.

The Table presents the resulting performance criteria for designing each type of wall.

The edge of slab tolerances were defined; in-out (plus or minus ½ inch), up-down (plus ½ inch up and 1 inch down). The edge of slab tolerance is tighter than typical construction but was agreed upon as the innovation of the connection evolved. The up-down tolerance included the effects of the building frame creep and shrinkage.

Building floors were 9-inch-thick post-tensioned slabs. Edge forms for post-tensioning typically have round holes for tendons.

Placing an embedded bolt in the slab, protruding through some of the holes to connect the brick ledger, would be easy. The bolt could be bent to engage the bottom of the slab form and a coupler added at the end to attach a bracket for a push-pull rod connection to the top of the floor below studs. This eliminated the need for the top of the stud system to have the questionable double-channel detail. The double channel detail is questionable because performance for both in-plane sliding and accommodating differential vertical movement requires careful installation.

The nominal 9 inches from the edge of Corner Design the slab to the face of brick provided the Meeting the operational and immediate opportunity for a custom ledger support Figure 2. Corner test for warping investigation. occupancy criteria for anticipated damage bracket. Friction bolt connections were at the corners of the building presented a preferred over welding. Vertical slotted design challenge. A typical BV/SS wall would not meet the criteria holes in the backup plate accommodated the vertical tolerance, and at the corner. Differential floor-to-floor displacement would break horizontal slotted holes in the ledger accommodated horizontal tolerthe rigid brick corner. Isolation of brick corners can be accomplished ance. Connections were typically spaced at 4 feet on-center, and the by various strategies: ledger angle was a 3×3×5⁄16 weighing 73 pounds for a 12-foot length 1) Eliminate the brick corner and substitute another element, (Figure 3). such as an aluminum plate.

Reinforced Window Lintels 2) Provide a large expansion joint at the corner.

3) Cantilever the backup system from one floor without attachThe design of window and door lintels provided another opportunity ment to the floor above. for innovation. Typically, bricks above a door or window are supported 4) Build a reinforced veneer or reinforced brick panel that is sup- on mild steel angles designed to a deflection limit of L/600. The angle ported on one floor without attachment to the floor above. is exposed to view and becomes an aesthetics issue. Also, there was 5) Warp the backup and brick system by placing the attachment the complexity of the doors and windows being inset 6 inches from to the upper floor at a defined distance from the corner. the face of the brick.

All options were considered. The decision was to use option 5.

Instead of the conventional steel lintel, a structural brick lintel was Options 1 thru 4 could have been developed by analysis without designed and specified. Figure 4 (page 26) shows a unique pistol-shaped additional technical information. But the information to accomplish Option 5 was not available. Warping the masonry (a panel with 3 corners fixed and the fourth lifted or pushed perpendicular to the masonry surface) is not a common design problem, and no information was available. Consequently, it was decided to test the corner. A mockup panel was constructed and tested to obtain the information (Figure 2). The test demonstrated that the corner criteria could be satisfied.

Brick-to-Building Connection Innovation For a BV/SS design, the edge of slab detail is important, not only to resist the required loads and accommodate expected differential deflections but also to minimize construction cost. There was a need to improve common details, which usually include slab embedded items that are often not properly located, tilted, and/or missing.

Figure 3. Edge of slab connection.

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Figure 4. Reinforced lintel.

brick that was fabricated and reinforced with a stainless steel (SS) allthread (much more economical than SS rebar). The lintel was shored for construction; a panelized lintel was not used due to project space restrictions and lifting limitations.

The Value of PBD The above is only a part of the innovation applied to these two buildings. There is more, like the design of the thin stone façade at the base of the building and much more on other parts of the City Creek Center project as a whole, including the reinforced veneer (structural brick veneer) on the retail portion of the project.

The City Creek project demonstrates the value of performancebased-design and designing the curtainwall system in coordination

with the rest of the design. For the two buildings, a total of 62 full-size curtainwall structural drawings were required. Costs were reduced and performance enhanced.

Most important, the inevitable conflicts between parties involved in a complex curtainwall construction project were significantly reduced. The project became a successful team effort.

Curtainwall structural engineering fees, per square foot, are commensurate with structural engineering fees for the primary structure, but the technology and materials can be more challenging. The initial curtainwall design fees may likely be the reason owners do not take advantage of the overall cost savings, which is a lost opportunity.■ This article originally posted as an online-only article in August 2020. The article is included here (May 2021 issue) at the author’s request.

John G. Tawresey is a retired CFO of KPFF Consulting Engineers in Seattle, WA. He is a past president of The Masonry Society, past editor of the Masonry Society Journal, past president of the Structural Engineers Risk Management Council (SERMC), past president of the Structural Engineering Institute of ASCE, and a current member of the TMS 402/602 Main Committee. He is a member of the National Technical Programs Committee for SEI and an adjunct professor at the University of Washington. (johntaw@aol.com)

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Earthquake Af termath

SALT LAKE CITY & COUNTY BUILDING

By Jerod G. Johnson, S.E., Ph.D., and Mason T. Walters, S.E. Courtesy of Preston Norris, VCBO Architecture.

28 STRUCTURE magazine

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n March 18, 2020, the historic Salt Lake City & County Building experienced shaking from an M5.7 event with an epicenter nearly 9 miles away. Damage experienced by this base-isolated, unreinforced masonry structure was hardly perceptible compared to other aging structures of the region and barely newsworthy…a considerable credit to the vision of the original stakeholders and the designers of its retrofit, which took place in the mid-1980s.

The unfortunate reality of structural engineering is that an entire career of work can often go unnoticed by the general public.

The exceptions to this are associated with projects that are exceptionally eye-catching or those for which things go horribly wrong.

Unfortunately, the latter type of project is most often deemed “newsworthy” by the media, whether they are “victims” of engineering mistakes, construction accidents, acts of terror, or acts of God. Projects that behave exactly as intended rarely make the 6 o’clock news. For most engineers, never having a newsworthy project will be counted as a “win.”

The Salt Lake City & County Building is a massive unreinforced masonry structure completed in 1894 as a regional government seat.

It is comprised of Richardsonian Romanesque Architecture, having 5 stories with a central clock tower rising over 250 feet from the ground level. It is heavily clad in sandstone; most masonry walls are load-bearing, having multiple wythes of unreinforced brick. The building is listed on the National Historic Register and complements the city block (Washington Square) it occupies. Washington Square Block was the original landing site of countless pioneers and immigrants arriving in the area after what, for most, was a very difficult and perilous migration.

By the 1970s, the building had fallen into severe disrepair. Years of neglect coupled with a sufficient lack of respect for its historic significance brought the building to the brink of demolition. Fortunately, sufficient interest in preservation led to a city bond enabling a $30M renovation effort completed near the end of the 1980s. In the years leading up to its renovation, the region’s seismic potential had been revealed. Hence, the renovation included engineering efforts to address the rather precarious unreinforced masonry. Prior earthquakes in other regions and other learning experiences had revealed performance expectations for this building that were dismal at best. Studies confirmed that the unreinforced masonry was ill-suited to resist expected earthquake demands. The clock-tower was particularly vulnerable due

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Figure 1. Seismic base isolator.

Figure 2. Masonry cracking manifested in plaster coating (inset).

to the increased seismic accelerations it was expected to experience • The potential of unreliable isolator test results due to visco-elastic over its height. (velocity-dependent) effects related to the extremely low rate Although seismic base isolation technology was in its infancy, project used for testing spare isolators. stakeholders saw its value as a means to reduce seismic demand on the • Presence of moat “bumpers” that had been introduced near the structure. The new technology would alleviate the need for massive end of the original renovation in an effort to arrest excessive deflection in the event of a very large earthquake. seismic retrofitting of unreinforced walls throughout the building. It would also make the Salt Lake City & County Building the world’s Approximately 10 years ago, stakeholders for the project comfirst seismic isolation retrofit project. As a result, 443 cubic isolators missioned a comprehensive assessment of the isolation system, now de-couple the building from the earth, thereby filtering much of including full-scale, real-time testing of isolators at UCSD’s Seismic the frequency content that would otherwise result in damaging accel- Response Modification Device. This testing included a loading eration in the building’s superstructure (Figure 1). Also, an armature regime consistent with earlier tests (for comparative purposes) and of red-iron bracing was developed and installed for the clock tower to a testing regime pushing the isolators to the brink of failure. The address its potential seismic accelerations, which, although reduced purpose of this was to understand the system’s ability to respond in by base isolation, were still large enough to be a concern. consideration of a large but rare (e.g., BSE-2/MCE) event.

Based on the isolation system’s engineering design, the building’s funWhereas initial testing included displacements to 4 inches (based on damental period was lengthened to approximately 2.5 seconds, a shift apparatus limitations), the modern testing allowed for displacements correlating to nearly an 80% reduction in spectral acceleration and to a nearly “rolled-over” condition, thereby enabling the characterizaforces when compared to the non-isolated condition. Such comparisons tion of the maximum-capable displacements of the isolation system. were derived from a site-specific spectra study developed as part of the The testing demonstrated maximum-capable (and at least somewhat renovation design. Interestingly, the 1985 Uniform Building Code (UBC) stable) isolator behavior to a full displacement of nearly 16 inches, prescribed lateral design accelerations of 0.1g for ductile structures in commensurate to approximately 70% of the maximum displacements the region of the building’s location. Owing to the project’s importance, expected in consideration of the large but rare Maximum Considered stakeholders increased this acceleration to 0.2g in the development of Earthquake. Tests clearly demonstrated the isolation system’s ability to spectra and time history records to use for design, taken as scaled versions perform as required for the smaller BSE-1 event or shaking intensities of the 1979 Imperial Valley Earthquake, Bond Corner Accelerograms. correlating to SDS or SD1 of modern codes.

Today, prescribed short-period spectral accelerations for this site are nearly 1.0g (Site Class D). Among other issues, the increase in expected seismic demand prompted current stakeholders to engage in a comprehensive study of the City & County Building’s base isolation system. Most notably, the isolators themselves were observed to be markedly smaller than those of another nearby structure that had recently been base-isolated, the Utah State Capitol Building. The observed differences reflect differing expectations of shaking intensity between the two projects, attributable to the rapidly advancing field of geoseismic engineering. Other issues raised concerning the isolation system at the City & County Building, based on modern standards, are the following: • Potential for hardening of isolator elastomers with time.

• The potential of skewed results for regular isolator testing by virtue of testing the same spare isolators Figure 3. Downtown Salt Lake City Acceleration Spectrum, Magna Earthquake and Code Spectra. every 5 to 7 years.

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Figure 4. Downtown Salt Lake City Displacement Spectrum.

Figure 5. Salt Lake City & County Building – displaced stone at isolated plane.

At approximately 7:09 am on March 18, 2020, the Magna Earthquake struck the Salt Lake Valley. While this alarming event drew significant notice and many calls to respond, it was a small earthquake by all accounts and metrics. A report of zero casualties was certainly good news for this event. Still, the M5.7 caused motions that were sufficiently intense to wreak havoc among the thousands of unreinforced masonry structures throughout the region. Many URM’s met the description of “substantial damage,” as outlined in the 2018 International Existing Building Code (IEBC). Numerous other URM’s suffered damage, meeting the IEBC’s definition of Disproportionate Structural Damage (relatively high damage for a small earthquake).

Not surprisingly, URM structures that had been previously retrofitted behaved comparably to more contemporary structures. Their performance was not considered newsworthy, and the Salt Lake City & County Building was among them. Damage manifest within the structure was comprised principally of cracked plaster due to in-plane action of shear walls and spandrels. Upon stripping cracked plaster, it became clear that most of the cracks were triggered by minor masonry bed-joint sliding that induced small permanent deformations (openings in joints) that are hardly perceptible (1⁄16 inch or less). Many such cracks were revealed only through the relatively sensitive and brittle plaster coatings (Figure 2) and are believed to have been pre-existing and were only exacerbated by the earthquake. Beyond the observed cracking, the isolation system was clearly mobilized, as evidenced by displaced stonework at the building entrances and moat covers.

Instrumentation records reveal that structures in Salt Lake City’s downtown area experienced about 20% of the shaking intensity of structures closer to the epicenter (about nine miles to the West). Figure 3 displays a spectrum developed from the event at a site not far from the Salt Lake City & County Building. When compared to the code-prescribed spectrum, this was clearly a small event. The fact that damage was limited primarily to archaic URM structures should not be surprising. Figure 4 displays the correlating displacement spectrum developed for the same site as Figure 3 (motions available through the Center for Engineering Strong Motion Data, CESMD; see www.strongmotioncenter.org). Interestingly, recordings from instrumentation on the site and in the building are either not available or non-existent.

The base isolation system design of the City & County renovation targeted a fundamental period of approximately 2.5 seconds.

Considering the displacement spectrum of Figure 4, peak displacements for the City & County Building with its isolation system should be approximately 2 to 2.5 inches. Examination of stonework abutting the isolated plane reveals that the displacement

experienced is consistent with the spectrum of Figure 4. In particular, displaced stones (Figure 5) indicate a maximum isolation system displacement slightly higher than 2 inches. A rough comparison of other spectral ordinates suggests that peak accelerations for the overall building were roughly 0.03g. A comparable, non-isolated building could have experienced accelerations of nearly 5 times higher or more.

The Salt Lake City & County Building stands as a literal manifestation of the benefits of modern engineering. Having suffered only cosmetic damage for a small event shows that seismic base isolation can be an extremely effective measure for addressing the seismic threat.

Furthermore, the consistency of observations of this building with the behaviors predicted using the equations of fundamental structural dynamics is striking and reinforces the concept that properly implemented engineering methods can satisfactorily predict the behaviors of systems subject to unusual conditions.■ Reference is included in the PDF version of the article at STRUCTUREmag.org.

Jerod G. Johnson is a Principal at Reaveley Engineers in Salt Lake City. He was the engineer of record for recent updates to the base isolation system for the Salt Lake City & County Building. He was the principal investigator of the comprehensive isolator testing of May 2011. (jjohnson@reaveley.com) Mason T. Walters is a Senior Principal at San Francisco Based Forell/ Elsesser Engineers. He played a key role in the Salt Lake City & County Building’s original base isolation design. (m.walters@forell.com)

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education ISSUES 2019 NCSEA Structural Engineering Curriculum Survey Results

By Scott M. Francis, P.E., SECB

T

he National Council of Structural Engineers Associations (NCSEA) is pleased to present the 2019 NCSEA Structural Engineering Curriculum Survey results. The survey is a triennial review of the recommended NCSEA Structural Engineering Curriculum at over 175 civil or architectural engineering schools throughout the country that offer educational opportunities for students desiring to become structural engineers.

For nearly 10 years, NCSEA has promoted the recommended NCSEA Structural Engineering Curriculum as the core subject matter deemed necessary by the profession for a sound educational background in structural engineering. The recommended curriculum consists of the following twelve courses: • Structural Analysis 1 – Determinate Structures • Structural Analysis 2 – Indeterminate Structures • Structural Analysis 3 – Matrix Methods • Steel Design 1 • Steel Design 2 • Concrete Design 1 • Concrete Design 2 (A-Advanced Reinforced or B-Prestressed Concrete) • Wood Design • Masonry Design • Structural Dynamics • Foundation Design/Soil Mechanics • Technical Communications Full course descriptions can be found on the Basic Education Committee

page of the NCSEA website (https://bit.ly/3taZ0M1). While these courses are recommended for a sound educational background, it is recognized by NCSEA that not all students will complete all of these courses before entering the workforce. Therefore, some Figure 1. Number of BEC recommended courses offered by post-graduate training will occur, Universities in 2019. whether formal (graduate school) or informal (on-the-job training), to further by ABET as Civil Engineering, Architectural develop the engineer’s skill set. Subsequent Engineering, and Structural Engineering. At to launching the curriculum survey, the com- this time, we do not include Civil Engineering mittee launched a Practitioner Survey, sent to Technology and Architectural Engineering all NCSEA members to gather information Technology programs. There are 311 degree about the structural engineering community’s programs in either civil or architectural engidesires from their new hires out of college. neering at 283 schools in the U.S. Of these, In this first of two articles developed by the there are 283 civil engineering programs NCSEA Basic Education Committee (BEC), and 28 architectural engineering programs. curriculum survey results are reviewed. A sub- If the school participated in the 2016 survey, sequent article will compare results where the same contact person was used for the academia and practitioners agree and disagree 2019 survey. However, if the school had not on courses that prepare students to transition responded to the previous survey, a contact to practicing engineers. was selected from information on the school’s website and was usually selected because they serve as chair of their department or taught Survey Process structural engineering-related courses.

The NCSEA BEC began planning for the The NCSEA BEC obtained survey results 2019 Curriculum Survey soon after the pre- from 176 of the 258 contacted schools. Please vious survey results were published in the note that the survey results herein are based on (September 2016, STRUCTURE magazine). self-reporting from the schools; BEC memThe list of schools contacted for participa- bers have performed no outside verification tion in the 2019 survey was first verified by of course offerings. reviewing all engineering programs accredited Graduation from an ABET-accredited program is the most common path towards achieving the educational requirement necessary for licensure. ABET does not require a set list of courses to be taught to engineering students at the undergraduate level. Instead, they provide a list of student outcomes and the number of credit hours that fall under the category of engineering topics, consisting of engineering sciences and engineering design appropriate to the student’s field of study.

For the 2019-2020 academic years, ABETaccredited schools were required to provide a minimum of 45 semester hours (or equivalent trimester or quarter hours) that fall in this category. Once students complete the necessary foundational (statics, dynamics, mechanics of materials, etc.) and breadth courses within Figure 2. Percent of engineering schools that offer the indicated recommended course. their area of study, undergraduate students are

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often left with only 16 to 24 semester hours of technical courses required to complete their degree. Therefore, at the conclusion of the average student’s undergraduate education, many have taken only four to six of the twelve BEC recommended courses.

Survey Results

The BEC sent out the practitioner survey to the members of NCSEA to collect information on the merit of the recommended courses and collect other data. The Practitioner Survey asked the same questions asked of the professors concerning the preparedness of students graduating with undergraduate degrees or graduate degrees to enter the workforce. Stay tuned for an upcoming article to review this survey’s findings and compare responses from consulting professionals and academia.

Survey results revealed that only 38% of the responding schools offer all twelve courses at either the undergraduate or graduate level Figure 3. Percent of Architectural and Civil engineering schools that offer the indicated (Figure 1). However, if course to undergraduate students. the scope is broadened to include all schools that offer at least 10 of the 12 recommended courses, universities, the student’s exposure to Conclusion 73% met this threshold. structural engineering can be augmented A breakdown of the percentage of schools by participation in student organizations. While NCSEA does not currently have a that offered each of the recommended twelve NCSEA member organizations have part- direct role at the table in formulating the courses (Figure 2) revealed nearly all respond- nered with schools to establish student academic requirements that students must ing schools offered Structural Analysis 1, Steel branches explicitly focused on structural meet for graduation, there are still opporDesign 1, Concrete Design 1, and Foundation engineering. To date, there are 25 stu- tunities for NCSEA members to support Design/Soil Mechanics courses. Likewise, a dent chapters of NCSEA. A brief check the outcomes for future professionals. We high percentage of schools offered seven out of ASCE’s website reveals over 300 ASCE encourage you to participate in your alma of the eight remaining courses. The only student chapters in the United States. The maters or other connected schools actively.

BEC recommended course that respondents Structural Engineering Institute, a subset There may be opportunities to serve on a indicated was offered by less than half of the of ASCE, also has student chapters for departmental advisory board and participate responding schools was Masonry Design. graduate students. Of the 176 responding in curriculum discussions. As alumni, you A further breakdown of materials courses universities, 11 had SEI student chapters. can collaborate with faculty to initiate and (Figure 3) reveals a wide discrepancy in the Other professional societies that have stu- cultivate a student chapter of NCSEA to augpercent of responding schools that offer dent chapters include ASCE’s Architectural ment the student’s experience. Alternatively, courses in each material at the undergraduate Engineering Institute (15 of 176), National you can volunteer to give a lecture/club level. As one would expect, Steel and Concrete Society of Professional Engineers (14 of presentation to the students to discuss Design 1 courses are nearly universal; how- 176), Earthquake Engineering Research the profession, the merits of NCSEA, and ever, the other design materials courses are Institute (40 of 176), Engineers without promote the recommended curriculum to offered at about half, or less, schools at the Borders (92 of 176), and the American students who are selecting technical elecundergraduate level.

Concrete Institute (3 of 176). tives. Finally, find ways to engage students For each of these materials courses, the directly as a mentor to promote NCSEA’s survey asked the schools to select no more recommended curriculum and positively Preparedness for than two of the following reasons why they affect their preparedness. With such efforts, Entering the Workforce did not offer a course in these materials: lack NCSEA’s members can support students’ of demand from students, lack of school We polled the professors who responded to preparedness as they enter the workforce support, lack of research funding for the our curriculum survey to see if they felt the and become professional engineers. material, lack of professors with familiarity students who completed their undergraduDo you have an interest in continuing in the material, imposed unit restrictions, ate degree were “adequately prepared when this important dialog between practitioor lack of professor interest. The three top entering the workforce.” Of the 176 respon- ners and academia, or would you like more answers, which were pretty evenly split dents to the survey, only 12% responded, ideas on engaging with academia? In that across the different materials, were: lack of “most definitely; they have the tools and case, we invite you to reach out to Judy demand from students, lack of professors skills, and are ready to perform structural Liu or Scott Francis, Co-Chairs with familiarity in the material, and imposed engineering.” The same question was asked of the BEC Committee, for more unit restrictions. regarding graduate students, and the per- information.■ centage responding “most definitely” rose Scott M. Francis is a Senior Structural Engineer to 80%. These responses show a very clear Student Organizations with Wiley|Wilson and is a co-chair of the delineation of where the academic world NCSEA Basic Education Committee.

In addition to academic opportuni- views the cutoff between having the basic (sfrancis@wileywilson.com) ties offered at the various colleges and tools and skills to enter the workforce.

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historic STRUCTURES West Hartford (Woodstock) Bridge Disaster

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

T

he Central Vermont Railroad was chartered on October 21, 1843, cars broke, the engine, baggage and smoking cars passed on to the to run from Burlington on Lake Champlain to Windsor on the bridge and over in safety but the other four cars bumped along over Connecticut River via St. Albans and Montpelier, the state Capital. the ties to the end of the bridge, knocked out the heavy timbers It was opened to traffic at various times which rested on the abutment, and then in 1849. It crossed the White River, a toppled over bridge, cars and human tributary of the Connecticut River about freight, fully eighty souls all told, falling four miles north of White River Junction with a tremendous crash down the jagged between Woodstock and West Hartford. precipice seventy feet striking upon the A major high bridge was built across the frozen surface of the river. river consisting of four spans of 140 feet Then followed a scene which beggars all and a short span of 70 feet, with a total power of human description. The splinlength of approximately 630 feet on a tered wreck took fire, and the dark gorge, large skew to the river. It was a 26-footfrom which the moon was hidden, was deep, single track, double Towne lattice soon lighted up by the glare of burning deck bridge, made of plank and strengthcoaches and bridge timbers. The detached ened by heavy timber arches with the piers portion of the train was stopped and run and abutments built of granite. It was back to the scene as soon as possible. covered on the sides and top to protect Those on board sprang into the deep the trussing from the weather. Its top was snow and made their way as best they also covered with iron plates to minimize could down the steep banks to assist any the chance of fire. From the track to the in the wreck who were alive…Many were surface of the ice (water) was 42 feet, with pinned beneath huge timbers, beyond all the bottom of the trusses 16 feet from the human aid.” river’s surface. The bridge was considered In those days, the cars were heated by by many to be a fine example of bridge coal-burning stoves placed in the cars design and construction. periodically and were lighted with sperm All was well with the bridge until the oil lamps. When the cars crashed onto the early morning of February 5, 1887, when Bridge in the White River; note iron rails and wrought ice, the stoves were upset and burned the what was called “the worst railroad acci- iron sheet intact. wooden coaches. The fire was so intense dent in Vermont history” occurred. The it ignited the wooden bridge above, and express train from Boston to Montreal, Canada, consisted of an engine, eventually the entire bridge collapsed into the river. one baggage car, one express car, one mail car, two ordinary passenger Fortunately, for the record, Robert Fletcher, a civil engineering coaches, the sleeping car St. Albans, and the Pullman sleeper Pilgrim, professor at Dartmouth College, visited the site on the next day and from Boston. The St. Albans carried about twenty-six passengers, and submitted a report published in the Engineering News on February 12 the Pilgrim about forty passengers. with illustrations. He wrote in part, Besides these passengers, there were “The testimony of escaped pasabout fifty people in the passenger sengers shows that the rear car, and coaches. The train was running late perhaps also the next one, became A few pounds of cast iron and a on a bitterly cold night, -20°F, and derailed when within a few rods of the reached the bridge around 3:00 AM. bridge. The testimony of the engineer few feet of timber . . . would have What happened next was told in the is to the effect that when the locomosaved every one of the forty or fifty Frank Leslie Illustrated Newspaper, tive was partway across the bridge the with an engraving of a burning car rope was pulled...The three cars in lives which appear to have been on the cover, as follows, advance of the rear one were turned “It was at this point that the train and dragged off in quick succession, lost, and the thirty or more injured. met its fate – a broken rail 200 feet rolling over and falling upside down from the bridge being the cause. on the ice, where they lay extended Whether the train broke the frosty from near the south abutment to a rail, throwing the cars from the track, whether the rail was broken point a little beyond the second pier… before the train arrived, or whether some wheel gave way and snapped Going to the embankment by which the south abutment is the rail is not known and may never be known. approached, it was found that the inside rail of the slight curve In an instant there was a jar, a bumping of tracks over the railroad leading toward the bridge had been entirely re-laid that morning ties. The coupling between the forward sleeper and the four following from the bridge to a point about 450 to 470 feet distant – sixteen

34 STRUCTURE magazine

or seventeen rail lengths. At intervals from this point, the ties were scratched and deeply cut nearly across the whole width, between the rails and also on the outside or to the right of the easterly rail, out to the ends of the ties, as the bridge was approached. Whatever marks may have been made in the snow and the frozen ground had been obliterated by the tramping of feet. Just outside the track, the snow was from 1 to 8 feet deep. On the east side, …was a pile of rail fragments, pieces of two or three rails, all but covered by snow.

A few feet farther on were more fragments of one, and perhaps of two, rails. Whether all of the pieces were due to the accident, or some resulted from the work of the section hands in re-laying the track, we have not ascertained…

These are the principal facts relevant to the case…However, in the opinion of your correspondent, the evidence seems to point to the failure of a rail about 450 feet from the end of the bridge as the beginning.

Whether this failed partly under the locomotive or under one of the forward cars, we can only guess, but the battered condition of the ends of some of the fragments indicates that they had been struck by two or more trucks or wheels, which may have succeeded in mounting and passing over them.”

Engineering News ran articles on the failure for months. They were promoting the Latimer re-railing system and wrote of the failure that it, “…belongs to the class of accidents which may be most easily, cheaply, and certainly prevented. That this particular disaster could, with certainty, have been prevented, it is beyond human knowledge to declare, but the multiplied instances in which trains derailed, as this one was, just before reaching a bridge, have been replaced upon the rails and all disaster avoided by the Latimer Broken rail by Fletcher. re-railing bridge safety-guard justifies a belief that the chances were many to one that a few pounds of cast iron and a few feet of timber arranged in the manner which we illustrate on another page, would have saved every one of the forty or fifty lives which appear to have been lost, and the thirty or more injured.”

They later wrote a long article entitled, The Cause of the Woodstock Disaster and its Lessons, and concluded, “Thus we find that this accident, in all human probability, was a case of broken rail derailment pure and simple, with or without the early breakage of a single axle of the rear truck, as a secondary consequence of its more erratic movements and more violent blows…The lesson of this disaster then is plain, and let us hope that it will sink into the minds and hearts of engineers so that they will never forget it. Good practice spreads slowly when it involves expense without immediate return or urgent necessity, and not wholly without reason. But he who studies the facts of this and other cases and then neglects hereafter to avail himself of that cheap defense of railroad trains, the cast iron watchman which we illustrate, which neither sleeps nor eats, nor asks for pay, but is always faithful and always on hand, will be little short of a criminal unless he can extract a very different moral from the facts from what we can.”

The Railroad Commissioners of the State of Vermont launched an in-depth study of why the accident occurred. They interviewed survivors, train crew, etc. They submitted their report later in February, and an extract was published in the Railway Review on February 26, 1887. They found, “The distance from White River Junction to the Hartford (formerly known as the Woodstock) bridge is about four miles. South of the bridge is a curve of 3 degrees 45 minutes in the track, which becomes straight again about 142 feet from the bridge and so continues for some rods beyond the bridge. From a point some 50 rods south of the bridge to a point about 142 feet therefrom, the grade is slightly downward when it becomes level and so continues to a point just beyond the bridge.”

They made the following recommendations, “Upon the subject of recommendations, the board is not prepared to report fully.

There is no ground of doubt that many who perished in this accident would have been released from the wreck alive had it not been for the stove fires and oil lamps which ignited the varnish, paint, draperies and other combustible material, almost as soon as the crash came, and caused the suffocation and burning of those who were pinioned beneath the rubbish before they could be extricated…Unless some better system is adopted by the roads of the state before another winter, as at present advised, the adoption of steam heating from the locomotives or other device without fires in the cars will be recommended by the board; and we do most earnestly recommend to the officials of the roads of the state in behalf of the corporate as well as public interests that they give especial attention to this subject by conference with officials of roads with which they have connection or business relations and by affording opportunities for experiment upon such inventions as seem to present promise of value in the direction of safety from fire and suffocation, in cases of derailment and collision…But it seems proper at this time, and upon this matter, to advise all the railroad companies of the state to take into consideration the most approved methods in vogue anywhere for the better equipment of their bridge approaches where needed, with strong buttresses, flaring safety beams, or other practical devices to diminish the fatality of this class of accidents. Such reasonable precautions should be taken before rather than after the occurrence of any fatal calamity.”

Lawsuits against the railroad went on for years, and it is not known what exact amount the line paid out. The greatest dangers to wooden bridges were fire, flood, decay, and derailment. In this case, it was a combination of fire and derailment. However, the fire was different as burning cars from below set the bridge on fire, and the derailment did not cause the bridge to collapse but only to result in the rear cars sliding off the bridge on the iron cover plates.■ 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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NCSEA

NCSEA News

National Council of Structural Engineers Associations

Board of Directors Transition | Announcing Officers for 2021-2022 NCSEA is pleased to announce the new Board of Directors for 2021-2022. Previously, the Board of Directors transition took place during the Structural Engineering Summit. In 2019, the Council made the decision to change the association's fiscal year and have the Board of Directors term follow the same schedule.

Ed Quesenberry (SEAO), Equilibrium Engineers, LLC., will serve as President replacing Emily Guglielmo (SEAONC), Martin/Martin, Inc., who will transition to Past President for one year before exiting the board. David Horos (SEAOI), Skidmore, Owings & Merrill, was named Vice-President, Ryan Kersting (SEAOC), Buehler Engineering, remains as Secretary, and Christopher Cerino (SEAoNY), STV, Inc., was named Treasurer. Two new directors have been welcomed to the Board: Sarah Appleton (SEAOG), Wallace Engineering, and Brian Petruzzi (SEA-MW), Facebook. Richard Boggs (SEC/CT) and Paul Rielly (SEAoT) have exited the Board. Jami Lorenz (SEAMT), DCI Engineers, also joined the board in 2020 when a change in personnel was required.

The Current NCSEA Board of Directors is: • President: Ed Quesenberry (SEAO) • Vice President: David Horos (SEAOI) • Secretary: Ryan Kersting (SEAOC) • Treasurer: Christopher Cerino (SEAONY) • Past President: Emily Guglielmo (SEAOC)

• • • •

Director: Eli Gottlieb (SEAoNY) Director: Jami Lorenz (SEAMT) Director: Sarah Appleton (SEAOG) Director: Brian Petruzzi (SEA-MW)

Ed Quesenberry, S.E. President

David Horos, S.E. Vice President

Ryan Kersting, S.E. Secretary

Christoper Cerino, P.E. Treasurer

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Jared will be giving a special introduction webinar for this terrific series; join us for this sneak peek by visiting www.ncsea.com to register.

Jared Jamison, founder and president of AE Ascend, is an accomplished A/E industry executive with over 20 years’ experience in A/E firm management, operations, financial management, and business strategy.

After spending the first half of his career in structural engineering, he shifted his focus to the strategy and management of A/E firms. He is a speaker, writer, and adviser on topics of business management and strategy with an emphasis on the A/E industry.

Registration includes 4.5 hours of expert-led education, the Strategic Action Playbook, access to the interactive forum, and the recordings for a full year. Register on www.ncsea.com.

36 STRUCTURE magazine

News from the National Council of Structural Engineers Associations

Secure Structural Engineer Specific Emergency Responder Training May 11 & 12, 2021 Speakers: Derek Hanson, P.E., and Klaus Perkins, P.E., S.E. NCSEA's structural engineering focused Emergency Response Course is built upon the CalOES Safety Assessment Program that is highly regarded as a standard to train emergency second responders. This FEMA reviewed and approved training course provides engineers, architects, and code-enforcement professionals with the basic skills required to perform safety assessments of structures following disasters.

NCSEA's course builds-in structural engineer focused education and lessons.

Licensed design professionals and certified building officials will be eligible for SAP Evaluator certification and credentials following completion of this program and submission of required documentation.

STRUCTUREsolutions: Software Webinar Series May 19 & 20, 2021 | 12:00 CST

A new webinar series demonstrating structural engineering software success from these participating vendors:

This two day course takes place Wednesday, May 11, and Thursday, May 12.

Learn more about the course at www.ncsea.com.

Excellence Awards Webinar Series June 9, 16, 23 & 30 Sponsored by Atlas Tube

Join this free event to learn all about the Outstanding Project Winners from the 2020 Excellence in Structural Engineering Awards. This program will highlight some of the best examples of structural engineering ingenuity throughout the world. Learn more by visiting www.ncsea.com.

Register at bit.ly/StructureSoftware

Connect with NCSEA on facebook, twitter, or linkedin for the latest news & events!

NCSEA Webinars

@NCSEA

on all the channels

Register on www.ncsea.com

May 4, 2021

Floor Vibration Design Methods for Timber, Steel, and Concrete Scott Breneman, Ph.D., P.E., S.E.

This presentation introduces floor vibration design methods for the many structural floor framing materials with an emphasis is on designing for human comfort and sensitive equipment to walking excitations. We will review guidelines available for wood, steel, and concrete framed floors.

May 18, 2021

Non-destructive Evaluation of Structural Concrete Nathaniel S. Rende, S.E.

This webinar will present the theory and practical applications of nondestructive evaluation methods used for the assessment of structural concrete and similar building materials, including methods to assess in-place concrete strength, characterize reinforcing placement, identify corrosion, and detect internal flaws.

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

M A Y 2 0 21

37

SEI Update Learning / Networking

Structures Congress 2022

Be part of the program and submit today.

SEI invites abstracts and sessions that support advancing the structural engineering profession, including leadership development, innovation and novel project solutions, emerging technologies, resilience, sustainability, function recovery, global climate change, and innovative research with practical application. Learn more at www.structurescongress.org.

NEW in the ASCE Bookstore and Library Hurricanes Irma and Maria in the U.S. Virgin Islands: Building Performance Observations and Recommendations for ASCE 7 William L. Coulbourne, P.E., Chrylyn Henry, P.E., Thomas L. Smith, AIA, RRC; Edited by Connor Bruns, S.E. 2021 / 112 pp.

American Society of Civil Engineers www.asce.org

Membership

SEI Local Chapter Activities and Best Practices 2020

SEI professional and Graduate Student Chapters serve 1,000+ members’ technical and professional needs through local events. SEI Chapters organize technical and social events, field visits, construction tours, scholarships, K-12 outreach, and more. Several SEI Chapters have shifted to virtual since COVID-19 and have increased attendance through this transition. Read more at www.asce.org/SEINews.

Call for Volunteer Members

The SEI Engineering Philosophy Committee’s goals are to investigate how philosophy is relevant to current issues in structural engineering within the broader engineering community and society. The committee has presented at Structures Congress and the Forum on Philosophy, Engineering, and Technology and submitted a proposal, “The Engineering Way of Thinking: Opportunities and Limits,” to the Society for Philosophy of Technology 2021 conference. They have published articles and papers in STRUCTURE, the ASCE Practice Periodical on Structural Design and Construction and the Journal of Professional Issues in Engineering Education and Practice (now Journal of Civil Engineering Education), and the international journal Science and Engineering Ethics. Apply to join the committee at www.asce.org/structural-engineering/sei-business-and-professional-activities-division. Questions? Contact Chair Bill Bulleit at wmbullei@mtu.edu.

Join us this year in celebrating 25 years of SEI – advancing and serving structural engineering!

Errata 38 STRUCTURE magazine

SEI Standards Supplements and Errata including ASCE 7. See www.asce.org/SEI-Errata.

If you would like to submit errata, contact Kelly Dooley at kdooley@asce.org.

News of the Structural Engineering Institute of ASCE Students and Young Professionals

Congratulations, New Graduates We are excited about all that is ahead for you!

• Make sure to take your SEI/ASCE member benefits with you - update your contact information 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 • Access Resources: Career Path Series, Career by Design, Mentor Match • Show your pride with an SEI t-shirt or necktie/scarf – www.asce.org/structural-engineering/sei-merchandise

SEI Online

SEI Virtual Events

www.asce.org/SEI/virtual-events • #SEILive Conversations with Leaders on Hot Topics in Structural Engineering: Diversity, Code Development, Licensure, Functional Recovery, and more • SEI Standards Series MAY 20 – ASCE/SEI 49 Wind Tunnel Testing for Buildings and Other Structures Join SEI Host Jennifer Goupil for a big-picture discussion of the state of wind tunnel testing with the chair of the ASCE/SEI 49 Committee Greg Kopp, P.E., M.ASCE, and committee member Forrest J. Masters, Ph.D., P.E. (FL), M.ASCE. The 2021 edition of ASCE/SEI 49, Wind Tunnel Studies for Buildings and Other Structures, provides the minimum requirements for conducting and interpreting wind tunnel tests to determine wind loads on buildings and other structures. Wind tunnel tests are used to predict wind loads and responses of a structure, structural components, and cladding to a variety of wind conditions. New to this version are requirements for wind loads on products, a critical design consideration for many mass-produced products constructed or installed at many different sites and in many different situations. Such products can be building-mounted (sunshades, solar racking, HVAC units, screen walls) or free-standing (ground-mounted solar trackers, gazebos, fences, communication towers). Also, commentary guidance is provided for determining wind loads on buildings and other structures in tornadoes, which is an area of current active research.

The expert panel will begin the session with an overview discussion on the state of the industry, the role the standard plays in the profession, and why it was updated. Then, Greg and Forrest will present the technical changes in ASCE/SEI 49, provide relevant design examples, and wrap up the session with live Q&A with the audience.

Individual session: Member $49, Nonmember $99. Student member: Free registration.

REGISTER NOW at https://cutt.ly/9hQDTEo Registration closes May 18 @ 10:00 pm US ET. • Structures Virtual – June 2-4, 2021 Join for skill-building, professional development, and fun networking. Technical sessions include Designing with Data, the ASCE 7-22 Preview, and Applications in Innovative Materials in Structural Engineering. Reconnect with friends and colleagues in the Mentoring Up and Down social event. Earn up to 10.5 PDHs. View the complete program and register by May 27 at www.structuresvirtual.org

SEI News Read the latest at www.asce.org/SEINews SEI Standards Visit www.asce.org/SEIStandards to view ASCE 7 development cycle M A Y 2 0 21

39

CASE in Point CASE Tools and Resources Did you know? CASE has tools to help firms address a wide variety of business scenarios. Whether your firm needs to establish new procedures or simply update established programs, CASE has the tools you need!

If your firm needs to update its current Risk Management Program or establish a program within the firm, the following CASE documents will guide employees: 962-H: National Practice Guideline on Project and Business Risk Management Tool 1-1: Tool 1-2: Tool 2-1: Tool 2-4: Tool 3-1: Tool 3-4: Tool 5-6:

Create a Culture for Managing Risks and Preventing Claims Developing a Culture of Quality A Risk Evaluation Checklist Project Risk Management Plan A Risk Management Program Planning Structure Project Work Plan Templates Lesson Learned

CASE Contracts Currently Available

CASE #1 – An Agreement for the Provision of Limited Professional Services CASE #2 – An Agreement Between Client and Structural Engineer of Record for Professional Services CASE #3 – An Agreement Between Owner and Structural Engineer as Prime Design Professional

Tools

CASE 5-1: A Guide to the Practice of Structural Engineering This tool is intended to teach structural engineers the business of being a consulting structural engineer and things they may not have learned in college. While the target audience for this tool is the young engineer with 0-3 years of experience, it also serves as a useful reminder for engineers of any age or experience. The Guide also contains a test at the end of the document to assess how much was learned and retained.

Additional sections focus on getting and starting projects, schematic design, design development, construction documents, third party review, contractor selection/project pricing/delivery methods, construction administration, project accounting and billing, and professional ethics.

CASE 9-2: Quality Assurance Plan This tool provides guidance to the structural engineering professional for developing a comprehensive, detailed Quality Assurance Plan suitable for their firm. From project initiation through construction completion, highquality client service is critical to project success and maintaining key client relationships. Elements of ensuring quality service include: • Client and project ownership by the individuals responsible for the project • Continual staff education including both leadership and technical skill development • Firm-wide Standard of Care • Quality control process with a complete communication loop • Written Quality Assurance Plan

You can purchase these and the other Risk Management Tools at http://education.acec.org/diweb/catalog/t/44250.

40 STRUCTURE magazine

News of the Coalition of American Structural Engineers CASE Guideline and Commentary on ASCE Wind Design Provisions The purpose of this Guideline is to provide guidance and commentary on the wind provisions of ASCE/SEI 7 and provide a brief overview of the changes from ASCE/ SEI 7-05 to ASCE/SEI 7-10 and again from ASCE/SEI 7-10 to ASCE/SEI 7-16.

The most recent revisions to the Standard have restructured the format of wind design procedures and added step-by-step checklists for each procedure to clarify how to use its provisions. The Standard is continually updating and editing its procedures based on the latest research, data, and studies.

You can purchase these and the other Risk Management Tools at http://education.acec.org/diweb/catalog/t/44250.

Donate to the CASE Scholarship Fund!

The ACEC Council of American Structural Engineers (CASE) is currently seeking contributions to help make the structural engineering scholarship program a success. The CASE scholarship, administered by the ACEC College of Fellows, is awarded to a student seeking a Bachelor’s degree, at minimum, in an ABETaccredited engineering program. Since 2009, the CASE Scholarship program has given more than $35,000 to help engineering students pave their way to a bright future in structural engineering.

We have all witnessed the stiff competition from other disciplines and professions eager to obtain the best and brightest young talent from a dwindling pool of engineering graduates. One way to enhance the ability of students in pursuing their dreams to become professional engineers is to offer incentives in educational support.

Your monetary support is vital in helping CASE and ACEC increase scholarships to those students who are the future of our industry. All donations toward the program may be eligible for a tax deduction, and you don’t have to be an ACEC member to donate!

Contact Michelle Kroeger at mkroeger@acec.org to donate.

CASE Member Firms Win Engineering Excellence Grand, Honor Awards

Congratulations go out to the following CASE Member firms for winning Grand Awards: • KPFF, OSU Marine Studies Initiative Building, Newport, OR • Magnusson Klemencic Associates, Chase Center, San Francisco, CA • Silman, The Heights, Arlington, VA • Walter P Moore, Bank of America Tower, Houston, TX These firms are finalists for the Grand Conceptor Award presented at the 54th virtual Engineering Excellence Awards Gala in Washington, D.C., being held June 17, 2021.

Follow ACEC Coalitions on Twitter – @ACECCoalitions.

M A Y 2 0 21

41

engineer's NOTEBOOK Error Checking and the Black Box Part 2

By Scott N. Jones, S.E.

I

n Part 1 of this series (STRUCTURE, April 2020), we discussed the Black Box and why it is critical to our work. We concentrated on how to control it in the areas of gross error (total load checks), boundary conditions, and deflections. We discuss load paths, connections, torsion, temperature and shrinkage, and dissimilar materials in this part.

Check Load Paths

most likely be fine. Step one, make sure the software is checking torsion!

Some do not. Step two, with reference back to deflected shapes, do you see the beam twisting under load? Dig into the internal stresses and the reactions being delivered to the supporting member.

Is there a moment being delivered due to the torsion?

Now, while you are at it, please do a quick check for the following often-overlooked torsional issues: 1) Beams radiused in the plane of the plan always have torsion, so do not calculate them as straight; 2) angles or other odd shapes taking gravity load in bending; 3) steel or concrete beams supporting outlookers; and, 4) sign and sign-like structures that need to receive the American Society of Civil Engineers’ ASCE 7’s, Minimum Design Loads for Buildings and Other Structures, unbalanced wind loading.

Extensive hand calculations forced the engineer to think more thoroughly about providing necessary continuous load paths. While a steel building that is entirely designed in RAM (structural analysis and design software) will have the general load paths taken care of by virtue of the model being correct (if you do not provide the path, the model will be unstable), that fixed-base steel moment frame is Temperature and Shrinkage Effects going to require some outside-of-the-[black]-box By virtue of their cross-sectional shape, calculations and detailing to get that taken care of. angles in bending are also in torsion.

Temperature and shrinkage make the most robust There will be no flashing red light reminding you concrete crack. They can shatter a weld or pop an to do so. Did that super-efficient Excel spreadsheet that the company embed like it was as brittle as glass. They are almost impossible to provided you actually check the shear around the window opening in resist, so make sure your strategy is to release the system instead. Does the shear wall? And if so, did it assume it will be strapped or bound- that big decorative steel beam that spans between two concrete walls ary fastened? Did the software assume the shear walls in the ends of have a slip connection at one end? Does the concrete reinforcing the U-shaped building are one line with a drag connector spanning extend uninterrupted through the control joint? What provisions have through thin air? Or separate shear walls? Follow the loads, from the been made at the reentrant corners? How much internal stress will diaphragm to the foundation, to make sure the black box did not build up in that long concrete deck with shear walls at both far ends? create a well-engineered fallacy.

Connections Connections are everything, right? And unless you are using a specialty connection program like RISAConnection, the program probably is not designing the connection. Further, any seasoned engineer (or steel detailer) can tell you that it is not uncommon for the connection’s geometric requirements to govern the size of the members being connected. Can those one-hundred 16-d nails fit in the connection without splitting the members being connected? Is the footing that “easily calcs out” for bending thick enough to develop the vertical bars extending into the structure above? The black box will not answer these questions for you, so beware!

Torsion Be afraid! Most of us do whatever we can do to make torsion a non-issue.

Just put in that extra bracing – it does not cost that much – and then we will not have to check torsion! That being said, torsion may be one area where the black box can actually do better than the average engineer with a pencil and paper. So while you should always keep an eye out for common conditions that can cause a torsional nightmare, if you just model it correctly (and check to make sure it is correct), you will 42 STRUCTURE magazine

Dissimilar Materials Lots of sleepers here, so when you have dissimilar materials, take the time to think it through. An obvious problem: corrosion. Galvanized steel-to-treated wood. Aluminum-to-stainless steel. But corrosion aside, let us look at how those materials work together. Whenever wood and steel are used together, we have to think about what happens when the wood shrinks (because it will) and the steel does not (because it will not). Big mistakes can be made when designing custom steel hangers for tall wood beams. It is essential to make sure the beam can shrink perpendicular to the direction of the grain without bolts or anything else resisting that shrinkage (and causing cracking). Is there a steel stair up to the 4th floor of that wood-framed apartment building? Or is there a concrete parking structure adjoining it at each floor? Or is there a masonry elevator shaft? The shrinkage of a wood structure can easily be on the order of several inches, depending on how things are detailed, and that tripping hazard could be enough to land a resident on the floor and you in court.

In the upcoming Part 3, we will discuss high-level strategies for the successful use of design software.■ Scott N. Jones is a Partner at Wright Engineers in Orange County, California. (sjones@wrightengineers.com)

M A Y 2 0 21

STRUCTURES VIRTUAL 21 JUNE 2 - 4 #StructuresVirtual21

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Oliver Broadbent

Stacie Sire

Ron Klemencic

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Receive 50% off membership using code STRM21 at go.asce.org/standards

WHERE VISION BECOMES STRUCTURE

RISA offers a comprehensive suite of design software that work together to simplify even the most complex projects. As a result, engineers can work efficiently on a variety of structures in a mix of materials including steel, concrete, wood, masonry and aluminum. risa.com

STRUCTURE M AY 2021

Bonus Content

construction ISSUES Damage Control for Deep Excavations Preventing Damage Caused by Ground Loss and Water Loss

By Hee Yang Ng, MIStructE, C.Eng, P.E.

D

eep excavation projects in highly urbanized and built-up environments must be designed and constructed with additional care. The design of these projects needs to deal with lateral wall movement, ground settlement, ground loss, and subsurface water pressure changes, all of which can lead to damage to adjacent structures (Figure 1). Damage can be in the form of cracks on non-structural walls (appearance), jammed windows and doors or dysfunctional services (functionality), and, in severe cases, cause structural members to be distressed (safety). Work suspension limits, usually for building settlement and inclinometers, may be breached. The project team could potentially face delays, claims, and additional costs due Figure 1. Example of damage due to deep excavation. to expensive rectification work.

The ground surrounding a deep excavation usually settles. This is have an angular distortion of 1/300. Therefore, a total settlement due to wall movement towards the excavation when soil is taken limited to 1 inch (25mm) for a footing can be considered a reasonout (Figure 2). In design, wall deflection may be controlled by able figure, assuming the differential settlement is a fraction of the applicable codes. In the author's region of practice (Asia-Pacific), total settlement. For piles, the settlement criteria are more stringent. deflections are limited to 0.5%H (excavation depth) based on Piles designed to CP4:2003 (Singapore Standard Code of Practice for BS8002-1994 (British Standard Code of Practice for earth-retaining foundations) have to meet a settlement criterion of 0.6 inches (15mm) structures). This also limits the ground settlement behind the at 1.5 times the working load. wall. However, for deep excavations, the wall deflection is usually controlled by a much smaller limit because of the need to prevent Soil Types and Behavior damage to adjacent properties. Another source of ground settlement is from ground loss and water loss resulting from wall leakage and There are generally two main types of soil behavior specific to clay seepage into the excavation. Similarly, a ground loss can develop and sand. In clay, the behavior is undrained. This means that, for an if fines are washed out when there are gaps in a retaining wall. unloading process during excavation, time is needed for the negative This article looks at ground settlement as a result of ground loss excess pore pressure to dissipate. Over time, the increase in the volume and water loss in deep excavations. of the soil is also accompanied by a reduction in strength. This is why the factor of safety decreases over time. For excavation in very soft soil, there is a tendency for the soil to heave up at the excavated area Building Damage because of the pressure relief and surcharge behind the wall. In such Building settlement and damage are related. Qualitatively, damage cases, it may be necessary to improve the soil locally at the excavation can range from negligible to very severe. Designers should always aim base or provide a deeper retaining wall. to have negligible damage caused to adjacent properties, although, in In sand, drained conditions are typically assumed. Due to the reality, it is sometimes complicated to achieve. It high permeability, water can flow easily. This high is necessary to control ground movement and difflow rate enables fines to be easily washed out of ferential settlement to achieve negligible damage. gaps in a retaining wall when soil is excavated.

During excavation, work suspension limits for When this happens, ground loss occurs and the building settlement may be set at 0.6 inches resulting settlement could cause damage to build(15mm) and 1 inch (25mm) for buildings on piles ings. Continued ground loss may even cause a and footings, respectively. Angular distortion is sinkhole to form (Figure 3). the differential settlement over length. Generally accepted limits are 1/500 for serviceability criteGround Loss ria and 1/150 for structural damage. The 1/500 target may be challenging to achieve; as a practical Two types of retaining wall systems that may compromise, a 1-inch total allowable settlement have gaps are sheet pile walls and soldier pile is commonly adopted for buildings on footings. walls. These are flexible walls and therefore are As an illustration, consider a low-rise house with more suited to excavations that are not too deep. a 20-foot width (6m) having a differential settleSoldier pile walls are preferred in good ground Figure 2. Wall movement and ground ment of 0.8 inches (20mm), which works out to conditions where the excavation is not too deep settlement affecting buildings.

STRUCTURE magazine

because they can be relatively economical, and fast and easy to install. However, the requirement to install lagging in a piecemeal manner means that weak soils and high water tables could pose issues. The soil should have sufficient stand-up time when excavated in small lifts to install the lagging (Figure 4).

For sheet piles, the interlocking joints can be dislodged during installation due to hard driving or obstructions. Re-used sheet piles may have difficulty interlocking Figure 3. Sinkhole due to ground loss. due to the distorted edge. In sensitive areas, where vibrations can be an issue (e.g., nearby houses or potential compaction of areas of loose sand), installation should be carried out using a pressed-in method. If hard ground is encountered, it may be necessary to first pre-bore a hole before installation. Any remaining gaps between the sheet pile and the hole should be backfilled with an appropriate material so that the wall can be firmly held in place. Sheet piles can be economical if they are extracted after completion of the excavation. However, it must be noted that extraction in certain ground conditions is difficult without causing excessive vibrations, and the resulting gaps in the ground after extraction need to be backfilled.

Another area where gaps are inevitable is at locations where the wall interfaces with an existing utility (e.g., water pipes, cables, etc.). To prevent washout of fines, it is prudent to provide localized ground improvement (e.g., grouting) or suitable localized lagging details (e.g., steel plate) to close up the gaps.

Figure 4. Gaps in soldier piles.

Water Loss The groundwater changes around an excavation as excavation proceeds. During excavation, soil and water are removed and the excavation has to be kept dry to facilitate construction. This means that a steep hydraulic gradient is created between the inside and outside of an excavation. Water will find a way to seep into the excavation. Typically, the piezometric levels outside the excavation drop in tandem with the excavation level. The deeper the excavation goes, the more the piezometric level drops. Sometimes, water level decreases may not be significant, so it is essential to look at the pore pressure readings. When there is a soft and compressible layer of soil, such piezometric drops can have very severe consequences. For example, in an area with 13 feet (4m) of peaty clay experiencing a pore pressure drop of 3 feet (1m), i.e., 10kPa, would result in a settlement of 2.5 inches (60mm), assuming mv = 1.5x0.001 kPa-1.

This approximation assumes consolidation settlement is the product of three components, namely, the coefficient of volume compressibility, change in stress, and thickness of soil. Volumetric strain is related to stress change by the coefficient mv. Settlement due to the consolidation of soft soils can have far-reaching effects. Experience has shown that houses located at a distance of more than 10 times the depth of excavation can be affected.

Wall Types Figure 5. Typical details to improve the water-tightness of CBP walls.

Figure 6. Typical D-wall installation sequence.

There are various types of walls used to support a deep excavation. However, it is essential to note that retaining walls need to be watertight to prevent water seeping into the excavation.

For deep excavation, rigid wall types are typically used, such as diaphragm walls (D-wall), contiguous bored piles (CBP), or secant bored piles (SBP). CBP walls are not watertight, and often secondary grout piles and a lagging system (skin wall) are required to plug the gaps between adjacent piles (Figure 5).

Secant piles provide much better water-tightness because of the overlap and overcutting process, but they are expensive.

For very deep excavations, very costly diaphragm walls (D-walls) are typically used. D-walls are watertight, but only if attention is paid to special aspects of the wall.

In a typical D-wall installation, the key issues to consider are the guide wall, bentonite slurry, panel size, and stop-end/waterstop (Figure 6 ).

The guide wall helps to provide correct alignment of the wall, and the slurry helps to prevent the trench from collapsing during excavation.

Joints between panels must be watertight.

continued on next page

M A Y 2 0 21 B O N U S C O N T E N T

Figure 7. D-wall water-stop mounted on stop-end.

To ensure water does not leak through the panel joints, waterstops are required. Typical details of a waterstop are shown in Figure 7.

These rubber strips cut across the direction of the wall joints, thereby preventing water flow. The waterstops are mounted on steel stopends during installation to provide rigidity and correct alignment.

The construction sequence involving the waterstop is critical.

First, the waterstop is attached to the stop-end. After excavation, the reinforcement cage and stop-ends are installed. Concrete is then cast. The stop-ends are removed after the concrete panel has set and gripped onto the waterstop. Lastly, the intermediate panels are cast and engage the waterstop where it was previously held by the stop-end. Occasionally, the waterstops do not go to the toe of the diaphragm wall. Some waterstops might be terminated at the base slab level or final excavation level. In certain ground conditions, this can be an issue.

There are two primary sources of water loss in a D-wall. First, water can seep through the panel joints below the water-stop. Second, water can also seep below the toe of the D-wall, especially if the soil or rock below the toe is permeable. Therefore, it is essential to understand the ground condition as much as possible when designing a deep excavation retaining system.

Site investigation is required to identify ground conditions and anticipate the issues that may be encountered during construction.

For example, in granitic rock and its residual soil, designers need to be aware that soil and rock may be highly permeable. Experience has shown that, at the soil and rock interface, permeability can be quite high. Even when rock is highly fractured with low RQD (Rock Quality Designation) values, water can flow through easily.

For such weak rocks, the behavior is dominated by the joints or discontinuities. Similarly, in areas where deltaic deposits are encountered, alternating layers of soft clay and sand may be encountered along with peat, which is very highly compressible. In such ground conditions, designers need to guard against water loss through under drainage and soft soil consolidation giving rise to excessive ground settlement.

It is very important to determine the ground permeability reliably.

There are many field tests to determine this, such as a pumping test, a rising head/falling head, and a packer test for rock. Also, a simple particle size distribution test (percentage by mass of particles) can reveal valuable insights about the permeability of a particular soil. A common rule of thumb is that the finest one-quarter of soil by percentage by mass of particles is likely to dictate how the soil behaves mechanically.

Mitigating Measures Three mitigating strategies to deal with ground settlement associated with water loss are customarily considered.

STRUCTURE magazine

The first strategy is grouting. Manchette tube or TAM (Tube-AManchette) grouting is often used to improve ground conditions. The process is carried out by drilling a hole in the ground and injecting grout, under pressure, in sections sequentially isolated by packers.

TAM grouting is carried out at locations where water leakage is suspected, for example, at D-wall panel joints below the waterstop and highly permeable ground below the wall toe. The purpose of the TAM grouting is to seal off leakage locations where water can find its way into the excavation.

In designing an effective grouting process, the designer has to consider the depth, grout type, spacing, pressure, and closely monitor the volume intake and ground movement. It is not uncommon to re-grout the drillhole several times or to have multiple closely spaced drillholes.

Close spacing of the drillholes is usually required, e.g., 3 to 6 feet (1 to 2m). TAM grouting is effective because the grout injection fills up voids and pores between solid particles in the ground. Permeability is reduced and groundwater flow is controlled. The strength and stiffness of the ground are also improved. This is why TAM grouting has also been used successfully to strengthen soil below foundations of buildings showing settlement.

Another important strategy to tackle piezometric drops is to activate deep recharge wells. Standard recharge wells are perforated at shallower depths. However, it has been found that deep recharge wells are more effective in channeling water into the deeper soil layers and fractured rocks. Deep recharge wells are drilled into the rock layer and have perforations only at the deeper end of the wells. Volume and recharge pressure is monitored continuously to ensure there is no anomaly. Designers need to be aware that the recharge capability deteriorates over time due to clogging. Therefore, it is important to monitor the performance and implement a periodic cleaning and maintenance regime.

The third strategy is to modify the construction sequence. From the author’s deep excavation experiences, piezometric recovery usually occurs only after the casting of the final level of the base slab. This is because water seepage from the base is cut off more effectively once it is completely sealed. For ease of construction, builders usually plan the excavation sequence such that a large area is excavated as much as possible to facilitate the reinforcement work and formwork before casting of the slab. However, if there is an issue with a piezometric drop around the excavation, then it would be necessary to limit the exposure of the open excavation, i.e., excavation and casting of the slab should be limited to smaller areas and done sequentially. Only after the first portion is fully cast should the second and subsequent portions be opened up in sequence.

Conclusion In conclusion, the prevention of damage requires stringent control of ground movement and retaining wall movement during excavation.

Also, ground loss and water loss should be minimized as much as possible by careful selection, detailing, and monitoring of the support of the excavation system considering the soil types present at the site.■ Hee Yang Ng is a Principal Engineer with a building control agency in the Asia-Pacific region.

SPOTLIGHT Virginia State Capitol Building Dome

T

he historic inner dome and the interior supporting walls of the West Virginia State Capitol Building were found to have structural deficiencies. Using 3-D finite element analysis modeling and laboratory testing to determine material properties for the unique coconut fiber reinforced plaster elements, repairs were designed to strengthen portions of the existing building and incorporate supplemental supports to preserve the integrity of this important structure.

Additionally, it was necessary to support the dome in-place while the walls beneath the dome were removed entirely and rebuilt.

The West Virginia State Capitol Building was constructed in 1932 and is a steelframed structure with masonry infill, interior plaster finishes, exterior limestone, and a 292-foot-tall gold leaf gilded dome.

The building features an inner dome that is constructed of two shells. The outer shell is constructed of metal reinforced cementitious plaster and is attached to a steel grid with hangers that suspend it from a concrete deck. The inner shell is constructed with coffers that were individually cast with gypsum plaster and coconut fiber reinforcement and attached to the framing grid for the outer shell using plaster wads reinforced with coconut fiber.

In some locations, the coconut fiber wads had begun to fail. There was also cracking observed within the lower portions of the inner shell. The clay tile walls beneath the inner dome that were intended to serve as furring for the interior plaster had displaced outward and upward through irreversible moisture expansion and had been laid in various orientations that compromised their structural integrity. Much of the inner shell’s weight was bearing on cracked clay tile walls, and the hangers for the dome had buckled.

Through structural modeling, the stresses within the existing dome were correlated to crack patterns and other failures. Laboratory testing of plaster samples containing coconut fiber reinforced gypsum from the interior rotunda was performed to determine key material properties for the analysis. At the inner shell, locations identified to have high stresses were retrofitted with glass fiber reinforced polymer (GFRP). This GFRP retrofit was also tested in a laboratory setting to evaluate the performance and increased strength of the composite system.

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Stainless steel plates connected with wire ties were also installed as supplemental supports to connect the inner shell to the outer shell to compliment the coconut fiber reinforced wadding used during the original construction. The GFRP system was

coated with a cementitious coating to offer fire resistance and mimic the appearance of plaster. A stand-alone mockup of the inner dome was constructed to work through constructability challenges before installing the components on the dome itself.

Many of the flat straphangers for the hanger system supporting the outer shell were buckled, and several of the concrete deck attachments were deficient. The repair design incorporated turnbuckles into each hanger such that the tension could be adjusted for each individual hanger. In-line data-logging load cells were installed at key locations such that the tension could be monitored during and after the retrofit process to ensure the loads within the hangers were properly distributed. This monitoring system will remain in place to provide a warning system if any of the loads become disproportionate.

Much of the inner shell’s dead load was bearing on the clay tile walls, which prevented them from simply being removed and replaced. A creative solution was developed to turn the cracked non-structural clay tile units into a reinforced horizontal bond beam element. The top two courses of existing clay tile would be grouted and reinforced to form a continuous beam around the base of the dome that would then be supported by new steel brackets connected to the main structural

framing of the building. A detailed sequencing plan was developed that created redundancy in the support of the dome as the walls were completely removed from beneath the dome.

First, slotted openings were made within the existing clay tile to insert the reinforcement and fully grout the outer cell of the existing clay tile. Once the bond beam was complete, discrete openings were made at each column to install the steel brackets; the reinforced clay tile beam was utilized to transfer the weight of the dome to the portions of the wall that remained. Finally, sections of clay tile were removed between brackets to install tube steel and angles to provide supplemental support below the clay tile bond beam. Flexibility was incorporated into the connections to the brackets using slotted holes and field welding to account for variations in the elevation of the bond beam and brackets. Once the steel framing was in place, all remaining clay tile was then removed.

This project presented many challenges. The coconut reinforced gypsum plaster required laboratory testing to determine properties for this unique material. Sequencing of the hanger re-tensioning, dome strengthening, and bond beam supports was necessary to maintain the dome in an equilibrium state without creating unbalanced stresses. Limited access and variability in existing conditions required coordination with the construction team and creative structural design to develop unique solutions.■ WDP & Associates Consulting Engineers, Inc was an Outstanding Award Winner for the West Virginia State Capitol Building project in the 2020 Annual Excellence in Structural Engineering Awards Program in the Category – Forensic/Renovation/Retrofit/Rehabilitation Structures under $20M. M A Y 2 0 21 B O N U S C O N T E N T