STRUCTURE magazine - April 2021

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APRIL 2021

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Contents APRI L 2021

Columns and Departments Editorial

The Future of Remote Work

By Bruce Burt, P.E., SECB

Structural Durability Improved Bridge Deck Performance

By Anton K. Schindler, P.E., Ph.D., William H. Wolfe, and Benjamin E. Byard, P.E., Ph.D.

Just the FAQs Frequently Asked Wind Questions

By Emily Guglielmo, S.E., P.E.

Structural Components Concrete Slabs-on-Ground, Crying for Attention

By TJ Cate, P.E., S.E.

Structural Rehabilitation Prestressed Concrete Bridge Girders – Part 2

By Dustin Black

Structural Repair Underwater Weld Repairs

By Uwe W. Aschemeier, et al.

Construction Issues Which Post-Tensioning Tendon?

By Bijan Aalami, Ph.D., S.E., C.Eng

Feature

Engineer’s Notebook Error Checking and the Black Box – Part 1

GRAND RAPIDS FLATIRON

By Scott N. Jones, S.E.

By Chad Van Kampen, P.E., Robert Szantner, AIA,

By Melissa T. Billig, Esq., and Maurizio Anglani, Esq.

and Barry McKinley, P.E.

InSights Reducing Potential Liability in Emergency Response

For 10-Ionia, the new architecture was

Structural Forum A Few Things Young Engineers Should Know!

chosen to complement the surrounding

By Steven G. Provenghi, S.E.

structures and conform to a unique triangular site. In early 2016, planning began on this new destination hotel designed as an icon for the City of Grand Rapids, MI, incorporating the architectural style from years past using today’s technology and materials.

In Every Issue Advertiser Index Resource Guide – Engineered Wood Products NCSEA News SEI Update CASE in Point

On the Cover Steels with chemistries more favorable to underwater wet welding, welding electrodes developed explicitly for underwater wet welding, managed training programs, and improved welding engineering techniques have elevated underwater wet welds to have comparable mechanical properties and nondestructive evaluation results to their top site welded counterparts. See full article here.

April 2021 Bonus Content

Additional Content Available Only at – STRUCTUREmag.org

Structural Design Design Strategies for 3-D Volumetric Construction for Concrete Spotlight Creating an Icon: The Dublin Link Pedestrian Bridge

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

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

A P R I L 2 0 21

EDITORIAL The Future of Remote Work

By Bruce Burt, P.E., SECB

F

or the first time in nearly a year, my firm recently hosted an in- retaining talent. But those policies must also allow the firm to function person employee gathering. It was a social event involving masks efficiently. Managers may need to be in the office more often than nonand social distancing. Everyone was quite happy and maybe a little managers. Entry-level staff may need to work almost exclusively in-office relieved to be together again. I had the opportunity to talk at length to gain the greatest benefit from the knowledge transfer that entails. with two recent hires, Shery and Jon. I had met Shery once, at her Rules regarding internal and external response time to emails, calls, or interview just before closing our physical office last March, and only team communications should be determined. The minimum amount knew Jon through his remote interview and videoconferences. Both of office presence must be established to maintain the cohesiveness of were thrilled to meet their co-workers in person, and they shared the workforce and continuity of corporate culture. their excitement at the prospect of one day soon joining them in The needs of the worker. Employees’ perceptions of their workspace an office environment. They were equally anxious to ask a nearby options have evolved. The changes employers made to keep the virtual, if teammate the basic questions not the physical, doors open have every new hire has. My experiallowed workers greater efficienence at this company outing cies when working away from was the strongest confirmathe office. Workers have gained tion possible that it was not a flexibility in work schedules to question of whether we should tend to childcare needs and other ever return to the office, but responsibilities. These benefits when and to what extent. should be preserved to the greatLike most engineering firms, est extent possible. For employees our work process has dramatiworking from home, necessary cally changed in the past twelve resources should be provided, months. A year ago, we had no along with technical assistance formal remote work policy and from the firm’s IT department.

Companies should carefully survey their were forced almost overnight With a return to the office, quick changing landscape and adopt policies to adapt as a matter of survival. surveys can be a valuable tool to that mutually benefit both company Fortunately, we had recently reveal attitudes toward flexibilmade investments in network ity, safety, technology, or even the and employee to address an increasingly capabilities and remote teamneed for more human interaction. mobile workforce’s varied needs. ing software. But the learning Once your remote work policy curve for the use of web-based has been established, make your communication tools rose steeply. Quality control procedures were firm’s expectations clear. Then communicate, communicate, and comadapted to assure that the product clients received differed little from municate some more. And offer a virtual suggestion box to employees what they expected. Some firms likely carried on with an office-bound, – an anonymous way for them to provide insights for improvement. business-as-usual approach, but that was not possible in our region of The conclusion I drew from our recent company outing is that the country. And with the benefit of over a year’s experience work- there is still a need for community within an organization, and the ing remotely, I had to ask myself whether a traditional, office-centric office is the locus of that community. This realization has probably strategy was the best approach in an engineering profession that has been reached in many engineering offices. A recent survey cited in fundamentally changed. a January STRUCTURE article indicated that only 12% of those My firm, like many others, is developing a remote work policy that surveyed wanted to work from home full-time, and 70% wanted to addresses the needs of the staff while maintaining and hopefully work in the office most of the week. That should not be surprising. It strengthening the company and its culture. Having spoken with many is in the office where the ties that bind people together are developed. of our staff, it will not be a one-size-fits-all policy. For some, freedom It is where synergies are achieved, ad hoc teams meet to creatively from long commutes and the ability to structure their workday around problem solve, and where new hires absorb almost by osmosis the other responsibilities have been a blessing; for others, the distractions of engineering craft and the company’s work ethic and esprit de corps. home life and the lack of daily interactions with co-workers have been The office-as-primary-workplace concept has not become outmoded, a curse. Companies should carefully survey their changing landscape but the workplace concept needs to adapt and become more flexible and adopt policies that mutually benefit both company and employee to address the post-pandemic realization that to efficiently to address an increasingly mobile workforce’s varied needs. provide a high-quality service, much more of the work can The needs of the firm. For an engineering firm to thrive, it must deliver be done outside the office than was once imagined.■ high-value services to its clients consistently, on-time, and at a profit.

Bruce Burt is Vice President of Engineering with Ruby+Associates, Inc., Attracting, developing, and retaining talent is the key to a firm’s suclocated in Bingham Farms, MI. He is a member of the CASE Contracts cess. Having policies that evolve with the needs and expectations of Committee. (bburt@rubyandassociates.com) existing and prospective workers is a vital component in recruiting and STRUCTURE magazine

APRIL 2021

structural DURABILITY Improved Bridge Deck Performance with Lightweight Aggregate Concrete

By Anton K. Schindler, P.E., Ph.D., William H. Wolfe, and Benjamin E. Byard, P.E., Ph.D.

E

arly-age cracking of concrete bridge decks, typically caused by drying, autogenous, and thermal shrinkage effects, can have detrimental effects on long-term behavior and durability. Darwin and Browning (2008) recently reported that “by controlling early-age cracking, the amount of cracking at later ages should remain low.”

They also reported that early-age cracking could significantly increase the rate and amount of chloride penetration (from deicing salts), which may accelerate the corrosion rate of embedded reinforcing steel.

Thus, it is essential for improved durability and sustainability that bridge deck concrete is proportioned and placed to minimize early-age cracking. Tensile stresses are induced in bridge decks when the girders restrain concrete volume changes. Early-age volume changes occur due Figure 1. Mechanical properties of expanded clay LWA concretes: a) compressive to the combined effect of temperature, autogenous shrinkage, and drying strength and b) modulus of elasticity development (Byard et al. 2012). shrinkage. The amount of stress produced when concrete volume change is restrained is a function of the extent of volume change, modulus of internally cured concrete can also provide concrete with the ability to elasticity, degree of restraint, stress concentrations, and relaxation of the undergo greater temperature variations before cracking.” concrete, which all vary with the maturity of the concrete.

Experimental evidence that supports that the use of LWAs effectively Lightweight aggregate (LWA) was evaluated to determine its benefits delays the occurrence of early-age cracking in bridge deck concrete is in bridge deck applications to mitigate early-age cracking. You might discussed below. Also discussed is the experience of using lightweight be wondering: Why do lightweight aggregates help to minimize cracking aggregates in various bridge decks in the State of New York. in bridge decks? The answer lies in the fact that, when using LWAs, the concrete’s modulus of elasticity and coefficient of thermal expansion Experimental Findings is reduced when compared to normalweight aggregates. Reducing the coefficient of thermal expansion will result in less strain from a tem- The effect of lightweight aggregate on the cracking tendency of perature change, and reducing the bridge deck concrete was evaluated modulus of elasticity will result in using cracking-frame testing techreduced stress when volume change niques. Cracking frames measure the effects are restrained. restrained concrete stress development Another reason to use LWAs in due to thermal and autogenous shrinkbridge deck applications is that they age effects from setting until the onset are pre-wetted during batching, of cracking under conditions that which allows them to provide intermatch those of in-place bridge decks. nal curing to the concrete. Internal In a large study (Byard and Schindler curing is provided as the absorbed 2010), expanded shale, clay, and slate water within the LWA is desorbed at lightweight coarse and fine aggregates early ages with the progress of hydrawere used to produce internal curing tion that needs and consumes water.

(IC), sand-lightweight (SLW), and The release of the internal curing all-lightweight (ALW) concretes. The water from LWA increases cementbehavior of these concretes containing ing material hydration and reduces these different types of LWAs was then capillary stress caused by self-desiccacompared to that of a normalweight tion. For more details about internal concrete in bridge deck applications. curing, the reader is referred to a 2012 This study evaluated concrete placed STRUCTURE article titled Internal during both fall and summer placeCuring: Constructing More Robust ment conditions.

Concrete (Weiss et al. 2012). That Internal curing (IC) concretes were Figure 2. Fall placement condition in bridge deck with expanded clay article states that, by “reducing the produced by replacing a fraction LWA concretes: a) modeled temperature profile and b) restrained stress autogenous and drying shrinkage, of the normalweight fine aggregate development (Byard et al. 2012).

STRUCTURE magazine

with lightweight fine aggregate. the use of pre-wetted LWA may be SLW concretes were produced using especially beneficial during summer lightweight coarse aggregate and placement conditions to minimize normalweight fine aggregate. ALW the occurrence of early-age cracking. concretes used both lightweight fine This figure also shows that, regardand lightweight coarse aggregate. less of the type of expanded LWA The compressive strength develand as more pre-wetted lightweight opment of some of these concretes aggregates are added to the concrete, are shown in Figure 1a. All conthe time to initial cracking is delayed, cretes exhibited 28-day compressive which will improve the in-place perstrengths above the 4,000 pounds per formance of bridge decks. square inch (psi) target. As shown in Figure 1a, the IC concretes exhibit Field Project Findings similar or slightly greater compressive strengths compared to the Control In 2010, the New York State mixture. The increased compressive Department of Transportation strength of the IC concretes is attrib(NYSDOT) constructed a singleuted to the additional hydration that point urban interchange (SPUI) IC concretes generally exhibit. The over Interstate 87 in Latham, NY. compressive strengths of the SLW conA portion of the bridge can be seen cretes are similar to or less than that in Figure 4. Because of the unique of the Control concretes (Figure 1a). geometry of the bridge, cracking The compressive strengths for the of the concrete bridge deck was a Figure 3. Time to initial cracking for different concretes placed under: all-lightweight concretes were approxiconcern. One method employed to a) fall conditions and b) summer conditions (Byard et al. 2012). mately 13% to 19% lower when reduce cracking was to utilize lightcompared to those of the normalweight control concrete. weight concrete. The deck was cast with sand-lightweight concrete The modulus of elasticity results of some of these concretes are shown with an equilibrium density of 110 pounds per cubic foot. After 10 in Figure 1b. The modulus of elasticity of the concrete depends heavily years of use, the exposed deck has performed excellently through the on the stiffness of the aggregate. It is clear from this figure that the severe winters experienced in Upstate New York. more LWA used in the concrete, the lower the concrete’s modulus To improve the durability of bridge decks, the NYSDOT has of elasticity. This effect can accurately be estimated by using the been utilizing internal curing as one of their crack reducing stratewell-known expression in ACI 318, Building Code Requirements for gies. The normalweight, high-performance concrete found in these Structural Concrete and Commentary, that indicates that the modulus decks contain supplemental cementitious materials (SCMs). While of elasticity is directly proportional to the unit weight to the 1.5 power these SCMs do a great job of reducing the permeability of the and the square root of the compressive strength. concrete, cracking of the deck is a concern as wide cracks provide Typical results of the in-place concrete temperature and the measured easy pathways for contaminants to reach the reinforcement quickly. restraint stress development for bridge decks placed under fall condi- The concrete’s susceptibility to cracking is improved by replacing tions are shown in Figure 2. The results from the rigid-cracking frame 30% by volume of the normalweight fines with an equal volume provide a relative index of cracking sensitivity, with a mixture with of saturated lightweight aggregate fines to cure the concrete interincreased cracking time being an indication of improved cracking per- nally. The NYSDOT evaluated internally cured concrete as part of formance in the field. This increased performance may be in the form a large research study on multiple bridge decks that included the of increased crack spacing, decreased crack widths, or fewer cracks. Court Street Bridge in Syracuse, New York, which is shown under Concrete made with LWA has a lower thermal diffusivity; therefore, construction in Figure 5, page 10. Current projects include the as shown in Figure 2a, this leads to higher temperatures compared to normalweight aggregate. However, the restrained stress development in Figure 2b indicates that the magnitude of the peak temperature alone does not provide a direct indication of the cracking tendency of the concretes. While the magnitude of the peak temperature is important, the decreased coefficient of thermal expansion of the LWA concretes causes a reduced strain per unit temperature change.

Furthermore, the reduced modulus of elasticity of the LWA concretes causes reduced stress for a given strain. Although the SLW and ALW concretes experience higher peak temperatures, the significant reduction in modulus of elasticity and coefficient of thermal expansion leads to a reduction in tensile stress and a significant overall delay in early-age cracking in bridge deck concrete applications.

The cracking times for all the concretes placed under fall and summer conditions are summarized in Figure 3. A comparison of the results provided in Figures 3a and 3b reveals that the time to cracking for all concretes made with LWA, when placed under summer placement conditions, is greater than the time to cracking of the normalweight concrete when placed under fall conditions. This indicates that Figure 4. SPUI Bridge deck utilizing lightweight concrete to reduce cracking.

A P R I L 2 0 21

elevated portion of the Bruckner Expressway in New York City. For this project, 12,000 cubic yards of internally cured concrete was implemented to reduce the cracking of the concrete and to improve the life span and sustainability of these bridge decks.

In Summary The use of lightweight aggregate decreases the modulus of elasticity and coefficient of thermal expansion of the concrete. Based on the experimental results and regardless of the type of expanded LWA used, as more pre-wetted lightweight aggregates are added to the concrete, the time to initial cracking is delayed, which will improve the in-place performance of bridge decks. State highway agencies have recognized the benefits of reducing cracking through the use of saturated lightweight aggregates. The NYSDOT’s most recent version of their bridge manual requires that high-performance, internally cured concrete be used on all continuous-span bridges and all simple-span prestressed concrete bridges using adjacent box beams or slab units.

Note that, while there are many benefits associated with internal curing, the recommended practice is that contractors continue to provide conventional (external) curing. As a result, by providing both internal curing with LWA and external curing, it is possible to greatly minimize the risk of unwanted early-age cracking, which will lead to improved bridge deck performance.■ References are included in the PDF version of the article at STRUCTUREmag.org.

Figure 5. Internally cured concrete being placed on the Court Street Bridge in Syracuse, New York.

Anton K. Schindler is a Professor and HRC Director in the Department of Civil Engineering, Auburn University. Anton is a fellow of ACI and ASCE, and he received ACI's Wason Medal for concrete materials research in 2006 and 2011. (schinak@auburn.edu) William H. Wolfe is a Senior Engineer with Norlite, LLC, in Cohoes, New York. He is active in several ACI and ASTM committees involving the use of lightweight concrete. (whwolfe@norliteagg.com) Benjamin E. Byard is a Bridge Program Manager with the Tennessee Valley Authority. He is a past president and treasurer of the ASCE Chattanooga Branch. (bebyard@tva.gov)

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just the FAQs

Frequently Asked Wind Questions

By Emily M. Guglielmo, S.E., P.E., F.SEI

T

his article is a compilation of questions that have been asked clearly inform owners and engineers about the storm intensities of the NCSEA Wind Engineering Committee. The referenced for which designs are performed. code is the American Society of Civil Engineers’ ASCE 7, Minimum SCE 7 says I cannot consider shielding. Do I have to design Design Loads for Buildings and Other Structures. The Figures noted each portion of a building with expansion joints as an indepenhere are found in ASCE 7-16. dent structure that receives both windward and leeward pressures? hat wind load should I use on a handrail of a balcony?

When considering a building with an expansion joint, it is important As with all things related to wind loads on buildings, it is to recognize that internal pressures do not cancel for the structure important to envision airflow around on each side of the joint in the direction the element. For a balcony, there are two perpendicular to the joint. In addition, important considerations when determinwhen applying the required wind load ing wind load on the railing. distribution on the building, the resistance The first consideration is the geometry is not transferred across the expansion on the leeward side of the balcony rail. joint. As a result of the unique geomWhen air flows up and over the railing, etry of an expansion joint building, it is a shallow balcony does not have enough important to consider future conditions depth to develop negative pressure on for the building. the backside of the railing. Thus, for a A large warehouse is a typical building balcony with a depth of 6 feet or less, for which an expansion joint is added the wind load on the railing is similar to to relieve stresses due to thermal expanthe load on the building windward wall sion or contraction. For structures with (Cp = 0.8, per Figure 27.3-1 in ASCE an internal expansion joint, where the 7-16). There is no need to consider a building must exist on both sides of the simultaneous leeward or suction pres- Wind loads on a building with an expansion joint. joint for the building to remain operasure on a railing of a shallow balcony. tional, it is reasonable to consider only However, suppose the balcony is deeper than 6 feet. In that case, it the windward loads on the windward wall and leeward loads on the is important to consider the effect of negative pressure building up leeward wall and ignore any external pressures at the expansion joint. on the backside of the railing in addition to the positive pressure on If something happened to cause the structure on one side of the the front face. As the balcony gets deeper, this combined pressure building to be damaged or removed, the structure on the adjacent begins to approach a parapet pressure. side of the expansion joint would also be demolished or reinforced The second consideration that affects wind load on a railing is the to act as a standalone structure. material of the railing. A solid glass railing has the capacity to develop However, if a structure is being built adjacent to an existing structure, negative pressures. A perforated or open railing cannot develop sig- it should be designed for the full wind loads assuming the adjacent nificant negative pressures. A solid surface is required to prevent wind structure is not there. For example, a parking deck entirely surrounded from flowing through the railing and force the flow to go up and over by residential units should be designed considering a future state for the railing in order to develop negative pressures. which the adjacent buildings are not present. hy did ASCE 7 decide to go from allowable to ultimate hat is the difference between wind maps in the ASCE 7 wind speeds? Why do we have multiple maps instead of Commentary to Appendix C and the IBC 0.42*W provision? a single map modified by an importance factor?

Unlike seismic drift, which is determined at the strength load level, Prior to ASCE 7-10, the Standard utilized a single map and adjusted wind drift is a serviceability concern and should be calculated at the the wind speed using a wind importance factor (Iw = 0.77, 0.87, 1.0, allowable load level. The ASCE 7 Appendix C Commentary presents 1.15) and a wind-load factor (1.6 for strength design). Beginning maps for return periods of 10, 25, 50, and 100 years (Figures CC.2-1 with ASCE 7-10, the Standard leverages three (four in ASCE 7-16) through CC.2-4). These maps adjust the Chapter 26 wind speeds in maps presented at a strength level (1.0 wind-load factor for strength two ways: 1) They reduce the wind loads from strength to allowable design) and eliminates the wind importance factor, Iw. level, and 2) They reduce the Mean Recurrence Interval (MRI 300, There are several reasons for these changes. First, multiple maps 700, 1700, and 3000 in Chapter 26) to 10, 25, 50, or 100 years. remove the inconsistencies inherent to the importance-factor However, the decision of which map to use is not explicitly stated approach. Importance factors must vary between hurricane and and is left to the engineer’s discretion. The building’s intended usage, non-hurricane zones, and even across the hurricane coastline, the type of cladding materials, and the detailing of the finishes are to provide equal risk. With multiple maps, a distinction may be important considerations when determining the appropriate return made based on location. The strength level maps establish a more period to use for drift calculations. uniform return period for the design-basis winds. Also, strength The International Building Code’s (IBC) Table 1604.3, footnote f, design wind speed maps bring the design approach used for wind permits the wind load to be taken at 0.42 times the “component and in line with that used for seismic loads. Lastly, the maps more cladding loads for the purpose of determining the deflection limits.”

A

W

W

STRUCTURE magazine

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For a Risk Category II Structure, the IBC 0.42 reduction is the equivalent of using the maps in ASCE 7 Commentary for a 10-year return period at a service level wind load. hat is the allowable drift for my Main Wind Force Resisting System (MWFRS) under wind loads?

ASCE 7 does not suggest an allowable drift limit for wind design as it does with a seismic design. However, ASCE 7-16 Appendix CC (Serviceability Considerations) notes that drift limits in common usage buildings should be on the order of 1/600 to 1/400 of the building or story height.

Designers often impose an absolute limit on the interstory drift in light of evidence that damage to nonstructural partitions, cladding, and glazing may occur if the interstory drift exceeds about 0.4 inches.

This absolute limit on story drift is often taken as ⅜ inches. Thus, for a building with a floor-to-floor height greater than 12.5 feet, the absolute limit of ⅜ inch would control.

Lastly, the ASCE/SEI Prestandard on Performance-Based Wind Design provides additional examples of drift limits for wind. hat is the difference between a parapet and a screen wall on a roof? How close does a screen wall need to be to the edge of the roof to be considered a parapet?

Mechanical equipment screens commonly are used to conceal plumbing, electrical, or mechanical equipment from view.

Historically, ASCE 7 has not provided guidance on what wind pressure to apply to these rooftop screens. Several approaches have been used within the industry, including applying parapet pressures, using the solid-freestanding wall provisions, and applying the rooftop structures and equipment provisions (discussed above).

Little research is currently available to provide guidance for determining wind loads on screen walls and equipment behind screens.

The ASCE 7-16 commentary to Section 29.5.1 suggests that the provisions for rooftop structures and equipment be used to generate wind forces on screen walls located away from the edge of a building.

Fh = qh(GCr)Af (ASCE 7-16 Equation 29.4-2) The commentary also alludes to the fact that screen walls located close to a building edge should be designed for parapet pressures. To quantify the appropriate distance from a building edge to differentiate between “parapet” and “rooftop structures and equipment” pressures, the boundary between corner and edge wind Zones (Zones 2 and 3) versus typical roof Zones (Zone 1) provides a reasonable delineation.

Therefore, a suggested practice would be that screen walls located in Zones 2 and 3 should be designed for parapet pressures, while screen walls located in Zone 1 can be engineered for a “rooftop structures and equipment” pressure. y Architect asked me to review a product that is rated for (XX psf or XX mph). How do I figure out if that is allowable or ultimate, or if the mph is ok for this project since it does not account for exposure or building height?

Most product ratings, including glazing, doors, and siding, are rated by the manufacturer using allowable wind pressures. By providing pressures, rather than miles-per-hour, the rating considers the building’s exposure, height, adjacent topography, elevation, and importance.

While most products still provide their ratings at an allowable level, the expectation is that they will adjust to ultimate pressures over time. A conversion of allowable to ultimate wind speeds is provided in Commentary Table C26.5-7.

Shingle ratings are a known exception to product data provided based on pressures. Due to the way roof shingles were originally tested and rated, typical product data for shingles is provided in miles-per-hour only. Thus, it is acceptable to approve shingles without consideration for the factors used to convert from milesper-hour to pressure.

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hat is the difference between a 3-second gust and fastest-mile wind speed? How are they related?

For years, engineers used the fastest-mile wind speed in ASCE 7, the average speed at which an imaginary airborne particle would travel when moving a mile downstream. Starting with ASCE 7-95, the Standard uses a peak three-second gust wind speed to define wind loads. Three-second gust is the highest average speed measured over a three-second duration.

The transition from fastest-mile to 3-second gust reflects the desire to report an engineering wind speed that more closely reflects the values quoted by a weather reporter, who tends to report the highest wind speed measurable.

ASCE 7 Commentary’s Table C26.5-7 provides a comparison of the strength design-based wind speeds used in the ASCE 7-10 and 7-16 basic wind speed (3-second gust) maps and the ASCE 7-05 basic wind speed (3-second gust), in addition to a comparison with ASCE 7-93 basic wind speeds (fastest mile). hat is the relation between hurricane wind speed and building design wind speed? Is my building designed for hurricane category 3, 4, or 5?

ASCE 7 Commentary Table C26.5-2 provides an approximate relationship between wind speeds in ASCE 7 and Hurricane Categories 1-5. hat wind load should I use on a temporary structure?

Many engineers attempt to reduce the wind loads applied to temporary structures due to their relatively short design life compared to regular structures. Common temporary structures include concert stages, tents, public art projects, shade structures, and bleachers. There currently is no nationally recognized Standard that specifies design wind loads for temporary structures.

The IBC addresses temporary structures in Section 3103. This section applies to structures erected for a period of fewer than 180 days, but it does not specify how to determine the design loads except to state that “temporary structures and uses shall conform to the structural strength, fire safety, means of egress, accessibility, light, ventilation and sanitary requirements of this code as necessary to ensure public health, safety, and general welfare.”

Some engineers attempt to use the maps for Serviceability in the Commentary to Appendix C to reduce the wind loads on temporary structures. While these maps do adjust for the return period, they specifically state that “the maps included in this appendix are appropriate for use with serviceability limit states and should not be used for strength limit states.”

Other engineers look to ASCE 37, Design Loads on Structures during Construction. ASCE 37 incorporates provisions for adjusting wind loads to reduce them for short-term exposure during construction for up to five years. This Standard provides reduction factors for the design wind speeds in ASCE 7 as a function of construction duration.

However, this Standard is intended for buildings under construction, not a temporary structure. It is important to recognize that many of the temporary structures noted above (concert stages, tents, public art projects, shade structures, bleachers) create areas intended for public assembly. The fact that these structures represent a significant risk to human life in the event of a failure is notably different from the expected usage of ASCE 37, which is intended for a construction site.

In light of the lack of specific direction, there is an intent to include guidance on loads on temporary structures for all hazards, including wind loads, in an Appendix in a future ASCE 7 Standard.■

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Emily M. Guglielmo is a Principal at Martin/Martin Consulting Engineers.

She is Past President of the NCSEA Board of Directors, Chair of the NCSEA Wind Engineering Committee, and a Voting Member of the ASCE 7-22 Wind Load Subcommittee. (eguglielmo@martinmartin.com) A P R I L 2 0 21

structural COMPONENTS Concrete Slabs-on-Ground, Crying for Attention

By TJ Cate, P.E., S.E., G.C.

C

oncrete slabs-on-ground are a prolific part of modern construction. Virtually every project, ranging from small single-family residences to monstrous manufacturing facilities, resort hotels, and everything in between, utilizes a concrete slab-on-ground in some fashion. Not surprisingly, therefore, is the prevalence of these slabs in construction disputes, construction defect allegations, and construction litigation. Unfortunately, the prevalence also creates complacency among design professionals. At times, concrete slabson-ground receive little more design attention than the inclusion of boiler-plate general notes and a “standard detail” or two, hastily inserted into construction drawings. In some cases, the notes and details have been a part of a firm’s standards for decades with no updates to account for current technologies, changes in construction methods, or industry standards.

Increased attention to concrete slabs-on-ground from design professionals, with an understanding of current concrete construction methods, can improve the quality of construction, decrease the occurrence of change orders and requests-for-information, and ultimately reduce the frequency of construction disputes, construction defect allegations, and construction litigation.

ACI 302.1R-15: Guide to Concrete Floor and Slab Construction (ACI 302.1R) states, “Designers also should understand slab construction to avoid building in problems for the contractor.”

The American Concrete Institute publication ACI 360R-10: Guide to Design of Slabs-on-Ground, (ACI 360R) paragraph 1.5 lists the minimum information that should be provided in the construction documents by the design professionals. a) Slab-on-ground design criteria b) Base and subbase materials, preparation requirements, and vapor retarder/barrier, when required c) Concrete thickness d) Concrete compressive strength, or flexural strength, or both e) Concrete mixture proportion requirements, ultimate dry shrinkage strain, or both f ) Joint locations and details g) Reinforcement (type, size, and location) when required h) Surface treatment, when required

Figure 2. Circular crack around the column block out (highlight added).

STRUCTURE magazine

Figure 1. Slab-on-ground detail at an interior footing.

i) Surface finish j) Tolerances (base, subbase, slab thickness, and floor flatness and levelness) k) Concrete curing l) Joint filling material and installation m) Special embedments n) Testing requirements o) Pre-construction meeting, quality assurance, and quality control These items serve as a starting point for the design and specification of a slab-on-ground. ACI 302.1R instructs that if any of those items are not provided, the contractor should request them from the designer.

The following are examples of slab-on-ground designs and specifications that negatively impacted the project’s outcome.

Example 1: Slab Restraint One of the fundamental axioms of concrete is that “concrete cracks.”

In conventional (non-post-tensioned) slabs, joints are used to limit the number and width of random cracks that result from normal curing shrinkage of the concrete. However, restraint of the slab from any source can render the joints ineffective and increase cracking potential. Consider the detail provided in the construction drawings for a light-use warehouse (no forklift traffic) slab (Figure 1). The slab-on-ground was detailed such that it was cast directly on top of the interior footing. The slab and footing were constructed as shown in the detail. Unfortunately, a mechanical bond developed between the footing and the slab, thereby restraining the slab. The result was a circular crack in the slab all around the column block out (Figure 2). Similar cracking developed around seven of the eleven column locations. The guidelines of ACI 360R state: “Every effort should be made to avoid connecting the slab to any other element

of the structure.” By simply lowering the footing and placing base material between the slab and the footing, or using slip sheets or other methods to prevent the bond between the slab and the footing, the cracking could have been prevented (see ACI 360R Figures 6.2 and 6.3).

Example 2: Boiler-Plate General Notes

Example 3: Construction Joints Construction joints are created whenever slabs are constructed in multiple pours rather than one continuous pour. Depending on the nature of the slab, it may be necessary to restrain vertical movement of the slab across the joint. A historical method to create a construction joint with vertical movement restraint was the use of a keyway.

The current preferred method is the use of dowels. The keyway construction joint is complex and expensive (Figure 3, page 16).

A keyway section must be constructed on-site or pre-purchased, then installed into the formwork. ACI 306R does not recommend keyways in areas of heavy load or wheeled traffic “because the male

The author was recently asked to evaluate a contractor’s decision to place a 35,000-square-foot slab-on-ground in a single day continuous pour operation. A general note in the construction drawings stated: Large areas of interior slabs on grade shall be placed in strips not to exceed 120 feet in length nor 30 feet in width which are subdivided into roughly squares whose sides shall not exceed 15 feet in either direction.

An opposing party contended that the work was defective because the slab was placed without regard to the pour limits shown. In this case, the contractor used a laser screed, power trowels, and early entry concrete saws to accomplish the slab construction. There is nothing in ACI limiting the size of a concrete pour. ACI 302.1R states, in paragraph 10.1.1.1: Large block placements are the most efficient way to place concrete in large Use for all types of concrete and grout applications, from slabs-on-grade to areas. Laser-guided equipment is most containment tanks, multi-story post-tension structures to bridge decks. often used for this configuration (Fig. 10.1.1a). Laser screeds provide accurate strike-off between construction ADVANTAGES joints. Strip placements are an accept¡ Maximize joint spacing (up to 300 ft, L/W 3:1) ¡ Enhance compressive and flexural strengths able alternative to block placements if a laser screed is not available or access ¡ Prevent shrinkage cracking and curling ¡ Eliminate pour/delay strips is inadequate. ¡ Thinner slabs and walls viable ¡ Reduce long-term relaxation of P/T tendons The size of a pour is limited only by what and shear wall stresses ¡ Reduce reinforcement requirements the contractor has resources and technol¡ Minimize creep and moment ogy to accomplish. Using modern tools ¡ Improve durability and lower permeability and techniques, contractors can success¡ Minimize waterstops ¡ Increase abrasion resistance 30-40% fully place large areas of concrete. It is not unheard of for contractors to place as much as 75,000 square feet or more of slab in a continuous single-day operation.

If the contractor had followed the general note, it would have taken at least ten separate pours to complete the slab. The cost of mobilizing, forming, placing, and post pour cleanup of ten individual pours would have been significantly more. It was not appropriate for the design professional to limit the contractor to methods that did not allow them to use modern equipment

by CTS Cement Manufacturing Corp. to perform efficient construction. In this instance, it was ultimately determined that the contractor’s decision had no detContact us for more information and project support at 800.929.3030. rimental effect on the slab and that there CTScement.com was no justification for the limitations of the general note.

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Figure 3. Keyway construction joint detail.

Figure 4. Dowel construction joint detail.

and female key components lose contact when the joint opens due to drying shrinkage. This can eventually cause a breakdown of the concrete joint edges and failure of the top side portion of the key.”

Keyway construction joints are especially problematic at thinner slabs because the slab must be thickened at the joint to provide enough space to construct the keyway. This increases the cost of the excavation, base and subbase preparation, and formwork installation. It also creates horizontal restraint to slab movement that increases the probability of cracking. The “standard detail” was provided in the construction drawings for a 4-inch thick slab-on-ground.

The contractor proposed using smooth dowels, consistent with ACI 360R Figure 6.5 guidelines, as shown in Figure 4. A dispute arose, and the contractor, facing the pressures of keeping the project on schedule, proceeded with slab construction using dowels. Had the design professional been familiar with current preferred construction methods, and updated their standard details accordingly, the dispute could have been avoided. Eventually, this dispute was one factor that led to the issuance of a stop-work order from the owner, culminating in costly litigation.

width, wire mesh reinforcing should be located within the top third of the slab, not in the middle of the slab as specified. The contractor’s proposal was rejected, and ultimately wire mesh reinforcing was installed. The owner was not satisfied with the cracks that developed and commissioned an investigation of the slab, including destructive testing, which revealed that the wire mesh reinforcing varied in its position within the slab, resting against the dirt in some locations and within 1 inch of the surface at other locations.

While the contractor was not relieved of his responsibility to place the wire mesh in the specified location, it is doubtful that the mesh placement at the center of the slab would have achieved results acceptable to the owner. ACI recognizes that both synthetic macro fibers and steel fibers can reduce plastic cracking and drying shrinkage cracking. In addition to reducing visible cracking, steel fibers can increase shear strength, and impact resistance and flexural toughness of concrete slabs. If fiber reinforcing had been permitted, perhaps a slab that satisfied the owner’s expectations would have resulted.

Example 4: Reinforcing

Both ACI 360R and 302.1R state that the design professional should provide the layout of joints and joint details. ACI 360R continues, “When the joint layout and joint details are not provided before project bid, the designer should provide a detailed joint layout along with joint details before the slab pre-construction meeting or commencing construction” (emphasis added). Does this boiler-plate general note adequately provide the layout of joints for a slab?

Control joint spacing rule of thumb is 24x slab thickness (4” slab = 8’-0” max O.C. each way) For a residential sidewalk, the note may be enough. For the 40,000-square-foot commercial slab-on-ground with a dog-leg geometry, where the note was the only information provided, a dimensioned control joint plan should have been created.

Design professionals have a direct impact on the outcome of concrete slab-on-ground construction. Whether that impact is positive or negative depends on the designers’ understanding of slab construction and the heed given to provide specifications and details consistent with current technology, techniques, and industry standards. If you have never seen a laser screed in action or never witnessed an early entry saw in use, talk to the contractor on your next project and get out to the site. Hit the books, visit concrete industry websites, and read concrete magazines. Do not leave your next concrete slab-on-ground crying for attention.■

A common misconception of project owners, some contractors, and the occasional design professional is that the use of reinforcing in a slab-on-ground prevents cracking. Reinforcement does not prevent cracks. Instead, it increases the number of cracks but reduces the crack widths. Options for slab reinforcing to limit crack widths include deformed bar reinforcing, wire mesh reinforcing, and fiber reinforcing. The design professional provided the following specifications for a large, indoor slab-on-ground: • Interior slabs on grade shall be 4 inches thick • Reinforce all slabs on grade with #4 bars at 18 inches on-center each way, or with 4x4 W2.9xW2.9 wire mesh.

• Reinforcing steel in concrete shall be securely anchored and tied in place prior to placing concrete and shall be positioned with the following minimum concrete cover: o Slabs on grade: Center of slab The contractor proposed using synthetic fiber reinforcing. He was concerned that #4 reinforcing in a 4-inch-thick slab would not provide adequate concrete coverage over the reinforcing. Both ACI 360R and 302.1R discuss the risk of bar shadowing cracking and/or subsidence cracking when cover over reinforcing is not sufficient in concrete slabs. 1½ to 2 inches of cover is generally considered the minimum allowable to reduce the risk of such cracking. The contractor was also opposed to wire mesh reinforcing because he intended to use laser screed equipment to achieve the desired floor flatness and levelness, and supporting wire mesh reinforcing to allow a laser screed to be driven over it is a daunting task. Also, to be effective at limiting crack

STRUCTURE magazine

Example 5: Joint Locations and Details

TJ Cate is the National Construction and Industrial Division Manager at Rimkus Consulting Group. (tjcate@rimkus.com)

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structural REHABILITATION

Prestressed Concrete Bridge Girders Part 2

By Dustin Black

T

his is the second of two articles discussing high-level design considerations of prestressed concrete girders. Part 1 (STRUCTURE, January 2021) provided an overview of the post-tensioning and pretensioning processes, described the common materials used in constructing prestressed bridge girders and discussed the time-dependent prestress losses inherent in their design. The discussion continues in Part 2 with fundamental design considerations for internal stress distributions within prestressed concrete girders and the methodology for application.

Raised, Draped, and Debonded Strands When straight strands are bonded for the full length of a prestressed girder, the tensile and compressive stresses near the ends of the girder will likely exceed the allowable service limit state stresses. This occurs because the strand pattern is designed for stresses at or near midspan, where the dead load moment is highest and can best balance the effects of the prestress. Near the ends of the girder, this dead load moment approaches zero and is less able to balance the prestress force. This results in tensile stresses at the top of the girder and compressive stresses at the bottom of the girder.

With a raised strand pattern, the center of gravity of the strand pattern is raised slightly and is a constant distance from the bottom of the girder for the girder’s entire length. Other strand configurations are available, as shown in the Figure. Draping some of the strands is another available method to decrease stresses from prestress at the ends of the I-beam, where the stress due to applied loads is at a minimum. Note that all the strands that lie within the “vertical web zone” of the mid-span arrangement are used in the draped group.

The designer may also use debonded strands. Partially debonded strands are fabricated by wrapping sleeves around individual strands for a specific length from the ends of the girder, rendering the bond between the strand and the girder concrete ineffective for the wrapped or shielded length. Preference for each of these methods is on a stateby-state basis. For example, Wisconsin’s order of preference for strand placement is straight, draped, and then partially debonded. For the Michigan Department of Transportation (MDOT), debonding is the preferred method of controlling stresses at the end of I-beams, and draped strands should be avoided where possible.

Strand Development, Transfer, Anchorage, and Spacing Development length is the shortest length of a strand in which the strand stress can increase from zero to the yield strength. If the distance from a point where the strand equals the yield strength to the end of the strand is less than the development length, the strand will pull out of the concrete.

Transfer length represents the first portion of the development length over which the prestressing strand should be bonded so that a stress in the prestressing strand at the nominal strength of the member may develop. A short transfer length increases stresses and the risk of cracking by concrete splitting, bursting, or spalling in the STRUCTURE magazine

General tendon profiles (Source: Raja, 2012).

end regions. A long transfer length reduces the available member length to resist bending moment and shear and therefore increases member cost. A pretensioned member’s design strength is taken as the nominal strength multiplied by the strength reduction factors in sections within the transfer length and the development length.

If a critical section occurs within these regions, where the strand is not fully developed, failure may occur by bond-slip (Building Code Requirements for Structural Concrete and Commentary, ACI 318-11).

Bond stresses are derived through a combination of adhesion, friction, and mechanical interlocking. It has been widely believed that a wedging effect, unique to pretensioned strands, creates significant bond stresses in the transfer zone. The effective prestressing force is transferred from the pretensioned strand to the concrete. In those same regions, slip occurs between strand and concrete due to the difference in strain condition. Research has indicated that the strand end slip can be used as a quality control measure for the bond of prestressing strands. Furthermore, the relative slip between strand and concrete virtually ensures that adhesion plays little or no role in transferring prestressing forces to concrete.

Historically, the American Association of State Highway Transportation Officials (AASHTO) limited the strand clear spacing to a minimum of three times the strand diameter. In bridge codes before the AASHTO Load and Resistance Factor Design (LRFD) Bridge Design Specifications, this provision was made an explicit part of the design code. This code provision likely mirrored the standard of placing 0.5-inch strands at 2.0-inch, center-to-center (c/c) spacing. If this provision were extended to the larger diameter 0.6-inch strands, then the 0.6-inch strands would have to be placed at 2.4-inch c/c. Nevertheless, using this strand spacing would cancel out the economic value inherent in using a 0.6-inch strand. It would also cancel out the most compelling reasons to use high-strength concrete in pretensioned girder applications. Russell (1994) showed that 0.6-inch strands must be placed at a spacing of about 2.0-inch c/c to enable designs to take advantage of high-strength concrete. Cousins et al. conducted a series of tests in 1993 and concluded that the strand spacing had no effects on the measured transfer and development lengths.

Design Methodology There are three primary stages to be addressed in prestressed girder design: transfer, service, and ultimate. The California Department of Transportation’s (Caltrans’) 2018 Bridge Design Practice Guide describes them succinctly: • Transfer refers to the stage at which the tensile force in the strands is transferred to the Precast (PC) girder by cutting

or detensioning the strands after a minimum girder concrete strength has been verified. Because the girder is simply supported and only self-weight acts with the prestressing at this stage, the most critical stresses typically occur at the ends of the girder or harping points (also known as drape points). Both tensile and compressive stresses should be checked at these locations against AASHTO LRFD stress limits.

• Service refers to the stage at which the girder and deck self-weight act on the non-composite girder, together with additional dead loads (e.g., barrier and wearing surface) and live loads on the composite section. This stage is checked using the AASHTO LRFD Service I and III load combinations, per Caltrans Amendments, Table 5.9.4.2.2.-1. The girder must also be designed to prevent tension in the precompressed tensile zones (“zero tension”) due to permanent loads.

• Ultimate refers to the Strength Limit State. Flexural and shear strengths are provided to meet all factored load demands, including the Caltrans P-15 design truck (Strength II load combination).

Generally speaking, the mechanics present in prestressed beams are similar to non-prestressed beams. The principles of flexural, shear, and torsional forces must be understood and accounted for in the design.

The inclusion of prestressing strands, however, modifies some of these characteristics. The modifications primarily occur in the flexural/ moment realm of element design through beam-end reactions and continuity moments. They can also affect the shear resistance of an element due to bursting considerations. A secondary design consideration is the prestressing forces at release, or transfer, which produce temporary tensile stresses at the top of the concrete member and need to be checked to ensure that they do not exceed the concrete’s tensile capacity. Additional longitudinal reinforcement in the bridge deck is

generally required near piers to provide adequate moment capacity for negative bending.

The amount of reinforcement is that which is sufficient to resist the total tension force in the concrete based on an uncracked section. For draped designs, the control is at the hold-down point of the girder. At the hold-down point, the initial prestress is acting together with the girder dead load stress. This is where tension due to prestress is still maximum, and compression due to girder dead load decreases. For non-draped designs, the control is at the end of the member where prestress tension exists but where dead load stresses do not occur.

As the bottom fibers of bridge girders encounter prestressing losses (this is where most of the prestressing strands are focused), the girders gradually encounter upward camber and beam-end moments. The beam-end rotations would continue to grow unless restrained. Current design methodology accounts for this phenomenon and restrains the beam-ends by inducing continuity through the spans of a bridge.

Deck-only continuity is when deck joints over the beam-ends are eliminated, and the deck itself acts as a hinge for the composite structure. In this case, the beams are designed as simple spans, and the reinforcing steel within the deck must be designed for compatibility with the girder rotations.

Full-section continuity is similar to deck-only continuity, except that the girders’ bottom flanges are tied together, and the space between the composite section of the deck and girders is sealed with a concrete closure pour. Full-section continuity allows a series of girders to act as a single girder placed continuously over a bearing.

AASHTO LRFD Article 5.14.1.4.5 defines two joint types: a fully effective joint, which treats the structure as continuous for all limit states, and a partially effective joint, which treats the structure as continuous for the strength limit state only.

continued on next page

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Effects on Design for Flexure Positive moment requires prestressing force below the section centroid; negative moment requires it above the centroid. The required eccentricity of prestressing tendons with the section’s centroid increases with increased applied bending moment. Curved tendon profiles that approximately follow the shape of the appropriate bending moment diagram are not easily feasible in pretensioned members, so general tendon profiles have been established for design purposes.

Effects on Design for Shear and Torsion Orangun et al. (1977) indicated that transverse reinforcement confines the concrete around anchored bars and limits the progression of splitting cracks.

Edge girders encounter torsion due to the eccentricity of their loading of the main deck slab, barrier and parapet walls, and/or cantilevered components. In some cases, edge girders are loaded in torsion by (floor) beams framing into them from the sides between the ends of the girder. The beam’s free-end rotation is retrained by the girder’s torsional stiffness and is, therefore, a design consideration.

Trends of Prestressed Girders The AASHTO LRFD Bridge Design Specifications contains restrictions on the use of high-strength concrete due to the lack of previous performance history and empirical data available. These restrictions limit the application of existing and new technology to bridges.

Concretes with design compressive strengths above 10 ksi shall be

used only when allowed by specific articles of the specifications or when physical tests are made to establish the relationship between the concrete strength and other properties. Revisions in design guidance will continue as data becomes increasingly available, leading to a broader array of high-strength concrete applications.

The lessons learned and knowledge base of prestressed concrete girders are being leveraged into other structural design aspects. Practitioners worldwide are beginning to realize the stakeholder benefits and timesaving possibilities of employing the principles of prestressed concrete in other, more irregular shapes. For example, Accelerated Bridge Construction (ABC) is predicated on the idea of designing elements to be constructed off-site using prestressed technologies and rapidly deploying them on site.

Conclusion Prestressing losses are characteristic of composite members once they are cast and affect the ultimate utility of the design. Elastic shortening, shrinkage, and creep all affect the design life and efficacy of prestressed concrete beams. The current methodology tries to consider and account for the interaction of these phenomena to develop an effective design. This highly complex process is the result of the bridge designer’s iterations over many years.■ References are included in the PDF version of the article at STRUCTUREmag.org.

Dustin Black is a Design and Operations Engineer at the Michigan Department of Transportation.

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By Chad Van Kampen, P.E., LEED AP BD+C, M.ASCE, Robert Szantner, AIA, NCARB, and Barry McKinley, P.E. History of Grand Rapids. Courtesy of Giboout.

As

new structures rise above city streets, the architecture is often in conflict with the historical style of surrounding buildings. This is not the case with the 10-Ionia project, located on a site with a deep history. For 10-Ionia, the new architecture was chosen to complement the surrounding structures and conform to a unique triangular site bound by three streets with very active traffic. Originally the Rindge Building site from the late 1800s, it was later used as a parking structure containing street-level commercial areas such as a candy store and cigar shop. This building was a structural steel building with a brick exterior and a unique geometric shape. The building was demolished on May 4, 1978. In early 2016, planning began on a new destination hotel designed as an icon for the City of Grand Rapids, MI, incorporating the architectural style from years past using today’s technology and materials.

Materials Selection The Architect of Record for the project, Yamasaki® Architects, proposed using precast concrete to the Owner as an alternate to cast-in-place concrete or structural steel to address various challenges, which were later validated in collaboration with the precaster, Kerkstra. First, the triangular building shape resulted in connection geometries that would have made connections using CIP concrete or structural steel complicated and expensive.

Second, the steel solution required a higher floor height, which meant an increase in 10-Ionia new structure. façade cost. Also, the CIP concrete solution was inefficient due to the formwork required for the triangular building and the need to clad the cast concrete to provide for a final finish. The precast solution provided the ability to use a thinner external bearing wall that maximized the depth from the exterior wall to the core; it was essential to adapt the Residence Inn® unit plans to the small footprint.

The precast solution also provided the structure with an architectural finished panel at the first two levels using special aggregates and a STRUCTURE magazine

special finish technique to provide the Limestone color. The upper floor facades constructed in structural precast concrete were stained for an economical solution. Taking advantage of the latest concrete precast technology and the economies of producing large-scale repetitive elements, the panels provide a sense of scale and quality generally associated with historic masonry structures.

Sufficient detail to match the surrounding historic district was achieved using a system of offsets, belts, and reveals.

Design Approach Originally conceptualized as a cast-in-place structure, maintaining a similar structural system depth at or near a post-tensioned slab was required. The selection of a hybridframed hollow-core system was made, allowing the floor plate to span 30 feet from the central core to the exterior wall without intermediate support using 8-inch hollowcore slabs with a 2-inch composite topping (Figure 1). Hybrid framing combines the benefits of three structural systems to provide a unique structural system of steel, cast-in-place, and precast. This U-shaped steel form provides a T-shaped beam and composite column design that bonds steel and concrete with significantly enhanced performance characteristics. The use of precast floor members, exterior architectural walls, and shear walls, along with a cast-in-place framing fill and plank topping, provide enhanced span-to-depth ratios, shallower beam depths, and smaller column footprints.

Following the building’s geometry, the structural core of 10-Ionia is also triangular, which provided challenges for the design. The geometry of the structure did not allow for a conventional shear wall or moment frame lateral resisting system, as is most common in total precast structures. Thus the triangular core was chosen to carry the lateral loads of the structure. The core was framed with twelve precast walls per level, connected at each vertical joint with a vertical cast in

place pour strip (full height of structure) and grouted mechanical dowels at each horizontal joint. The result was a structure that behaves monolithically and was designed and analyzed as an emulated cast-in-place triangular tube.

RISA 3D, a structural engineering design and analysis program, was used to model the emulated structure to determine behavior, shear flow through the vertical pour strips, and flexural and shear design of the members (Figure 2).

Numerous block-outs through the precast walls for other trades further complicated the analysis.

The detailing of the structural geometry and intricate architectural façade (also load-bearing) Figure 1. Hybrid steel framing. was accomplished utilizing three-dimensional modeling by both the Architect and Precast Design Team. The Architectural Team utilized Revit, and the Precast Design as the structure went vertical. This allowed for the occupancy of the Team utilized Edge (a three-dimensional modeling software that sits floor below by other trades during non-erection times. The Erector on top of Revit, designed especially for precast concrete detailing) to had a conventional ground-control lattice boom crane with a luffer create the construction model, precast concrete structure drawings, and in a tower configuration. This allowed the crane to move along the production formwork details. The BIM coordination and modeling of East side of the structure to access each of the North and South the structure with Edge proved vital in the final structural design of the side corners and remain within the chart of the crane. This crane core and procurement of the complex forms (Figure 3) required for the did not have two lines, restricting the method in which the panels exterior façade. Many openings in core walls had to be coordinated to were shipped. Each panel had to be shipped vertically such that the either miss critical reinforcing in the panel or design the reinforcing around crane could pick the piece directly from the trailer. Also, most of them. This was accomplished with weekly coordination meetings where the exterior walls had windows pre-installed in the precast, adding the openings were reviewed in the model and decisions made as to the to the gentle nature of the installation. The precast plant rolled all correct course of action to ensure functionality and structural integrity. panels vertically before final finishing and racking. Radius panels Implementing BIM at the beginning of the project helped successfully had special attention paid to the picking inserts’ location to ensure coordinate the complicated geometry and needs of other trades that they did not roll out of vertical such that they could not be installed without additional effort.

Erection Considerations The building was built such that each floor was plated, composite topping added, and connections and steel member infill completed

Lessons Learned Project coordination between the design team, precast fabrication plant, and the Erector was key to this project’s success. During the planning of this project, the precast fabrication labor leads were deeply involved in the means and methods of the forming system to be used, how to remove the pieces, roll the pieces, and eventually ship pieces with minimal detrimental impacts. Plant logistics, labor balancing, and yard logistics were part of the success of a completed panel that was an architectural precast fully-glazed wall section. The precast manufacturing staff also had to engage and manage structural components not typically used with precast structures. This integration and assumption of responsibility opened the project to the systems selected for the core walls, exterior walls, and floor plate. Had the hybrid framing not been introduced, a typical precast beam and column system would have proved too invasive to the building’s floor floor-to-floor height.■ The Project Team and references are included in the PDF version of the article at STRUCTUREmag.org.

Chad Van Kampen currently serves as Chair of the PCI FRP Committee, Vice Chair of PCI Hollow-core Committee, and a PCI Total Precast Committee member. (cvankampen@kerkstra.com) Robert Szantner is a Principal and Lead Designer at Yamasaki Architects. (rszantner@yamasaki.com)

Figure 2. RISA 3D model of the core deflection.

Figure 3. Complex precast radiused piece and form.

Barry McKinley is President of PTAC Consulting Engineers, Monroeville, Alabama. He is active with PCI and currently is on the Industry Handbook Committee and the Technical Activities Council. (barry@ptac.com) A P R I L 2 0 21

structural REPAIR

Underwater Weld Repairs

By Uwe W. Aschemeier, Axel Mutzeck, Kevin S. Peters and Jarrad A. Schmerl

D

ue to damage or deterioration, structures, infrastructures, and even vehicles may require in-situ repairs and modifications. This is true for structures that are on land but also for those underwater.

Piers in ports deteriorate or get damaged by impact or operations over time, and ships and vessels require maintenance and repairs below the waterline.

Topside (on land) welding codes like the American Welding Society’s AWS D1.1, Structural Welding Code, strictly prohibits welding and thermal cutting on wet surfaces or in the rain. Accelerated cooling rates could result in faulty welds with discontinuities in the weld that could lead to the failure of the structure.

This article introduces the basics of underwater wet welding, including two case studies for underwater repair – a pier in the port of Kiel in Germany and the underwater repair of two rudders on a cruise ship in various ports in the Caribbean.

Underwater Wet Welding Per AWS D3.6, Underwater Welding Code, underwater wet welding is performed at ambient pressure with the welder diver in the water without any physical barrier between the water and the welding arc (Figure 1).

Konstantin Konstantinovich Khrenov, an engineer from the Soviet Union, is credited with the first underwater wet weld performed in 1932. During WWII, underwater wet welding gained acceptance by the U.S. NAVY as an emergency repair method and for salvage operations. In the late 1960s and early 1970s, underwater wet welding gained new momentum with the emergence of the offshore oil and gas industry.

Quality of Underwater Wet Welding Five major factors influence the quality of underwater wet welds.

With the introduction of the following key factors, underwater wet welding can be consistently performed for permanent repairs on all types of underwater structures.

Wet Weldability of Materials Wet welding is completed in an environment that is hostile to both the weld and the welding process. Welding in a wet environment results in greater cooling rates and produces significant amounts of martensite (a very hard and brittle form of a steel crystalline structure) in the heat-affected zone (HAZ) for nearly all low-carbon steels. Materials with a higher carbon equivalent (CE) may exhibit a high martensitic microstructure and become increasingly crack susceptible.

The International Institute of Welding (IIW) formula is used to calculate the CE to determine the wet weldability of the base metal.

The generally accepted CE upper limit for wet welding using ferritic electrodes is 0.4%. The ideal materials for wet welding exhibit a CE ≤ 0.38 percent with a carbon content C ≤ 0.16 percent.

The Shielded Metal Arc Welding (SMAW) process using a coated electrode (Process 111 in ISO 4063) is the most common welding process used in underwater wet welding.

Environment The welding process and/or the quality of the weld being produced can be severely affected by environmental issues.

Poor underwater visibility can affect the welder Figure 1. Welder diver performs underwater wet welding. diver’s ability to see the arc and to inspect the resultant weld. Swells, waves, water currents, or vessel movements can have a detrimental effect on the welder diver’s ability to remain stable. Contaminated water may require special diving suits, which may restrict the movement of the welder diver.

Contamination of the water or materials may affect the weld properties. Depth of water has a direct effect on weld quality, in particular porosity and ductility.

Personnel Welder divers should be experienced commercial divers holding an applicable national or international commercial diving qualification and be suitably trained, competent, and experienced in wet welding.

Historically, it was assumed that qualified and experienced topside welders could easily apply their skills underwater without the need for further training. In practice, individuals can rarely adapt without additional training. It is well noted that most experienced top site welders find it difficult to break habits that work fine on the surface but are difficult, if not impossible, to apply underwater.

Wet Welding Electrodes Welding electrodes are comprised of a filler (core) wire, which has a flux covering extruded along its length, except for approximately 1 inch left bare to allow contact in the electrode holder. The filler wire is of high quality but lean in terms of alloying elements. The flux coating is a mixture of compounds and elements that contain all the alloying elements that change the ‘as welded’ weld metal to give the weld metal the desired properties. The flux coating also provides slag and gas formers, de-oxidizers, chemical cleaners, and other additions as applicable to the type of electrode.

Wet welding electrodes are manufactured in the same way as conventional surface welding electrodes. However, they include a waterproofing or additional protective coating. Historically, wet welding was completed using surface welding electrodes either covered in insulation tape or dipped in wax, lacquer, or paint. While some of them produced reasonable looking welds, their mechanical properties and user-friendliness were comparatively poor. Weld quality has significantly improved with the development of new purpose-built welding electrodes.

Equipment Finally, the equipment used in underwater wet welding can have an impact on the quality of the final product. There are basic wet welding equipment requirements that should be met to comply with safety regulations and welding procedure requirements. continued on next page

A P R I L 2 0 21

Underwater Welding Codes The worldwide established underwater welding code for hyperbaric dry and wet welding is AWS D3.6M, which specifies three weld classes (A, B, and O), encompassing a range of quality and properties. Class A welds are intended to be suitable for applications and design stresses comparable to conventional surface welded counterparts. In contrast, Class B welds are intended for less critical applications where lower ductility, moderate porosity, and other limited discontinuities can be tolerated. Class O underwater welds must meet requirements of another designated code/standard, as well as additional requirements specified in AWS D3.6, to cope Figure 2. Condition of the starboard side rudder.

Figure 3. Condition of the port side rudder. with the underwater welding environment.

Following are two case studies: one describes the underwater wet Completion weld repairs of erosion/cavitation damage of the side rudders of a cruise ship performed during regular port stops. The second case A total of approximately 4,500 underwater wet welding electrodes study introduces the renovation of a sheet pile pier at a ferry terminal were used to restore the cavitated/eroded sections of the rudders. In using complex underwater wet welding techniques. some locations, the original plate thickness of 19⁄16 inches was deteriorated to 5⁄16 inches. It took the dive teams six attendances, each lasting 2 weeks, to repair the damages. Diving was performed during Cruise Ship Repair scheduled port stops without interrupting the itinerary of the vessel.

An in-water survey, performed by divers on the underwater part of a Figure 4 shows the completed build-up welds in a particular area 317-yard-long and 125,323-gross-tonnage cruise ship revealed heavy on the port side rudder. cavitation and erosion damage caused by galvanic corrosion on the inboard and outboard side of the port and the starboard rudder. The Pier Renovation in Kiel, Germany port side rudder revealed a total section loss in some areas of the rudder plating, which caused the rudder to flood with seawater (Figures 2 and 3). The sheet pile wall of the Stena Line pier was found to be heavily corroded After the damage was observed, a repair plan was developed that over a length of approximately 1,000 feet. Typically, corrosion damage would keep the vessel out of dry dock and not interfere with the itin- on sheet pile piers is contained within the splash or tidal zones, 3 feet erary of the vessel. The divers would sail with the vessel and perform above and 5 feet below the waterline. However, this sheet pile wall was the work during regularly scheduled port stops. different and exhibited corrosion holes with total section loss into the mudline, to a depth of approximately 36 feet. One of the reasons for the Repair Plan unusual corrosion pattern was due to the daily arrival and departure of The first step was to determine the extent of the damage via an the largest Stena Line ferries. The propellers on the three bow and two inspection dive. Material certificates for the steel used on the rud- stern thrusters of these vessels stir up the water, bringing oxygen to the ders were not available. Since a chemical composition is required bottom and causing heavy corrosion to the sheet pile wall (Figure 5). to determine the wet weldability of the material, steel samples were Repair Plan retrieved from the rudder during the inspection dive. The results of the analysis were within the limits of wet weldability. A repair Two different solutions were discussed for the repair of the pier. The plan was developed to clean and restore the deteriorated areas to first discussion was a completely new construction of the entire pier the original plate thickness. Some of the rudder plating in specific over a length of approximately 1,000 feet, which required the pier to locations could be cropped. The leading edge of the rudder with be closed during construction. All daily handling activities, including total section loss was repaired with a doubler plate that matched trucks, trailers with containers, as well as cars and passenger traffic, the curvature of the leading edge. would have to be diverted to another pier, an almost impossible task in the relatively small port of Kiel.

Welding

All welding was performed underwater employing underwater wet welding techniques following the submitted and approved repair procedure and with the class approved Procedure Qualification Records (PQR) and Welding Procedure Specifications (WPS).

Underwater welding was performed employing the wet SMAW with Hydroweld FS underwater wet welding electrodes.

The finished welds met the requirements of Class A underwater welds per ANSI/AWS D3.6M:2010.

The mechanical properties of Class A welds per AWS D3.6 are comparable to topside welded connections.

STRUCTURE magazine

Figure 4. Build-up weld on port side rudder.

Figure 5. The propeller of the ferries swirls the water.

Figure 6. Detailed view of a sheet pile field.

of each sheet pile field to guide and secure the seal plates into position. The T 120 structural profiles were welded with shear tabs 6 x 3 x ½ inches to the seaside face of the existing sheet pile wall (4,611 pieces) (Figure 7 ).

Adjustment tabs were used to fine-tune and to align the T 120 structural profiles and to support the sealing plates (Figure 8).

567 pieces of sealing plates (Item 1 in Figure 6) fabricated from ½-inch-thick higher tensile steel S355 were installed. The sealing plates were installed from the bottom to top elevation in between the T 120 structural profiles. They were secured in position with stitch welds between the plates and the T 120 on the sides, and with weld tabs on the top and bottom. Three seal plates were used to achieve the required height. The height of the plates varied between 94.5 inches and 102.4 inches.

A total of 1,850 feet of linear welds were produced by underwater wet welding techniques to connect all structural components. All structural welds were executed as multiple layer fillet welds by welder divers certified to EN ISO 15618-1.

The approximately 1,850 cubic yards of C35/45 underwater concrete was poured in three separate lifts using the tremie method. After each pour, the concrete was allowed to cure completely. Prior to the next pour, the laitance layer on top of the concrete was removed with a highpressure cleaner.

The second option considered the renovation of the sheet pile wall during day-to-day operations by divers, mostly during night shifts. This solution was chosen since it did not affect the operation of the pier.

The most significant advantage of the renovation compared to a new build was reduced cost, despite relatively expensive labor costs resulting from diving activities performed primarily during night hours. Additionally, the operation of the ferries was not affected.

The approach for the repair of the pier was planned and executed as follows: 1) Installation of prefabricated reinforced steel forms that wrapped around the existing sheet pile wall, 2) Installation of support brackets (Item 5 in Figure 6) inside the sheet pile fields between the upstream and downstream wall, Figure 7. Aligned and welded T 120 structural profiles.

3) Installation of structural T-profiles Approximately 14 feet of linear weld was placed for the (Item 2 in Figure 6) welded with shear installation of each structural profile T 120. tabs to the seaside face of the existing sheet pile wall, and 4) Installation of sealing plates (Item 1 in Figure 6).

The Repair To gain access to the bottom of the sheet pile wall, approximately 3,200 cubic feet of mud, silt, and scrap had to be removed and relocated by divers, with the help of pumps, from the base of the sheet pile wall.

The next step was the cleaning of the existing Figure 8. Adjustment tabs. sheet pile wall. Approximately 32,000 square feet of the existing sheet pile wall had to be cleaned from marine growth, loose rust, and other foreign material by divers with a pressure washer at an operation pressure of 4.650 ksi.

A total of 20 U.S. tons of reinforcing steel, including 660 pieces of prefabricated reinforced steel forms that wrapped around the existing sheet pile wall, were installed and followed the contour of the existing sheet pile wall, held in position with welded shear tabs.

Structural U 140 channels (5.5 x 2.5 x ¼ inches) were used to fabricate the braces to be installed between the upstream and downstream walls of the existing sheet pile wall. A total of 890 braces (5 U.S. tons) were installed at different elevations.

39 U.S. tons of structural profiles T 120 (Item 2 in Figure 6), each approximately 25 feet in length, were welded onto the seaside face

Summary Today, the introduction of steels with chemistries more favorable to underwater wet welding (C ≤ 0.16 percent and C.E. ≤ 0.38 percent), welding electrodes developed explicitly for underwater wet welding applications, managed training programs, and improved welding techniques have greatly improved options for underwater welding. Underwater wet welds in groove and fillet welds can achieve comparable mechanical properties and nondestructive evaluation (NDE) results as their top site welded counterparts.

The two case studies show that complex underwater projects can be completed meeting top site engineering requirements without building expansive habitats, which would allow the work to be performed under dry conditions.■

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

Uwe W. Aschemeier is Senior Welding Engineer of Subsea Global Solutions, Miami, FL.(uwe@sgsdiving.com) Axel Mutzeck is Chief Executive Officer, Unterwasserkrause – Mutzeck GmbH, Kiel, Germany. (info@unterwasserkrause.de) Kevin S. Peters is Director of Technical Sales & Repair & Environmental Services of Subsea Global Solutions, Miami, FL. (kevin@sgsdiving.com) Jarrad A. Schmerl is Senior Project Manager of Subsea Global Solutions, Long Beach, CA. (jarrad@sgsdiving.com) A P R I L 2 0 21

construction ISSUES Which Post-Tensioning Tendon?

Which End to Stress First?

By Bijan Aalami, Ph.D., S.E., C.Eng

I

n post-tensioned floors, tendons are stressed after cast concrete gains adequate strength. The stressing operation is monitored to ensure that tendons receive their designintended force.

Figure 1. Post-tensioned member at stressing.

Three questions on tendon stressing are often raised among post-tensioning crews. These are: To fully appreciate the significance of the above instructions and 1) What is the right sequence of tendon stressing when the their applicability, two considerations are important. structure has multiple tendons with different arrangements, First, in standard construction, the entire floor is resting on forms such as common two-way slab systems.? at the time of stressing. Post-tensioning results in bending stresses in 2) When long tendons are marked for stressing at both ends, the slab or beam only if the uplift from the post-tensioning tendons which end should be stressed first and what is the right order is greater than the self-weight of the member being stressed. Bending for correct elongation measurement? stresses result from a change in the curvature of the member. If the 3) How are the stressing records reconciled when the elongation member being stressed remains in contact with its support, its curvameasurements are outside the target limits? ture does not change – hence no bending stresses (Figure 1).

This article covers the first two questions. A follow-up article will Second, in standard building construction, post-tensioning for slabs address the third. or beams is typically selected to provide uplift between 60% to 80% of the member self-weight, or, in the case of beams, the self-weight of the beam and the reaction from members that shed load on the Stressing Sequence beam. Again, if, at stressing, the post-tensioning is not large enough When there is more than one tendon or one tendon group in the to lift the members that load the beams, the distribution of load construction, one question is – which tendon group to stress first? among the members’ envisaged in-service conditions does not occur.

The following text is from the General Notes of a typical construcThe following illustrates that, for common conditions, standard tion document for post-tensioned floors. design parameters do not favor a specific stressing sequence.

The general tendon stressing sequence for one-way systems shall be as follows: Beam and One-Way Slab Construction First: uniformly distributed tendons; Second: beam tendons; Figure 2 shows a post-tensioned beam and one-way slab construcThird: girder tendons. tion. The slab and beam tendons are each profiled to counteract their The General tendon stressing sequence for two-way systems shall be respective self-weights. as follows: Apart from its own weight, the beam carries the reaction from the First: uniformly distributed tendons; slab it supports. The question on stressing sequence reduces to the Second: banded tendons. following: Does stressing of the slab before or after beam stressing Continuous tendons in multi-span beams (girders) shall be stressed impact the beam’s performance either during the construction or before any non-continuous tendons. when in service?

Figure 2. Post-tensioned beam and slab construction; single and multi-span slabs.

STRUCTURE magazine

Figure 3. Reactions of a single-span member.

For the single-span slab shown in Figure 2a, the slab reaction on the beam does not depend on whether or not the slab is stressed and how much of the slab weight is balanced by post-tensioning. The slab reactions, as shown in Figure 3, remain unchanged. The load on the beam is independent of the state of stressing in the slab. For this reason, it is immaterial whether the beam or the slab is stressed first.

Likewise, the stressing of the beam does not impact the response of the slab. The sequence of stressing can be based on the expediency of construction. The other consideration is that the beams are rarely designed to provide uplift beyond their own weight and the reaction of the slabs they support. The beam is not likely to lift off its support when stressed first.

Figure 2b shows a multi-span member, such as slabs over parallel beams. In this case, the slab reactions on the beam depend on whether the slab tendons are stressed or not – tendon stressing changes each of the reactions from gravity by the hyperstatic forces from prestressing. The question is whether the change in the slab reaction on the beam arising from slab stressing is large enough to impact the beams’ response.

Referring to Figure 2b, where the slab spans over several beams, the load on the beams will be different depending on whether the slab is stressed. In this case, in principle, the sequence of stressing matters. But, since the hyperstatic reactions from prestressing are generally much smaller than the reactions from self-weight, the sequence of stressing is not viewed as consequential. Again, the question is addressed since, at stressing, the slab and beam are both supported.

In summary, for either single or multispan beam and slab construction, the sequence of stressing may be based on the convenience of construction. Each member may be stressed in full before moving to the next.

Hence, the slab will not lift off the form support. Extreme fiber bending stresses will form only if stressing results in a change in slab curvature.

The common practice is that distributed tendons are stressed first.

As an example, for the condition shown in Figure 4 (page 30) with interior spans at 29 feet and slab depth at 7.5 inches, the uplift from the post-tensioning of banded tendons must exceed 80% of the slab’s self-weight before the slab partially lifts off the form. The sequence of stressing is not critical to the stress condition of the slab either at stressing or when in service. The convenience of construction favors stressing sequence. continued on next page

MAPEI STRENGTHENS.

Two-Way Slab Construction There are several options in the tendon layout of two-way slabs. Figure 4 (page 30) is a partial view of a two-way slab illustrating the typical banded-distributed layout of tendons.

Two questions arise in connection with the sequence of stressing: first, whether the sequence of stressing will result in local over-stress at construction. Second, whether the sequence will change the resulting load-carrying characteristic of the floor system when in service.

In practically all constructions, the post-tensioning is fine-tuned to balance a fraction of the member’s self-weight, typically 50% to 80% – not more than the member’s total weight at stressing.

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A P R I L 2 0 21

Point A marks the tendon force at maximum jacking pressure. Once seated, the retraction of the wedges into the anchor cavity results in some stress loss. Once fully seated, the tendon force diagram is shown by line DCB. ACD is the stress loss from the retraction of the wedges.

The extension length of the strand after it is seated is proportional to the area below the curve DCB. The larger the area, the longer is the extended length of strand protruding out of the anchor block.

The extended length beyond the anchor block is referred to as tendon “elongation.”

The direct correlation between the force in the tendon, given by the area below the force diagram, and the elongation of the strand, evidenced by its extended length out of the member, is used to verify the successful jacking operation. For successful stressing, the tendon elongation is matched against the area below the stressing Figure 4. Partial view of slab showing the banded-distributed layout of tendons. curve (Figure 5b).

Figure 5c shows the force diagram after stressing and seating of the Stressing Sequence second end. BEFH marks the change in the tendon’s force diagram When a long tendon is marked to be stressed at both ends, the fol- resulting from stressing and seating of the second end. lowing questions arise.

The extension length of the strand after stressing of the second end is (i) Which end should be stressed first? proportional to the area BEFH. Comparing the area below the force (ii) Should one end be stressed to diagram associated with the stressing of the full value and seated before first end and that of the second end, it is stressing the other end? evident that the elongation of the tendon Or, should one end be parat the second end is less than that of the tially stressed, be seated, and first end. the second end stressed to The final force diagram of the tendon, completion? when stressed in full and seated, is shown (iii) Should both ends be stressed in Figure 5d. Irrespective of the stresssimultaneously? ing sequence, if during the stressing Briefly, at stressing, the jack pulls the operation, at some point and each end, prestressing strand out of the anchor the tendon is pulled to the design force piece. Once the jacking force reaches (points A and G), the final force diathe design value, the strand is anchored gram will be DCEFH. And the sum of at the face of the member. The common the elongations of the two ends will be anchoring mechanism is by way of proportional to the area below DCEFH. wedges that grip the strand. The retracFrom the preceding, the successful tion of the wedges into the conical cavity stressing operation of the tendon is of the anchorage grips to lock the pulled measured by matching the sum of the strand into position. elongations at the two ends against In Post-Tensioned Buildings: Design and the calculated value based on the area Construction (2014), the author provides below the diagram DCEFH. the details of the stress loss in the tendon In summary, the sequence of stressing a and other considerations that govern the tendon (or whether the stress at one end force in the tendon at stressing and longis applied partially and followed by stress term effects. The focus of the following is at the other end before completing the a sequence of stressing and measurement stress at the first end) does not impact the of tendon elongation. total elongation and the force distribuFigure 5a shows a post-tensioned tion in the tendon. This is on the premise member to be stressed at both ends. that the stressing operation is concluded The common practice is to stress one when each end, at some point, has been Figure 5. Force diagram of a two-end stressed tendon. end to its design force, seat it, stress the stressed to the full value.■ second end to its design force, and seat the second end. Tendons are simultaneously stressed at both ends A full reference is included in the PDF version only in special circumstances. The common practice is to stress the of the article at STRUCTUREmag.org. tendon ends to full force, one end after the other.

Under full force at the left end, the distribution of force in the Bijan Aalami is a Life Member of ASCE, a Principal of PT-Structures Inc., an tendon is shown by line AB (Figure 5b). The distribution is shown Emeritus Professor at San Francisco State University, and a former Professor by a straight line to explain the concept. The force drop along the at Arya-Mehr University (now the Sharif University of Technology). tendon length is governed by the tendon’s geometry and the tendon’s (bijan@pt-structures.com) other properties. It is close to a straight line.

STRUCTURE magazine

engineer's NOTEBOOK Error Checking and the Black Box Part 1

By Scott N. Jones, S.E.

T

he “black box.” We have all heard the phrase, maybe to the point of nauseum.

By necessity, as code requirements continue their evolution into ever-increasing complexity, we depend more and more on software to do the calculations that some of us and our predecessors used to do by hand. Not only can the intimate knowledge of how to run the calculation be lost in this trade-off, An incorrectly modeled continuous beam (top) demonstrating an inverted “V” at the interior support. A correctly that move from pencil, paper, and calculamodeled continuous beam (bottom) demonstrating an inverted “U.” tor (or slide rule!) to computer software, the old-fashioned “gut feeling” can be lost as well. Unfortunately, these model failed to work, the culprit was usually in the area of boundary essential tools of the wise engineer are quickly disappearing from conditions. Similarly, the culprit with a part of the structure being our profession. under-designed was also often due to improper boundary conditions The author understands that it is not economically feasible to aban- and member-to-member fixities. The secret to success: the animated don the black box in favor of the wisdom gained from a lifetime of deflected shape. Deflected shapes tell a story, and animated shapes hand calculations. This three-part series provides you with a set of write a novel! Is the end of that cantilever deflecting as it should? Do tools and principles that will allow you to error-check your results you have a point on the structure flying off the screen? Does the conwith confidence. tinuous beam look like a sharp inverted “V” (mistake) or a swooping inverted “U” (correct) at the interior support? (see Figure) Make sure to view it from different planes, with isometric often being the most Total Load Checks useful. Read the story that the deflected shape tells.

You likely had a college professor or two suggest this method of error checking your schoolwork. Use a good old-fashioned calculator and Deflection add up the shear of each story and make sure it matches the total base shear. Add up the axial load of all columns supporting the level and We understand it well, but the general public and even many architects check it against the total dead load plus live load (DL+LL) on the do not know that a beam must deflect in order to support a load. floor. Similarly, do this at the foundation. Note that, in some cases, But one more thing is for sure: if your beam deflects enough such doing this type of check will require you to create a new load case in that it can be seen by the naked eye, you will be left trying to teach a that black box to spit out service-level loads. Factors and reductions course in structural design of beams to an uneducated audience who tend to skew the results! does not care about engineering principle. They just want the beam Now consider doing a 180 from the total load check. What if you to be straight. To error check for this, look for long-span beams that started with approximate methods and rough calculations? Do enough are lightly loaded – beams where the self-weight is significant. This work upfront to establish what the answer should look like – just is especially true for beams that can be seen, like those being used as approximately. That way, when the black box starts spitting out a decorative eyebrow or a storefront element. Check the total deflecinformation, you can know right away if it seems reasonable – or if tion. You will be surprised to see how often you will want to stiffen you have a big problem that needs to be resolved before you start the it, even if it does meet a reasonable deflection ratio, i.e., an “L-over.” tedious work of refinement. There is nothing worse than being asked Do any of the beams support masonry? Is the ceiling a flexible T-bar, by a superior, “do the results make sense?” And then having nothing or did the architect slip a rigid hard lid ceiling in there? Make sure the to support your obligatory answer, “I think so…” appropriate L-over has been used. Now, do you want to thicken the plot? Consider that which many an engineer often ignores – cumulative deflections. Here is an area where modeling a beam network in Deflected Shapes a program shines and hand calculations lag behind.

Early in the author’s career, he became a self-professed guru of There you have it, Part 1 of three of Error Checking and the RISA-3D (a structural engineering analysis and design software). Black Box. Part 2 will discuss load paths, connections, torAn expertise in the software offered the ability to model complex sion, temperature and shrinkage, and dissimilar materials.■ structural networks, frames, etc. – a valuable bag of tricks for a young, aspiring engineer. Many colleagues at the firm had a hard time Scott N. Jones is a Partner at Wright Engineers in Orange County, properly setting the boundary conditions with what was, at the time, California. (sjones@wrightengineers.com) fairly new software. The author’s experience taught him that when a

STRUCTURE magazine

A P R I L 2 0 21

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INSIGHTS Reducing Potential Liability in Emergency Response

By Melissa T. Billig, Esq., and Maurizio Anglani, Esq.

A

rchitects and engineers have become more reluctant to render services over the last 20+ years in response to disaster emergencies out of fear of liability exposure.

Before doing so, A/E professionals should ensure that their actions do not present a liability disaster to themselves and their firms.

This article briefly reviews lessons learned in the aftermath of emergency responses by the A/E community and addresses ways that A/E professionals can manage risk to continue providing vital emergency response services.

Lessons Learned The scores of architects and engineers that have been called upon to respond to disasters – such as the World Trade Center terrorist attacks, hurricanes Ivan, Katrina, Sandy, and Maria, and several earthquakes – have, unfortunately, been exposed to varying degrees of liability in providing their well-intentioned emergency services.

In response to the WTC disaster in New York, A/E professionals were retained to analyze the stability of impacted buildings and debris piles to facilitate recovery and cleanup efforts by the emergency responders, many of whom fell ill due to exposure to various toxins during these efforts. To their surprise, the A/E professionals found themselves engulfed in multi-year litigations filed by the responders who claimed that the involved professionals were responsible for their safety. After many years of litigation, the claims against these professionals were ultimately dismissed. Since then, despite being “victorious” (years of litigation is not victorious to anyone), A/E professionals have hesitated to respond to subsequent disasters such as earthquakes and hurricanes.

A/E professionals should be able to respond to emergencies without fear of placing themselves or their firms at risk. While Good Samaritan laws may provide some protection (for purposes of this article, these laws refer to statutes that provide certain liability protections and/ or immunity to A/E professionals performing emergency services in disaster response situations), these laws are not perfect or consistent across jurisdictions. Also, design professionals face the same liability for the services they provide – whether they are compensated or volunteer to provide them pro bono.

Limiting Liability Exposure A/E professionals can effectively reduce their liability exposure by including the following STRUCTURE magazine

provisions in their agreements and/or waivers for the emergency services. Note that while these provisions may reduce risk with their contracting party, they may not guard against potential liability to third parties such as a member of the public.

1) Scope of Services and Standard of Care. A welldefined and narrow scope of services should be included that states the services are being rendered on an emergency (and not-comprehensive) basis. Services not being rendered should be excluded (e.g., site safety, environmental, health-related, air quality, inspection, or protection services). In addition, the standard of care should be that of an A/E professional performing emergency services under similar circumstances in the applicable jurisdiction and under a compressed schedule.

2) No Responsibility for Means and Methods of Construction or Safety.

Including a provision that confirms that the A/E professionals are not responsible for site safety, the means and methods of the emergency work, or other factors relating to the project (environmental, air quality, protections, etc.) can help to make frivolous litigations infinitely less costly (stating this in the contract may help the A/E professional obtain an early dismissal from the litigation). Of course, this requires design professionals to ensure that their services do not run afoul of the terms of these provisions.

3) Limitations of Liability. Suppose A/E professionals are providing disaster response services free of charge. In that case, they should still attempt to limit their liability to the available insurance covering the services – to the extent its insurer provides coverage for such pro bono work. If compensated for the services, they should try to limit their liability to the lesser of (i) the amount of its fees under the contract or (ii) the available insurance proceeds. Again, as mentioned above, while limitations of liability are valuable to reduce risk exposure with the contracting party, they may not reduce risk exposure to non-contracting parties such as the general public.

A/E professionals should be able to respond to emergencies without fear of placing themselves or their firms at risk.

4) No Liability for Consequential Damages. Including a waiver of consequential damages can shield A/E professionals from being exposed to remote – and potentially high – contractual damages, such as lost profits and loss-of-use damages of the contracting party. However, as with the limitations of liability provisions, this provision would not apply to claims by non-contracting parties.

5) Indemnity. Indemnity provisions are crucial to hold harmless, defend, and indemnify the A/E professional from third-party claims (claims from persons or entities who are not parties to the contract), including those arising from personal injuries or property damage. A properly drafted indemnity provision should also include reimbursement of reasonable attorneys’ fees for the counsel of the A/E professional’s choice.

Conclusion Providing design services in disaster response situations can be a significant risk to architects and engineers. Including the above-referenced contractual provisions can help reduce risk and enable A/E professionals to continue to provide vital emergency responses. However, given the potential risk exposure to third-parties, A/E professionals would be well-advised to provide emergency response services with the same care and diligence as they would for their best corporate client.■ Melissa T. Billig is a Partner with Ingram Yuzek Gainen Carroll & Bertolotti, LLP. (mbillig@ingramllp.com) Maurizio Anglani is an Associate with Ingram Yuzek Gainen Carroll & Bertolotti, LLP. (manglani@ingramllp.com) A P R I L 2 0 21

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NCSEA

NCSEA News

National Council of Structural Engineers Associations

Foundation Funds Diversity in Structural Engineering Scholarship NCSEA is proud to announce a new scholarship opportunity funded by the NCSEA Foundation. The NCSEA Diversity in Structural Engineering Scholarship was established as part of ongoing initiatives to strengthen equity, diversity, and inclusion programs.

This scholarship will provide funding to students who have been traditionally underrepresented in structural engineering, including but not limited to Black/African Americans, Native/Indigenous Americans, Hispanics/Latinos, and other people of color.

The Diversity in Structural Engineering Scholarship is open to any currently enrolled junior college, undergraduate, or graduate-level student who will be registered in the 2021 fall term and has a focused career interest in structural engineering and/or is actively pursuing a structural engineering degree. Multiple scholarships up to $5,000 each will be presented annually.

NCSEA encourages SEA members to share this scholarship opportunity with individuals, firms, and schools in their area. Together with the support of the SEAs, we can strengthen the future of the structural engineering profession.

For full eligibility and the application, visit www.ncsea.com. Applications are due April 30, 2021.

READ.WATCH.LISTEN. NCSEA’s Structural Engineering Engagement and Equity (SE3) Committee has been releasing monthly Diversity, Equity & Inclusion Resources in the e-Newsletter, Structural Connection. Each month, the committee curates a series of articles, audio-visual and digital media to facilitate self-education in matters that affect our professional practice as structural engineers. The purpose of READ.WATCH. LISTEN. is to share and promote conversations on diversity, equity, and inclusion within the structural engineering profession. The library of resources can be found under the DEI Resources page on www.ncsea.com. Whether you choose to read, watch, or listen (or all three!), you are invited to join us in this important conversation.

Diversity, Equity & Inclusion Webinar Series NCSEA's Diversity, Equity & Inclusion Resource page is home to the three part DEI Webinar Series that took place in 2020.

This series, like the above initiatives, is part of NCSEA's work to identify and eradicate behaviors that perpetuate racism and inequality within the structural engineering profession. For this series, NCSEA partnered with a strategic diversity and inclusion practitioner to develop the webinars to introduce attendees to diversity, equity, inclusion, and speak to creating a multicultural organization via inclusive policies, programs, and practices. The webinars are listed in the recommended, chronological viewing order on www.ncsea.com/resources/dei.

NCSEA also has established a Memorandum of Understanding with The National Society of Black Engineers (NSBE) to collaborate on several initiatives, including membership, pre-college and college student outreach, professional education, and local, state, and regional connection. NSBE is hosting its 47th Annual Convention (#NSBE47) virtually on April 5-9, 2021. The Convention demonstrates NSBE’s commitment to lead the United States to graduate 10,000 Black Engineers annually by 2025. As one of the largest student-governed organizations based in the United States, the NSBE supports and promotes the aspirations of collegiate and pre-collegiate students and technical professionals in engineering and technology.

Visit convention.nsbe.org to register!

STRUCTURE magazine

News from the National Council of Structural Engineers Associations 2021 Virtual SEA Leadership Retreat Join us April 20-22 for the 2nd Annual Virtual SEA Leadership Retreat that features a customizable schedule of 21 sessions over 3 days to help you and the leaders and members of your SEA. Registration is free. Click here to register for each session you want to attend. Please invite others from your SEA. Everyone is welcome!

In addition to the opening keynote on what association leaders can expect in the future and six other education sessions, this year’s Retreat includes opportunities to engage with NCSEA Committee Chairs through 12 Interactive Committee sessions. Learn more and register for any of the events that interest you by visiting www.ncsea.com/events/leadership.

The Retreat includes: • NCSEA 101: Everything You Want to Know or Ask • Welcome and Opening Keynote: Mark Engle from the Association Management Center shares his unique insight on what leaders of membership-based associations can expect in 2021 and beyond.

• 6 Interactive SEA Education Sessions: Tips and ideas to help your SEA. • 12 NCSEA Committee Interaction Sessions: Learn about the goals of the committees. Ask questions and get feedback on what you need.

• Closing Celebration: Prize drawings and more!

Become a Corporate Member NCSEA offers two levels of Corporate membership that provide unique benefits and opportunities to increase your engagement within the structural engineering profession. Both levels include various advertising capabilities, event discounts, access to NCSEA's Webinar Subscription, and more! The Associate Membership is for nationally recognized bodies that are associated with the practice of structural engineering, or are companies who provide supplies or services to structural engineers. The Sustaining Membership is for structural engineering firms, firms that employ structural engineers, and practicing structural engineers. Secure visibility and enhance your recognition in the industry by becoming a member today! Learn more at www.ncsea.com.

NCSEA Webinars

Register on www.ncsea.com

April 6, 2021

The State of the Art in Existing Masonry Structural Testing Donald Harvey, P.E.

Whether adding stories to an existing structure or removing walls to open up interior space, structural engineers are often asked to modify loads to existing masonry walls. Fortunately, there are many tools available for the evaluation of existing masonry and existing masonry distress. This webinar will discuss the latest and greatest in both destructive and nondestructive methods for existing masonry structural evaluation.

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 available guidelines 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.

A P R I L 2 0 21

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Celebrate 25 years advancing and serving structural engineering

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Earn PDHs and have fun networking with your community and global colleagues!

Earn PDHs and have fun Celebrate 25 years networking with your advancing and serving community and global structural engineering colleagues!

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Advancing the Profession

Congratulations!

Performance-Based Structural Fire Design of Tall Buildings: Exemplar Design using ASCE 7-16 Appendix E has won an “Award of Excellence” from the Council on Tall Buildings and Urban Habit in the Innovation Category. This free publication is a joint effort of SEI and the Charles Pankow Foundation. https://bit.ly/3qsa6dg

Errata STRUCTURE magazine

NEW Electrical Grid Collection

In response to the severe winter storms and failure of the electrical grid in Texas, the ASCE Library has assembled papers highlighting the limitations of current electrical grid infrastructure. Open to registered ASCE Library users through April 23 at https://ascelibrary.org/electricgrid2021.

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 Membership

The Power of Invitation

How did you join SEI – did someone take the time to invite you when you started out and share why involvement is valuable – for the learning…resources…connections? Remind your colleagues about SEI member benefits and invite them to join www.asce.org/structural-engineering/sei-membership.

Take the challenge and participate in the ASCE Member-Get-A-Member program – making it easy to extend the invitation and earn rewards. https://bit.ly/3kVLcBy

Thank you to 2021 SEI Sustaining Organization Members

SEI Sustaining Organization Members support the mission and goals of SEI to advance and serve the structural engineering profession. Learn more at https://bit.ly/30q4TIu SEI Elite Sustaining: ALFRED BENESCH & COMPANY SEI Sustaining Organization Members: BOSWELL ENGINEERING SIMPSON STRONG-TIE HARDESTY & HANOVER WALTER P MOORE SCHNABEL FOUNDATION COMPANY

SEI Online

SEI Virtual Events

www.asce.org/SEI/virtual-events • #SEILive Conversations with Leaders on Hot Topics in Structural Engineering Join us Wednesday, April 7, 12:30 pm US ET on Engaging with the ASCE Infrastructure Report Card • SEI Standards Series Join live, virtual sessions for exclusive interaction with expert ASCE/SEI Standard developers on state-of-the-market updates.

Participants will learn about technical revisions and review a design example. Attendees are encouraged to join and participate in live Q&A. Each session is LIVE and only available 1:00 - 2:30 pm US ET. MAY 20 – ASCE/SEI 49 Wind Tunnel Testing for Buildings and Other Structures JULY 15 – ASCE/SEI 72 Athletic Field Lighting SEPTEMBER 16 – ASCE/SEI 59 Blast Protection of Buildings NOVEMBER 18 – ASCE/SEI 8 Specification for the Design of Cold-Formed Stainless Steel Structural Members Individual session: Member $49, Nonmember $99. Student member: Free registration.

REGISTER NOW at https://cutt.ly/9hQDTEo

In Case You Missed It

View the latest SEI YouTube videos, including the Joint CASE, NCSEA, SEI Town Hall and the Future Impact of COVID-19 on the Commercial Development Market – http://bit.ly/30wffGN

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

CASE in Point Did you know?

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

The following documents/templates are recommended to review/use if your firm needs to update its current Quality Assurance Program or incorporate a new program into the firm culture: 962: 962-B: 962-C: 962-D: Tool 1-2: Tool 2-1: Tool 2-4: Tool 4-1: Tool 4-2: Tool 4-3: Tool 4-4: Tool 4-5: Tool 9-2: Tool 10-1: Tool 10-2:

National Practice Guidelines for the Structural Engineer of Record (2018) National Practice Guideline for Specialty Structural Engineers Guidelines for International Building Code Mandated Special Inspections and Tests and Quality Assurance Guideline addressing Coordination and Completeness of Structural Construction Documents Developing a Culture of Quality Risk Evaluation Checklist Project Risk Management Plan Status Report Template Project Kick-off Meeting Agenda Sample Correspondence Letters Phone Conversation Log Project Communication Matrix Quality Assurance Plan Site Visit Cards Construction Administration Log

You can purchase these and the other Risk Management Tools at http://bit.do/CASEDOCS or www.acec.org/bookstore.

CASE Winter Member Meeting Update

In the past, CASE has brought their committee members together for a day-long working session with a roundtable the night before. This year, along with other ACEC Coalitions, CASE developed a virtual education program with committee meetings. Held February 24-26, this meeting had several education sessions ranging from Effective Communication and Documentation and Navigating Cyber Security in Engineering to Maximizing Client Relationships at a Physical Distance and Remote Work Tax Implications.

During the afternoon of the 26th, CASE Contracts, Guidelines, Toolkit, and Programs and Communications Committees met to work on advancing some strategic initiatives and engaging members even further in discussions. If you have an interest in joining any of these committees, please contact the chairs listed below.

Contracts Committee – Bruce Burt (bburt@rubyandassociates.com) Guidelines Committee – Kevin Chamberlain (kevinc@dcstructural.com) Programs and Communications Committee – Nils Ericson (nericson@m2structural.com) Toolkit Committee – Roger Parra (rparra@degenkolb.com) The next Member meeting will be held summer of 2021; please contact Marie Ternieden (mternieden@acec.org) to be added to the pre-registration listing.

Follow ACEC Coalitions on Twitter – @ACECCoalitions.

STRUCTURE magazine

News of the Coalition of American Structural Engineers CASE Practice Guidelines Currently Available CASE 962-I – Structural Engineer’s Guide to Working with a Geotechnical Engineer Structural engineers charged with designing a structure depend on the input of geotechnical engineers to determine the appropriate foundation type for a site and building design. Practicing Structural Engineers are faced with educating their clients about services needed from the Geotechnical Engineer, how to retain the right Geotechnical Engineering firm for the project, and how best to implement the recommendations of the Geotechnical Report on their project. This Guide was created to discuss the structural engineering business practice aspects of working with a Geotechnical Engineer.

CASE 976-C – Commentary on Code of Standard Practice for Steel Buildings and Bridges The 2010 COSP addresses many recent changes in the practice of designing, purchasing, fabricating, and erecting structural steel. Therefore, the document is a continuation of the trend of past improvements and developments of this standard. It is important to note the Structural Engineer can change any of the requirements of the Code of Standard Practice by specifying an alternative in the Contract Documents. This document discusses the list of changes published in the preface of the 2010 Edition and provides some commentary on these changes. It also addresses areas of the COSP that may not be well understood by some SERs but will likely impact the structural engineer’s practice of designing and specifying structural steel.

CASE 976-D – Commentary on 2010 & 2015 Code of Standard Practice for Steel Joists and Joist Girders This commentary provides observations and analysis of the revisions and additions in both documents and discusses specific aspects of the COSP that have a direct impact on the structural engineer’s practice of specifying steel joists. A familiarity and understanding of the entire SJI COSP is necessary to ensure the proper design and documentation of steel joists and Joist Girders. However, the Commentary discussion highlights sections of interest to the specifying structural engineer.

CASE 976-E – Commentary on ASCE Wind Design Procedures 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. One helpful aspect of the restructured wind provisions is that each part of each analysis procedure contains a step-by-step checklist of items that need to be determined for that given procedure, along with references to Figures, Tables, and Equations in which those parameters can be determined. The changes in wind design procedures and chapter formatting from ASCE/SEI 7-05 to ASCE/SEI 7-10 were very extensive. The changes from ASCE/ SEI 7-10 to ASCE/SEI 7-16 were minor in comparison and were noted with solid grey lines in the margins of ASCE/SEI 7-16.

You can purchase these and the other Risk Management Tools at http://bit.do/CASEDOCS or www.acec.org/bookstore.

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 ABET-accredited 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 Marie Ternieden (mternieden@acec.org) to donate.

A P R I L 2 0 21

structural FORUM A Few Things Young Engineers Should Know!

By Steven G. Provenghi, S.E., M.ASCE

H

aving graduated from college into my first job as an engineer a mere 45 years ago, I experienced the things I write about here.

Although not easy, I am happy to say that I still love to come in to work and perform engineering. I have worked for four Companies over my career, all good Companies. I started with an ENR Top 10 Engineering firm and have moved to smaller and, for me, more intimate Companies each time I took a new position. I felt that by working for smaller companies, my contributions would have more impact on my clients. Here are a few things that served me well as I began my career and have held up as I advise young engineers on traditional structural engineering career paths. This article was initially prepared and delivered to Dr. Joseph M. Plecnik’s Professional Practices Class (CE481) at California State University, Long Beach, in March 2007 and has been updated for publication here. Dr. Plecnik was the author’s favorite Professor at CSULB. That first job. The first obligation of an engineer is to hold paramount the public’s health, safety, and welfare. Your first responsibility as an engineer will be to find a job.

As a graduate engineer, understand that you will essentially be an intern until you become licensed. You need to be willing to do any and all assignments which can broaden your overall knowledge of the field, including working as a CAD specialist or measuring or monitoring. This will make you a more valuable employee to your company, and it will make you a more rounded and capable engineer with many more future options! Also, stress any past work history. And remember, you are a professional; dress like one.

Organize and be organized. When assigned projects, break the projects into manageable tasks that can be completed one by one. Many engineers at all levels find it helpful to prepare a daily to-do list. Remember, the Pyramids were constructed block by block, and projects are completed on a task-by-task basis.

Once projects are broken up into manageable tasks, they become less daunting, and it is easy to enlist the help of other engineers in your firm. When I get bogged down or feel overwhelmed, I fall back to my to-do lists and complete one task at a time. I continuously tell myself that, by the end of this week or next week, I will have completed all the necessary tasks and the crisis will be over. For me, it is very reassuring.

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Engineering license. Pursue becoming a licensed professional – PE/civil engineer (CE)/ structural engineer (SE). Remember that only 20% of all engineers get their licenses, and it is that 20% that get the best jobs and largest salaries. Many states also require Continuing Education Units (CEU) or Professional Development Hours (PDH), leading up to and following licensure. While many companies financially support attendance, be willing to spend some of your own time and money for outside seminars or courses as well. Your education does not stop when you graduate.

Be proactive, especially with communications. If you are waiting for the client or contractor to complete part of the job before you can observe or complete the next step, mark your calendar for a follow-up email or call. If you need something from a co-worker, set clear expectations on the timeline and follow-up per your original plan.

Learn the system. Remember, you are walking into a new office environment that is a welloiled machine. They have procedures in place for timecards, billing, reports, inspections, etc.

Your task is to learn the system that is in place before offering to reinvent the wheel. Your way may be better; it may not. Insisting that your system be used in place of the existing system will not create allies, and you will not get very much cooperation. This is called inertia!

Expectations. Your new boss will have expectations of you even before you start.

They will expect you to be technically competent. They may expect you to be in the office or at the job-site early, and they may expect you to stay late when needed. Try to be aware of the expectations and be prepared to go the extra mile when possible. Even if you think they do not notice, they do! If the boss is willing to work extra hours, a team player will consider doing the same.

Check your work. Complex computer analyses can sometimes be a bit of a black box.

Learn to do manual checks, being aware of the order of magnitude, sign, and other basics to determine if the results are reasonable. Your boss will soon become wary of your abilities if they regularly need to do extensive work to check your calculations and models. Make sure you examine your work for accuracy and dimension errors. When using standard details or notes, it is easy to overlook the obvious and include something that is not particular to the job you are working on,

i.e., City of XXX, or using wood notes for a concrete masonry job. No one likes sloppy work, and when others see mistakes, they may feel the need to scrutinize the whole project for errors. You must learn to review your work before calling in the Cavalry for help!

No whining. When you are assigned a task, do not whine. If you have a legitimate situation that will prevent you from completing the task on time, let the boss know. You can learn even from tasks you do not enjoy.

On the flip side, you should always be a team player to help the team achieve more. I cannot stress how important it is to close or complete a given project; never leave a task 90% complete and expect it to be handled by others.

Invest in others. Be sure to be appreciative of your co-worker’s contributions to your projects. A word of appreciation at the appropriate time boosts attitudes and reinforces work efforts.

Keep your Commitments. Regardless of how you feel about keeping commitments on your personal time, it is essential to uphold those made in your professional position! Some of these commitments will be made for you by your boss or supervisor. If you cannot complete the assignment by that date, you must let your boss or supervisor know early.

When you let them know, it is even better if you have at least one idea that would allow you to complete the assignment within the allotted commitment time or near the commitment time. There are other times when you will have made the commitment, and it is even more imperative that you keep those commitments! There are few things that a boss or supervisor dislikes more than being told that the subordinate has made a commitment (promise) and they are not going to keep it.

Keep within the budget. Find out from the boss how much time they think the project or your assigned task will take. Remember, they set the budget and may have made promises to the client. If you are having a hard time completing the project within the guidelines, alert the boss early. They can give you advice and/or assign someone else to do part of the project. Your boss must also keep the client in the loop.■ Steven G. Provenghi is the Managing Principal at Mackintosh & Mackintosh. (provenghi@mackintosh-mackintosh.com) A P R I L 2 0 21

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Project: Cal Poly Pomona Student Services Building Engineer: John A. Martin & Associates Architect: CO Architects Photos: Bill Timmerman

STRUCTURE A PR I L 2021 |

Bonus Content

structural DESIGN Design Strategies for 3-D Volumetric Construction for Concrete

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

T

hree-dimensional (3-D) volumetric construction is also known as concrete modular construction or Prefabricated Prefinished Volumetric Construction (PPVC). This construction method involves the stacking of rectangular factory-finished modular components onsite to form a complete building, similar to Lego® bricks. Joints are typically grouted with special interfacing details. To achieve speed and high productivity, the components have to be substantially completed with minimal site work. This article looks at some of the key design considerations and strategies that designers need to think about when using this type of construction method.

Overview 3-D volumetric construction (3DVC) evolved from traditional precast concrete construction where precast components in two dimensions were assembled on site. Traditional precast concrete construction typically consists of slabs, beams, columns, and wall components connected on-site via bearing, tension, or shear joints (Figure 1).

Moment joints are less common. Connection strategies may involve using a corbel, half-joints, or open pockets for in-situ grouting with placed-on-site reinforcement bars. An opening in a structural element may be formed by pipes allowing room for lapping of reinforcement bars; the remaining voids can then be grouted with high strength grout. Other connecting joints may consist of steel plates with simple on-site bolting. Shear resistance at interfaces may be achieve using corrugated or roughened concrete surfaces (acting as a shear key) or C-shaped links with cottering bars (using reinforcement bar dowel action). To resist tension forces, the use of high-strength steel wires can be an effective solution.

When precast slabs, beams, columns, and walls are assembled in the factory, instead of on-site, this becomes 3-D modular concrete construction. Considerably more effort is required to produce a 3-D component

Figure 1. Typical connection mechanisms for precast components.

in a factory. The builder needs to be aware that this 3-D component is also larger and heavier to maneuver, hoist, store, and transport.

One advantage of using 3DVC is the increase in speed and productivity on-site, especially when time constraints are tight and labor costs are prohibitively high. Also, as the prefabrication is carried out in a factory-controlled environment, prefabricated modules can be manufactured to a higher-quality standard. To take full advantage of modular construction, the building itself must be modular and repetitive so that the number of different modules can be minimized to achieve economies of scale. Remember that portions of the structure which cannot be modularized will have to be cast in situ, on-site. It is not uncommon to see 3DVC technology for high-rise residential projects where there are a large number of repetitive residential units, such as condominiums and apartment flats. The bedrooms and living rooms of such residential units are typically standard within the same stack and with minimal variations across the stacks. Also, residential developments require much less MEP (mechanical, electrical, and plumbing) input compared to other building types, making it favorable for minimizing connecting works across the modules.

Typical Modular Units

Figure 2. Typical tower floor plate.

A typical high-rise residential tower block is shown in Figure 2. Each residential unit can be compartmentalized into several rectangular box modules. Each bedroom is typically one module. The living room modules might be larger and sometimes integrated with a kitchen or a cantilevered balcony. There will still be areas on a floor plate which require in-situ casting work, such as the elevator lobby and corridor access into the dwelling units.

Figure 3 shows some typical five-sided or six-sided 3DVC modules.

The six-sided modules have the advantage of a working platform in the form of a ceiling slab, adding to its rigidity as a complete and fully enclosed box. However, the double slab (ceiling and floor) at every level will consume valuable headroom and the dead space between slab is wasted. Therefore, five sided modules are often preferred (i.e., without ceiling slab). However, temporary rigidity, especially in A P R I L 2 0 21 B O N U S C O N T E N T

Figure 3. Typical modular units.

Figure 4. Diaphragm to transfer lateral loads to shear corewall.

torsion, needs to be considered and a temporary working platform needs to be provided for workers working on top of the module during installation.

In modern concrete construction for residential homes, beams are not preferred for aesthetic reasons. The presence of beams would allow kinks (i.e., beam profile) to be visible at the wall and ceiling slab interface, especially where false ceilings are not constructed.

Therefore, the strategy would be to design one-way spanning slabs to be supported by walls and omit the beams. In residential units, as the span of slabs are typically 9.8 to 13 feet (3-4 meters), it is possible to have relatively thin slabs of approximately 6 inches (150mm) for slabs supported by walls. In areas where beam support is unavoidable, hidden beams (i.e., localised strengthened strip of slab) or band beams (i.e., shallow and wide beams) are introduced. Similarly, columns are not a preferred option for the same reason and long slim walls are often used in place of columns for residential construction.

Therefore, it is sensible to design a box 3DVC module using only structural walls and slabs, without the need for beams and columns.

The walls need to be strategically located to avoid coinciding with openings required for doors and windows, and they can be designed to a suitable length to meet strength and stiffness requirements.

Non-structural walls can be used to fill up remaining voids to compartmentalise spaces. Walls are made as thin as possible to reduce overall weight but also need to cater to fire resistance and slenderness considerations.

must satisfy strength and stiffness requirements in accordance with the applicable design code. For serviceability, the building cannot crack, deflect, or vibrate excessively to an extent noticeable and alarm the user. The building aspect ratio (e.g., height/breadth greater than 7 might be considered slender) and structural element sizes need to be adequate to address these concerns.

However, there are several peculiar issues associated with modular construction that designers need to be aware of. One, diaphragm action is often assumed for a cast-in-place concrete slab. Besides resisting vertical gravity loads, slabs play an important role in transmitting the lateral loads to the vertical structural elements such as walls and columns, using diaphragm action. Therefore, it is important to ensure that slabs between modules are tied together adequately so that they act integrally and do not fragment under load. Tie reinforcement between slabs of abutting volumetric modules must be able to take the minimum tying force specified by the relevant codes.

Next is the treatment of abutting walls. Should the twin wall configuration be modelled individually or as a composite wall? The answer will depend on how the vertical joint connection is being designed and detailed. For simplicity, it is easier to model them as individual walls with ties at appropriate discrete locations. Normally, the design force to tie the two walls will not be excessively large. High-strength non-shrink grout filling the voids between the abutting walls will primarily act as a void-filler and not be required to ensure composite action. If the twin walls were designed as a composite wall, then the connection details would need to ensure adequate shear transfer in the vertical and horizontal directions.

Thirdly, the volumetric units must be able to resist forces induced during the hoisting operation when supported at the lifting points as well as any other temporary condition during transportation, handling, and storage. Lifting hooks should be planned such that they have sufficient development length and produce uniform load distribution. In addition, the module should be stable and not tilt during hoisting. It is also good practice to use a steel lifting frame to lift the modules, instead of inclined wires which would introduce inclined forces into the module. If necessary, additional temporary supports may need to be introduced during the lifting operation.

Lateral loads for a high-rise building may typically be comprised of wind, notional, imperfection, and seismic loads (Figure 4). For a tall building, the wind load may be critical, especially in areas where wind speeds are high. Imperfection loads are lateral loads imposed due to the out-of-plumb of vertical members (i.e., 1/200 for Eurocodes) or geometric imperfections. In the legacy British Standards, reinforced concrete structures must withstand a minimum notional load of 1.5% of dead load to account for imperfections.

For seismic regions, there are specific requirements on horizontal diaphragms and shear walls. First, the designer needs to consider whether

Transfer Deck A transfer deck is a transfer slab required to support the walls and columns above it with walls and columns below it which are not aligned, such as in the case of a basement garage located below a tower block. In addition, there may be instances of requiring large drops in the slab structure, such as accommodating planting boxes or swimming pools. The transfer slab needs to be designed for moment, shear, and punching shear arising from usually high loadings from the superstructure, in addition to meeting deflection requirements.

To provide for large drops, a double slab may be useful. A double slab transfer deck (box deck) may achieve significant savings, especially if regions of low loading demands are hollowed out, instead of casting a massive block of concrete, which adds to dead weight. The transfer deck also acts as a launching platform for the commencement of modular stacking of the volumetric units.

Analysis and Design The structural analysis for modular construction is not too different from that of a conventional reinforced concrete building. The building STRUCTURE magazine

Figure 5. Typical vertical and horizontal connection for abutting walls.

floor diaphragms are topped or untopped, as there are restrictions on the use of untopped diaphragms. Next, the American Concrete Institute’s ACI 318 Building Code Requirements for Structural Concrete and Commentary, also has specific requirements for using friction coefficient μ = 1 when relying on shear friction. Similarly, for shear walls, there are provisions to allow yielding only at the steel and to achieve satisfactory post-yield performance. Localized regions in the diaphragm and shear walls might require additional strengthening, such as at the edges and offsets. As such, it might not be easy for volumetric construction to comply with the onerous requirements required at highly seismic regions.

Connections The connections of a volumetric construction are the most critical factors in determining the success or failure of the project. There are primarily three types of connection: the wall-to-wall connections between abutting modules (vertical joint type) and between upper and lower modules (horizontal joint type) (Figure 5) and the slab-toslab connection (horizontal joint type) at the floor level (Figure 6).

Slab joints are designed to take tension to meet the minimum tying force, ensuring integral diaphragm action to prevent fragmentation.

A pocket is deliberately left opened at the slab edge which interfaces with an adjacent slab. Linking C-shaped reinforcement bars are then placed in situ to stitch across the two slabs with cottering bars, ensuring adequate anchorage. The joint is then cast with high-strength grout. It is important to note that a rigid floor diaphragm needs to be tied internally in two orthogonal directions and externally at the peripheral of the building.

Figure 6. Typical floor connection.

Vertical wall-to-wall joints will take nominal tying force if the walls are designed as individual, separate walls.

The outer wall surface which faces an adjacent wall from the abutting module would typically be roughened and corrugated. This enhances the shear transfer, even if such shear transfer is not required by design. High strength wire ropes from both modules overlap with an additional reinforcement bar inserted into the loop. The void is then filled with high strength grout.

Horizontal wall-to-wall connection will be required to take tension resulting from lateral forces causing overall overturning moments. The tension is taken by reinforcement bars which extend from the lower module to the upper module with adequate development length. These reinforcement bars are inserted in oval-shaped (preferred over circular) corrugated pipes which run throughout the height of the wall. Grouting is carried from floor level to two-thirds of the story height (e.g., approximately 6.6 feet or about 2 meters).

This is to facilitate the insertion of the next level reinforcement bars which can extend into one-third of the ungrouted story height. This would also ensure continuity of vertical ties along the walls throughout the entire height of the building superstructure.

Other Issues One of the key concerns in volumetric construction is the seepage of water through the joints. It is important to ensure that the volumetric modules are manufactured to high precision with strict control on tolerances. This is so that gaps between modules are consistent and do not exceed design specifications. At the outermost interface, a sealant will need to be applied. Beneath the sealant, a soft compressible backer rod (approximately 1.2 inches or about 30mm) is placed. This compressed backer rod, when uncompressed, should be larger than the gap. When modules are abutting and cause the backer rod to compress, it should snugly plug the gap between adjacent modules. In addition, the joint can be detailed such that a small kerb prevents direct water ingress.

Conclusion Three-dimensional (3-D) volumetric construction is a viable construction method in urban contexts, especially when speed and productivity are key concerns. Besides dealing with the technical considerations, designers should examine the merits of each project to assess whether adopting volumetric construction can achieve economies of scale and is worth the effort. Initially, there would be a learning curve during the installation of the first few batches of modules. When the installation team is well-versed with the sequence of work, the installation would gain efficiency and speed.

The project team, including the design engineer, when applying for building approval, would need to convince the local building authority (Authority Having Jurisdiction, AHJ) on the safety and viability of any newly proposed volumetric construction system, including compliance with relevant codes. A design based on sound engineering principles with a proven track record of successfully implemented methods or systems would be helpful in giving confidence to authorities when proposing a new volumetric construction solution for a project.■ Hee Yang Ng is a Principal Engineer with a building control agency in the Asia-Pacific region.

A P R I L 2 0 21 B O N U S C O N T E N T

SPOTLIGHT Creating an Icon

The Dublin Link Pedestrian Bridge

T

he Dublin Link was designed to create an iconic destination for Dublin, Ohio, activate the Scioto River corridor, and literally tie together the Eastern and Western sides of the city. The formal aesthetic and structural methods were developed simultaneously to create a single coherent vision. The resulting bridge is the longest single-sided suspension S-curve bridge in the world. It is supported by an expressive central pylon that the bridge deck passes through, conceptualized as the gateway between the historic town center and the newly developed mixed-use district on the East bank. The form and arrangement of the tower and bridge cross-section were developed through theoretical stress-shaping and refined through digitally-driven optimization.

The Scioto River bisects the City of Dublin, resulting in a shift in the urban fabric from East to West. This offset became the impetus for the bridge’s unique S-Shape in plan, directly tying together the historic downtown and the newly revitalized East Bank. This formal solution has its historical precedent in the S-Bridges used to efficiently cross streams in eastern Ohio during the construction of the National Road in the early 19th Century.

At flood stage, the river inundates much of its western riverbank, rising nearly twenty feet around the base of the central pylon.

The pylon’s cross-section subtlety twists as it descends below the bridge deck in order to minimize fluid drag during 100-yr events.

The bridge’s slenderness resulted in a susceptibility to lateral vibration in strong wind events. Also, the central pylon’s unique form required resolution of the thrust created as it flows around the bridge deck. Both problems were addressed by including two viscous, tuned-mass dampers as a method of dissipating energy while maintaining stability.

Wind tunnel testing revealed the possibility of instability in the triangular box girder caused by vortex shedding under constant wind stream. An inverted wind vane was installed along the bottom of the section to provide stability. This not only stabilized the bridge section but provided a surface for indirect lighting of the bridge’s underside.

The Dublin Link was designed to present a bold form while simultaneously evoking a

STRUCTURE magazine

feeling of lightness for pedestrians and cyclists as they cross above the riparian corridor.

To accomplish these two goals, the design team utilized geometric optimization and stress-shaping principles to create maximum impact with minimal material.

The expressive form of the tower was only possible by using the bridge deck to tie the sides of the needle’s eye together. The cross-section area stays nearly constant from the top of the tower to the bridge deck, resolving the increasing flexural demand by morphing the cross-section and precisely locating It was designed to present a bold form it relative to the central axis.

With the basic form of the while simultaneously evoking a feeling of tower set, a digital optimilightness for pedestrians and cyclists. zation process was used to further refine the flowing concrete form.

The single-sided stay cables were critical to creating open, shifting views of While the Dublin Link is a showcase the river and surrounding town. To minimize of leading-edge design and construction the impact of this spatial goal, the bridge’s techniques, its role is also as a symbol for cross-section morphs as it approaches the a historic town with a long connection to the tower, thereby aligning the section’s shear surrounding land. Merging these two identicenter with the line of action of the stay cable. ties was primarily the work of the material By keeping the cables installed on the inside of the bridge. The concrete was sourced of each curve, the primary forces are fixed in from a nearby white-sand quarry, the lightthe bridge deck, and the incidental torsion colored aggregate was sourced, and white created through construction tolerances and GGBC pozzolan was used as a cementitious installed material weight is balanced. replacement instead of dark silica fume. The The complexity and required precision of the effect of this was to connect the concrete of central tower for both aesthetics and structural the deck, central pylon, and abutments to performance posed one of the biggest construc- nearby limestone cliffs that are endemic to tion challenges. Hundreds of precisely milled the region. This limestone is also CNC form inserts were created from the digital used in many historic buildings in model and installed in a reusable outer form. the adjacent downtown.■ The design team used the model to precisely lay out every piece of rebar for the central tower Endrestudio was an Outstanding Award Winner to aid in the speed of placement during confor the Dublin Link Pedestrian Bridge project in struction. The constructor developed their own NCSEA’s 2020 Annual Excellence in Structural model independently, which was compared Engineering Awards Program in the Category – directly to the design team’s model as part of New Bridges or Transportation Structures. the quality control program.

A P R I L 2 0 21 B O N U S C O N T E N T