STRUCTURE magazine | September 2017 by structuremag

A Joint Publication of NCSEA | CASE | SEI

STRUCTURE

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September 2017 Concrete 2017 NCSEA Structural Engineering Summit, Washington, D.C. October 11-14

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CONTENTS Columns and Departments EDITORIAL

7 ASCE 7-16 and Beyond By Ronald O. Hamburger, S.E., P.E., SECB CONSTRUCTION ISSUES

9 Concrete on Metal Deck By William H. Wolfe and John P. Ries, P.E. SPECIAL SECTION

12 NCSEA Structural Engineering Summit

36 – Cover Feature

55 Hudson Yards

BUILDING BLOCKS

By Jeffrey Smilow, P.E., Ahmad Rahimian, Ph.D., P.E., S.E., and Lan-Cheng (Peter) Pan, Ph.D., P.E. This 14-acre site, in an urban environment on the west side of Manhattan, required unique solutions including the use of high performance structural materials and post tensioning with light weight concrete.

14 Cold and Hot Weather Concrete By Cawsie Jijina, P.E., SECB and J. Benjamin Alper, P.E., S.E. STRUCTURAL SUSTAINABILITY

CODES AND STANDARDS

18 Optimizing Concrete for More Sustainable Bridges

43 What is the Performance Method trying to do?

By Jennifer McConnell, Ph.D., Eric N. Stone, P.E.,

SPOTLIGHT

59 Protecting a Landmark By Stephen. K. Harris, P.E., S.E. and

By David Bonowitz, S.E.

Benjamin A. Mohr, P.E., S.E.

BUSINESS PRACTICES

STRUCTURAL FORUM

and Joseph Robert Yost, Ph.D. STRUCTURAL ANALYSIS

47 Advice for the First-Time (or Experienced) Manager

22 Vibration Excitations – Part 1 By David A. Fanella, Ph.D., S.E. and

66 How Big is Big? By Dilip Khatri, Ph.D., S.E.

By Jennifer Anderson

Michael Mota, Ph.D., P.E., SECB LEGAL PERSPECTIVES

EDUCATIONAL ISSUES

49 A Final Look at Consent to Assignment Agreements

27 Bridge to the Past By Mark Kanonik, P.E.

By Gail S. Kelley, P.E., Esq.

STRUCTURAL DESIGN

PROFESSIONAL ISSUES

30 Low-Slope Roof and Deck Design Considerations – Part 2 By Scott D. Coffman, P.E., SECB

By Mike Olson and Brett Stewart

INSIGHTS

40 Structural Impacts of Low-Energy Buildings By Neil Steiner

53 SB 496 and Design Professional Indemnities in California

CASE BUSINESS PRACTICES

54 The Good and the Bad with Delegated Design By Kevin H. Chamberlain, P.E., SECB

STRUCTURE magazine

September 2017

IN EVERY ISSUE 8 Advertiser Index 56 Resource Guide – Anchors 60 NCSEA News 62 SEI Structural Columns 64 CASE in Point Publication of any article, image, or advertisement in STRUCTURE® magazine does not constitute endorsement by NCSEA, CASE, SEI, C 3 Ink, or the Editorial Board. Authors, contributors, and advertisers retain sole responsibility for the content of their submissions.

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Editorial

new trends, new techniques and current industry issues ASCE 7-16 and Beyond

ASCE 7-16 Now Available

By Ronald O. Hamburger, S.E., P.E., SECB, F.SEI

fter six years of intensive effort by nearly 350 volunteers, the 2016 edition of the ASCE 7 Standard on Minimum Design Loads and Associated Criteria for Buildings and Other Structures (ASCE/SEI 7-16) is now complete and available for purchase through the ASCE bookstore (www.asce.org/asce-7). The new standard incorporates many improvements over the previous edition, including: updated hazard maps for atmospheric icing, earthquake, snow and wind; a new appendix on performancebased fire resistive design of structures; a new chapter on design for tsunami effects; new criteria for linear and nonlinear dynamic seismic analysis; new criteria for mounting PV units on roofs; and, significantly improved rain load criteria. The updated standard has been formally adopted for reference in the 2018 International Building Code (IBC) but is already being used by some designers to take advantage of the improved criteria and, in some cases, to reduce loading below levels currently specified. The IBC adoption of ASCE 7-16 also eliminates much of the redundancy and differences between the code and the standard. The traditional paper edition is available in a 2-volume set, Volume 1 containing the provisions and Volume 2 the extensive commentary. As an alternative to the paper copy, many users will prefer the new web-based electronic access to the standard: ASCE 7 Online. This platform allows users to make custom notes, either on a company or individual basis, to view standards requirements and commentary side by side, to copy and paste provisions to facilitate reference to the standard in project documents and other useful features. Also new and available separately, the ASCE 7 Hazard Tool is a web-based tool that provides geo-based lookup of all hazard data – seismic, ice, snow, tsunami, flood, and wind parameters – based on user input of latitude and longitude or selection from a map. For tsunami loads, the new ASCE Tsunami Design Geodatabase application provides users with the necessary data required by the provisions, including the ability to draw transects through project sites.

In addition to these technical goals, we hope to significantly broaden the structural engineering profession’s access and impact on the standards development process. Following completion of ASCE 7-16, we dismissed the committee, with thanks for their efforts, and put forward a call for volunteers to serve on the reconstituted committee. To produce the ASCE 7-22 standard, we will need a large group of practicing engineers with broad experience and technical backgrounds including designers, building officials, researchers, contractors, and product suppliers. In particular, SEI has made engagement of younger engineers in the process a priority and has established a fund to provide financial support for younger participants. I encourage all engineers with interest in improving the standard and having an impact on the profession’s future to apply online for membership at http://bit.ly/2wAjqjB. The membership application is available in two grades, including voting and associate membership of the main committee as well as each of the subcommittees on General Requirements, Load Combinations, Dead & Live Loads; Flood, Tsunami; Rain & Snow; Atmospheric Icing; Seismic; and Wind. Typically, the committee membership includes several hundred participants in the process including persons with broad experience and background, as well as those with expertise in specialized areas. Applications will be accepted through September 30, 2017.

Theme Sessions at Summit and Congress As we start the new cycle, we will also seek input from the profession on improvements to the standard at the 2017 NCSEA Summit in Washington D.C., October 11-14 (www.ncsea.com/meetings/annual conference/) and also during the 2018 SEI Structures Congress in Fort Worth. The Summit technical program will include a 2-hour panel discussion on means by which use of the standard can be simplified on Thursday, October 12. Committee leadership will put forward ideas and solicit direct feedback from NCSEA Summit session attendees. A similar session will be held next April at the SEI Structures Congress (www.structurescongress.org).

Call for New Members Even as the ink dries on ASCE 7-16, we are starting work to produce the 2022 edition. Over the years, the engineering community has demanded simplification of the standard to make it easier to use on routine projects. As the continuing Chair of the committee, I hope to make this a focus of the committee’s work. Other important goals include continued reduction in the duplication and contradictions between IBC and the standard; coordination with ACI, AISC, AWC and TMS to further harmonize our standards; and further facilitation of performance-based procedures. Of note, there is significant interest in formulating performance-based wind STRUCTURAL ENGINEERING design procedures paralleling those INSTITUTE already available for earthquake, fire, and tsunami.

Conclusion

a member benefit

STRUCTURE

As ASCE 7 Chair for both the 2016 and 2022 cycles, I wish to note my personal appreciation of the extraordinary efforts of the more than 300 volunteers who contributed to the development of ASCE 7-16. I also hope that each of you reading this article will seriously consider becoming involved in the important work of producing ASCE 7-22, either by participating on the committee, joining us in one of the upcoming sessions at the NCSEA Summit and SEI Structures Congress, and submitting proposals for change to the standard online. For any questions regarding ASCE 7, contact Jennifer Goupil at jgoupil@asce.org.▪

STRUCTURE magazine

Ronald O. Hamburger is a Senior Principal with Simpson Gumpertz & Heger in the San Francisco office. He can be reached at ROHamburger@sgh.com.

September 2017

ADVERTISER INDEX

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train is placed on several building materials as construction schedules compress to accommodate building occupancy “as soon as possible.” Add pressures to utilize environmentally responsible products and suddenly traditional construction practices have the potential for problems. A classic example is the interaction between floor coverings, adhesives, and the floor substrate. One of the most common floor systems used in multi-story commercial construction is concrete placed on metal decking. The concrete is typically covered with some type of flooring material based on the building’s end use. The floor covering is adhered to the concrete with one of many readily available flooring adhesives. This system was successfully used for many decades. However, during the late 1990s and early 2000s, many things changed. Environmentally friendly water-based adhesives were introduced; many floor coverings changed and became less permeable. This combination increased the potential for floor failures because of moisture vapor trapped below the floor coverings. As a result, more attention is now focused on floor coverings, adhesive, and concrete drying times.

Floor Coverings The type of floor covering used affects the system’s capability to handle moisture. The covering’s ability to allow moisture vapor to pass through can determine what the concrete moisture content needs to be before installing the floor covering. Vinyl composition tile (VCT), vinyl sheet goods, and the type of carpet backing can all have different tolerances to moisture within a flooring system. It is best for the designer to check with the manufacturer prior to specifying flooring materials.

Adhesives Flooring adhesives are the vital link that connects the flooring to the substrate. There are several choices that vary in composition, quality, and cost. Since the late 1990s, adhesive manufacturers have been forced to reduce emissions when the adhesives are curing. Government agencies mandated that volatile organic compounds (VOCs) be limited in flooring adhesives (Rule 1168) to improve indoor air quality. These mandates sent shock waves through the flooring industry and prompted the development of many new adhesives that were low in VOCs. These low VOC adhesives are water-based as compared to the previous solvent-based adhesives. The new adhesives are more susceptible to degradation from alkalinity that is carried to the surface of the substrate by moisture. Recommendations were established and published concerning the moisture vapor emission rates (MVER). These rates were established at 3 pounds per 1,000 square feet per 24 hours as measured by ASTM method F 1869 Standard Test Method for Measuring Moisture Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride, and 75% relative humidity (RH) as measured by ASTM method F 2170 Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes. Recommendations have also been established concerning pH of concrete floor slabs. Readings of pH below 7.0 and in excess of 10.0 have been known to have a deteriorating effect on flooring adhesives (ASTM F710-08).

ConstruCtion issues discussion of construction issues and techniques

Concrete on Metal Deck

continued on next page

Mitigating Flooring Issues By William H. Wolfe and John P. Ries, P.E.

William H. Wolfe is a Senior Engineer for Norlite, LLC in Albany, NY. He can be reached at bwolfe@norliteagg.com. John P. Ries is the President/ Technical Director of the Expanded Shale, Clay and Slate Institute. He is active in ACI, ASTM, and several other national concrete related associations.

Figure 1. Concrete is placed on an elevated floor metal deck.

STRUCTURE magazine

Table 1. Concrete assembly properties.

UL D916 Concrete Assembly Properties Slab Number

Concrete Type

Concrete Density (Required)

Concrete Density (Actual)

Deck Type

Total Concrete Depth (at deepest point)

Assembly Fire Rating

107 – 116

111.7

2” deep Fluted

5.25”

2 hour

147 – 153

147.7

2” deep Fluted

6.5”

2 hour

107 – 116

111.7

2” deep Fluted w/ hanger tabs

5.25”

2 hour

Recently, flooring adhesive manufacturers have developed products that can be placed at higher substrate moisture contents. These products have different bases that can handle higher relative humidity (RH) concrete and higher pH surfaces. It is best to check with the individual adhesive manufacturers for their product’s capabilities.

utilizing a truck-mounted pump to convey the concrete up to the elevated slab where the concrete is placed on the corrugated metal decking. The concrete must be proportioned appropriately to meet strength, density, workability, and conveyance requirements. This typically involves adding water and chemicals to the concrete to obtain the desired properties. At a designed water cement ratio (w/c), a known volume of water reacts with the cement and supplementary cementitious materials to hydrate these materials. The water above this amount is called water of convenience and is added to the concrete mixture to improve its pumpability and workability. This water needs to evaporate out of the floor slab to a point where the slab achieves an equilibrium with the ambient conditions. As these conditions change, so does the relative humidity of the slab. The moisture that leaves a slab can bring alkalinity to the surface and interact with some flooring adhesives. The Expanded Shale Clay and Slate Institute conducted a study in conjunction with adhesive manufacturer representatives

Concrete The Portland Cement Association describes concrete as a mixture of paste and aggregates or rocks. The paste, composed of Portland cement and water, coats the surface of the fine (small) and coarse (larger) aggregates. Through a chemical reaction called hydration, the paste hardens and gains strength to form the rock-like mass known as concrete. Concrete placed on metal decks (Figure 1, page 9) is either normal weight concrete, typically weighing from 145 to 155 pounds per cubic foot, or lightweight concrete, typically weighing 110 to115 pounds per cubic foot. The structural design and fire rating of the floor assembly dictates the concrete density, strength, and slab thickness. Commonly, concrete is placed

LW Slab 1 NW Slab 2

MVER (lb/1000 sq ft/24 hr)

LW‐Slotted Slab 3 10

3 lb/1000 sq ft/24 hr

6/6/2008

6/20/2008

5/23/2008

5/9/2008

4/25/2008

4/11/2008

3/28/2008

3/14/2008

2/29/2008

2/1/2008

2/15/2008

1/4/2008

1/18/2008

12/7/2007

12/21/2007

11/9/2007

11/23/2007

10/26/2007

9/28/2007

10/12/2007

9/14/2007

8/31/2007

8/17/2007

8/3/2007

7/20/2007

Figure 2. Moisture vapor emission rate from concrete slabs in ESCSI Floor Moisture Study.

STRUCTURE magazine

September 2017

to determine the drying time of commonly used floor assemblies. The assemblies were 12 feet by 12 feet and had a minimum of 10 inches of airspace beneath the metal decking. The frames were constructed from steel to replicate suspended floors in a steel frame building so that the assemblies complied with requirements found in construction today. The testing assemblies were constructed to comply with a commonly used Underwriters Laboratories Design No. D916 for a two-hour fire rated assembly. The design requirements are listed in Table 1 (Fire Resistance Directory, Volume 1, UL, 2009). The testing frames were constructed by a local contractor using full-size metal decking. Slabs 1 and 2 were constructed with metal decking with 2-inch deep flutes. Slab 3 was constructed with metal decking with 2-inch deep flutes and hanger tabs rolled into the decking. The tabs are approximately 1½ inches long and 3⁄8-inch wide and rolled-in every 16 inches. These tabs, used to secure hanger wire for ceiling tile grids, exposed small areas of the underside of the concrete to ambient air. The tabs were evaluated to see if there was any improvement in slab drying because of moisture moving to both the top and bottom of the slabs. No improvement in drying was seen for the deck with rolled-in hanger tabs. The testing frames were placed in a nonconditioned warehouse in Dalton, GA, under a roof to protect the slabs from rewetting. Large roll-up doors were opened on a daily basis to allow cross ventilation of outside air to simulate conditions found on a job site that is under a roof but has not been enclosed. The results of this study showed that both the normal weight and lightweight concretes’ Moisture Vapor Emission Rate (MVER) dried to 3 pounds per 1000 square feet per 24 hours (Figure 2) when tested in accordance with ASTM F1869. The internal relative humidity for both the normal weight and lightweight concrete never dipped below 80% (Figure 3). These

100.0%

LW Slab 1 NW Slab 2 LW‐Slotted Slab 3 75% RH

95.0%

90.0%

85.0%

80.0%

75.0%

Protection Enclose the building to prevent rewetting of the slab caused by rainwater, runoff from adjacent slopes, or landscaping water. Exposure to water from outside of the structure as well as interior sources, such as curing, wet grinding or sawing, and cleaning, will add water absorbed by the concrete and reset the drying clock. Enclosing the building also allows for HVAC systems to be utilized to assist with the drying of the slabs. Since ambient conditions have a great effect on the drying rates of the concrete, exposure to warmer, less humid air aids in drying. Moisture Mitigation

70.0% 07/16/07

08/16/07

09/16/07

10/16/07

11/16/07

12/16/07

01/16/08

02/16/08

03/16/08

04/16/08

05/16/08

06/16/08

Figure 3. Internal relative humidity from concrete slabs in ESCSI Floor Moisture Study.

values were measured in accordance with ASTM F2170. Both the MVER and the internal RH of both types of concrete declined and increased based on the ambient conditions to which the slabs were exposed. The slabs had the lowest MVER and RH in mid-winter when the ambient air was at its driest. As the ambient RH increased coming out of winter, the slab’s moisture content also increased. When comparing the normal weight and lightweight test assemblies, the lightweight slabs had a slightly higher moisture content throughout the testing period because of the absorbed moisture in the pre-conditioned lightweight aggregate. This additional water is added to lightweight aggregate to prevent slump loss as the concrete is pumped. Since 29.5% more normal weight concrete is needed to achieve the same 2-hour fire rating, the additional water-of-convenience needs to evaporate from the normal weight test assembly. This brought the total water included in the concrete test assemblies closer together, as shown in Table 2. Although there was more water in the lightweight slabs, the drying times were similar to the normal weight slabs.

Recommendations Now that the individual components of the flooring system have been analyzed, what processes should be followed to work with the pieces and get a successful project that

performs as desired? The American Concrete Institute’s document 302.2R-06 Guide for Concrete Slabs that Receive Moisture-Sensitive Flooring Materials offers several suggestions.

Because some time-critical schedules do not allow for concrete to dry fully, a moisture mitigation system may be specified. These systems are externally applied to the surface of the concrete to produce a moisture state that allows the adhesive to bond to the substrate surface (ACI 302.2R-06).

Concrete Properties ASTM F710 for the Standard Practice for Preparing Concrete Floors to Receive Resilient Flooring suggests concrete with a moderate to moderately low water cement ratio (0.40 to 0.45) can be used to produce floors slabs that are easily placed, finished, and dried. Workability of the concrete can be improved by utilizing water reducing admixtures. With respect to surface finish, specify a finish that is appropriate for the flooring placed and the minimum appropriate flatness. Do not over-finish the slab. At times, floor finishers can burnish the floor by repeated hard troweling. This densifies the surface and can slow the moisture leaving the slab. Curing Slabs should not be wet cured by ponding water or by placing wet burlap over the concrete. Curing compounds or cure-and-seal materials also should not be used. These products slow the initial drying, resulting in longer drying times. Also, the products typically have to be removed prior to placing the floor adhesive. The slab should be cured by utilizing waterproof paper, plastic sheets, or a combination of the two, for 3 to 7 days.

Conclusions The specification and construction of a properly designed elevated floor system is possible if a number of factors are considered. The materials and design of the substrate, flooring adhesive, and flooring materials must all be evaluated to check for compatibility with each other and their ability to perform within the desired construction schedule. The timeline must be realistic for the materials involved. If the materials cannot perform in the desired timeframe, other strategies must be incorporated to improve the performance. This is a team effort. It is important to communicate during the design, planning, and construction phases of the project. A pre-construction meeting with all involved parties present is recommended to communicate the expectations of everyone involved. With proper planning and a well-informed team, a successful project can easily be accomplished.▪ The online version of this article contains references. Please visit www.STRUCTUREmag.org.

Table 2. Water in pounds batched in concrete test assemblies. Concrete in Test Assembly Assembly cubic yards

Mix Water per cubic yard

Sand per cubic yard

Sand Moisture

Water from NW NW Water from Sand per Stone per Stone HW Stone per cubic yard cubic yard Moisture cubic yard

NWC

2.46

275

1469

58.76

1840

LWC

1.90

270

1480

59.20

STRUCTURE magazine

0.5%

9.20

LW LW Water from Total Aggregate per Aggregate LWA per Water per cubic yard Moisture cubic yard Assembly 0 1075

September 2017

843.68 18%

163.98

937.04

National Council of Structural Engineers Associations

2017 STRUCTURAL ENGINEERING SUMMIT October 11–14, 2017 · Washington Hilton · Washington, D.C.

Join us in Washington D.C. as we celebrate 25 years of bringing together structural engineers!

REGISTER NOW ON WWW.NCSEA.COM! The 2017 Structural Engineering Summit features two days of education sessions specific to structural engineering, including 3 specialized panels, an entire track dedicated to young engineers, and the opportunity to earn over 10 PDHs. Complimenting the education at this year’s Summit are several networking events including the Welcome Reception on Wednesday night, A Celebration of Structural Engineering at the National Building Museum on Thursday night, as well as the Trade Show that runs from Thursday to Friday afternoon. View the complete schedule on www.ncsea.com. Keynote Panel: Shaking Up DC - The Insiders’ Story Martina Driscoll, P.E. & Terrence Paret of Wiss, Janney, Elstner Associates

ASCE Panel: How to Improve ASCE 7 ASCE/SEI 7 Committee Leadership: Ron Hamburger, P.E., S.E., SECB, John Hooper, P.E., S.E. & Don Scott, S.E.

NCSEA Committee Meetings Open to all SEA members, the complete schedule can be found on www.ncsea.com. Welcome Reception Join us Wednesday night to kickoff the 2017 Summit and toast as we celebrate 25 years of bringing together structural engineers! Young Engineer Track • Young Member Mentor Roundtable • Wind Design Considerations for Joist/Joist Girder Structures • Basics of Shear Wall Design Trade Show & Trade Show Reception The largest Trade Show in NCSEA history! Turn to page 60 to view the 2017 Summit Exhibitors & floor plan. Product Presentations These sessions, held from 8–9:50 am on Friday, are presented by exhibitors from this year’s Trade Show.

A Celebration of Structural Engineering Hosted by Computers & Structures Inc. at the National Building Museum Join CSi on Thursday evening at the National Building Museum for a unique celebration of the structural engineering profession, including full dinner, champagne, finely crafted cocktails, and live music. Come celebrate the immeasurable contributions of the structural engineering profession to all generations and the ways in which structural engineers are essential to the progress of humanity! Mingle and unwind with your fellow conference attendees as you take in the stunning architecture of America’s leading cultural institution devoted to interpreting the history of architecture, engineering, and design. STRUCTURE magazine

September 2017

Building Blocks updates and information on structural materials

iscussions of cold and hot weather concrete procedures do not occur until the five-day forecast calls for extreme weather. At that point, everything becomes a rush. Special concrete mixes, as required, need to be submitted for approval at the beginning of a project. Time is required to test and practice with different temperatures and dosages and to adjust the cocktail of admixtures that form the basis of high-performance concrete today. The right time to discuss these procedures is in the Pre-Concrete Conference before placing the first yard of concrete. This ensures adequate time for the structural engineer, contractors, and concrete producer to properly coordinate and prepare for inclement weather conditions. At 70 degrees Fahrenheit, life is good. However, temperatures below 50 degrees and above 85 degrees require different concrete mixes and different weather related procedures. It is not just the cocktail mix or the temperature that

Cold and Hot Weather Concrete By Cawsie Jijina, P.E., SECB and J. Benjamin Alper, P.E., S.E.

Cawsie Jijina is a Principal at Severud Associates and serves as the Deputy Technical Director for Severud Associates’ inspection services. He can be reached at cjijina@severud.com. J. Benjamin Alper is an Associate at Severud Associates and serves as the Quality Control Officer for Severud Associates’ inspection services. He can be reached at jalper@severud.com.

is the variable. Traffic conditions, plant conditions, distance from plant to site, and finishes required all play a part in the determination of the correct concrete for a given structural element. It is also important to note that each site is different. For example, even within the same city, a site that is near the concrete supplier may have different needs to achieve proper placement than a site that is farther away. As such, each site needs to have its own cold and hot weather concrete procedures. This should be submitted by the concrete contractor, in conjunction with the concrete plant and the various suppliers, for review by the engineer of record. High-performance concrete is very dependent on the source of the aggregate, its gradation, and continued availability. Delivery of the aggregate to the plant is also a consideration. The ACI provisions most applicable to this topic are ACI 305 Guide to Hot Weather Concreting and ACI 306 Guide to Cold Weather Concreting. For more in-depth discussions of any of the items addressed in this article, these reports contain detailed information on the topics.

one and four days for concrete with accelerators mixed in (Table 7.1 of ACI 306). Prior to ACI 306R-10, cold weather concreting was defined as “a period when, for more than 3 consecutive days, the following conditions exist: 1) the average daily air temperature is less than 40⁰F and 2) the air temperature is not greater than 50⁰F for more than one-half of any 24-hour period. The average daily air temperature is the average of the highest and lowest temperatures occurring during the period from midnight to midnight” (ACI 306R99). The newer definition requires cold weather concrete procedures to be enforced more often. Minimum Concrete Temperatures ACI 306 provides recommended concrete temperatures as shown in Table 5.1 of ACI 306. For a typical slab placement on a freezing day, the concrete needs to be at least 65 degrees while mixing and then needs to be maintained at 55 degrees once placed. For thicker elements, these temperatures are reduced as the exothermic reaction of the concrete itself generates additional heat. These temperatures should be maintained for the required protection period, as shown in Table 7.1 of ACI 306. The temperatures and time periods for protection are to prevent damage to the concrete by early-age freezing of the water required for cementitious hydration. Maintaining the required concrete temperatures during cold weather periods can be done in multiple ways. The most common methods used include: • The use of hot water in the concrete mix • The use of steam heated aggregate • The introduction of an accelerating admixture • Heating and tenting the area of concrete placement

When Cold Applies The definition of when to apply cold weather concrete provisions, per ACI 306, differs based on the ACI edition referenced by the applicable building code. Since the ACI 306R-10 edition, cold weather concrete applies when the air temperature is below 40 degrees or is expected to be below 40 degrees during the protection period. The protection period is defined as being between

Tarping to protect rebar prior to cold weather placement.

14 September 2017

More exotic methods include the introduction of a network of pipes that control the internal temperature of concrete by the flow of temperature-controlled water through this network. Depending on the ambient temperatures, only some or all of these methods are utilized. It is common for the accelerating admixture dosage to vary depending on the ambient temperature and other factors. The concrete producer can provide a chart of accelerating admixture dosages for a given project, based on member size. Trowel finished slabs must be “normal setting.” A delayed set can result in surface drying that initiates the finishing process. Retarded setting can cause delayed bleed which leads to surface delamination.

Practical Field Issues

It is imperative that the concrete temperatures of all trucks arriving at the site be constantly monitored. Assuming that if the first few trucks have temperatures within the specified limits, then the remainder of trucks should be acceptable, is not a good assumption and increases the risk of a possible failure. While temperatures typically rise as the day goes on, other external factors can reduce concrete temperatures. For example, a plant may only be able to preheat a certain amount of aggregate the night before. Once the pre-heated aggregate is used up, the next trucks would see a temperature drop which likely would not be sufficiently compensated by the slight increase in ambient temperature. Similarly, a plant may reduce the amount of accelerator in the concrete later in the day which can also reduce the concrete temperatures. Accelerating admixtures should be properly dosed based on ambient temperatures and should be adjusted throughout the concrete placement if needed. The overuse of accelerators can result in concrete that sets before it is finished and can cause cold joints in the concrete. As a general rule, if ambient temperatures are below freezing during concrete placement or the protection period, then some means of externally applied heat will almost always be required when placing slabs. This is often achieved by tenting and covering the concrete with thermal blankets and utilizing heaters. Direct heat should be avoided.

Even with the best planning, things can and do go wrong in the field. A member of the construction team needs to be constantly monitoring the concrete trucks and keeping an eye on things. For example, when trucks start backing up due to a broken pump or for any other valid reason, sending one or two trucks away from the site may mean that the next dozen trucks stay within specifications. Problems compound when they are not continually monitored.

As with all rules, there are always exceptions. Freeze resistant concrete is concrete that utilizes an accelerating admixture that allows for lower concrete temperatures at the time of placement. While these chemicals provide a slight increase in the cost of the concrete, they can significantly reduce some of the costs associated with external heat and other methods. Most manufacturers of admixtures have dosing tables which can be used as a baseline

Minimum Temperatures for Everything Else ACI recommends that any surface that comes in contact with the concrete during placement should have a surface temperature above freezing. This includes reinforcement and formwork. ACI considers this a best practice. For “massive metallic embedments” (typically pipes, or wide flange beams), it is important that these embedded elements be heated before concrete placement to bring the element as close to the concrete temperature as possible. This avoids the potential of the metal locally causing the concrete to freeze and thereby locally reducing the bond strength.

Calibrated thermometer checking the temperature of in situ concrete.

when beginning to implement a cold weather mix on site. As exacting procedures of freeze resistant concrete are not yet included in ACI, testing of all mixes under field conditions (with field cured cylinders) is typically required to ensure that the concrete is reaching the proper required strength. Concrete placement with the correct, well-designed concrete mix in ambient temperatures as low as 15 to 20 degrees, without externally applied heat, is a fairly common practice on many projects. Monitoring Temperatures The temperature of the concrete must be monitored closely. When the concrete truck arrives at the site, the concrete temperature should be recorded immediately to make sure it meets the minimum criteria specified by the Structural Engineer of Record. The designated Special Inspector, an extremely valuable part of the process, must use a calibrated thermometer. It is recommended that spare calibrated thermometers be kept on site; nothing is more damaging than finding out that trucks were deemed non-compliant

Freeze Resistant Concrete

Table 5.1. Recommended concrete temperatures. Section size, minimum dimension < 12 in. (300 mm) Line

Air temperature

–

Above 30°F (-1°C)

12 to 36 in. (300 to 900 mm)

36 to 72 in. (900 to 1800 mm)

> 72 in. (1800 mm)

Minimum concrete temperature as placed and maintained 55°F (13°C)

50°F (10°C)

45°F (7°C)

40°F (5°C)

Minimum concrete temperature as mixed for indicated air temperature* 60°F (16°C)

55°F (13°C)

50°F (10°C)

45°F (7°C)

0 to 30°F (-18 to -1°C)

65°F (18°C)

60°F (16°C)

55°F (13°C)

50°F (10°C)

Below 0°F (-18°C)

70°F (21°C)

65°F (18°C)

60°F (16°C)

55°F (13°C)

–

Maximum allowable gradual temperature drop in first 24 hours after end of protection 50 °F (28°C)

40°F (22°C)

30°F (17°)

20°F (11°C)

*For colder weather, a greater margin in temperature is provided between concrete as mixed and required minimum temperature of fresh concrete in place.

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September 2017

due to faulty equipment or the tester’s lack of knowledge in the use of testing equipment. If the tester finds trucks beyond the range of the specifications, the plant needs to be informed immediately. This allows the plant to make corrections and adjust the design mix, thereby containing the number of trucks that arrive beyond the specification range. The concrete temperature should also be monitored when the concrete reaches the deck/point of placement. After placement, during the protection period, the concrete temperature must again be verified to ensure that the concrete remains at the minimum required temperatures. A simple way to monitor these temperature changes is by using a calibrated maximumminimum thermometer. Thermometers are available with a probe on one end that can be tucked under a blanket or in the concrete form. The probe tracks the approximate temperature on the concrete surface during curing. In addition to properly curing horizontal surfaces, required concrete cylinders need to be properly stored in an insulated curing box. During cold or hot weather days, the maximum-minimum thermometer can be used to monitor the temperature of the curing box. If there are low breaks during hot or cold weather, one of the first suspects is the curing method of the cylinders. By monitoring these temperatures, this concern is proactively eliminated. Checking the temperature of the concrete upon arrival, and then again in place in 45 to 60 minutes, gives the concrete team guidance on slump and setting time adjustments if needed. (The above paragraph is an excerpt from “Ensuring Quality Concrete”(Alper, et. al), a Construction Issues article published in the June 2016 issue of STRUCTURE magazine.)

Hot Weather Concrete Unlike Cold Weather concrete, the ACI Code does not provide specific temperature requirements for when hot weather concrete criteria apply. ACI 305 defines hot weather as any combination of conditions that “impair the quality of freshly mixed or hardened concrete

by accelerating the rate of moisture loss and rate of cement hydration.” As a general rule, structural engineers in the Northeast require hot weather concreting procedures to commence when ambient temperatures exceed 85 degrees. This may vary based on location and other relevant conditions. ACI 305R does not provide maximum temperatures for concrete during hot weather placement. The emphasis is on maintaining proper workability without affecting concrete strength. During hot weather, the rate of cement hydration and increased evaporation create issues with the workability of the concrete. The addition of water to the mix would typically mitigate these issues. Unfortunately, additional water also reduces the concrete strength, increases shrinkage, and causes additional cracking. Maintaining the required concrete temperatures during hot weather periods can be done many ways without reducing the concrete strength. The most common methods include: • The use of cold water in the concrete mix • The use of ice in the concrete mix • The use of water reducing admixtures (instead of using water on site) • The addition of retarding admixtures and hydration set control admixture • The use of fly ash and slag to reduce the cement content • Cooling of the aggregate by sprinklering and shading It is important for field personnel to ensure that cold joints do not form during concrete placement, as they can cause significant structural problems with the introduction of shear failure planes between the layers of concrete.

when the concrete finishers can start their work. Accordingly, the careful calibration of the amount of admixture used as concrete placement progresses reduces setting times towards the end of a concrete placement. Whereas the idea is theoretically sound, there are as yet no metrics and no big data available to provide ground rules and basic parameters for this calibration. The result can be problems for workability and cold joints. While it is not unreasonable to reduce the admixture dosage, it still is a judgment call based on the structural engineer’s and contractor’s experience; it needs to be done in a cooperatively controlled manner. Placing concrete at reduced ambient temperatures, such as night placements, can significantly ease the problems related to hot weather concreting. Mass concreting during hot weather can be extremely challenging and is the scope of a future article. Proper curing is extremely important to reduce cracking. The use of foggers and wet burlap are effective ways to cure slabs. Slabs that are not properly cured can exhibit significant cracking. This cracking is more pronounced on slabs that are restrained on the ends, such as a ground floor slab restrained by the foundation walls. As cold joints are more prevalent when the concrete sets faster, consider the addition of preformed construction joints within the slab, even when the slab is scheduled to be placed in a single day. This allows the concrete placement to be stopped at strategic locations without compromising the integrity of the slab. If the construction joints are not needed, they can simply be removed during the placement before the concrete reaches that area.

Practical Field Issues

Monitoring Concrete Temperatures

A smooth continuous flow of work that constantly engages all personnel is something that every contractor strives for; that process maximizes efficiency and therefore increases profitability. The use of retarding admixtures, while excellent during the travel time from the plant to the site, delay

The monitoring of concrete during hot weather is similar to that of cold weather concrete. In addition to monitoring temperatures, the monitoring must include oversite to ensure that cold joints do not form.

Maximum Temperatures

Table 7.1. Length of protection period for concrete placed during cold weather. Protection period at minimum temperature indicated in Line 1 of Table 5.1, days* Line

Service condition

Normal-set concrete

Accelerated-set concrete

No load, not exposed

1 2

No load, exposed

Partial load, exposed

Full load

4 Refer to Chapter 8

*A day is a 24-hour period.

STRUCTURE magazine

September 2017

Conclusion As with everything in construction, a smooth and continuous flow of operations is the difference between a project being successful and profitable or otherwise. With proper planning and execution, quality concrete can be placed under almost any inclement weather conditions. The use of heated/cooled materials, properly dosed admixtures, and effective curing of the concrete are critical in making sure that the concrete placement is setup to succeed.▪

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Structural

SuStainability sustainability and preservation as they pertain to structural engineering

he built infrastructure is one of the largest contributors to various measures of environmental impacts, e.g., energy consumption and carbon dioxide (CO2) emissions. There is sustained momentum towards decreasing this impact. This momentum is consistent with the ASCE Code of Ethics which states, in part, that engineers “shall strive to comply with the principles of sustainable development in the performance of their professional duties.” Furthermore, several states require analyses of environmental impacts for some transportation projects (FHWA, 2016). For example, in Massachusetts, certain projects must quantify CO2 emissions and “document the … plans to avoid, minimize or mitigate Damage to the Environment to the maximum extent feasible” (Commonwealth of Massachusetts, 2010). In addition to a similar air quality analysis, New York requires an energy impact analysis for some projects (NYSDOT 2010).

challenge for those with little education on the topic of sustainability. The purpose of this article is to offer specific strategies for achieving the overarching sustainability goals contained in sustainability guidelines, with a focus on concrete.

Significant Environmental Impacts It is useful to understand which phase of a bridge’s life-cycle contributes the most to environmental impacts to identify what strategies are most effective for reducing environmental impacts. Typical bridge life-cycle phases include: manufacture/production, construction, operation/maintenance, and end-of-life. The impacts of each phase can be quantified using life-cycle assessment (LCA) methodology. In short, the LCA quantifies environmental impacts in categories such as global warming potential using quantities for a well-defined system boundary (a defined portion of the supply chain for a particular project). Numerous LCA studies have analyzed the emissions associated with bridge projects. Some studies report results for each life-cycle phase. Others focus on comparing emissions of different bridge alternatives for a given phase. Many of the studies evaluate various superstructure alternatives, assuming the substructure stays the same. Project-specific LCA studies could be used to quantitatively balance both superstructure and substructure alternatives to minimize environmental impacts. LCA studies can help uncover which project elements have the greatest impacts and which elements have an outsized impact relative to their size or cost. Figure 1 shows example LCA results for global warming potential on a

Optimizing Concrete for More Sustainable Bridges By Jennifer McConnell, Ph.D., Eric N. Stone, P.E., ENV SP, and Joseph Robert Yost, Ph.D.

Jennifer McConnell is an Associate Professor at the University of Delaware in Newark. Eric N. Stone is a Structural Engineer at HNTB Corporation in Milwaukee, WI. Joseph Robert Yost is a Professor at Villanova University in Villanova, PA. The authors are members of the ASCE/ SEI Sustainability and Steel Bridge Committees. The authors can be reached at righman@udel.edu, estone@hntb.com, and joseph. yost@villanova.edu, respectively.

Various sustainability guidelines like Envision, Greenroads, and Leadership in Energy and Environmental Design (LEED) are available for engineers or owners interested in reducing the environmental impact of structures. These guidelines primarily list credits that identify broad goals. To focus on Envision’s Resource Allocation Category as one example where structural engineers have a large influence on this rating system, listed goals include: reduce net embodied energy and support sustainable procurement practices. Clearly, prescriptive approaches for achieving these goals are not the intent, which poses a

ECL Phase Maintenance Phase Construction Phase Manufature Phase

kg COs equiv.

Figure 1. Example LCA results for global warming potential.

18 September 2017

(a)

(b)

(c)

Figure 2. Example bridges designed for extended service life: a) Second Gateway Bridge, Brisbane, AU, designed for 300-year service life (© Murarrie, 2016), b) St. Anthony Falls Bridge, MN, USA, designed for 100-year service life (©RJ Watson, Inc., 2016) c) New Tappan Zee Bridge, NY, USA, designed for 100-year service life (© HDR, Inc., 2016)

hypothetical project. Other environmental impact categories can be studied as well. The studies that compare emissions throughout the life-cycle agree that the manufacture/ production phase creates the largest emissions. Martin (2011), Hammervold et al. (2013), and Du et al. (2014) report greater than 84% of the total global warming potential is due to the initial phase, and Dequidt reports 64%. Du et al. (2014) also reports that the initial phase accounts for the vast majority of CH4, SO2, NOx, and NH3 emissions. The magnitude of the emissions is largely sensitive to the cement quantity, steel recycling rate (which is high in the U.S.), and the electricity source used in the steel production. Therefore, the initial manufacture/production phase should be targeted by bridge engineers looking to reduce the most emissions of their projects.

Strategies Alternative Cementitious Materials One means of reducing the environmental impact of highway structures, and other structures containing concrete, is to replace all or part of the Portland cement content of concrete with alternative cementitious materials (ACM). These materials are also frequently referred to as supplementary cementitious materials (SCM). While the two terms are essentially synonymous, the term ACM better highlights that some of these materials can be used in place of Portland cement to a significant extent, or even completely in some cases. Because many of the ACM materials are manufacturing byproducts, the environmental impact of structures can be dramatically improved through their use. Reduced thermal cracking can be another benefit of the use of ACM, which is particularly advantageous for large concrete members. Furthermore, ACM can improve long-term mechanical performance and reduce durability concerns, such as resistance to chloride permeation and resistance to alkali-silica reaction. Thus,

there are secondary environmental benefits due to increases in service life, which can be considered in general terms or explicitly considered through a service life design. In fact, through proper mixture design and use of ACM, the expected service life can be significantly extended beyond that conventionally achieved. Example structures designed for extended service life are shown in Figure 2. A wide variety of possible ACM exist. Those with the lowest environmental impact would intuitively be those that are waste products from other sectors (although, to fully assess whether this is true, full consideration of environmental impacts is necessary). Fly ash, blast furnace slag, and silica fume are the manufacturing byproducts most commonly used as ACM at present. Other products that have been used as ACM include other industrial, agricultural, and municipal wastes and natural pozzolans such as clays and volcanic ash. The proportioning of ACM is critical. Slower strength gains experienced with some ACM dictates that attention is given to ensuring sufficient strengths at early ages. Workability can also be a concern for several potential ACM. Thus, there are few materials available that can be wholly substituted for Portland cement at present. Numerous and varied combinations of ACM have been used to successfully meet strength, serviceability, and constructability requirements while also reducing environmental impacts. In Table 1, a synthesis of selected infrastructure owners’ specifications (for normal, i.e., not high-performance concrete) gives the typical proportions of the commonly used ACM that are permitted while innovative

designs and research projects inform higher proportions of ACM that are possible. The percentage of fly ash allowed in the standard specifications of owners varies widely but is often up to 30% replacement (expressed as the weight of fly ash relative to the total weight of cementitious material). Some agencies place additional requirements on use, such as specifying the type of fly ash that may be utilized (with Class F regarded as the highest quality and Class C also commonly used). Other agencies are less restrictive of fly ash use. For example, the Texas Department of Transportation (DOT) allows up to 35% fly ash (assuming other restrictions are met), which is the same proportion at which Bentz et al. (2011) were able to achieve no compromise in early-age properties. An additional example is the Delaware DOT’s specific designation for fly ash type concretes, which specifies a minimum of 20% fly ash and no maximum other than a demonstration of acceptable performance. Furthermore, multiple projects have exceeded the upper bounds on fly ash cited previously. These include the Sunshine Skyway Bridge in Florida and Cooper River Bridge in South Carolina utilizing 50% and 43% fly ash, respectively. Even higher percentages of fly ash have been used by Bouzoubaâ and Lachemi (2001), who developed a highvolume blend containing 70% fly ash. Field trials conducted by Cross et al. (2005) used 100% Class C fly ash for foundations and walls where the compromise in early strength gain resulting from the high proportion of fly ash was insignificant. Similar levels of ACM could routinely be used for members that are

Table 1. Comparison of standard and innovative ranges of ACM used.

Material

Representative Range Generally Permitted Successfully Demonstrated High Volume Ranges

Fly Ash

Up to 30%

Up to 35% w/o strength decrease 100% Class C 70% Class F

Slag

Up to 50%

Up to 75%

Silica Fume

Up to 10%

STRUCTURE magazine

September 2017

Table 2. Example projects using corrosion resistant reinforcement in bridge construction.

Bridge and/or Owner Sustainable Component Woodrow Wilson Bridge, MD SHA/Virginia DOT 1100 tons of stainless steel on the bascule spans of the bridge to prevent corrosion. US 2 Bridge over Winooski River, Vermont DOT

Bridge deck with high-performance concrete and stainless steel reinforcement.

Wenzlick, 2007. Missouri DOT New York State DOT

Constructed its first cast-in-place bridge deck using stainless steel reinforcing bars in 2006. Stainless steel reinforcing in the decks of several bridges. Offset some cost of solid stainless steel by design efficiencies (CITRE 2006). Concrete deck steel-free and reinforced with carbon fiber reinforced polymer (CFRP) grid. Long-term performance modeling. Select stainless steel reinforcement in highly corrosive locations of the bridge. Commonly uses corrosion-resistant reinforcing (such as stainless steel) in bridges.

Rollins Road Bridge, New Hampshire DOT Haynes Inlet Slough Bridge, Oregon DOT Virginia DOT not schedule-critical when these components do not need to achieve their full design strength until later in the construction process. Higher proportions of slag are permitted compared to fly ash. For example, Wisconsin DOT is one of the several agencies allowing up to 50% slag. Up to this threshold, the final structural and environmental benefits are advantageous. Elahi et. al (2010) showed increased strengths at 28 days and later and significantly decreased chloride permeability, although reduced strength at 3 and 7 days is the tradeoff in performance as compared to the 100% Portland cement alternative. Considering the advantages of combining structural performance demands and environmental goals, there are innovative examples where the use of slag as an ACM has been maximized, including 75% slag employed in the James River Bridge in Virginia and 69% slag used in combination with 16% fly ash in the I-35W St. Anthony Falls Bridge in Minnesota (Figure 2b). Silica fume is used less extensively than fly ash and slag and is limited in proportion. Thus, the benefits of using it are presently more restricted than other ACM. While the majority of the case studies and research performed to date focus on the utilization of a single ACM, combinations of various ACM are also possible. An ASTM standard specification for blended ACM exists for this purpose (ASTM C1697, 2010). In addition to minimizing environmental impacts, two critical challenges continue to spur research and development of ACM. One is the compromised early-age strength mentioned previously. Various strategies have been used and are under consideration to mitigate these effects. For example, one technique is the “filler effect,” which can be achieved through the addition of materials such as fine limestone. Another concern is that the local demand for ACM can exceed the available supply. Availability of high-quality fly ash is a problem at times and could be a bigger problem in the future due to changes in regulations and the phasing-out of coal power plants.

This issue provides strong motivation to pursue research into fly ash alternatives which are currently available and to evaluate emerging technologies. Texas DOT (Seraj et al., 2014) sponsored a study to look at possible alternatives for Class F fly ash. Six ACM were identified. However, the material costs at the time of the study were typically more than double the fly ash cost and greater than the cement cost. Increased use of slag from steel making processes, which are sustained in part by “Buy America” requirements, may help to fill the gap. Cement Production Efficiency Sourcing cement from energy-efficient plants is a sustainable strategy. A 2008 report conducted by the Lawrence Berkeley National Laboratory investigated over 40 improvements plants can implement to increase energy efficiency and to reduce CO2 emissions. Ideally, cement plants would be able to provide an environmental product declaration to detail their product’s environmental impacts. However, in the absence of a detailed report, specifications could be written to favor cement suppliers who have made investments in energy-efficient and environmental impact reduction technologies. Other Emerging Technologies Emerging technologies have been developed to improve reinforced concrete’s sustainability performance. For example, replacement of steel reinforcement with corrosion-resistant materials can significantly improve the lifespan of concrete bridges. Corrosion-resistant reinforcement materials include stainless steel (SS), fiber reinforced polymer composites (FRP), galvanized reinforcing steel (GRS), and low carbon chromium reinforcing steel (LCC). Proper use of these materials alone may extend maintenance-free service life when compared to conventional steel rebar (CS). Another strategy is using waste CO2 as an admixture in producing PCC. In this application, the upcycled waste CO2 is mineralized within the concrete thereby significantly reducing the material’s carbon footprint

STRUCTURE magazine

September 2017

(http://carboncure.com). Alkali Activated Fly Ash Concrete (AAFAC) is another option, which uses fly ash as a 100 % replacement for Portland cement. AAFAC relies on industrial byproducts to significantly reduce its carbon footprint, while also being very resistant to many of the durability issues that can plague PCC. Other emerging technologies related to improving concrete’s sustainability rating include completely recyclable concrete (CRC), calcium sulfoaluminate (CSA) cement, and incorporating recycled concrete aggregates into concrete. This latter strategy has been successfully applied for concrete pavements at present, and some recent research provides favorable results in structural applications involving recycled aggregates. Case Studies Presented in Table 2 are examples of corrosion resistant reinforcements that have been used in components of bridges to extend the service life of the structure.

Conclusion As the sustainability knowledge-base continues to grow, engineers can expect new alternatives and increasing application of existing methods for reducing the environmental impact of highway infrastructure. Methods such as LCA and service life design offer many benefits presently, and it is expected that the ease and preciseness of quantifying outcomes through these methods will only increase. The available data suggest that ACM, other improvements in cement production processes, and more corrosion-resistant reinforcements are among the present best practices for reducing the environmental impact of concrete used in highway bridges, and thus the overall environmental impact of these structures.▪ The online version of this article contains detailed references. Please visit www.STRUCTUREmag.org.

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Structural analySiS discussing problems, solutions, idiosyncrasies, and applications of various analysis methods

ibration analysis has become more commonplace in everyday practice. In office environments, the need for longer spans continues to increase. Open spaces, with minimal or no partitions and no filing cabinets, are becoming even more prevalent (typical electronic office), which translates to less damping to counteract vibrational effects. Amenity centers that include fitness equipment, hospitals, and microelectronic processing facilities are just a few examples of where stringent vibration requirements need to be satisfied for the comfort of building occupants and for the proper functioning of equipment. Because of their inherent mass and stiffness, reinforced concrete floor systems routinely provide adequate resistance to vibration caused by a variety of sources. Unlike other types of structural framing systems, utilizing reinforced concrete member sizes that satisfy only minimum deflection requirements are more than adequate to satisfy vibration acceptance criteria for walking excitations. For other than walking excitations, the choice of a reinforced concrete floor system may not be obvious. It is important to have sufficient information to select an economical system that satisfies all necessary vibration requirements. The purpose of this article is to present guidelines to assist design professionals to quickly ascertain when a flat plate system and a wide-module joist system are adequate for various types of vibration excitations. Flat plate voided concrete systems and two-way joists (waffle slabs) will be covered in Part 2.

Vibration Excitations Part 1: How to Select a Reinforced Concrete Floor System By David A. Fanella, Ph.D., S.E., P.E., F.ACI, F.ASCE and Michael Mota, Ph.D., P.E., SECB, F.ASCE, F.ACI, F.SEI

David A. Fanella is Senior Director of Engineering at the Concrete Reinforcing Steel Institute and can be reached at dfanella@crsi.org. Michael Mota is Vice President of Engineering at the Concrete Reinforcing Steel Institute and can be reached at mmota@crsi.org. David and Michael are co-authors of the CRSI publication, Design Guide for Vibrations of Reinforced Concrete Floor Systems.

Sources of Vibration In general, vibrations can be generated from both internal and external sources. People walking across the floor (walking excitations), people dancing or doing aerobics (rhythmic excitations), and mechanical equipment vibrations are typically classified as internal forms of vibration. Transportation-related sources (such as traffic or trains), construction activities (for example, pile driving), or industrial activities are examples of external sources. The focus here is on internal sources. Walking excitations are typically modeled as impulsive loads that occur and dissipate very quickly. Heel-drop impacts from a person walking or the impact from a single jump are examples of impulsive loads on a floor system. Periodic loads are caused by rhythmic human activities such as dancing and aerobics or by impactive

machinery, whereas harmonic (or equivalently, sinusoidal) loads are commonly used to represent the forces produced by rotating machinery. The vibration source plays an important role in the overall vibration analysis.

Acceptance Criteria Acceptance criteria for vibration are specified for human comfort and sensitive equipment. Many criteria have been proposed through the years for human comfort related to vibration of floor systems. However, no universally accepted criteria exist to date. For walking excitations, the peak accelerations recommended by the International Organization for Standardization (ISO) in 1989 have been successfully implemented in a wide variety of situations. For floor vibrations to be acceptable, the peak acceleration of the floor system, which is a function of the floor system’s natural frequency, damping, and effective weight, must be less than or equal to the recommended acceleration, which is equal to 0.5 percent of the acceleration of gravity for office and residential occupancies. The ISO criteria are also used for rhythmic activities. For dining and dancing and other rhythmic activities (such as aerobics), the recommended accelerations are 1.5 and 5.0 percent, respectively. The acceptance criteria for sensitive equipment is typically provided by the equipment manufacturers, with regards to limiting vibrational velocities. Vibrational acceptance criteria for sensitive equipment are satisfied for fast, moderate, and slow walking paces when the expected maximum velocity of the floor system (which is related to the natural frequency/stiffness of the floor system) is less than or equal to the limiting velocity given by the manufacturer. The smaller the limiting velocity, the more sensitive the equipment is to vibration. For example, operating rooms in hospitals may require a limiting velocity of 4,000 μin./sec while a facility that manufactures microelectronic equipment may require 130 μin./sec. The microelectronic facility will require a significantly stiffer floor system than the one for the operating room.

Vibration Characteristics As noted previously, the stiffness of a floor system plays a key role in its ability to combat vibrations. The main component of deflection in a reinforced concrete floor system, which is inversely proportional to stiffness, is from flexure. As such, stiffness can be calculated using the modulus of elasticity of the concrete, Ec´, and the effective moment of inertia, Ie. Because of the transient nature of vibration, the dynamic modulus of elasticity can be used to calculate floor stiffness, which

22 September 2017

(a critically damped system is one in which the motion decays without oscillation) and is commonly referred to as the damping ratio, β. Electronic offices and floors with few nonstructural components are usually assigned a value of β equal to 0.02. For office spaces with partial-height partitions, a value of 0.03 is commonly used, while a value of 0.05 would be assigned where fullheight partitions are utilized. In the case of rhythmic activity, additional damping is provided by the occupants and β = 0.06 can be used. The natural frequency, fn, of a floor system, which is related to mass and stiffness, is utilized in all vibration analyses, including checking that applicable acceptance criteria are satisfied. In lieu of performing a finite element analysis, simplified techniques can be utilized to determine fn. In the case of flat plates, the system can be modeled as a thin, isotropic plate, which is free to deflect at any point except the columns. The equation for fn can be found in the STRUCTURE magazine article titled Vibration of Reinforced Concrete Floor Systems, April 2015, by the authors. A simplified equation for wide-module joist systems can be found in the same article. In

both cases, the approximate fn are at most 10% less than those from a finite element analysis. These equations can be utilized in the preliminary design stage to quickly ascertain whether the floor system is best suited to satisfy required vibration criteria. Table 1. Minimum slab thickness/maximum span lengths for flat plate systems subjected to walking excitations.

Minimum Slab Thickness (in.)

Maximum Span (ft)

7.0

22.4

7.5

23.9

8.0

24.0

Table 2. Minimum total thickness/maximum span lengths for wide-module joist systems subjected to walking excitations.

Minimum Total Thickness (in.)*

Maximum Joist Span (ft)

18.5

36.0

20.5

39.0

24.5

45.0

28.5

50.0

* 4.5-in. slab thickness plus thickness of joist rib

continued on next page

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can conservatively be taken as 1.2 times the static modulus given by ACI 318-14 Equation (19.2.2.1.a). For flat plate systems, Ie´ of a panel section can be calculated based on the average effective moments of inertia of the column and middle strips that make up the panel. ACI Equation (24.2.3.5a) for Ie can be used to determine these quantities for nonprestressed systems; that equation is a function of the cracking moment, Mcr, which in turn is a function of the modulus of rupture, fr. Because of the relatively low reinforcement ratios in flat plates (generally less than 1%), it is recommended to use fr = 4.5λ√fć instead of fr = 7.5λ√fć [ACI Equation (19.2.3.1)] to account for shrinkage restraint when determining Mcr. In the equation for fr, λ is the modification factor that reflects the reduced mechanical properties of lightweight concrete and fć is the specified compressive strength of the concrete. In the case of wide-module joist systems, the same equation for flat plates can be used to calculate Ie with the exception that Mcr is determined using fr = 7.5λ√fć. Because of the inherent continuity of wide-module joist systems, ACI Section 24.2.3.6 permits Ie of a joist or beam (girder) to be taken as the average of the values calculated at its critical negative and positive locations. The effective mass (or weight) of a floor system is required when determining its natural frequency. In general, this quantity is equal to the dead load of the floor system plus any superimposed dead and live loads supported by the floor. When considering the supported live load, it is important to include in the analysis an estimate of the live load that is expected to occur; the code-prescribed live load is only to be used when determining the required reinforcement for the member. For office and residential occupancies, live loads of 11 psf and 6 psf are recommended for vibration analysis, respectively. For all other occupancies, the live load can be taken as zero unless specific information on the actual live load is available. Damping is a measure of how quickly vibrations subside and eventually stop, and plays a key role in vibration analysis. It is greatly dependent on the nonstructural items that are supported by the floor; for example people, partitions, file cabinets, bookshelves, furniture, and suspended ceilings. Th e amount of damping in a system is usually expressed as a percentage of the critical damping

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Selecting a System Parametric studies were performed to determine the conditions under which vibration acceptance criteria were satisfied for flat plate and wide-module joist systems. The following assumptions were used in the analyses: • Normal weight concrete with fć = 4,000 psi • Grade 60 reinforcing bars • Superimposed dead load = 10 psf • Live load varies from 40 to 100 psf • Actual live load from 6 to 11 psf • Damping ratio = 0.03 For both floor systems, acceptance criteria for walking excitations are easily met. Maximum span lengths that satisfy the acceptance criteria for walking excitations for flat plate systems, as a function of slab thickness, are given in Table 1. A 7-inch-thick slab can satisfy acceptance criteria for walking up to about a 22-foot span; slabs thicker than 7 inches can satisfy the criteria for spans up to 24 feet. Maximum span lengths for widemodule joist systems based on total thickness (4.5-inch slab thickness plus thickness of rib) are given in Table 2. In all cases, the girders can span up to 30 feet. In short, acceptance criteria for walking excitations are satisfied for typical flat plate and wide-module joist systems that meet the minimum requirements for deflection in ACI Tables 8.3.1.1 and 9.3.1.1 for non-prestressed members, respectively. Maximum span lengths for flat plate systems based on three types of rhythmic excitations are given in Table 3. Table 4 contains maximum joist and girder spans for wide-module joist systems subjected to jumping exercises and aerobics. Flat plate and wide-module joist systems can typically satisfy the acceptance criteria for sensitive equipment that have limiting vibrational velocities greater than or equal to 2,000 μin./sec. Table 5 contains a summary of the maximum span lengths and required slab thicknesses for flat plate systems assuming a fast walking pace. Given in Table 6 are the maximum span lengths of wide-module joist systems as a function of minimum overall thickness and maximum girder spans with limiting vibrational velocities greater than or equal to 2,000 μin./sec. and a fast walking pace. For vibrational velocities less than 2,000 μin./sec, flat plate and wide-module joist systems will not work for other than slow and moderate walking paces; in such cases, it is recommended to use a stiffer reinforced concrete system.

The information presented in these tables can be used to quickly ascertain whether a flat plate or wide-module joist system is suitable for a given set of constraints. Note that the results from the parametric study are not meant to

take the place of a more refined analysis; the main purpose of the study is to provide information that assists the design professional in making a rational decision on a suitable reinforced concrete floor system for vibrations.▪

Table 3. Minimum slab thickness/maximum span lengths for flat plate systems subjected to rhythmic excitations.

Maximum span (ft)

Minimum Slab Thickness (in.) 7.0

Dancing and Dining

Lively Concert / Sporting Event

Jumping Exercises / Aerobics

23.9

22.4

19.6

7.5

24.0

23.5

20.6

8.0

24.0

21.4

8.5

24.0

22.2

9.0

24.0

23.0

9.5

24.0

10.0

24.0

Table 4. Minimum overall thickness/maximum joist span lengths for wide-module joist systems subjected to jumping exercises or aerobics.

Minimum Total Thickness (in.)*

Maximum Girder Span (ft)

Maximum Joist Span (ft)

18.5

20.5

24.5

28.5

* 4.5-in. slab thickness plus thickness of joist rib

Table 5. Minimum slab thickness/maximum span lengths for flat plate systems as a function of limiting vibrational velocities V.

V (μin./sec )

Minimum Slab Thickness (in.)

Maximum Span (ft)

7.5

8,000

4,000 2,000

8.5

9.5

10.0

8.0

9.0

9.5

Table 6. Minimum overall thickness/maximum joist span lengths for wide-module joist systems as a function of limiting vibrational velocities V.

V (μin./sec )

Minimum Overall Thickness (in.)*

Maximum Joist Span (ft)

Maximum Girder Span (ft)

20.5

24.5

28.5

24.5

28.5

8,000

4,000 2,000

* 4.5-in. slab thickness plus thickness of joist rib

STRUCTURE magazine

September 2017

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n the spring of 2016, a group of eight students completing their master’s degrees in structural engineering at Rensselaer Polytechnic Institute in Troy, New York, were led out of the classroom and into the great outdoors for a very atypical educational experience. After years of calculus and physics, it was time for a much-needed mental break, where textbooks and exams were set aside for tape measures and cameras. The students headed 30 miles north to Shushan, New York, a quaint hamlet of about 800, to explore and document the historic Shushan Covered Bridge in accordance with the Historic American Engineering Record (HAER) (Figure 1). Why such a radical departure from the traditional engineering education experience? It sprang from a class on 19th-century Pennsylvania architecture, attended by the author while studying Architectural Engineering at Penn State University. There were no books, no homework, and no exams, but the class helped to progress the restoration of an 1850s log cabin using period tools. Lessons even included how to “square up” a round log using an ax. The experience left lasting memories and sparked an interest in historic preservation. Seeing firsthand how structures were built and how they had fared over time reinforced the engineering concepts learned in the classroom. Calculations are important, but they are also abstract and without form. The ability to see and touch the end result helps in understanding the theory within the calculations. So many of the structures that structural engineers work with are already built, yet very little classroom time is devoted to existing structures. Classes such as

The Historic American Engineering Record was established in 1969 by the National Park Service, the American Society of Civil Engineers, and the Library of Congress to document historic sites and structures related to engineering and industry. HAER developed out of a close working alliance between the Historic American Buildings Survey (HABS) and the Smithsonian Institution’s (SI) Museum of History and Technology (now the Museum of American History). From its inception, HAER focused less on the building fabric and more on the machinery and processes within, although structures of distinctly industrial character continue to be recorded. As the most ubiquitous historic engineering structure on the landscape, bridges have been a mainstay of HAER recording. HABS also documented more than 100 covered bridges before 1969. Source: National Park Service www.nps.gov/hdp/haer. these reduce this disconnect between theory and practice. Constructed in 1858, the Shushan Covered Bridge is a two-span, 161-footlong Town Lattice covered bridge, so named after Ithiel Town, a Connecticut architect who patented the design of the lattice bridge in 1820. Town lattice bridges quickly became very popular because they were relatively easy to build. The genius of the Town lattice truss is that the trusses are constructed of sawn planks and not of heavy timber; consequently, no complicated timber joinery was required and the planks were connected with simple wooden

Education issuEs discussion of core requirements and continuing education issues

Bridge to the Past Field Work Complements Classroom Learning By Mark Kanonik, P.E., F.ASCE

Mark Kanonik is a Senior Structural Engineer with EYP Architecture & Engineering, PC in Albany, NY. He is also an adjunct faculty with Rensselaer Polytechnic Institute in Troy, NY, where he teaches graduate-level classes in steel and masonry. He can be reached at mkanonik@eypae.com.

Figure 1. Overall elevation of the Shushan Covered Bridge.

STRUCTURE magazine

Figure 2. The underside of the bridge; note that the ends of the lattice planks are uneven and without special joinery.

dowels called trunnels or “tree-nails” (Figures 2 and 3). Some have estimated that several thousand Town lattice-truss bridges were constructed worldwide, and well over 100 remain in the United States today. Like many Town lattice-truss bridges, the trusses of the Shushan Covered Bridge were assembled on the ground adjacent to the bridge site and then joined on-site atop falsework. The bridge served the transportation needs of Shushan for 100 years until a “modern” steel bridge was built adjacent to it, at which time the Shushan Covered Bridge was abandoned in-place. After years of neglect and near collapse, the bridge was purchased from Washington County for $1 by the Shushan Covered Bridge Association, who converted the bridge into a museum showcasing farm tools (many of which were donated by local farmers), clothing, and other items of that time period (Figure 4). Documenting any bridge in accordance with HAER is a very intensive process that involves both time in the field measuring the

Figure 3. Close-up of the Town lattice truss; openings were frequently framed into the siding of covered bridges to allow natural daylight into the bridge.

bridge and archival research to gain insight into the bridge that cannot be obtained from the field. The students spent several days at the bridge, measuring and documenting nearly every accessible structural member. Unfortunately, much of the underside of the bridge could not be accessed because of the Batten Kill below, but representative members of the bridge floor system were observed. The bottom of one of the abutments was not accessible, also due to the flowing Batten Kill, but the top of the abutment could be measured with reasonable accuracy. Throughout the time studying the bridge, non-original and/or non-historic structural members (such as boards that had been replaced) as well as missing members (such as missing trunnels) were noted. The students completed archival research on Ithiel Town, the Town lattice bridge type, and the Shushan Covered Bridge. Furthermore, they prepared drawings, a short report, and photography, all in strict accordance with the mandates of HAER.

Not surprisingly, the students thoroughly enjoyed the whole process (Figure 5). None of them had ever studied a covered bridge before, so this was a new experience for them. Also, seeing and touching the bridge brought abstract engineering concepts to life. An unexpected benefit was that the time spent at the bridge, away from the classroom and the books, relaxed and rejuvenated the students, giving them some much-needed energy for their other courses. The process of documenting a historic structure is an activity that engineering schools should consider as part of their curriculum. From experience in northern climates, professors may want to consider scheduling the course for the fall rather than the spring. The Shushan Bridge field work began in February, the middle of winter. The students tried to stay warm as best as they could, but several hours in below-freezing temperatures does make field work difficult. Then again, it may prepare the students well for the construction sites they may face in their careers ahead.▪

Figure 5. Enjoying a rare warm day at the bridge. Left to right – Antonio D’Elia, Jonathan Schmierer, Jacqueline Sanchez, Jenna Hastings, Brandon Samardich, Navid Safaie, Lucas Deyglun, Noel Gorab, and the author.

Figure 4. The museum inside the bridge.

STRUCTURE magazine

September 2017

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he author’s company, a forensic engineering and architecture firm, has investigated hundreds of low-slope roof and exterior deck applications with water stains, ponding, framing damage, and structural collapse. The first article, Part 1: ¼ in 12 Design Slope and Water Drainage (STRUCTURE, August 2017), examined two building code parameters that contribute to lowslope roof and deck serviceability issues. This article identifies design and construction practices that limit or prevent free drainage. Potential solutions are presented to mitigate ponding that contributes to serviceability issues and structural framing damage. The goal is to raise awareness in the construction industry of typical practices that may cause harm to structural members and the building envelope.

may create a negative slope or a “bowl” condition at the low end that limits or prevents free drainage. The condition is exacerbated for materials susceptible to creep deflection, such as wood. Beam members designed and installed to the ¼-inch per foot slope should be considered a susceptible bay. Readers are encouraged to visit the first article for additional information and potential solutions. Field observations have identified common design practices that contribute to serviceability issues. These design blunders limit or prevent free drainage and result in unsatisfactory building envelope performance. Additionally, the absence of specific design details and reference to a “best practice” often result in typical construction practices that may meet the general intent of the building code but limit free drainage.

Background

Design Blunders

The 2015 International Building Code (IBC) establishes minimum parameters for building design and construction. A member or system that satisfies applicable individual code parameters may

When design professionals specify framing members to minimum building code parameters alone, it is possible for the constructed roof to have inservice low-slope issues related to ponding or drainage of the system. Five common design blunders that contribute to low-slope issues and potential solutions are summarized below.

Low-Slope Roof and Deck Design Considerations Part 2: Mitigate Ponding and Water Intrusion By Scott D. Coffman, P.E., SECB

Scott Coffman is a Forensic Engineer with Construction Science and Engineering, Inc. in Westminster, SC. He can be reached at scottcoffman@ constructionscience.org.

create a less than ideal condition when multiple Intersecting Planes minimum code parameters are combined. For example, the combination of the ¼-inch per foot Building offsets are common and create interdesign slope and a maximum permitted deflec- secting planes that contribute to drainage issues tion ratio can create a condition that inhibits free for low-slope applications. Design professionals drainage. The IBC, however, does recognize this frequently specify the minimum code-permitted potential condition in Table 1604.3 footnote “e” slope with little, if any, consideration of the resuland instructs a building designer to investigate tant valley slope created by the intersecting planes applications with insufficient slope or camber (Figure 1). Ponding water is commonly observed for ponding. at valley intersections for low-slope roof and deck Design professionals, contractors, and perhaps (balcony) applications. code officials have come to believe a roof or exterior deck surface designed to the ¼-inch per foot slope is satisfactory because it meets building code intent. However, member deflection ROOF DRAIN creates an average slope that limits free drainage and contributes to ponding toward the low end. The “average slope” is the slope of a line from the low-end support to the point of maximum deflecPLAN VIEW tion for a member. Members optimized to a code-permitted deflection ratio further reduce the average slope and Figure 1. Common minimum roof slope plan with valley.

30 September 2017

SLOPE SLOPE

BALCONY

SLOPE

LIVING ROOM

PLAN VIEW Figure 2. The design slope is reduced at sloped plane intersections.

The diagonal distance between two fixed elevation points is less than the design slope. This principle can be illustrated by two sloped planes that intersect at a right angle (Figure 2). The eight-foot wide balcony with a specified ¼ in 12 slope has a twoinch elevation drop from the wall to the free drainage edge. The diagonal distance denoted in red has the same two-inch elevation change. However, the elevation change occurs over a distance of approximately eleven feet four inches, creating a slope less than the ¼ in 12 minimum slope. The 2010 edition of the Minimum Design Loads for Buildings and Other Structures (ASCE 7), published by the American Society of Civil Engineers (ASCE) and referenced by IBC, states in part “surfaces with a slope of at least ¼-inch per foot toward points of free drainage need not be considered a susceptible bay.” Therefore, roof and balcony surface areas designed to a ¼ in 12 slope that intentionally direct water to a valley should be considered a susceptible bay. A potential solution is to assign the ¼ in 12 slope to the valley and calculate the

Figure 3. Integrated balcony column without drainage provisions.

associated roof or balcony slope to be shown on the construction documents.

Integrated Columns Building codes and accepted design practice incorporate “crickets” to divert water for effective drainage. Balcony support columns present conditions that are rarely detailed within the construction documents. Design professionals routinely design integrated exterior balcony columns that serve as “dams” that inhibit water from flowing toward points of free drainage (Figure 3). Columns are frequently located at the balcony perimeter and contain interior edges or corners. Water becomes trapped at the interior edges and often contributes to damage. Design professionals should provide clear details that divert or allow free drainage at these locations throughout the life of the building.

Sloped Concrete Surface Many design professionals specify horizontal framing members with a sloped topping

surface for drainage (Figure 4). The topping surface is typically a lightweight concrete product installed in a semi-fluid state. Specifications for a “stiff” slump test or to install with a stipulated slope are difficult at best, rarely achieved, and often result in a constructed level surface. Water percolates the permeable topping surface to the horizontal plane created by the support members. Free drainage rarely occurs since the support member is level or deflected vertically downward, allowing water to pond. Forensic investigations often encounter damage to support members when water finds a breach in the protective membrane between the topping surface and structural framing. Structural members should be designed and oriented with a positive slope toward points of free drainage for water discharge. The topping surface should conform to the structural member slope to maintain positive drainage. Water that permeates the topping surface encounters the sloped surface and is directed toward the desired free drainage location. continued on next page

Δ1 Δ2

SLOPE

Figure 4. Slope obtained with finish topping.

Figure 5. Differential deflection adjacent to wall.

STRUCTURE magazine

September 2017

NOTCH DECK FOR FLASHING THICKNESS DROPPED FASCIA TO MAINTAIN SLOPE OF DECKING

Figure 6. Fascia detail to maintain slope to free drainage edge.

Wood Framing Members The use of ripped, solid sawn framing members is a common design and construction practice to achieve a desired slope. Lumber grade marks are assigned in accordance with criteria outlined in the code referenced by the American Softwood Lumber Standard (PS 20). The standard specifically states that the remanufacture (ripping) of a graded or grademarked wood member negates the mark and associated design values of the original product. The “ripping” of lumber members can be eliminated by modifying the framing detail. One option is to install each end of the lumber member at two distinct elevations to achieve the desired slope. A ceiling joist or furring may be required to obtain a “flat” ceiling. A second option is to specify a truss with the desired top chord slope for drainage and horizontal bottom chord for ceiling attachment. Another common framing technique orients the framing member perpendicular to the free drainage slope direction. Forensic investigation of this condition typically finds water accumulation toward the center as the member deflects downward. Framing members should be oriented and installed to promote water flow toward points of free drainage.

Differential Deflection A system of members with the same span are anticipated to deflect a similar amount. Adjacent members with different spans, however, deflect a different amount; the longer span member deflects more, relative to a shorter span member, and creates a “bowl” that retains water. The Truss Plate Institute (TPI) recognized this phenomenon and identified differential

Figure 7. Component thickness prevents free drainage.

deflection as a design parameter for metal plate connected wood trusses. TPI Section 2.3.2.4 (g) (4) specifically requires the building designer to specify differential deflection design limits. Differential deflections, however, are not limited to wood trusses and this practice should be adopted to other building components. The design professional should consider material properties of the cover, framing, and ceiling when evaluating an acceptable limit to evaluate differential deflections. A similar condition exists for structural members installed parallel to a wall supported by a foundation. The design intent is for the wall to be a free drainage location; however, the wall is “rigid” and does not displace downward under load (Figure 5, page 31). The structural member adjacent to the wall deflects downward creating a “bowl.” For lowslope members adjacent to a “rigid” member, water may begin to accumulate inward of the intended free drainage point.

Construction Practices Construction practices also contribute to ponding for minimum slope applications. Fascia members are often installed flush to the top edge of the framing member to create a horizontal surface. Detailed fascia members should be shown “dropped” to maintain the slope of the plane (Figure 6). Flashing is often installed at the boundary, with one leg placed on top of the roof or deck substrate. The material thickness at the boundary impedes water discharge (Figure 7). The substrate should be notched to receive flashing members and accommodate material thickness. These are two examples of a common framing practices that may be found within

STRUCTURE magazine

September 2017

construction standards and implemented in the field. Material installation or thickness impact low-slope drainage and are often neglected at the time of design or during construction. A design professional should recognize the limits of building code requirements, standard details or practices; In these cases, it is important to provide “best practice” details within the construction documents to mitigate potential ponding and serviceability issues.

Conclusion Accepted design and framing practices often contribute to serviceability issues with lowslope roof and deck applications. Practices or conditions that inhibit or prevent the flow of water toward free drainage should be identified during the design phase and changed. Design professionals have the ability to create in-service conditions that diminish ponding and promote free drainage. Slopes should be increased to maintain a sufficient slope to drain at intersecting planes. Framing members should provide the drainage plane and not rely completely on the slope of the topping or finish surface. Additionally, differential deflection of adjacent structural members should be investigated and the appropriate limit assigned to mitigate low areas for water retention. Framing practices and standard construction details often create high points that inhibit water drainage in low-slope applications. The design professional is encouraged to detail boundary conditions to promote drainage.▪ The online version of this article contains references. Please visit www.STRUCTUREmag.org.

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udson Yards, located on the west side of Manhattan, is a 14-acre area experiencing a major expansion. This renewed interest in the parcel is spearheaded by Related/ Oxford Properties, who acquired the overbuild rights of the yards of the Long Island Railroad. Considered the largest private real estate development to date in the United States, the Related/ Oxford Properties Hudson Yards development will house about 19 million square feet of retail, commercial, residential, and mixed use spaces over the course of the next decade. Working on new projects within urban areas is always a challenge considering the numerous existing constraints, including subway infrastructure, underground utilities, neighboring buildings, and difficult construction logistics. Although the 55 Hudson Yards site initially appeared to be a sparsely developed block, in reality, the site presents complex and difficult site conditions. It partially sits on top of the 34th Street-Hudson Yards subway station for the new No. 7 New York City subway and is occupied by a 6-story subway ventilation building. Another unique aspect of the 55 Hudson Yards structure was the use of a post-tensioned lightweight concrete flat slab floor system for a high rise commercial office building.

55 HUDSON YARDS

By Jeffrey Smilow, P.E., F.ASCE, Ahmad Rahimian, Ph.D., P.E., S.E., F.ASCE, and Lan-Cheng (Peter) Pan, Ph.D., P.E.

Project Evolution The developer behind 55 Hudson Yards, Related/Oxford Properties, acquired the 55 Hudson Yards site from another major New York City (NYC) developer in early 2000. While the initial developer pursued potential tenants, early designs for a steel diagrid building were proposed, and negotiations with the Metropolitan Transit Authority (MTA) took place to accommodate the infrastructure for the new No. 7 subway line. The negotiations resulted in an agreement to build the 34th Street-Hudson Yards subway station with portions directly below the office tower, including two major entrance escalators and a 6-story ventilation building. The agreement included a provision for the ventilation building and the below-grade subway station to have adequate capacity to support the proposed vertical and lateral loads imposed by the commercial office tower. Towards the end of 2007, construction of the station and adjoining ventilation building began and, by the fall of 2015, they were in service. During the construction of the MTA infrastructure below and next to 55 Hudson Yards, the original developer sold the property and air rights to Related/Oxford Properties, which was constructing high-rise buildings adjacent to the 55 Hudson Yards site. Related/Oxford Properties immediately pursued the modification of the original tower design to meet their vision of a 51-story tower with 1.3 million square feet of gross area for commercial use. These changes resulted in architectural and

Figure 1. Existing site with structural rendering of the tower (background image obtained from Google Earth).

STRUCTURE magazine

structural modifications specific to the tenant base considered by the new developer. The redesign of the 55 Hudson Yards tower included two key structural considerations: • Use of alternative structural materials, namely cast-in-place concrete, to meet the financial requirements of Related Properties. (This original concept was converted into lightweight concrete to reduce the loading upon the existing subway structure.) • Redirecting the loads of the new tower to meet acceptable vertical and lateral load limits of the already built underground subway station and the 6-story subway ventilation building.

Project Description The 55 Hudson Yards project is located in the western half of the block limited by West 34th and West 33rd streets to the north and south, respectively. To the east, the project neighbors Hudson Park, where the main entrance of the 34th Street-Hudson Yards subway station is located (Figure 1). Furthermore, the new building neatly extends above and along the north side of the existing 6-story subway ventilation building in the southwest corner of the lot. The architects of the project are Kohn Pedersen Fox and Kevin Roche (John Dinkeloo and Associates). WSP USA Inc. provided integrated engineering services for structural and MEP systems. The building is 51 stories high, divided into a 10-story podium and a 41-story tower, reaching 760 feet. The geometry of both the podium and the tower is approximately rectangular with column-free spans reaching almost 50 feet in the podium and more than 40 feet in the tower. The combination of site-related constraints triggered by the MTA infrastructure and the limitations associated with the original design posed engineering challenges addressed by unique and highly innovative structural solutions.

Foundation System The main objectives of the foundation system envisioned for 55 Hudson Yards were to address the large interaction with existing MTA underground facilities and to minimize the impact in terms of loads transmitted to those facilities. The vertical load demands of the building were divided into three major components to provide adequate load-carrying capacity. Approximately 20% of the vertical load was transferred to the rock substrate by means of a reinforced concrete mat and footing foundations, about 40% was transferred using 10 reinforced concrete

drilled caissons, ranging from 4.5 to 5.5 feet in diameter, and the remaining 40% was carried by the existing 6-story MTA ventilation building. The caissons were the primary support system for the concrete core and shear walls, carrying loads ranging from 10,000 to 33,000 kips. Complicating the foundation design was the fact that the location of the caissons was limited to the area between the two previously constructed subway entrance escalators (Figure 2). Figure 2. Foundation overview with The acceptable diameter and detail of caissons between escalators. location for the caissons required the reinforced concrete core and shear walls to cantilever beyond the caissons, resulting in a 20-foot deep caisson cap. All caissons utilized 12,000-psi grout and were embedded into rock sockets below the escalator base to adequately transfer their loads into the rock below the escalator shafts (Figure 3). The length of rock sockets ranged from 35 feet to approximately 100 feet. To limit the caissons diameter and maximize their load-carrying capacity, No. 28 reinforcing bars with 75-ksi yielding strength were used. In exceptional cases, a solid steel section was provided in lieu of reinforcing bars.

Superstructure The superstructure of the 55 Hudson Yards project is based on a central reinforced concrete shear wall core and exterior moment frames connecting perimeter columns with a spandrel beam around the slab edge of the tower. It is complemented by a partial outrigger and belt system at the top (Figure 4, page 38). The structural solution provided by WSP addressed three main design objectives: • To follow the main architectural intent, which required longspan, column-free spaces. • To minimize the structural weight and thus meet the loadcarrying limitations of the existing 6-story MTA ventilation building and the underground subway station. • To provide flexibility for future modification to the floor plans, specifically allowing for additional slab openings in selected locations. continued on next page

Figure 3. General layout of foundation system with details of caissons and caisson sockets.

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September 2017

The floor system was solved using a post-tensioned lightweight concrete flat slab. Floor slab thickness was primarily 9 inches in the tower and 10 inches in the podium, with 18-inch thick floors in areas of major load transfers. The compressive strength of concrete for slabs was increased during the construction phase from 7,000 psi, as required by structural analysis and design, to 8,500 psi to accommodate the required construction schedule by applying post-tensioning as early as possible. The increase in compressive strength allowed for the early strength required prior to the post tensioning operation. Despite the use of a post-tensioned system, which is somewhat unusual in the New York City market, this solution was more attractive financially than the original steel frame solution. It also was able to accommodate both the load limitations imposed by the existing subway structure and the floor plan flexibility for tenants. The post-tensioned concrete slabs utilized blended lightweight concrete, which went through an extensive pre-construction sample mix design study to assure its suitability, pumpability, and constructability under the project conditions. Three-dimensional laser scans were carried out at each floor level to determine the precise final location of post-tensioning strands (Figure 5) to address floor plan flexibility for future tenants. The post-tensioned slab utilized a banded system, thereby grouping the post-tensioning bands on either side of potential future slab openings. The 9th floor is used as the primary mechanical floor and also serves as the interface between the podium and the tower floors. At this level, all the perimeter columns of the tower were transferred from a smaller tower footprint to the larger podium footprint. The transfer was accomplished with a walking column system where the 10th-floor slab carries in-plane compression loads, and the 9th-floor level carries in-plane tension loads. The 9th-floor tension forces were efficiently counteracted with undraped horizontal post tensioning (Figure 6). Finally, to limit lateral deflections and accelerations due to wind events, a full-story height, partial outrigger and belt system connecting the central shear wall core with a number of perimeter columns on the north end of the tower was incorporated at the 51st floor of the tower.

Figure 4. Structural overview. 3D view from Northwest.

Figure 5. Example of 3D laser scan of as-built post-tensioning layout in a podium floor.

Wind Tunnel Testing Comprehensive and project-specific wind tunnel tests were performed at the boundary layer wind tunnel lab at RWDI Consulting Engineers to determine direct wind pressures and wind-induced building accelerations. The industry-recommended comfort level for human occupancy in terms of acceleration due to wind for office buildings was achieved.

Figure 6. Load transfer mechanism using walking columns and post-tensioned slab.

The structural modeling of 55 Hudson Yards included full three-dimensional sequential construction analysis with consideration to time-dependent changes in material properties and loading demands. This analysis was necessary to assure the short-term and long-term loading on all existing subway foundation elements was kept within acceptable limits. Two unique methods of load redirection were used to remove excessive loads from existing foundation elements or superstructure components of the subway structure. The

Figure 7. Post-tensioning scheme and detail of construction of transfer wall.

STRUCTURE magazine

Structural Modeling and Innovation in Construction Sequencing

September 2017

Figure 8. 55 Hudson Yards tower under construction as of May 2017.

first method was the innovative use of construction sequencing devised to prevent overstressing of critical elements of the MTA structure. A multi-strand, grouted posttensioned system was used as a transfer wall to bridge loads away from the MTA structure (Figure 7). The second method employed temporary loadpath bridging to avoid specific locations with limited support capacity. After the appropriate amount of load had bypassed a specific location, the supporting structure was engaged using steel plate shims which then created a new continuous load path through the initially bypassed support points.

Considering the load-carrying capacity limitation of the existing 6-story MTA ventilation building, it was crucial to reduce the weight of the structure without jeopardizing its structural integrity. With the use of 9-inch thick blended lightweight concrete slabs, the appropriate building weight was attained. The concrete supplier was able to develop, test, and successfully produce a mix design for a pumpable, lightweight concrete with a specified compressive strength of 8,500 psi at 56 days. The blended lightweight concrete was produced using both traditional lightweight and smaller diameter coarse aggregates and had a volumetric weight of approximately 130 pcf instead of the 150 pcf associated with normal weight concrete. In conclusion, this project demonstrates unique and innovative solutions to overcome existing site constraints. These constraints are becoming more prevalent in cities as developers seek out previously undeveloped land within urban centers. Also, the use of post tensioning, coupled with light weight concrete, a rarity on commercial high rises, is proven to be an economical alternative to structural steel framing. See Figure 8 for an overall view of the project under construction as of May 2017.▪

Material and Construction Innovations High-performance structural materials were used in 55 Hudson Yards to provide adequate stiffness, strength, and resilience. For columns and shear walls, a concrete compressive strength of 12,000 psi was used in all podium floors, which gradually reduced to 7,000 psi at the 51st floor.

Jeffrey Smilow, P.E., F.ASCE, is Executive Vice President and USA Director of Building Structures at WSP USA, Principal in Charge of the project. Ahmad Rahimian, Ph.D., P.E., S.E., F.ASCE, is Technical Director and USA Director of Building Structures at WSP USA. Lan-Cheng (Peter) Pan, Ph.D., P.E., is Senior Associate of Building Structures at WSP USA, Project Manager.

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STRUCTURE magazine

September 2017

InSIghtS new trends, new techniques and current industry issues

nergy efficiency continues to weave itself into the expectation of building design. Building trades, including Mechanical, Electrical, Plumbing, and Structural Engineers, are having to modify their general practices. The new requirements of building codes, namely energy codes, are subsequently forcing ageold structural detailing into a new realm. Moreover, as designers seek to get the most bang for their buck in their building designs, the integration of structural design with mechanical, electrical, and plumbing (MEP) infrastructure becomes paramount. No longer can an architect throw a building design over the fence to structural and MEP consultants with the expectation that they simply “make it work.” Rather, high-performance building design requires everyone be around the same table from day one, constantly evaluating the tradeoffs from one concept to another. This article discusses three of the most prominent relationships between structure and energy efficiency: structural type, application of rigid insulation, and the elimination or reduction of thermal bridging.

Structural Impacts of Low-Energy Buildings By Neil Steiner

Neil Steiner is an Energy Analyst and Project Manager at Glumac, a full-service MEP consulting firm that specializes in highperformance buildings. If you are interested in continuing the conversation, reach out directly at nsteiner@glumac.com.

The first step in the design conversation involves the selection of structural materials. In most cases, this is a question of steel or concrete, although highperformance building designs are driving other solutions as well, such as straw-bale construction, structurally-insulated panels (SIPs), phase-change materials (PCMs), and polymer-based 3-D printing. Regardless of the structural material, the concerns associated with thermal massing, thermal breaks, and application of insulation remain the same. For example, a concrete structure has an inherently larger thermal mass capacity than steel-framed structures (because heat dissipates from steel faster than concrete). Thus, concrete can provide exceptional thermal comfort, reduced/delayed peak loads (Figure 1), and improved energy efficiency when applied

properly. Of course, span length, seismic weight, commodity pricing, and other issues can override these concerns, which forces decisions regarding structural type. Design teams must consider their local climate to drive material selection. Milder climates, for example, tend to support low thermal mass, while more extreme climates tend to support (properly applied) high thermal mass. The default response is to add more insulation to drive down energy usage in buildings. The traditional application of fiberglass batt insulation becomes limiting because increased thermal resistance values (R-values) demand thicker walls. A point of diminishing return is reached quickly as usable floor area is decreased and wall thicknesses require significantly more structure to stand up. Theoretically, the “silver bullet” solution to this challenge is the application of rigid insulation. It has about twice as much R-value per inch as traditional batt insulation and can maintain a relatively similar thermal performance, regardless of the structure type, due to its theoretically “continuous” application that is not disrupted by structural members. On paper, this is easy to visualize, and the R-value numbers look great; however, the reality of its application in the field is not so straightforward. For example, the 2012 International Energy Code (IECC) expects a U-value of 0.079 Btu/hr-sf°F as a prescriptive minimum for metal stud walls. With a 2x6 metal stud wall, this equates to approximately R-21 batt insulation between studs with 1.5 inches of rigid insulation applied to the exterior. Not only can this target U-value not be reasonably achieved in metal stud walls without the integration of rigid insulation, but the choice of exterior finish can have additional structural challenges. If plaster is applied as the exterior finish, then all is good. If brick is applied, then the face of the wall is upwards of 5 to 6 inches away from the supporting studs, with the heaviest part (brick) at the furthest point (Figure 2). This conundrum either forces the designers to dramatically increase the structural elements, such as shelf angles and the detailing associated with this assembly, or select different materials and assemblies altogether. off-peak demand

outdoor temperature extreme outdoor temperature swing

indoor temperature

reduced HVAC load

moderate indoor temperature swing

Figure 1. Thermal mass response.

40 September 2017

HSS SHELF ANGLE INSTALLED OVER INTERMITTENT KNIFE PLATE

In addition to increased insulation values, appropriate attention to thermal bridging and the incorporation of thermal breaks are becoming important considerations. The objective is to eliminate or minimize all direct connections for heat transfer between the outside and the inside of the building structure. Although relatively small in area, the number of connection points is high across an entire building, which ultimately degrades the theoretical performance of the building envelope assembly in a big way. Imagine a metal screw or pin that holds the rigid insulation to the metal structure. This point is a direct pathway for the outside temperature to influence the inside temperature, which defeats the whole purpose of applying insulation in the first place. This phenomenon (Figure 3) is inherent in numerous envelope assemblies: metal-framed windows, rigid insulation applied to metal stud, and concrete structures where floors and walls meet (to name a few). The challenge with these thermal pathways is that they de-rate the U-value of the overall assembly, which requires the application of more (or higher performance) materials/systems to offset this deficiency. In metal stud framing without continuous insulation, the performance of the wall can degrade upwards of 50% from the prescribed insulation R-value due to thermal bridging of the studs passing through the batt insulation. For this reason, the 2016 California code now mandates (i.e. not allowed to circumvent with whole-building energy analysis) rigid insulation on metal-framed 2x6 stud walls. Furthermore, if not addressed appropriately, the impacts of thermal bridging can cause moisture problems within the assembly, which has a multitude of problems (e.g. mold, structural degradation, etc.). Infiltration can be avoided when a tightly sealed continuous insulation layer is applied to walls. To avoid the pitfalls associated with thermal bridging, structural engineers must envision how the envelope and structural assembly come together in a way that maximizes the application of continuous insulation. This evolution in the building industry requires unprecedented integration and coordination during the design process. All parties need to be engaged early in the project, and decisions cannot be made in a vacuum. Energy modeling from concept through completion is necessary to realize the high-performance expectations of the industry and codes. By asking the questions early on and modeling the energy/cost performance of multiple scenarios, the chances of delivering the product that the client envisions (and codes expect) becomes much more feasible.▪ STRUCTURE magazine

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updates and discussions related to codes and standards

Codes and standards

What is the Performance Method Trying to Do? NCSEA’s Position on Some Confusing Code Provisions By David Bonowitz, S.E.

s states adopt the 2015 International Existing Building Code (IEBC), more engineers are learning the differences between two of its compliance methods – the Prescriptive method and the Work Area method. (See “What Happened to Chapter 34?” in the June 2016 issue of STRUCTURE.) But there is a third set of code provisions for existing buildings. The Performance method is less commonly used, but it’s available both as Chapter 14 of the IEBC and as Section 3412 of the 2012 International Building Code (IBC). The Existing Buildings Subcommittee of NCSEA’s Code Advisory Committee has done a lot to improve the IEBC since 2006, but it has mostly stayed away from the Performance method, which appears to have limited application to structural issues. Some code officials and engineers, however, have a completely different interpretation of how the Performance method should be applied, and the differences have important implications for engineers, their clients and their communities.

Which projects are exempt? Two provisions from the 2015 IEBC Performance method are most likely to affect structural work. First, Section 1401.2: 1401.2 Applicability. Structures existing prior to [DATE TO BE INSERTED BY THE JURISDICTION. Note: it is recommended that this date coincide with the effective date of building codes within the jurisdiction.], in which there is work involving additions, alterations or changes of occupancy shall be made to conform to the requirements of this chapter or the provisions of Chapters 5 through 13. The provisions of Sections 1401.2.1 through 1401.2.5 shall apply to existing occupancies that will continue to be, or are proposed to be, in Groups A, B, E, F, I-2, M, R and S. These provisions shall not apply to buildings with occupancies in Group H or I-1, I-3 or I-4. The first sentence says the Performance method, (or, optionally, the Work Area method in Chapters 5 through 13) applies to buildings older than a date to be provided, raising some questions:

• What about newer buildings? Are addition, alteration, and change of occupancy projects in newer buildings completely exempt from any regulation? Or does this mean that a newer building must comply with the Prescriptive method in Chapter 4 instead? • For a project in a newer building, does the second sentence still apply? The only reasonable answer (discussed below) is yes, but that is not how the provision reads. • Does the last sentence mean those H and I occupancies are exempt from the limits imposed by 1401.2.1 through 1401.2.5? That would seem backward for such sensitive uses as high-hazard facilities, jails, and daycare centers. Or does it mean those occupancies need more restrictive regulations not specified? • What about repairs? Chapter 14 mentions repairs in several places, but not here. So in newer buildings, are repairs subject to the Performance method while additions and major alterations are exempt?

Does a 70-year old structure need to be checked? In terms of code clarity, Chapter 14 is not exactly off to a good start. The big question in 1401.2, however, is about that DATE TO BE INSERTED. The note recommending “the effective date of building codes within the jurisdiction” is printed with the text of the code. What date does that mean? Does it mean the date on which the current code edition replaced the previous edition, a date that shifts every time a state or city adopts a new set of I-codes? Or does it mean the date when the jurisdiction started enforcing its first building codes, which for many east coast cities was before 1900? Consider that latter interpretation. Say a city started enforcing building codes in 1947. Then any addition or alteration project in a building up to 70 years old would be exempt from regulation. It seems nonsensical. This interpretation is supported by the ICC’s commentary, but that commentary

STRUCTURE magazine

September 2017

has scarcely changed since it was written for the 1993 BOCA code. That doesn’t make it wrong, but it does mean that neither the provision nor the commentary has been updated for a quarter century, even while codes for existing buildings have otherwise evolved dramatically. NCSEA’s Existing Buildings Subcommittee interprets the “cutoff date” provision the first way, expecting states and cities to insert a recent date within the last three, maybe six years. The idea is that the Performance method is an option for buildings that are essentially new. If the building is more than a few years old, one may either show that it is still just as good as a recent building (as discussed below) or use either of the IEBC’s other two methods. This interpretation makes sense when you consider what the Performance method appears to be about. Chapter 14 has one page of generic requirements followed by ten pages of detailed rules, tables, and equations all about fire safety and egress. Clearly, this method is meant to provide an alternative way to assess room layouts and corridors that do not quite match what the IBC requires for new buildings. That is a valuable tool, as long as the structure and other systems are unaffected. But if a project affects the structure, as most additions and many alterations do, how can the code exempt the whole project from review just because of the age of the building? One way around this is to assume that the second sentence of 1401.2 applies even to newer buildings that the first sentence appears to exempt. Of course, that is not what the code (or the 1993 commentary) says. Plus, the hypothetical 70-year old building would still be exempt from fire, maintenance, flood, and structural provisions in 1401.3 and 1401.4. Is that the intent? So there are two, maybe three, different interpretations. How are actual cities and states setting their cutoff dates? The Subcommittee canvassed jurisdictions across the country and found some interesting answers. A number of jurisdictions insert an early date. For example: • St. Louis: December 19, 1951 • Michigan: November 6, 1974

• Denver: The date it adopted the 1976 Denver Building Code • Ohio: July 1, 1979 • Portsmouth, New Hampshire: September 14, 2003 Others share the Subcommittee’s understanding that the cutoff date is meant to be recent. California no longer allows the Performance method, but when it did, the 2013 state code set January 1, 2014, as the cutoff. Oregon sets it at July 1, 2014. The cities of Seattle and Dallas and the states of Florida and Virginia delete the entire “existing prior to” phrase, effectively making today the cutoff and removing any date-based exemption. Interestingly, many jurisdictions – including Rhode Island, Pennsylvania, Utah, Montana, North Carolina, South Carolina, and Washington state – fail to insert any date, either because they are reserving judgment or because they did not realize the blank needed to be filled. When we spoke to a member of Utah’s building commission, he read the ICC note and assumed it meant 2016. In any case, implementation of the Performance method is inconsistent from state to state, and sometimes even within a state.

Should a deficient structure be upgraded? The other critical provision involves structural checks for any building that is not exempt by age: 1401.4.1 Structural analysis. The owner shall have a structural analysis of the existing building made to determine adequacy of structural systems for the proposed alteration, addition or change of occupancy. The analysis shall demonstrate that the building with the work completed is capable of resisting the loads specified in Chapter 16 of the International Building Code. The first sentence is about the proposed project itself, and it is sensible. The second sentence is about the building as a whole – or at least it seems to be, and that is how the Subcommittee understands it. The building, not just the parts affected by the proposed project, must satisfy requirements for new construction. That is a high bar, but it makes sense together with our understanding of the cutoff date. An effective code sets the same standards for all buildings in similar situations. If the Performance method is to be effective, then any building subject to Chapter 14 should be as good (or should be made as good) as any building automatically exempted due to age. So if the cutoff date in 1401.2 is recent, then

the requirement to satisfy IBC Chapter 16 is appropriate. Taken together, the two provisions effectively say that if the building is recent, or if you can show by analysis that it’s as good as a recent building, then the building is eligible for the Performance method, and you may use the rest of the chapter to check the fire safety and egress. If your building has an obsolete structural system, however, you must use one of the IEBC’s other methods, which account more completely for structural issues. This interpretation is consistent with similar eligibility rules in Section 1401.3.2: Any building using the Performance method must comply with, or must be upgraded to comply with, the current International Fire Code and International Property Maintenance Code. But some read 1401.4.1 differently. They say the first sentence sets the required scope: Check the effects of the proposed project, and ignore the rest of the structure. The second sentence then merely gives the criteria and loads for this limited structural review. Again, that is not what the plain language of the code says, but maybe this interpretation is correct. Maybe it is the intent of the Performance method never to trigger an upgrade of a deficient structure, no matter how extensive the addition, alteration, or repair. Of course, if this is the intent, it is contradicted by Section 1401.3.3, which uses typical IEBC language to trigger flood upgrades based on the cost of the proposed work. It would also deviate from the IEBC’s other two methods, allowing permit applicants to “game the code,” or take advantage of an unclear or inconsistent provision to avoid certain requirements.

The NCSEA Existing Buildings Subcommittee Position Vagueness, illogic, incompleteness, and inconsistency in the text of Chapter 14 thus give rise to two opposite interpretations of when and how the Performance method should be applied: • Read one way, projects in buildings 30, 50, or even 100 years old are exempt from a thorough structural review, and perhaps from any regulation at all. Older buildings subject to review are still never required to be structurally upgraded, no matter how extensive the project. • As NCSEA’s Existing Buildings Subcommittee reads it, the Performance method is for checking fire safety and egress in otherwise reliable, typically recent, buildings. Structures of obsolete design may use the Performance method if they can

STRUCTURE magazine

September 2017

show equivalence to newer buildings or if they are structurally upgraded. It is only because of confidence in this second interpretation that the Subcommittee has left Chapter 14 alone over the last several code cycles. As we work to refine the Prescriptive and Work Area methods, having a more conservative third option has not been a concern. If the Subcommittee’s interpretation has been wrong, however, then it will have more work to do. The NCSEA Existing Buildings Subcommittee’s position regarding the IEBC’s different methods is 1) that the structural provisions should not differ to the degree that the differences encourage gaming, and 2) that it is not only reasonable but advisable for extensive projects to trigger structural upgrades. These positions have informed the Subcommittee’s work on the Prescriptive and Work Area methods, whose structural provisions and upgrade triggers will be practically identical with the 2018 edition. The Subcommittee sees no reason why the Performance method should be so different. The Subcommittee will rely on these positions, and revisit them as needed, when developing proposals for the next cycle and working with jurisdictions adopting the 2015 code.

What should engineers do? Until the Performance method can be clarified in the next code cycle, the NCSEA Existing Buildings Subcommittee’s recommendation is to: • Know your local code. If the jurisdictions you work in allow the Performance method, know what cutoff dates they have selected and how they interpret the two key sections discussed above. • Advise and educate your staff, your code official colleagues, and your clients about the options presented by the IEBC, and how some of them might lead to unexpected results depending on how they are interpreted. • Send comments and questions to your SEA delegates to NCSEA’s Existing Buildings Subcommittee, or to the author of this article.▪ David Bonowitz, S.E. chairs the Existing Buildings Subcommittee of NCSEA’s Code Advisory Committee. Contact him at dbonowitz@att.net. Committee members representing NCSEA Member Organizations are listed at www.ncsea.com/committees/ existingbuildings.

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business issues

Business Practices

Advice for the First-Time (or Experienced) Manager By Jennifer Anderson

ongratulations – you landed your first role in leadership! Welcome to the beginning of significant personal and professional growth. To help you navigate the immense learning curve, here are the top 12 areas that all managers need to navigate to achieve success as a leader. For those who have been in a management role for years, this article provides helpful reminders of how to be a great manager. All managers – experienced and new – should read on and apply this information in their daily roles. 1) Keep Learning – Even though you have moved up the ladder into management, that does not mean that you should stop learning your trade. Continue to find ways to blend learning into your work. Attend conferences (such as the upcoming NCSEA Summit in D.C. in October), read trade publications and magazines, and follow subject matter experts on social media. You will also want to include time for learning how to be a manager. There are more books on management than you can read in a lifetime; just ask your favorite manager what leadership books they enjoy and start there. 2) Communication Skills – If you do not know how to communicate effectively, you will never succeed as a manager. The key to communicating is first to understand the other person’s perspective; you should ask questions and seek to understand their point of view. Listening skills are critical in all areas of life, and now, as a manager, it is critical that you learn how to listen well. 3) Personal Brand – Your personal brand is the mark you want to make or the “legacy” that you want to create. Start with answering the question: “When you get to the end of your life, what do you want to be known for?” The answer will help you to know how you want to interact with people today. As a manager, you will impact people – it is important that you are clear about how you want to influence them. 4) Self-care – You are busy, who isn’t these days?! Find ways to incorporate exercise, hobbies, and fun time into your life. Anyone who has had health concerns will quickly tell you that if you do not have good health, you have nothing at all. New managers must find time to take care of their health – mentally, physically, and emotionally. 5) Time Management – Learning how to manage your calendar will be vitally important

to long-term success as a leader. The best way is to set aside time each Friday to review your week. Look for things that you could have deleted, delegated, and deferred. For example, start with an evaluation of your meetings. Were the meetings effective? Were the right people in the meetings? Did the meetings even need to happen? Maybe there are meetings that could have been canceled, freeing up an hour to work on building relationships with your new direct reports instead. 6) Mentor – Today you may not be able to fathom how important a mentor will be. Ask anyone who is a good manager about what has made the difference in their career, and they will all tell you how someone else helped them in their career. Having a mentor that you can go to for advice and help is invaluable. At some point, you will have the opportunity to mentor others as well. 7) Interview Skills – Work with your HR team to understand how they want you to interview for the company; they will help you to avoid getting sued for asking the wrong kinds of questions. But, as an individual, look for unique ways to personally connect with applicants. Recruits are not just evaluating the firm; they are evaluating you as their potential new boss. Help them get to know you, and you will get to know them better too. 8) Hiring and Firing – As with interview skills, work closely with your HR representatives to ensure that you are following legal guidelines for hiring and firing employees. Learning how to navigate hiring of an employee is not just an HR function. In fact, during the onboarding process, it’s vitally important that you are involved in welcoming the new hire effectively. They will be working with you more than anyone else, so get involved in the process. Getting comfortable with firing people takes a long time. Hopefully, as you get better at developing relationships with your direct reports, you will find firing to be at a minimum in your tenure as a leader. 9) Online Reputation – Social media is here to stay, folks. If you do not embrace it, you will become antiquated as a professional. I am not suggesting that you spend hours each day on Twitter or Facebook unless that makes sense for your firm’s marketing and recruiting efforts. I do recommend that you find time each week to use LinkedIn to connect with other professionals, join in group conversations, and read articles. LinkedIn is the best

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September 2017

place for engineers to spend their social media time, so make time for it. 10) In-person Networking – Attending national conferences is well worth your time for gaining CE credits, but make time for connecting with professionals in your local market too. It’s remarkable how regular networking can help you meet interesting and helpful people in your immediate community. (For more helpful details on networking, refer to the June 2017 STRUCTURE magazine article, Effective Networking: 4 Techniques to Successfully Navigate Networking.) 11) Relationship Building – You will go farther and accomplish more if you go with people. Develop relationships with your boss and with direct reports. Take time for oneon-one meetings to really get to know people. Getting out of the office for lunch is a great way to build relationships, just do not go as a group. A group setting leads to playful banter, whereas one-on-one time will help you to get to know the people with whom you work. Knowing people leads to respect and trust, which in turn leads to the critical foundation of a strong team. Make sure to have regular meetings also, because one lunch will not lead to strong relationships. 12) Service Mindset – Approaching interactions with a service mindset with peers, your boss, direct reports – not to mention interactions with vendors, partners, and customers – will help you to have successful relationships and realize that your role as a leader is not about you; leadership is about helping others to be successful. Take the opportunity to focus on how you can help others. Do not do their work but look for ways to be supportive and encouraging. The number one piece of advice for a new or experienced manager is to have fun being a leader! Performing as a good manager impacts both personal and professional aspects of one’s life, and makes a difference in the lives of team members and other business associates. Keep these 12 suggestions in mind, and you will be well on your way to becoming (or staying) a solid manager.▪ Born into a family of engineers but focusing on the people side of engineering, Jennifer Anderson (www.CareerCoachJen.com) has nearly 20 years helping companies hire and retain the right talent. She may be reached at jen@careercoachjen.com.

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discussion of legal issues of interest to structural engineers

LegaL PersPectives

A Final Look at Consent to Assignment Agreements By Gail S. Kelley, P.E., Esq.

n engineer’s Consent to Assignment, also referred to as an Acknowledgement and Consent, is usually drafted by the bank providing construction financing for a project. A typical consent requires the engineer to agree that the bank can exercise the rights it has acquired under an assignment from the borrower (the owner); among these rights will be the right to assume the design agreement if the borrower defaults on its loan. Articles in the June, July and August editions of STRUCTURE discussed some of the concerns with respect to consent agreements, specifically whether the lender is required to pay outstanding amounts due to the engineer, whether the lender has the right to use the plans and specifications if it does not assume the design agreement, and to what information or certifications the lender is entitled. This article takes a final look at some of the provisions commonly found in consent agreements.

Approval of Modifications and Changes Often the consent will state that no modifications or changes can be made to the design agreement without the lender’s consent. This is not unreasonable, as the assignment gives the lender an interest in the design agreement. Some consents also restrict the engineer’s right to perform work on change orders, using provisions such as: Engineer shall not perform work pursuant to any change order without first securing Lender’s written consent to such change order unless the cost of any single change does not

exceed _____, and the aggregate amount of all changes does not exceed _____. Generally, the dollar amounts specified are fairly high, so the minor change orders that often occur towards the end of the design process are not affected. However, if the engineer is just beginning the design and there are likely to be significant changes, a provision like this can affect the engineer’s ability to meet the owner’s deadlines. In such cases, it is advisable to discuss the provision with the owner. One option would be to edit the provision to include wording such as: If Engineer has not received a response to a properly provided notice of a change order within ten (10) days of such notice, Engineer shall be entitled to proceed with such work. This requires the lender to act promptly if it receives notification of a change order. Nevertheless, the engineer must remember to provide notice to the lender. Alternatively, the engineer can require the owner to obtain the lender’s consent. The following wording could be used in place of the original provision: Engineer shall not perform work pursuant to any change order unless Borrower has certified to the Engineer that it has received consent from the Lender.

Notice to Lender of Termination

Most consents will prohibit the engineer from terminating the design agreement for default without giving the lender notice and an Demos at www.struware.com opportunity to cure Wind, Seismic, Snow, etc. Struware’s Code Search program calculates these and the default, which other loadings for all codes based on the IBC or ASCE7 in just minutes (see online in almost all cases video). Also calculates wind loads on rooftop equipment, signs, walls, chimneys, will be the owner's trussed towers, tanks and more. ($195.00). failure to pay the CMU or Tilt-up Concrete Walls Analyze solid walls for out of plane loading and engineer. While panel legs next to or between openings by automatically calculating loads to the wall this is a reasonable leg from vertical and horizontal loads at the opening. ($75.00 ea) provision, often the Floor Vibration Program to analyze floors with steel beams and/or steel joist. consent will give the Compare up to 4 systems side by side ($75.00). lender an unreasonConcrete beam/slab Program to provide bending, shear and/or torsional reinforcing. Quick and easy to use ($45.00). ably long time to cure the default. STRUCTURE magazine

September 2017

As a general rule, the engineer should not be required to allow the lender any more time to cure the default than the owner is allowed under the design agreement. The following is reasonable wording and timing for a termination provision: If Borrower defaults under the Design Agreement, Engineer shall not exercise any remedies, including any right to terminate the contract, unless Engineer gives notice to Lender, and Lender fails to either (i) remedy the default within thirty (30) days after receipt of the notice or such longer period as is allowed to Borrower under the Design Agreement; or (ii) deliver to Engineer within such thirty (30) day period an agreement to remedy such default at Lender’s expense and, thereafter diligently pursue such remedy. If the engineer wants to continue working on the project, the engineer may choose to suspend performance rather than enforcing its right to terminate, as this will allow the entity that takes the project over to assume the design agreement. Nevertheless, having the right to terminate can be an advantage in negotiations.

The Consideration Clause One of the basic principles of contract law is that a contract is generally not enforceable in court unless both parties receive something of legal value, referred to as consideration. Thus, a court will typically not enforce a contract for a gift or services of an uncompensated worker. In most contracts, the consideration is a promise from each party. For example, under a design agreement, the owner receives a promise of the engineer’s services; the engineer receives a promise of compensation. However, under an engineer’s

Consent to Assignment, the lender receives a promise that the engineer will continue to provide its services if the owner defaults on its loan, but the engineer receives nothing in return. In fact, many consents explicitly state that engineer is not a beneficiary of the assignment and the lender has no obligations to the engineer unless, and until, it assumes the design agreement. To avoid a challenge that the consent is unenforceable, lenders will sometimes include a clause stating that the engineer has received consideration. Standard wording for the clause is: This Consent is given by Engineer for good and valuable consideration, the receipt and sufficiency of which is hereby acknowledged by Engineer. This is a so-called “legal fiction” often used in contracts where one of the parties is not actually receiving anything of legal value; the parties agree that consideration has been provided so that the contract will be enforceable. If a party to a contract agrees it has received consideration, a court will generally take the statement as true; it will not ask what the consideration was, or whether it was in fact received. In some cases, the Recitals at the beginning of the consent may state:

Engineer will benefit if the Loan is made to Borrower, as a portion of the proceeds of the Loan will be used to pay Project costs, including amounts due and payable to Engineer under the Contract. Whether or not the Recitals include such a statement, the consent will invariably include a disclaimer such as: Nothing herein shall be construed to confer any present benefits on Engineer or to create any duty upon Lender to see to the application of the proceeds of the Loan or to give any notice of any type to Engineer. In other words, while the proceeds of the loan might be used to pay the engineer, the lender does not assume any responsibility for the payment.

from an engineer, the main purpose of the consent is to guarantee that the party has the right to assume the design agreement if the owner defaults on its agreement with that party. Consent to Assignments should always be read carefully, as they often contain provisions completely unrelated to the design agreement. Engineers who are not careful may find that, by signing the consent, they have given up valuable rights or agreed to unreasonable obligations.▪ Disclaimer: The information in this article is for educational purposes only and is not legal advice. Readers should not act or refrain from acting based on this article without seeking appropriate legal or other professional advice as to their particular circumstances.

Conclusion

Gail S. Kelley is a LEED AP as well as a professional engineer and licensed attorney in Maryland and the District of Columbia. Her practice focuses on reviewing and negotiating design agreements for architects and engineers. She is the author of “Construction Law: An Introduction for Engineers, Architects, and Contractors,” published by Wiley & Sons. Ms. Kelley can be reached at Gail.Kelley.Esq@gmail.com.

Although Consent to Assignments are most often used by banks providing construction financing, they are sometimes used when a development is being done pursuant to a ground lease. In such cases, the owner of the land may want the right to take over the agreements for the project if the developer defaults. Regardless of the party requesting a Consent to Assignment ADVERTISEMENT–For Advertiser Information, visit www.STRUCTUREmag.org

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issues affecting the structural engineering profession

Professional issues

SB 496 and Design Professional Indemnities in California Not a Free Pass, but a Major Step Forward By Mike Olson and Brett Stewart

alifornia Governor Jerry Brown recently signed into law Senate Bill 496 (SB 496) representing a major step forward in reducing the uninsurable burden of indemnity provisions and the duty to defend for most public and private contracts signed by design professionals in California. SB 496, which amends California Civil Code Section 2782.8, is the result of several years’ effort spearheaded by ACEC California, including direct support by the authors of this article whose companies focus exclusively on providing insurance and risk management solutions for design professionals The genesis of SB 496 was the watershed 2010 California Court of Appeals case, UDCUniversal Development v. CH2M Hill, which held that an engineer who agrees to contractually defend its client from a third-party lawsuit owes an immediate defense obligation – even if the engineer was ultimately found not to be negligent. The case expanded upon the 2008 Crawford v. Weather Shield Manufacturing decision rendered by the California Supreme Court, which held that a contractual duty to defend arises immediately when a claim is made for contractual indemnity. In 2010, much was publicized about the landmark case, bringing further attention to the increased risk in contractual indemnity clauses. Aside from representing a significant risk for design professionals, indemnity provisions in design professional agreements have created insurability issues. The duty to defend a third-party is uninsurable, as are most obligations not founded upon negligence. After the 2010 decision, ACEC California and other industry groups, including the Structural Engineers Association of California (SEAOC), embarked on a mission to address the indemnity laws in California. Both SB 972 (2010) and SB 885 (2016) represented mixed efforts on behalf of these industry groups to enact change in California. Senate Bill 496 built upon the success of these earlier bills, culminating in the successful passage in 2017. The resulting legislation is good news for design professionals, as it limits the contractual duty to defend to the comparative fault of the design professional. For contracts entered into on or after January 1, 2018, SB 496 amends California Civil

Code Section 2782.8 and contains the following key provisions: 1) Civil Code Section 2782.8 now applies to all contracts (except those involving State of California entities). Previously, it only applied to public contracts. Under the amended code section, indemnification clauses in both public and private contracts are unenforceable except to the extent they arise from, pertain to, or relate to the negligence, recklessness, or willful misconduct of the design professional. 2) Design professionals can no longer be obligated to pay an indemnitees’ defense costs beyond the design professional’s proportionate share of fault. However, design professionals may still be legally responsible to pay for the up-front duty to defend an indemnitee. 3) The restrictions on the duty to defend do not apply to members of a designbuild joint venture. Under California law, members of a joint venture are jointly and severally liable for the acts and omissions of the other members. Contractor industry groups understandably wanted to avoid situations where SB 496 shifted all of the risk to them. 4) The duty to defend provisions of SB 496 also do not apply where there is a project-specific general liability insurance policy, such as an Owner Controlled Insurance Program (OCIP) that insures all project participants for general liability exposures on a primary basis and also covers all design professionals for their legal liability arising out of their professional services on a primary basis. 5) And, under the newly enacted SB 496, if multiple parties owe a contractual defense obligation, and one of those parties is bankrupt or dissolved, then the design professional is obligated to meet and confer with the other parties regarding unpaid defense costs. What is the practical impact for design professionals? Insurability of contractual indemnification clauses has been a critical risk management issue for design professionals. Civil Code Section 2782.8 now applies

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September 2017

to all contracts (except for State of California entities), and indemnification obligations must be founded upon negligence, an essential element triggering professional liability insurance coverage. Some professional liability insurers provide coverage for reimbursement of an indemnitee’s reasonable defense costs to the extent caused by the design professional’s negligence as determined by a court of competent jurisdiction. The authors of this article are not aware of any professional liability policy which will insure the immediate duty to defend a third party. As noted, SB 496 does not address the upfront duty to defend demands and therefore needs to be the subject of careful negotiation. We are hopeful that, with this new law, contract negotiations will be more productive relative to this issue. The best negotiation result would be a contractual declaration that the design professional has no obligation to defend a third party. A fallback position could be contract language clarifying a design professional’s defense obligation that applies only “after the fact.” For example: Consultant has no obligation to pay for any of the indemnitees’ defense related costs prior to a final determination of liability or to pay any amount that exceeds Consultant’s finally determined percentage of liability based upon the comparative fault of Consultant. SB 496 is a big step in securing a fair allocation of risk between design professionals and their clients. While other states have enacted more stringent anti-indemnity legislation, California has long remained one of the most difficult contracting environments for design professionals. Design professionals are pleased with this outcome and are optimistic that the passage of SB 496 will ultimately reduce design professionals’ future uninsurable risk.▪ Mike Olson is a Vice President at Dealey, Renton & Associates, Inc., an insurance agency serving the interests of design professionals. Mike can be reached at molson@dealeyrenton.com. Brett Stewart is a Risk Manager in the Design Professional Unit of XL Catlin, a specialty provider of professional liability insurance for design professionals. Brett can be reached at brett.stewart@xlcatlin.com.

CASE BuSinESS PrACtiCES

business issues

The Good and the Bad with Delegated Design By Kevin H. Chamberlain, P.E., SECB, CASE Guidelines Committee

tructural engineers of record (SER) are always under some external pressure on projects. Is there enough fee? Is there enough time to complete the work? Is the project becoming more difficult and complex because of bad decisions made without SER input? Are the Owner’s expectations unrealistic? Is there scope creep because other team members are passing the buck? Delegated design – deferring the structural design of certain components or systems on a project to a specialty structural engineer (SSE) hired by the contractor – fills a role in today’s complex projects that have tight schedules and aggressive budgets. There are plenty of good reasons why it might make sense to delegate the design but, regardless of the motivation, it is important to be aware that delegated design is not a panacea. Before deciding to delegate a design, consider the pros and cons, the good and the bad.

The Good

The Bad Most contractors are good people with reputable business practices. But they typically work for a fixed price and are motivated to shop around for sub-contractors to get the best pricing. If the lowest price subcontractor uses an ethically-challenged engineer who is

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There are structural engineers who are expert in specialized aspects of the profession, so why not tap that resource? For example, consider the office known for expertise in the design of timber frame structures using traditional joinery with wood pegs. Bringing a true expert onto the team can improve the whole project.

If the structural system to be used is commonly something the contractor will want to design, why not let him? Why waste precious time and resources on a design that is just a starting point for someone else? Sometimes it does make sense to lay down ground rules (design criteria and boundary conditions) and then get out of the way. It probably also does not make much sense for the SER to engineer the design for a proprietary product when the manufacturer provides a design anyway, even when the SER does not ask for one. Structural components or systems for which the design is commonly delegated to an SSE include: • Metal-plate connected wood trusses • Light gage metal framing and trusses • Precast/prestressed concrete elements • Joist girders • Steel connections • Heavy timber connections • Modular retaining wall systems

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September 2017

willing to “stamp” anything, the SER may wind up with a worthless submittal that does not work or has not been thoroughly developed. Then the design becomes the SER’s problem. Some SERs will delegate the design of a system because they do not understand it, or have never designed it before. But what if this delegated portion of the project is most of the project? The SER ought to ask himself if he is the right structural engineer for that project. Sometimes it is best just to step aside. If delegating a design is a business practice undertaken with the thought of increasing profit and minimizing risk, do not kid yourself. The ultimate responsibility for the safety and adequacy of the design of a structure rests with the SER. Coordination of the delegated design with the overall project remains the SER’s responsibility. Adding a specialty structural engineer or two to the project may not reduce risk, it may increase it. As for cost, how much time will it take to review, coordinate, and possibly correct the delegated design? If the SER can design it, the SER probably should.

Gray Areas What is the right amount of information to show on a set of structural drawings for a design which will be delegated to an SSE? Not all structural engineers provide the same amount of information for the SSE to work from. Best practice will include design criteria and loads, primary structural system design, and relevant notes, specifications, or design standards to be used in the design of the delegated item so that the SSE can figure out the work required. Also, the SER needs to prepare or edit the relevant specification sections carefully. Do not leave that to the architect. Sometimes there is no structural engineer of record on the project. That may seem like an uncommon occurrence, but it happens. For example, pre-engineered metal buildings are typically designed by the manufacturer’s engineer, who is probably located out of state and may not have professional liability insurance. Or, perhaps it is a design-build project with no SER and a hodge-podge of isolated SSEs. Who is providing the structural design criteria and ensuring the overall stability of the

building? Probably no one. If asked to be an SSE for only a piece of the project, while the rest of the project has no SER, think twice. Even if the SSE’s work is perfect, but the rest of the project goes awry, the SSE may be left with the lion’s share of liability for a mouse’s share of the fees. Is the SER really asking for a delegated design? It is not unusual for an SSE to receive a set of structural drawings for a project with every single piece, nut, and bolt sized and drawn by the SER. Nothing left to design, so no SSE is needed, right? Not so fast. Often the specifications or drawing notes, or the construction manager’s bid instructions, will still require a delegated design by an SSE. Sometimes it is a simple oversight, as when default text in a standard document did not get deleted. Other times it is a conscious decision by the SER to use the SSE as a second set of eyes, or again under the misguided pretense of shedding

liability. What should an SSE do in this case? The SSE’s first reaction ought to be to tell the client (usually a subcontractor) that a delegated design is not needed and ask for clarification. If the answer comes back that a delegated design is indeed required, other questions may arise. What does the SSE do if the SER’s design does not work? Is the contractor entitled to a change order if something gets bigger, heavier, or more laborious? Has the Owner now just paid twice for the same design work?

Communication, communication, communication. Protocols should go out the window here, and the SER and SSE need to communicate directly. Picking up the phone to straighten out an issue can save weeks of back and forth. Keep the team apprised of developments.▪ Kevin Chamberlain is CEO and Principal of DeStefano & Chamberlain, Inc. in Fairfield, CT, and serves on the CASE Guidelines Committee. Kevin can be reached at kevinc@dcstructural.com.

Five Takeaways for the SER:

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• Delegating design does not necessarily reduce liability and risk. • Only delegate design work when it makes sense to do so. • Think like the SSE and provide the required project information. • Do not let other team members delegate aspects of the structural design. • Communicate directly with the SSE during the course of the work.

Five Takeaways for the SSE: • Learn enough about the project to understand the risk. Charge accordingly. • Get a signed contract in advance, and a retainer if appropriate. • Be prepared to deliver on deliverables (sealed calculations, connection details, etc.) • Speak up about unreasonable requests. • Communicate directly with the SER during the course of the work.

For Further Reading CASE Document 962B – National Practice Guidelines for Specialty Structural Engineers STRUCTURE magazine

September 2017

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Phone: 800-423-8140 Email: info@halfenusa.com Web: www.halfenusa.com Product: HZA Anchor Channels Description: Ideal for seismic conditions as they provide load resistance in tension as well as transverse and longitudinal shear. HZA Anchor Channels are an ideal cast-in solution to replace post installed anchors and weld plates. Available in stainless steel and hot dip galvanized.

Kelken Construction Systems

Product: HTA Anchor Channels Description: An ideal cast-in solution to replace post installed anchors and weld plates. HTA Anchor Channels are ICC Approved and provide a high level of adjustability to accommodate large concrete tolerances. Load resistance provided in tension and shear. Available in stainless steel and hot dip galvanized.

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IN PRINT

STRUCTURE magazine

September 2017

Submit your free listings for this year’s issue!

Visit www.STRUCTUREmag.org and click on the Resources tab.

Anchor Bolts, Concrete, Façade, Geotechnical, Masonry, Post-Tensioning, Reinforcing and Utility Anchors, and General Hardware & Ties

Anchoring guide

RISA Technologies

Simpson Strong-Tie

Trimble

Phone: 949-951-5815 Email: info@risa.com Web: risa.com Product: RISAConnection Description: Get baseplate and anchor design for RISA-3D and RISAFloor models using RISAConnection. Column base reactions, including biaxial moments, are sent for each load combination to RISAConnection. More accurate than traditional methods of designing for envelope reactions. With RISA-3D and RISAFloor integration, RISAConnection is the new standard for baseplate design.

Phone: 800-925-5099 Email: web@strongtie.com Web: www.strongtie.com Product: Strong-Bolt® 2 Wedge Anchor: Type 304 SS Description: The Strong-Bolt 2 is designed for use in cracked and uncracked concrete, as well as uncracked masonry applications. It is now available in Type 304 stainless steel for all standard sizes. The additional product offerings make it easier for you to choose the best material solution for your job.

Phone: 770-426-5105 Email: kristine.plemmons@trimble.com Web: www.tekla.com Product: Tekla Tedds Description: Automating your everyday structural designs, Tedds’ broad library includes anchor bolt design per ACI 318 Appendix D. The calculation includes comprehensive checks for tensile and shear failure of anchors and is available as part of a free trial at the website.

Product: Stainless-Steel Titen HD® Heavy-Duty Screw Anchor Description: The new stainless-steel Titen HD screw anchor can now be installed in exterior and corrosive environments. Its innovative design effectively cuts the concrete while significantly reducing carbon steel in the anchor to maximize corrosion resistance.

Standards Design Group, Inc

Product: Tekla Structural Designer Description: Revolutionary software that gives engineers the power to analyze and design steel and concrete buildings efficiently and profitably. Physical, information-rich models contain all intelligence needed to fully automate the design and document your project, including all end force reactions communicated with two-way BIM integration, comprehensive reports and drawings.

Phone: 806-792-5086 Email: info@standardsdesign.com Web: www.standardsdesign.com Product: Wind Loads on Structures 4 Description: Performs computations and computes wind loads by analytical method, provides basic wind speeds from a built-in version of the wind speed, allows the user to enter wind speed.

Product: Tekla Structures Description: Tekla is an Open BIM modeling software that can model all types of anchors required to create a 100% constructible 3D model. Anchors can either be created inside the software or imported directly from vendors that have 3D CAD files of their products.

S-FRAME Software Phone: 604-273-7737 Email: info@sframe.com Web: s-frame.com Product: S-FOUNDATION 2017 Description: A complete foundation management solution. Use as standalone application or integrated within S-FRAME Analysis. Easily import support data from any 3rd party analysis program. Automatically generates and manages the underlying model as you optimize your design. Includes advanced customization and integration features.

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STRUCTURE magazine

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September 2017

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award winners and outstanding projects

Spotlight

Protecting a Landmark

The War Memorial Veterans Building

By Stephen. K. Harris, P.E., S.E. and Benjamin A. Mohr, P.E., S.E. Simpson Gumpertz & Heger Inc was an Award Winner for its War Memorial Veterans Building project in the 2016 NCSEA Annual Excellence in Structural Engineering Awards Program in the Category – Renovation/Rehabilitation.

he War Memorial Veterans Building was built in 1932 and is one of the last major American buildings constructed in the Beaux-Arts Style. It stands across the street from San Francisco City Hall and is a designated historic landmark. It houses various City offices, spaces for veterans, and several performance and event rooms. The centerpiece of the building is the Herbst Theater. The walls of the theater incorporate eight 25-foot high murals, which were originally created by the renowned artist Frank Brangwyn for the 1915 San Francisco International Exposition. In 1945, President Truman signed the United Nations Charter on the stage of the Herbst Theater. Despite its illustrious history, however, the Veterans Building suffered from neglect for many years. By 2011, the building was plagued by extensive roof leaks, inoperable elevators, and inadequate building systems. Also, the building had significant seismic deficiencies, which constituted a life safety hazard in a major earthquake. For these reasons, the City and County of San Francisco initiated a $156 million project, driven by the need for a seismic upgrade and incorporating complete replacement of building systems. Simpson Gumpertz & Heger (SGH) acted as the prime consultant for the project, as well as structural engineer of record and waterproofing consultant. The original structure comprises a steel frame, encased in concrete, with reinforced concrete walls and slabs. It is clad in terra cotta and granite panels. The original lateral system was weak, torsional, and discontinuous. It was unable to resist the seismic demands created by the building’s significant mass and its proximity to the San Andreas Fault. Also, the finishes – both interior and exterior – are ornate and brittle, capable of sustaining only small lateral drifts. Early on, the project team decided on shotcrete shear walls as the seismic upgrade solution. Shotcrete shear walls have two distinct advantages: first, their stiffness was compatible with the concrete walls in the existing structure. Second, they could be placed strategically

throughout the building to provide acoustic isolation between performance spaces and administrative offices. Unfortunately, these shear walls also come with disadvantages. Construction of concrete walls introduces moisture, heat, and heavy equipment in the building, all of which put the building’s historic finishes at risk. Specifically, new concrete walls were placed in the wall cavity directly behind the Brangwyn murals. This required waterproof formwork for the shotcrete walls, as well as ventilation and monitoring of the space between the formwork and the murals. The walls are constructed as infill within the existing steel frames. This naturally increases the flexural strength of the walls, causing their behavior to be governed by brittle shear failure. A traditional, fixed-base linear-static analysis resulted in excessively thick shear walls and very large foundation hold-down forces at the wall ends. Clearly, the project required a more innovative approach. A system of rocking concrete shear walls was implemented to improve the building’s seismic performance while maintaining reasonable wall proportions. In this way, the need for deep foundations was eliminated and the maximum shear force imparted to each wall was limited. In order to allow the walls to rock but still transfer shear to the foundation, SGH designed a “shear lug” connection at the base of each wall. The shear lugs are constructed of steel pipes cast into the new walls and grouted into a greased sleeve in the existing foundation – they resist lateral movement but do not resist uplift. A total of 250 shear lugs were incorporated at the bases of the shear walls. The design was validated with nonlinear response-history analyses in both SAP and CSI Perform. The final thickness of the walls was 10 inches, with modest reinforcing and no hold-down requirements at the wall ends. Heavy steel coupling beams were designed to connect the walls across the building’s main corridors, while maintaining the shape of the vaulted ceilings, to control the rocking

STRUCTURE magazine

September 2017

behavior of the shear walls. A system of extensive steel bracing in the building’s attic space was also designed, essentially providing a supplemental horizontal diaphragm where none existed before. All this allowed SGH to limit the expected drifts in the Maximum Considered Earthquake to less than 0.5%. The final design meets the Enhanced Rehabilitation requirements of ASCE 41 – equivalent to a new Risk Category III structure designed per ASCE 7. The design also included a new 70-foot span truss to allow removal of two columns at the Herbst stage, a new gridiron above the stage, new steel catwalks throughout the attic, and seismic bracing of historic hollow-clay-tile partitions and heavy plaster ceilings. After a two-year construction effort, the building is now open and hosting performances again. The project is on track to achieve LEED Gold certification and has received awards from the American Public Works Association, SEAOC, the American Institute of Architects, the California Preservation Foundation, NCSEA, and ASCE. The completed project is a great source of pride for the entire project team.▪ Stephen K. Harris is a Principal with Simpson Gumpertz & Heger in San Francisco. He can be reached at skharris@sgh.com. Benjamin A. Mohr is a Project Engineer at Mar Structural Design. He is a past chair of the Seismology Committee for the Structural Engineers Association of California. In 2017, Benjamin was awarded the Ed Zacher Award for Outstanding Service to the Structural Engineers Association of Northern California. He can be reached at ben. mohr@marstructuraldesign.com.

NCSEA to Host Largest Trade Show in Summit History! Denotes NCSEA membership

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American Welding Society

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American Society of Civil Engineers

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News form the National Council of Structural Engineers Associations

This year’s Structural Engineering Summit marks 25 years of bringing together structural engineers, and will also host NCSEA’s largest Trade Show to date. The 2017 Summit Trade Show will include over 55 exhibitors!

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CoreBrace

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Prime loca�on available

Available

CTP Anchors DACS Inc. DeWalt/Powers

Visit www.ncsea.com for more information on exhibitors and exhibit opportunities.

Dlubal Software, Inc

MiTek USA/MiTek Builder Products

EM-BOLT

New Millennium Building Systems

Euclid Chemical Company

Nucor

Fabreeka International

Performance Structural Concrete Solutions

Freyssinet, Inc.

Piekko USA

Fyfe Co./Fibrwrap Construction

RISA TECHNOLOGIES

Geopier Foundation

SCIA a Nemetschek Company

Giza Steel

SidePlate Systems, Inc

Graitec, Inc.

Simpson Strong Tie

Hayward Baker

Stabil-Loc

Headed Reinforcement

Steel Deck Institue

Hexagon

Steel Joist Institute

Hilti

Steel Tube Institute

ICC-ES

Strand7

LafargeHolcim

Structural Engineering Engagement & Equity (SE3) Comm.

Lindapter

Trimble

Meadow Burke LLC

USG

Menard USA

Valmont

MIDASoft

Vector Corrosion STRUCTURE magazine

September 2017

Current as of 8/17/2017

Since 2012, NCSEA has awarded Scholarships to Young Members to attend the NCSEA Structural Engineering Summit. For the second year, NCSEA was able to award a total of eight scholarships. Congratulations to the young members listed below! Visit the Awards tab on www.ncsea.com to read the essays that won them their spot at the 2017 Structural Engineering Summit.

James Foreman

Isabella Horton

Lori Koch

Eric McElrath

A Project Engineer with Martin/Martin, Lakewood, Colorado, James is a member of the Structural Engineers Association of Colorado.

A Structural Designer at FSB, Oklahoma City, Oklahoma, Isabella is a member of the Oklahoma Structural Engineers Association.

The Manager of Educational Outreach for the American Wood Council, Leesburg, Virginia, Lori is a member of the Structural Engineers Association of Virginia.

An Engineer-in-Training with Mattson Macdonald Young, Inc., Minneapolis, Minnesota, Eric is a member of the Minnesota Structural Engineers Association.

Kyle Palagi

Mary Shinners

Rajesh Vuddandam

A Structural Engineer with TD&H Engineering, Great Falls, Montana, Kyle is a member of the Structural Engineers Association of Montana.

An Associate III with Sargent and Lundy, LLC, Chicago, Illinois, David is a member of the Structural Engineers Association of Illinois.

An Engineer-in-Training with PES Structural Engineers, Atlanta, Georgia, Mary is a member of the Structural Engineers Association of Georgia.

An Assistant Professor at Tarleton State University, Stephenville, Texas, Rajesh is a member of the Structural Engineers Association of Texas.

Young Member Group of the Year Finalists Announced Each year, NCSEA recognizes an outstanding Young Member Group from one state SEA at the Structural Engineering Summit. The emphasis of this award is to recognize Young Member Groups that are providing a benefit to their young members, member organization, and communities. Eleven YMGs applied for the award this year and five finalists have been chosen to attend the 2017 Summit: Colorado (SEAC), Georgia (SEAOG), Metro Washington DC (SEA-MW), Minnesota (MNSEA), and Oklahoma (OSEA). Each of the five groups will send a member to represent their YMG during the Summit. The final winner will be announced during the Young Member Reception on Wednesday, October 11th. Visit www.ncsea.com to view their applications.

This year’s Summit benefits Young Engineers like never before! Each year NCSEA offers discounted registration for young engineers, special activities including a Young Engineers’ Reception, and resources. For the 2017 Summit, an entire track dedicated to Young Engineer education and growth has been included! Learn more at www.ncsea.com.

NCSEA Webinars September 12, 2017 ASCE-7-16 Wind: How it Affects the Practicing Engineer Donald R. Scott, S.E., F.SEI, F.ASCE October 24, 2017 Understanding and Interpreting Geotechnical Reports Trent Parkhill, P.E. Visit www.ncsea.com for full descriptions and registration. Courses award 1.5 hours of continuing education after the completion of a quiz. Webinars run at 10:00 am Pacific, 11:00 am Mountain, 12:00 pm Central, and 1:00 pm Eastern. Diamond Review approved in all 50 States.

STRUCTURE magazine

September 2017

News from the National Council of Structural Engineers Associations

David Nauheimer

NCSEA News

Young Members Awarded Scholarships to Summit

The Newsletter of the Structural Engineering Institute of ASCE

Structural Columns

Save the Date

Exhibits & Sponsorships

The Premier Event for Structural Engineering to be held in Fort Worth, Texas, April 19 – 21, 2018. The technical program is being finalized and will include new session and plenary formats. Visit the Congress website, www.structurescongress.org for more information.

Increase your company’s visibility and reach hundreds of industry professionals at this premier event for structural engineers. Contact Sean Scully at sscully@asce.org or 703-295-6154, for exhibiting and sponsorship opportunities.

New SEI Program for Young Professionals SEI is currently seeking young professionals interested in serving on SEI standards committees to apply to participate in the SEI CSAD – YP program. (http://bit.ly/2hCkiSh). The SEI Board of Governors has approved a new program to fund young professionals to participate in SEI standards committees who apply for and are accepted to the specific positions on standard committees such as secretary, balloteer, or historian. Currently,

ASCE 7-22 is seeking YP applicants to participate in this new program. If you are interested in applying for the committee, please submit an application via the online form by selecting SEI from Institute drop down and then Minimum Design Loads (ASCE/SEI7). Carefully indicate the committee for which you are applying (main or sub). Contact Jennifer Goupil at jgoupil@asce.org or 571-421-3998 with any questions.

New Code Cycle for ASCE/SEI 7 The ASCE/SEI 7 Committee Leadership will be presenting a special panel session on How to Improve ASCE 7 at the 2017 NCSEA Structural Engineering Summit on October 12, 2017, at the Washington Hilton, DC. Attend the morning session and learn about the new cycle of ASCE/SEI 7. If you are interested in applying for the new ASCE/SEI 7-22 committee, please submit an application at http://bit.ly/2flOEYo by September 30, 2017; on the online form select SEI from

Institute drop down and then Minimum Design Loads (ASCE/ SEI 7). Carefully indicate the category of membership for which you are applying (Voting or Associate) for each of the main or subcommittees and if you are applying for the Young Professional program. Associate members can be accepted until balloting begins. Eligible regulatory members can qualify for travel reimbursement per ASCE Travel Policy. Contact Jennifer Goupil at jgoupil@asce.org or 571-421-3998 with questions.

2018 Ammann Fellowship Call for Nominations The O. H. Ammann Research Fellowship in Structural Engineering is awarded annually to a member or members of ASCE or SEI to encourage the creation of new knowledge in the field of structural design and construction. All members or applicants for membership are eligible. Applicants will submit a description of their research, an essay about why they chose to become a structural engineer and their academic transcripts.

The deadline for 2018 Ammann applications is November 1, 2017. For more information and to fill out the online application visit the SEI website at www.asce.org/structural-engineering/ ammann-research-fellowship.

ASCE Week – Las Vegas

SEI Collaboration

There is still time to register for ASCE Week, September 24 – 29, 2017, at the Green Valley Ranch Resort Spa & Casino in Las Vegas and earn up to 42 PDHs in one week. Register now at www.asce.org/event/2017/asce-week.

The SEI Technical Committee on Advances in Information Technology, in collaboration with National Academy of Sciences, Engineering, and Medicine, is hosting a workshop on September 26, 2017, in Irvine, CA, on The Role of Advanced Technologies in Structural Engineering For More Resilient Communities. Register now at www.nationalacademies.org/PGA/ResilientAmerica The SEI Global Activities Division is presenting a special panel session at the 2017 IABSE Symposium in Vancouver on September 23, 2017, on Structural Engineering Global Interoperability in addition to sponsoring the Friday Evening Gala Dinner. View the Symposium schedule and register at www.IABSE2017.org.

ASCE 2017 Annual Convention October 8 – 11, 2017

New Orleans, Louisiana Registration is now open www.asceconvention.org. STRUCTURE magazine

This fellowship award is at least $5,000 and can be up to $10,000.

September 2017

Join/Renew for ASCE/SEI Member Rate

 www.asce.org

A faster, easier way to work with Standard ASCE 7

ASCE 7 Online ASCE 7 Online is a user-friendly web-based application that provides digital access to both Standard ASCE 7-10 and 7-16. Unique features and functionality include: • Side-by-side display of Provisions and Commentary • Real-time updates of supplements and errata • Redlining to track changes • Corporate and personal note taking features 12-Month Subscription Information:

Watch Video www.asce.org/asce7

Individual User License: List: $200 | ASCE Member: $150 Corporate Single Site License: 3 Concurrent Users: $450 5 Concurrent Users: $720

Corporate Multi-site licensing is available

ASCE 7 Hazard Tool ASCE 7 Hazard Tool is a quick reliable way to look up key design parameters specified in ASCE 7-10 and 7-16. Easy-to-use mapping features quickly retrieve your choice of hazard data such as wind, seismic, ice, rain, flood, snow, and tsunami. Generate and download a PDF report with your results to include in your engineering proposals.

12-Month Subscription Information:

List Price: $60

ASCE Member: $45

Corporate Multi-site licensing is available

Visit Booth 139 on Oct. 12-13, 2017 at NCSEA 2017 Structural Engineering Summit for product demonstration and information about ASCE 7 Online and Hazard Tool. Minimum Design Loads and Associated Criteria for Buildings and Other Structures, ASCE/SEI 7-16 2017 l 888 pp. (Two-volume set) List: $240 l ASCE Member: $180 Soft Cover ISBN: 978-0-7844-1424-8 l Stock #41424 eBook PDF ISBN: 978-0-7844-7996-4 l Stock #47996

For information about all ASCE 7 Products, visit www.asce.org/asce7 or call 1.800.548.2723 or 1.703.295.6300. STRUCTURE magazine

September 2017

The Newsletter of the Structural Engineering Institute of ASCE

One Site. Precise Data. Fast Results.

Structural Columns

ASCE/SEI 7-16 Standard Available

CASE in Point

The Newsletter of the Council of American Structural Engineers

CASE Summer Planning Meeting Update On August 2-3, the CASE Summer Planning Meeting took place in Chicago, IL, with over 35 CASE committee members and guests in attendance, making this a well-attended and productive meeting. Included in the planning meeting was a roundtable discussion lead by members of the CASE Executive Committee. During the meeting, breakout sessions were held by the CASE Contracts, Guidelines, Membership, Toolkit, and Programs & Communications Committees. Listed below are the current initiatives being developed by the committees: Contracts Committee • Looking at all the Terms and Conditions for the contracts, making sure the language is standard in all contracts • Working on what “must have” language needs to be standard within the documents • Next outside legal review of contracts will be 2019 Guidelines Committee – Kirk Haverland (khaverland@larsonengr.com) • Working on updating the Code of Standard Practice document to reflect updated information • Working on updating the Guideline on Special Inspections to reflect updated information • Working on the following new documents: – Commentary on ASCE-7 Wind Design Provisions – Commentary on ASCE-7 Seismic Design Provisions – Guideline on Geotech Reports

Membership Committee – Stacy Bartoletti / Win Bishop (sbartoletti@degenkolb.com) • Will be surveying the membership in preparation for strategic plan update Programs and Communications Committee – Nils Ericson (nericson@m2structural.com) • Confirmed/finalized sessions for 2018 ASCE/SEI Structures Congress • Working on session for 2018 AISC Conference and the ACEC Annual Convention • Putting together the editorial calendar for articles to STRUCTURE magazine from CASE • Worked on putting together list of topics/sessions for next year’s Business & Risk Management Seminar Toolkit Committee – Brent White (brentw@arwengineers.com) • Working on the following new tools: – Project Management Training Tool – Multi-Disciplinary Project Coordination Tool – Contract Clause Tool – Short Term Staffing Tool The 2018 CASE Winter Planning Meeting is scheduled for February 1-2 in Austin, TX. If you are interested in attending the meeting or have any suggested topics/ideas from a firm perspective for the committees to pursue, please contact Heather Talbert at htalbert@acec.org.

ACEC Fall Conference Features CASE Risk Management Convocation and More! October 15-18 ACEC is holding its Fall Conference at The Hilton Bonnet Creek, Orlando, FL. CASE will be holding their convocation on Monday, October 16. Sessions include: 10:45 am Did I Say That? Managing Claims and Liability in Electronic Business Communication Speakers: Karen Erger, Lockton; Eric Singer, Ice Miller, LLP 2:00 pm Dangerous Contractual Terms Speakers: Ryan Harley, Collins, Collins, Muir + Stewart; Tom Bongi, Brit Global Specialty Insurance 3:45 pm Managing Risk in the Design and Construction of Property Line Building Structures Speakers: Benjamin Cornelius, Leslie E. Robertson Associates, RLLP; Kriton Pantelidis, Welby Brady & Greenblat, LLP 5:00 pm ACEC / Coalition Meet and Greet The Conference also features: • CEO roundtables • Exclusive CFO, CIO, Architect tracks • Numerous ACEC coalition, council, and forum events • Earn up to 21PDHs

STRUCTURE magazine

Additional Risk Management Strategies for Bottom-Line In addition to the CASE sponsored sessions, the ACEC Fall Conference will feature more than 30 advanced business programs, including the following sessions, focused on managing a firm’s liability and risk: Cybersecurity, Terrorism and Crisis Management Michael Hayden, Former Director of the CIA and NSA (Keynote Address) CEO Panel: Contracting Trends – How to Protect the Role of the Engineer Reducing Information Risk: Lessons Learned from Recent Construction Legal Tangles Tyler Ferguson, Newforma, and a Panel of Experts Funding Future Capital Claims/Risk Mitigation Rich Chapman, Chartwell Cybersecurity for Critical Infrastructure Dee Brown, Brown Engineers, LLC

September 2017

generation is, what motivates them as consumers and workers, and how they will shape our national future. • Michael Hayden, former Director of the CIA and NSA, will dissect hot spots around the world, analyzing the tumultuous global environment, the impact of the recent U.S. election, and what it all means for Americans and America’s interests. For more information and to register www.acec.org/conferences/fall-conference-2017.

Foundation 9: Contract Documents – Produce Quality Contract Documents Understand the definition of contract deliverables • Include staff in the work planning effort • Develop written design criteria • Capitalize on similar designs without starting over on each project • Establish reasonable schedule expectations • Share agreement/contract information with staff • Tailor project documents to project delivery method • Integrate the BIM/CAD team Tool 9-1: A Guideline Addressing Coordination and Completeness of Structural Construction Documents It is recommended that engineers read this Guideline and take the test at the end of the document. More experienced engineers should then sit down with the engineers to go over the various subjects and answer any questions. The CASE Drawing Review Checklist will be a valuable tool to take away from this experience and implement for regular office use. Tool 9-2: Quality Assurance Plan High-quality client service – from project initiation through construction completion – is critical to both project success and maintaining key client relationships. Elements of ensuring quality service include: • Client and project ownership by the individuals responsible for the project • Continual staff education including both leadership and technical skill development; Firm-wide standard of care • Quality control process with a complete communication loop • Written Quality Assurance Plan As part of the Ten Foundations of Risk Management, CASE Tool No. 9-2: Quality Assurance Plan guides the structural

engineering professional toward developing a comprehensive, detailed Quality Assurance Plan suitable for their firm. Foundation 10: Construction Administration – Provide Services to Complete the Risk Management Process Train staff for the CA work • Clarify SE’s role during submittal review and construction site visits • Get to know the Superintendent and other important players • Document efforts well • Make site visits and reports meaningful • Follow up on changed construction tasks • Strive toward the goal of a successful project Tool 10-1: Site Visit Cards (Updated November 2016) This tool provides sample cards for the people in your firm who make construction site visits. These cards provide a brief list of tasks to perform as a part of making a site visit – What to do before the site visit; What to take to the construction site; What to observe while at the site; What to do after completing the site visit. The sample cards include several types of structural construction, plus a general guide for all site visits. Tool 10-2: Construction Administration Log Construction administration is a time when good record keeping and prompt response are essential to the success of the project and to limit the risk of the structural engineer. For this reason and many others, a well-organized and maintained construction administration log is essential. You can purchase these and the other Risk Management Tools at www.acec.org/bookstore.

Follow ACEC Coalitions on Twitter – @ACECCoalitions. STRUCTURE magazine

September 2017

CASE is a part of the American Council of Engineering Companies

CASE Risk Management Tools Available

CASE in Point

The Conference will also feature: • Martin Ford speaking on robotics, artificial intelligence, and the impact on the workforce and economy • Robert O’Neill, former Seal Team Six leader, speaking on high-impact, actionable insights on leadership, decisionmaking, operating in uncertain environments, and how to become the “best of the best.” • Neil Howe, a world-renowned expert on generations and demography, will give powerful insights into who today’s

Structural Forum

opinions on topics of current importance to structural engineers

How Big is Big? By Dilip Khatri, Ph.D., S.E.

would like to discuss the idea of “big” structures and explore some interesting facts about these landmark structures that have defined the egocentric ambitions of countries, rulers, and people. What is Big? Developers, builders, architects, engineers, and the public have been pondering that question for thousands of years. From the Egyptian Pharaohs that wanted us to remember them through their Pyramids to today’s modern high rise buildings, there is apparently a demand within the human psyche to be the “Biggest.” This race started in the 1930s with the Chrysler Building (1,046 feet height, 1.195 Million square feet [MSF], built in 1930) and the Empire State Building (1250 feet, 2.25 MSF, built in 1931), to the current title holder, Burj Khalifa (2717 feet tall, 3.31 MSF, completed in 2010). But wait, the race never ends! There is a host of new contenders on the horizon to take the title of “World’s Tallest” every year. I became intrigued with the topic of “Big, Tall, Large” after a recent trip to Romania, where I visited and toured the Palace of Parliament Building (Parliament Building) in Bucharest. The City’s claim that this is the second largest government building (3.95 MSF) peaked my curiosity, and I had to go for a tour of the facility. I learned that the U.S. Pentagon tops out at 6.5 MSF and is the world’s largest government building. Comparing these two structures with the Burj Khalifa and several other high-rise buildings, the concept of “Big” gets blurred very quickly. If we look at floor area, the largest building is the New Century Global Center (18.9 MSF) in Chengdu, China, with the Dubai Airport Terminal (18.44 MSF) not far behind. So why this fascination with being the “Biggest?” The simple answer: Ego. Governments around the globe are no different from children at play with blocks. As children, we used to play with Legos®/blocks and try to make a really tall tower and proceed to show off our “Tallest” tower. Then, someone would walk by and knock the tower down. This almost sounds like the real world! Today, we hold Bridge Building Contests, with popsicle sticks and glue as building

materials, to see which is the strongest and can hold the most weight. Awards are handed out for these competitions. This seemingly standard educational exercise actually fosters the idea that Big is Best. However, it did not start with our generation; it has been with us for centuries. The Pharaohs built their Pyramids, not for eternal fame, but to take their treasures with them into the next life. After a few thousand years of pillaging the Great Pyramids, Egyptians learned that making a big statement about your wealth and not following up with security is a bad idea. So, they decided to hide their treasures inside a Tomb, instead of making big landmarks which stood as advertisements to thieves. “Hey, I’ve got a lot of treasures and they are right here.” No doubt, this was a fundamental shift in philosophy from “Bigger is Better” to “Less is More.” I learned these facts after making a personal visit to Cairo, Luxor, and the Valley of the Kings. Fast forward to the modern world. The developers of the Burj Khalifa and the soon to be constructed Jedda Tower are in direct compe- The Burj Khalifa. tition with each other to outdo Bigness. The remember us when we are no longer here Chinese aren’t far behind, with massive con- and say, “Oh wow, that was a Good Dude,” struction projects underway in Shanghai, or something like that. Beijing, and Hong Kong. Governments, Nowhere does this become more relnations, and wealthy individuals have an evant than in personal residences. One of urge to create their “space,” an area that the world’s largest residential spaces is the defines their existence and proclaims their Royal Palace of Caserta (2.53 MSF, Naples, identity for future generations. In my opin- Italy) and outstrips the Palace of Versailles ion, it’s an attempt at immortality. There (720,000 sq. ft.) by three times, even though are a few of us who would like someone to it is not as famous.

Structural Forum is intended to stimulate thoughtful dialogue and debate among structural engineers and other participants in the design and construction process. Any opinions expressed in Structural Forum are those of the author(s) and do not necessarily reflect the views of NCSEA, CASE, SEI, C 3 Ink, or the STRUCTURE® magazine Editorial Board. STRUCTURE magazine

September 2017

It’s hard to believe that one palace for one family is larger than the Empire State Building. They must have a lot of cousins! In contrast, the White House is just a tiny 54,900 square feet. As I went on my tour of the Parliament Building in Bucharest, I was amazed that out of almost 4 MSF, only about 20% of the building was in actual use. The rest of it was cold, uninhabited, and suffering from neglect. The tour guide admitted that the expense of heating and lighting the building exceeds the Government’s budgetary capacity. I learned that the purpose of the Parliament Building was to satisfy the egocentric objectives of its former ruler, Nicolae Ceausescu, who wanted to make this building his personal residence. Unfortunately, he was a brutal dictator that exploited his country’s resources to complete his fascination and, in the end, was overthrown and shot by firing squad. In the final analysis, what does this all mean for Structural Engineers, Architects, Designers, Builders, and Design Professionals? We all know that each one of these examples creates a plethora of design issues, problems to be solved, and construction difficulties that have been resolved through years of toil, torment, and thought-provoking Saturday evenings by many engineers, architects, and

builders several times over. People have an urge to be famous for one reason or another, whatever that may be. And, successful people have a special desire to out-do, out-build, out-perform, and outshine other successful people in whatever category they can find. After many international tours to some fascinating destinations and visits to Royal Palace of Caserta. landmark structures, I have learned that it is not what is “Big” that matters; instead, it is what purpose your life served in the grand plan of making this world a better place. That’s my personal lesson. However, I have some positive thoughts for developers of large buildings. I want to say Thank You. Thank You to all those egomaniacal personalities that want to create, with their personal signature, the most unique space. If it were not for these individuals, design professionals would simply be busy designing more hospitals and schools. Instead,

you make our profession challenging while we fulfill your dreams of immortal grandeur, even though others are always working on something Bigger, Taller, or Larger.▪ Dilip Khatri is the Principal of Khatri International Inc, Civil and Structural Engineers, based in Las Vegas, NV, and Pasadena, CA. He was a Professor of Civil Engineering at Cal Poly Pomona for ten years. He serves a member of the STRUCTURE Editorial Board and may be reached at dkhatri@gmail.com.

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STRUCTURAL FORUM

SPOTLIGHT

STRUCTURAL DESIGN

BUILDING BLOCKS

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EDUCATIONAL ISSUES

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