9 minute read
NCSEA News
Excellence, Creativity and Innovation
By Gerald Epp, M.Eng, P.Eng, Struct.Eng, P.E.
Engineers rose to the demand for an exuberant free-form expression with practical and elegant solutions to the project’s complex technical challenges (and shrinking budget). Two major elements enclose the existing theaters, isolating them from the noise of the busy capitol:
High Roof
Courtesy of Bing Thom Architects. Fast + Epp was an Award Winner for the Arena Stage at the Mead Center for American Theater project in the 2011 NCSEA Annual Excellence in Structural Engineering Awards Program (Category – New Buildings over $100 Million)
The initial fully-designed and peer-reviewed scheme had the roof cantilevering 180 feet, supported by harped steel cables anchored into a custom steel space truss. Post-9/11 funding shortfalls necessitated redesign. A 500-foot-long steel truss structure was introduced, retaining drama by bringing a support wall only part way along the back side, leaving an 85-foot cantilever which appears similar to the original. The large, complex roof that spans over the theaters – cantilevering toward the Washington monument – required close collaboration between engineer and erector. Every nine-foot-deep truss is unique, has one-of-a-kind support conditions, and spans up to 170 feet. A sharp edge was efficiently achieved with light steel framing. To accommodate the multiple orientations of the leaning support columns, a single 2½-inch diameter bolt connection was devised, speeding erection of the large trusses. A simple “bounce test” (one engineer, all alone) was used to test the stiffness of the cantilever tip, resulting in an instruction for minor stiffening in the secondary trusses.
Timber Façade
The roof is supported by large engineered timber columns, which also serve as backup to a sinuous 650-foot long, up to 58-foot tall suspended glass façade; the acoustical and environmental barrier. The geometric complexity is exacerbated by a four degree tilt from vertical. For efficiency, two-thirds of the double-glazed facets were designed to be identical in size. The remaining bays unnoticeably take up the irregular geometry. The timber columns are set back, receiving tapered timber arms that reach out to laterally support timber muntins, to which the glazing units are clipped. All of the timber is engineered wood – parallel strand lumber (PSL). The 20-inch by 30-inch elliptical timber column is designed to carry axial forces (up to 400 kips) and out of plane near hurricane wind-forces while minimizing the amount of PSL used. Deflection actually controlled the design. The column cross-section was designed with a partially restrained relief joint through the neutral axis to manage the strong potential for movement and checking. The base connector for the columns visually references a lightly touching ballet slipper, heightened by “pencil-sharpened” tapering on the bottom nine feet of the timber. Non-linear 3D solids finite element analysis and full scale load testing were performed to minimize the weight of the ductile iron casting. The roughly 16- by 12-foot glazing units and shaped PSL muntins are suspended from 5/8-inch diameter stainless steel cables via fully adjustable connectors. In order to accommodate erection tolerances and ensure tension during the life of the building, a carefully calculated assembly of three plates as “leaf springs” was installed at the top of each cable. The entire façade is structurally complex; lateral deflections occur in both the span of the columns and in the slender spans of muntins between the columns, which gain their stiffness through a combination of bending and axial forces (catenary action). Analysis determined stiffness in the system was quite dependent on bending moments being carried through every PSL muntin-to-muntin joint, making the connection an important part of the overall design. Research and load testing was carried out on a tight-fitting multi-pin connector which would allow the connection to be virtually invisible, yet efficiently carry the high forces. It was further proved on a system basis when a full size 50- by 60-foot mock up was constructed and tested with full design wind forces. All the lateral forces from the roof and façade were focused into the petal-shaped architectural concrete walls of the new “Cradle” theater, eliminating the need for visually disturbing bracing along the length of the façade. This is believed to be the tallest free-span timber-backed glass façade in the world.
Complexity and Design Sustainability
This was in every respect a complex structure to build. The general contractor and the entire team of consultants were highly engaged with the project, paying close attention to detail in planning, 3D-modeling and shop drawings. To ensure smooth execution of the timber façade, a related design-build company took on the contract for its design, testing, fabrication and installation. This project involves the unprecedented use of architecturally exposed engineered wood. Such wood is far and away the most sustainable structural material. Extensive use of efficient architecturally-exposed structural materials starkly contrasts with the heavily clad structures of Washington. Persistent “value engineering” eliminated superfluous finishes and put pressure on the structural engineer to provide an aesthetically satisfying structure. For example, the custom timber façade is not only structurally unique, but also uniquely serves as acoustical barrier and roof support, yet costs less than commercially available alternatives.▪ Gerald A. Epp, M.Eng, P.Eng, Struct.Eng, P.E. is a founding partner at Fast + Epp. He has been President of the design-build company, StructureCraft Builders Inc., since 1998.
NCSEA News
By Thomas A. Grogan Jr., P.E., S.E., SECB
Many states already have some form of structural engineering (SE) practice restriction, but if you live in a state without one, you might be asking yourself what it takes to start one. The Florida Structural Engineers Association (FSEA) decided just over two years ago that Florida should look into SE licensure, but only after some very healthy debate among our board members. We asked ourselves: Would our membership support this worthy cause? What would it take to get there? From whom could we elicit help? Though we had the backing of NCSEA, if our members were not in favor of SE licensure, then we would be dead in the water. We did several things to address this concern. First, we had two of our board members join the NCSEA Licensing Committee and spend some time learning what other Member Organizations around the country were doing in this area. Of particular interest were the recent successes in Washington and Utah. Those committee members also attended national licensing summits to learn which other professional organizations were for and against SE licensure. Second, we polled our membership and were pleased to learn that over 80% were in support of SE licensure. This provided the impetus to start the process in earnest. Third, we formed our own state SE licensure committee, which I currently chair, and it now has over a dozen members. At the first meeting of this committee, which was attended by over twenty engineers, we heard from two members of the NCSEA Licensing Committee: Susie Jorgensen, the chair, and Barry Arnold, the person behind the scenes who helped get the SE practice restriction in Utah. Susie shared with us the status of SE licensure and its importance in our profession’s over-arching mission to protect the safety, health and welfare of the public. This was followed by Barry’s passionate presentation on why he became involved and the work of his committee in Utah. Several of the FSEA committee members were not enthusiastic about SE licensure at first; but they ended up becoming vital contributors to our progress. They played devil’s advocate and challenged Susie, Barry and the other members, to the point where we were not sure that we had enough support from within our committee. Though initially quite frustrating, this became valuable over the next several months as we worked hard to convince these individuals that this was a worthy cause for which we needed not only their support, but also their help. That initial meeting unified the committee; and we voted to continue pursuit of SE licensure. We have continued to meet almost every month since. Using Utah as a model, we decided that we needed to write a white paper outlining why SE licensure was important for Florida. We gathered all of our facts, including information on several collapses in Florida over the past 20 years, attributed to poor design. We also learned that over 80% of the complaints filed with our Florida Board of Professional Engineers (FBPE) were structural issues. This was very enlightening and motivated the committee to take their work to the next level. From the white paper, we drafted and published a one-page document that explained very briefly why we were pursuing this goal. In addition, we formed several subcommittees: legislation, advocacy, and marketing. Momentum was starting to build. When pursuing any legislation, it is important to know who will support you and who might be against you. The Florida Engineering Society (FES), an affiliate of the National Society of Professional Engineers (NSPE), is the predominant engineering organization in our state, with strong connections to the legislature and paid lobbyists. We realized that having their support would be critical to our success. Two of our licensure committee members are also members of FES, so they began the discussion of separate licensure at FES state meetings. The FES board believed that our request warranted their review and charged their Professional Concerns Committee with carrying it out. After careful assessment of our white paper and one-page document, the Professional Concerns Committee wrote a position paper indicating that they were in complete support of this initiative, as long as it applied only to Threshold Buildings as already defined by state law and would be a post-licensure credential. This was extremely good news, and several of us were invited to their next state board meeting to participate in a discussion, after which they voted on the issue. Although the discussion was heated and contentious, at the conclusion, the FES board passed a motion indicating they would support SE licensure and offered support to FSEA in working with the legislature. FSEA has reciprocated by offering to support FES on other initiatives that they are pursuing. Since that time, FSEA has written a revision to the state law that would recognize structural engineering as a specific discipline and give the FBPE the ability to define the types of structures to which practice restrictions would apply. However, at the moment we are in a holding pattern, as Governor Rick Scott has asked that no new legislation be initiated unless it immediately creates jobs. We intend to begin again in earnest toward the middle of 2012; but we will be contacting FES to determine what lobbying support they are willing to provide. Though we still have a ways to go, we believe that our committee is motivated and ready to address all the obstacles to obtaining SE licensure. It is this motivation that will be critical to our success in this endeavor.
NCSEA Continuing Education in February
February 7 webinar: Design Considerations for Exterior Wall Interfaces in Steel-Framed Buildings by Patrick McManus February 10-11: NCSEA Winter Institute on Soft Soil – Water and Wind, Hotel Monteleone, New Orleans, LA
