As tilt-up construction continues to grow in popularity around the country, it's only natural the impact made by changes to design standards be balanced by general economics and constructability concerns. This clearly is evident with the issue of the design for high seismic regions. The evolution of regions considered high risk continues to expand the significance of designing for higher forces. As seen in the most recent maps from the United States Geological Survey, regions with the greatest risk exist along the West Coast, as well as near Memphis, Tenn., and Charleston, S.C. These regions have seen the most changes to design standards regardless of the construction method. However, several construction methods already have been dominant in high seismic regions for decades and have proven performance, yet they are being challenged to produce evidence of their performance.
Building on the innovations developed by the collaborative efforts of the experienced designers and contractors, and recognizing the need for a definitive base of knowledge regarding seismically effective tilt-up design, the Tilt-Up Concrete Association (TCA) formed a seismic design task force in 2005 to increase the knowledge base and promote the performance evidenced by the thousands of successful projects already placed in these regions.
Composed of TCA member contractors and engineers along with other key industry professionals—all with a wide range of expertise in the area of seismic construction—the group continues to review current tilt-up design procedures and standards for seismic performance as a means to improve current building code provisions. Already the group has begun to create models for current design practice, as well as solutions for detailed dynamic modeling to better understand seismic behavior as well as proposed detailing provisions for submittal to code writing organizations such as the Structural Engineering Institute and the American Concrete Institute.
Innovation in progress
An important concentration of the group's study will be the structural connections required in these areas of high seismic risk and the performance of the building diaphragms under severe seismic loads. In a report commissioned by this Seismic Task Force, the background of this issue can be more clearly understood.
Tilt-up buildings today exhibit a structural diversity as varied as the applied market types. Single-story warehouse or “big box” projects remain an important segment of the tilt-up industry and are used widely to illustrate tilt-up design principles in engineering handbooks. This building type is characterized typically as rigid wall/flexible diaphragm. There are currently two basic types of roof framing systems that are used most often for this project type. These include steel deck/steel bar joist and wood deck/steel bar joist or “hybrid.” Historically, along the seismically active West Coast, tilt-up roof systems consisted of all-wood construction utilizing a plywood deck on wood subpurlins, purlins, and glue-laminated beams.
In general, building envelopes designed wholly as tilt-up construction or integrating components of tilt-up with other systems occur across the broad spectrum of project classifications. However, from this general statement and the detail that follows, the focus of this effort can be seen on perhaps the most simple, and in history the most common form for the use of tilt-up construction: the “big box.” The increasing use of architectural features in tilt-up construction has presented additional challenges in the realm of addressing seismic safety. This is especially true in cases where tilt-up walls are joined to other types of structures.
The path ahead
Innovative solutions, such as those found on these projects, are already quite common and successful. Strong tilt-up construction is well documented in seismic regions and yet increasingly conservative design standards can restrict the creative design of buildings with improving tilt-up technologies. The objective of TCA's Seismic Task Force is to create rational code provisions based on true seismic performance and to further develop the documentation to support a more favorable position for tilt-up systems in the design codes and standards. The evolution of the industry has proven competent to transition to better detailing, roof structures, and analysis. The now popular hybrid roof combining steel joist and girder members with a traditional plywood roof diaphragm has improved the capacity of these buildings to withstand severe seismic loads. Detailing of the connections to the foundation system (footings, walls, or slabs) has improved the reliability of the analysis methods to predict the performance under seismic events. Finally, research underway both analytically and experimentally to affirm the behavior and provide alternative solutions promises to further support better recognition for the performance of these tilt-up structures.
The months ahead show a strong plan for the TCA Seismic Task Force. Efforts include the finalization of seismic impact maps that will allow designers and contractors to quickly determine the need to consider additional design provisions in these strengthened regions. The development history of design standards affecting tilt-up will be finalized to support the positions of the Task Force. Further efforts will be made for proposed language in design standards most often cited for their impact on tilt-up design. These revised provisions will be supported by research as well as empirical evidence. In its effort to standardize the design of seismically safe tilt-up structures, the TCA Task Force is committed to maintaining the legacy that tilt-up has created for more than two decades as a building method that is cost-effective, efficient, and full of architectural possibilities.
—Jim Baty is technical director of the Tilt-Up Concrete Association.