Shakey Ground
By: Clare Martin
Constructive Communication, Inc. cmartin@constructivecommunication.com
As Tilt-Up construction continues to grow in popularity around the country, it’s only natural that the impact made by changes to design standards be balanced by general economics and constructability concerns. This is clearly 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 those higher forces. As seen in the most recent maps from the United States Geological Survey (USGS), regions with the greatest risk exist up and down the Pacific coast as well as near Memphis, Tennessee and Charleston, South Carolina. These regions have seen the most changes to design standards regardless of the construction method. However, several construction methods have already 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 begin increasing the knowledge base and promoting 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 (SEI) and the American Concrete Institute (ACI).
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 widely used to illustrate Tilt-Up design principles in engineering handbooks. This building type is typically characterized as rigid wall/flexible diaphragm (RWFD). There are currently two basic types of roof framing systems that are most often used 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 in this report, 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”.
A few Tilt-Up contractors have taken the lead in designing panels and connections capable of withstanding high levels of seismic force. When constructing the Carson Tahoe Hospital in Carson City, Nevada, Degenkolb Engineers worked with the Tilt-Up contractor, Tedesco Construction, to design custom- welded hold-down details capable of both transmitting high shear and overturning forces and allowing for necessary erection tolerances. These connections consisted of threaded bar connections at the bottom of the panels, as well as embed plates that were welded to embed plates in the foundation, thereby allowing seismic forces to be transferred into the foundation.
Other Tilt-Up projects have required solutions that go beyond the scope of panel design—the Mendota Federal Correctional Institution is a Tilt-Up structure that sits in California’s San Joaquin Valley Basin. To address poor soil conditions (soil in the valley can liquefy when confronted with seismic forces), Tilt-Up contractor Dick Pacific Construction (DPC) had to install liquefaction relief drains 50 feet below the structure before slab work began. To help the panels resist seismic forces, DPC designed hold-down details consisting of a vertical plate with vertical hold-down bars welded to the backside lap and spliced with full-wall-height vertical bars. To protect the sandwich panels’ 3-inch architectural wythe from crushing during a seismic event, DPC set the bottom of the walls 3⁄4-inch above the 28-inch-thick mat foundation, and non-shrink grout was installed only under the 7-inch structural wythe.
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, such as the Spectrum Academic Center and Library at National University in San Diego. The design by Architects Delawie Wilkes Rodriguez Barker in San Diego called for two Tilt-Up wings joined by a 170-foot-long steel frame galleria. To ensure a proper seismic connection between the flexible steel frame and the rigid concrete walls, the architects worked with Hope Engineering, Inc. of San Diego, the structural engineer, to develop a structural steel horizontal truss system designed to transfer diaphragm forces between the two wings.
According to Michael B. Wilkes, FAIA, Principal of Architects Delawie Wilkes Rodriguez Barker, a horizontal bracing system was provided below the atrium trusses. The bracing system consists of tube steel “X” braces in the plane of the main roof steel. The design forces for the braces were scaled up by an overstrength factor because of their critical nature in replacing the diaphragm at the atrium opening, and the stiffness of the braces was sufficient to limit the differential drift across the atrium to an acceptable level.
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 sometimes 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, better roof structures and better analysis. The now popular hybrid roof combining steel joist and girder members with a traditional plywood roof diaphragm has vastly 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 “simple” 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 over more than two decades as a building method that is cost-effective, efficient and full of architectural possibilities.
