Study analyzes Seismic Force Resisting Systems for steel structures in high seismic regions.
By Tyler B. Sease, P.E.
The Seismic Force Resisting Systems typically utilized for steel structures in high seismic regions are Special Concentrically Braced Frames (SCBF) or Special Moment Frames. Steel Special Plate Shear Walls (SPSW) are gradually gaining traction among structural engineers due to superb performance in laboratory tests and improved codification. This article is based on a study that sought to analyze and design a common structure using both the SCBF and SPSW systems and to evaluate the seismic performance and economy of SPSW systems in relation to SCBF systems. The SCBF system referenced was detailed by the author in the February 2013 issue of Structural Engineer (“Performance-based earthquake engineering,” page 26, http://cenews.com/article/9185/performance-based-earthquake-engineering) and thus only the SPSW design will be described.
SPSWs have been used in the U.S. since the 1970s, but were not a codified system until incorporation into AISC’s Seismic Provisions in 2005 (AISC 341-05). Early use of SPSWs was primarily for seismic retrofits, as practicing engineers are often reluctant to use systems that are not well codified or do not have a clear history of performance.
Several SPSW structures have endured major seismic events with minimal damage. Two such structures are the six-story Sylmar Hospital, which withstood the Northridge earthquake, and the 35-story Kobe office building, which survived the Kobe earthquake.
A thin steel plate called a web plate is the main component of an SPSW system. The web plate is attached to Horizontal Boundary Elements (HBE) and Vertical Boundary Elements (VBE) with welded or bolted connections (Figure 1). The web plates are extremely thin and slender, thus only capable of resisting tensile forces. The HBEs are connected to the VBEs with rigid connections, which provide additional stiffness and redundancy to the system.
The superb hysteretic performance of the system is due in large part to the stiffness of the SPSW in combination with the ductile moment frames. SPSWs have a high initial stiffness, which results in excellent drift ratios, similar to those of an SCBF. SPSWs can withstand significant deformation and drift ratios up to 4 percent and continue to dissipate seismic energy. Although some tearing and yielding will take place in the web plate at high drifts, this does little to degrade the strength of the SPSW. Laboratory studies have shown that SPSW systems can undergo numerous cycles of inelastic deformation before sustaining detrimental damage.
SPSWs are typically analyzed with either a strip model (Figure 2) or a finite element mesh model (Figure 3). The finite element mesh model, when evaluated in laboratory studies, has been known to overstate the stiffness of the SPSW. Strip models consist of discretizing the plate into 10 or more bar elements, which are modeled based on their tributary area. Strip models are simpler to analyze and provide sufficient accuracy. Both procedures were utilized to analyze this structure, but ultimately the strip model was used for the final design because of its conservative nature.
The location selected for this study is Charleston, S.C., due to its high seismicity. The five-story, multi-use commercial building has a seismic design category of “D,” an occupancy category of III, and an importance factor of 1.25. This 80-foot-tall structure consists of steel framing and composite slabs at each level. Typical dead and live loads were used in the gravity design of this 200-foot by 200-foot structure. SPSW systems have a response modification factor of six, which equals the “R” value for SCBF systems.
The web plate design utilized Grade A36 steel plate, which has excellent tensile strength at very thin thicknesses. The bottom level was designed as 3/16-inch plate and incrementally gets thinner before transitioning to 0.0747-inch (14 gauge) plate at the fifth level. Small differential thicknesses between levels results in large HBEs due to the design tensile forces not canceling each other out. SPSWs are designed with a capacity-based approach and thus the HBEs and VBEs must be able to resist the full tensile strength that the web plate could exert on it. This approach resulted in a W14x211 VBE and W33x152 HBE at the lower level (Table 1).
With SPSW systems, a structural member must be used at the foundation level to transfer the forces in the bottom web plate into the foundation. The web plate is designed to attach to a WT8x44.5, which is anchored to a concrete grade beam. AISC’s Seismic Provisions require the HBE to be rigidly connected to the VBE. Reduced beam section moment connections were used to reduce the design moment approximately 33 percent.
According to studies, web plates typically account for 60 percent to 80 percent of the story shear, with the remainder being resisted by the moment frame of the HBEs and VBEs. In the strip model analysis, the web plate resisted 80.9 percent of the story shear; however, in the finite element analysis, the web plate resisted 95.5 percent of the story shear. This further validates the theory that finite element models exaggerate the strength and stiffness of web plates.
Based on the estimation of an AISC certified structural steel fabricator, the SPSW design would cost approximately 39 percent more than that of the SCBF design. This would result in a cost premium of approximately $169,155 for this structure. However, the fabricator and the erector had no previous experience with SPSW systems and thus likely priced the system high due to uncertainty with fabrication and erection means and methods. Primary constructability concerns include material handling of large thin plates, potential splicing requirements, and availability of large thin plates. The bay width and story height were not ideal for a SPSW system, and smaller bays and shorter heights would have resulted in much smaller HBEs and VBEs, which may have made the system more competitive with the SCBF system.
SPSWs may not have been the most cost-effective solution for this structure, but could prove to be a worthwhile system in certain structures. The extremely high redundancy and hysteretic performance of this system are particularly helpful in high-seismic regions where a structure may be subjected to multiple seismic excitations over its lifespan. SPSWs merge the ductility of moment frames with the stiffness of a web plate to form a system with tremendous energy dissipation capabilities. These characteristics, combined with the increased codification and superb performance in seismic events, will likely lead to increased utilization of this system by structural engineers in the future.
Tyler B. Sease, P.E., is a structural engineer at CMC Cary Engineering in Greenville, S.C. He specializes in structural steel design and completed the research for this article while completing graduate studies at the University of South Carolina. He can be reached at firstname.lastname@example.org.