TESTING OF SPECIAL LYS STEEL PLATE SHEAR WALLS

An experimental program of steel panel shear walls is outlined and some results are presented. The tested specimens utilized low yield strength (LYS) steel infill panels and reduced beam sections (RBS) at the beam-ends. Two specimens make allowances for penetration of the panel by utilities, which would exist in a retrofit situation. The first, consisting of multiple holes, or perforations, in the steel panel, also has the characteristic of further reducing the corresponding solid panel strength (as compared with the use of traditional steel). The second such specimen utilizes quarter-circle cutouts in the panel corners, which are reinforced to transfer the panel forces to the adjacent framing. INTRODUCTION The selection of Steel Plate Shear Walls (SPSWs) as the primary lateral force resisting system in buildings has increased in recent years as design engineers discover the benefits of this option. Its use has matured since initial designs, which did not allow for utilization of the post-buckling strength, but only elastic and shear yield plate behavior. This design approach typically resulted in the selection of a relatively thick panel for the infill. A large plate thickness, while producing a stiff structure that would reduce displacement demand during a seismic event, would also induce relatively large forces on the surrounding frame members, which must be detailed accordingly to ensure adequate performance. Research conducted by Thorburn et al. (1983) supported the SPSW design philosophy that reduced plate thickness by allowing the occurrence of shear buckling. After buckling, lateral load is carried in the panel via the subsequently developed diagonal tension field action. Smaller panel thicknesses also reduce forces on adjacent members, resulting in more efficient framing designs. Research programs at various universities have furthered the understanding of thin plate SPSWs (e.g., Lubell et al., 2000; Driver et al., 1997; Caccese et al., 1993). However, some obstacles still exist that may impede further widespread acceptance of this system. For example, using the yield stress for typically available steel material, the panel thickness as required by a given design situation may often be much thinner than plate typically available from steel mills. In a case such as this, using the minimum available plate thickness would result in a large difference in panel forces from that required by calculations. Attempts at alleviating this problem were recently addressed by the use of light-gauge, cold-formed steel panels, in a new application by Berman and Bruneau (2003). Xue and Lu (1994) suggested additional means of reducing demand on framing adjacent to an SPSW, including the connection of the infill panel to only the beams in a 1 Ph.D. Candidate, Department of Civil, Structural, and Environmental Engineering, University at Buffalo, Buffalo, NY, USA. Email: vian@eng.buffalo. edu 2 Professor, Department of Civil, Structural, and Environmental Engineering, University at Buffalo, Buffalo, NY, USA. Email: bruneau@mceermail.buffalo. edu moment frame. However, more work is required to ensure the viability of the SPSW system in a wide range of situations. The University at Buffalo (UB) and the Multidisciplinary Center for Earthquake Engineering Research (MCEER) initiated a co-operative experimental program with National Taiwan University (NTU) and the National Center for Research on Earthquake Engineering (NCREE) in order to further address the above issues with regards to SPSW performance. A description of the test program and presentation of results follows below. EXPERIMENTAL PROGRAM A total of three single bay, single story LYS SPSW specimens were designed by the researchers at UB, fabricated in Taiwan, and subjected to quasi-static cyclic testing in the NCREE laboratory at NTU. The frames measured 4000mm wide and 2000mm high between member centerlines, and consisted of 345MPa steel members. The infill panels produced by China Steel were 2.6mm thick, LYS steel plates with an initial yield stress of 165MPa, and ultimate strength of 300MPa, important properties that may aid in alleviating over-strength concerns mentioned above. All specimens also have a beam-to-column connection detail that includes reduced beam sections (RBS) at each end. This detail was designed to ensure all inelastic beam action would occur at these locations, with the intention of efficient anchoring of infill panel tension field forces, as required at the extremes (roof and basement level beams) of a multistory SPSW-retrofitted/designed steel frame. A solid panel specimen is shown schematically in Fig. 1. Figure 1. Typical specimen dimensions. Two specimens tested had solid panels while the remaining two provide utility access through the panels using cutouts. One specimen consisted of a panel with a total of twenty 200mm-diameter holes, or perforations, in an arrangement shown in Fig. 2. Roberts and Sabouri-Ghomi (1992) conducted research investigating the effects of a single perforation in an unstiffened shear panel, leading to some reduction factors that could be applied to the properties of a solid panel, conservatively reducing the stiffness and strength to account for the presence of the perforation. The multiple perforations present in the tested specimen share the common goal of utility access in order to make the SPSW system more acceptable, while also serving as a method of reducing the panel strength and therefore the demand on the surrounding framing. This latter characteristic may prove beneficial in markets that do not have LYS readily available for structural applications. Figure 2. Specimen P before testing. The other specimen allowing for utility penetration is a solid panel, with the top corners of the panel cutout and reinforced to transmit panel forces to the surrounding framing, as shown in Fig. 3 below. This specimen would allow utility access through the wall, while also transmitting forces near that of the solid panel counterpart. Figure 3. Specimen CR before testing. All specimens were tested using a cyclic, quasi-static loading protocol similar to ATC-24. In agreement with the typical testing procedure at NCREE, a displacement-controlled scheme was selected for the entire experimental program. Based on estimates of yield from SAP2000 pushover analyses, the displacement history shown in Fig. 4, was developed and applied horizontally to the center of the top beam using four actuators, as shown in the figures above. The same displacement loading history was used for actuator control of all the specimens tested. -160 -120 -80 -40 0 40 80 120 160 0 3 6 9 12 15 18 21 24 27 30 33 36 Number of Cycles, N In te rs to ry D is pl ac em en t ( m m ) -8% -6% -4% -2% 0% 2% 4% 6% 8% In te rs to ry D rif t ( % ) 2 cycles per amplitude 3 cycles per amplitude