Recent Developments in Ductile Steel Design Concepts

This paper reviews selected recent innovations that expand the range of applicability of a number of new and emerging structural steel systems that can provide effective seismic performance. Focus is on recent developments on: (a) Steel Plate Shear Walls having light gauge infill plates; (b) Perforated Steel Plate Shear Walls; (c) Buckling Restrained Braced frames designed to meet Structural Fuse objectives; (d) Tubular Eccentrically Braced Frames, and; (e) Rocking braced frames. INTRODUCTION A recently published paper has provided a brief review of selected recent work on the development of solutions for the seismic design and retrofit of steel structures by various members of the U.S. research community (Bruneau et al. 2005). That previous paper focused on research on Retrofit of Beam-to-Column Moment Connections, Frame Modifications at Beams’ Mid-Span, Self-Centering Systems, Zipper Frames, Buckling-Restrained Braced Frames, Steel Plate Shear Walls, Plastic and Rotation Limits for Buildings and on Shear Links and Truss Piers for Bridges. That research has resulted on the development of valuable concepts for enhancing the seismic performance of steel structures. Here, information is presented on selected subset of recent innovations that expand the range of applicability of some emerging systems that have seen a significant increase in interest by the practicing engineering community over the past few years. In a first part, this paper focuses on Steel Plate Shear Walls (SPSW) designed to rely on the development of diagonal tension yielding for seismic energy dissipation, and Buckling Restrained Braces (BRB) which are special braces that can develop their full axial yield strength both in tension and compression. SPSW were first proposed by Canadian researchers and the Canadian standard “Limit States Design of Steel Structures” (CSA 2001) was first to implement specific seismic design provisions for this system. BRB were originally developed by Japanese researchers in the early 1980’s, and North American requirements for their design were first specified by the “Seismic Provisions for Structural Steel Buildings” of the American Institute of Steel Construction (AISC 2005). Both SPSW and BEB are highly ductile systems that make it possible to design structures with high lateral stiffness, thus indirectly limiting some of the non-structural damage that can be suffered during earthquakes. Passage of the California Senate Bill 1953 that mandates that all health care facilities providing acute care services be retrofitted to a life-safety performance level by 2008, and a full-serviceability level by 2030, has partly played an important role in raising awareness that extensive non-structural damage is undesirable and detrimental, as it can render buildings unusable for extended periods of time following earthquakes. A latter part of the paper focuses on innovations recently developed as design strategies for large steel bridges, but that can also have important applications in buildings. Important seismic evaluation and retrofit of major crossings have occurred in North America since a span of the San Francisco–Oakland Bay Bridge collapsed during the 1989 Loma Prieta earthquake. Large steel truss bridges were evaluated and in some cases retrofitted in most states where these important lifelines exist, including California, Washington, 1. Director, MCEER, and Professor, Department of Civil, Structural, and Environmental Engineering, University at Buffalo, Buffalo, NY 14260 XVI Congreso Nacional de Ingeniería Sísmica Ixtapa-Zihuatanejo, Guerrero, 2007 2 Oregon, New York, and the Mid-West States. The systems described here would be applicable for these types of retrofit as well as for new designs. STEEL PLATE SHEAR WALLS The selection of SPSW 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. Research conducted by Thorburn et al. (1983), Lubell et al (2000), Driver et al. (1997), Caccese et al (1993), Berman and Bruneau (2003b, 2004) (among many) supported the SPSW design philosophy that reduced plate thickness by allowing the occurrence of shear buckling. After buckling, the 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. Understanding of the seismic behavior of thin plate SPSW has significantly improved in recent years. Yet, 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 the minimum hot rolled steel plate thickness typically available from steel mills. In the perspective of capacity design, this will increase the necessary sizes of horizontal and vertical boundary members as well as foundation demands. To alleviate this concern, recent work has focused on the use of light-gauge cold-rolled and low yield strength (LYS) steel for the infill panel (Berman and Bruneau, 2003b; Vian and Bruneau, 2004), and also by placement of a pattern of perforations to decrease the strength and stiffness of the panel by a desired amount (Vian and Bruneau, 2004). In addition, the use of reduced beam sections at the ends of the horizontal boundary members is being investigated as a means of reducing the overall system demand on the vertical boundary members (Vian and Bruneau, 2004). These efforts are briefly summarized below: SPSW WITH LIGHT-GAUGE INFILL A SPSW test specimen utilizing a light-gauge infill (thickness of 1.0 mm, 0.0396 in) is shown in Figure 1 (Berman and Bruneau, 2003b). The specimen used W 310 x 143 (US W 12 x 96) columns and W 460 x 128 (US W 12 x 86) beams. This test was performed using quasi-static cyclic loading conforming the recommended Applied Technology Council (ATC) loading protocol of ATC 24 (ATC 1992). Hysteretic results are shown Figure 2 along with the boundary frame contribution. After subtracting the boundary frame contribution, the hysteresis of Figure 3 is obtained. This specimen reached a ductility ratio of 12 and drift of 3.7%, and the infill was found to provide approximately 90% of the initial stiffness of the system. Ultimate failure of the specimen was due to fractures in the infill propagating from the welds which connected it to the boundary frame. Figures 4a and 4b show the buckling of the infill plate at the peak displacement of cycle 20 (ductility ratio of 6, 1.82% drift) and the fracture at the infill corner during cycle 26 (ductility ratio of 10, 3.07% drift) respectively.