Seismic Demands on Steel Braced Frame Buildings with Buckling-Restrained Braces by
Rafael Sabelli, Stephen Mahin and Chunho Chang
This paper highlights research being conducted to identify ground motion and structural characteristics that control the response of concentrically braced frames, and to identify improved design procedures and code provisions. The focus of this paper is on the seismic response of three and six story concentrically braced frames utilizing buckling-restrained braces. A brief discussion is provided regarding the mechanical properties of such braces and the benefit of their use. Results of detailed nonlinear dynamic analyses are then examined for specific cases as well as statistically for several suites of ground motions to characterize the effect on key response parameters of various structural configurations and proportions. Introduction
Steel moment-resisting frames are susceptible to large lateral displacements during severe earthquake ground motions, and require special attention to limit damage to nonstructural elements as well as to avoid problems associated with P-∆ effects and brittle or ductile fracture of beam to column connections [FEMA, 2000]. As a consequence, engineers in the US have increasingly turned to concentrically braced steel frames as an economical means for resisting earthquake loads. However, damage to concentrically braced frames in past earthquakes, such as the 1985 Mexico [Osteraas, 1989], 1989 Loma Prieta [Kim, 1992], 1994 Northridge [Tremblay, 1995; Krawinkler, 1996], and 1995 Hyogo-ken Nanbu [AIJ/Kinki Branch Steel Committee, 1995; Hisatoku, 1995; Tremblay, 1996] earthquakes, raises concerns about the ultimate deformation capacity of this class of structure. Individual braces often possess only limited ductility capacity under cyclic loading [Tang, 1989]. Brace hysteretic behavior is unsymmetric in tension and compression, and typically exhibits substantial strength deterioration when loaded monotonically in compression or cyclically. Because of this complex behavior, actual distributions of internal forces and deformations often differ substantially from those predicted using conventional design methods [see, for example, Jain, 1979 and Khatib, 1987]. Design simplifications and practical considerations often result in the braces selected for some stories being far stronger than required, while braces in other stories have capacities very close to design targets. This variation in story capacity, together with potential strength losses when some braces buckle prior to others, tend to concentrate earthquake damage a few “weak” stories. Such damage concentrations place even greater burdens on the limited ductility capacities of conventional braces and their connections. It has also been noted that lateral buckling of braces may cause substantial damage to adjacent nonstructural elements.
1. Director of Technical Development, DASSE Design, Inc., San Francisco, CA 2 Nishkian Prof. of Structural Engineering, Univ. of Calif, Berkeley, CA 3. Visiting Scholar, Pacific Earthquake Engineering Research Center, Univ. of Calif, Berkeley, CA
Prompted by these observations and concerns, seismic design requirements for braced frames have changed considerably during the 1990s, and the concept of special concentric braced frames has been introduced [AISC, 1997; ICBO, 1997]. Considerable research has also been initiated improve the performance of concentrically braced frames through the introduction of new structural configurations [see, for example, Khatib, 1987] or the use of special braces, including those utilizing composite action [Liu, 1987], metallic yielding [Watanabe, 1992; Kamura, 2000; and others], high performance materials [Ohi, 2001], friction and viscous damping [see, for example, Aiken, 1996]. During the past decade, there have also been parallel advances in research related to characterizing the seismic hazard at a...
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