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Seismic Demands on Steel Braced Frame Buildings with Buckling-Restrained Braces by 1

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Rafael Sabelli, Stephen Mahin and Chunho Chang

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Abstract
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

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