The 14 th World Conference on Earthquake Engineering October 12-17, 2008, Beijing, China Seismic Performance Assessment of Concentrically Braced Steel Frame Buildings Chui-Hsin Chen 1 , Jiun-Wei Lai 1 and Stephen Mahin 2 1 Graduate Student, Dept. of Civil and Env. Eng., Univ. of Calif., Berkeley, CA, USA 2 Professor, Dept. of Civil and Env. Eng., Univ. of Calif., Berkeley, CA, USA Email: chchen@berkeley.edu, adrian.jwlai@berkeley.edu, mahin@berkeley.edu ABSTRACT: Three-story tall, model building systems are analyzed using OpenSEES to improve understanding of the behavior of conventionally braced and buckling restrained braced frames, and to identify improved performance-based design and analysis procedures. The analyses demonstrate that statistically, the three-story model building designed following 1997 NEHRP and that following ASCE 7-05 have similar performance in terms of the damage concentration. If the R value is reduced from 6 to 3, the demands on the braces and framing components are reduced as well as the tendency to form a soft story. However, stronger structure results in higher floor level accelerations, possibly creating greater falling hazards or increasing the costs of protecting nonstructural components and hazards. For the three-story BRBF model the tendency to form a soft story is less than SCBF. Nonetheless, the BRBF model has the largest residual drifts. A preliminary examination of floor level accelerations shows that the peak accelerations appear to be limited by the strength of the structure. The testing protocols are evaluated based on the analytical results, but further experiments are needed to verify the results. Based on the parametric study, four large-scale two-story braced frames with different brace types and different bracing configurations are designed to be tested in the University of California at Berkeley. Test results will be used to refine the analytical models and validate the seismic performance of SCBF and BRBF. KEYWORDS: Performance-based design, Special Concentrically Braced Frame, Buckling Restrained Braced Frame 1. INTRODUCTION A series of nonlinear dynamic analyses are presented that extend earlier work by Uriz (2005). In addition to updating the building code used, the configuration of the structure is altered from a stacked chevron arrangement to one where the braces are arranged in a double story X configuration. The model buildings are three-story tall SCBF and BRBF structures. A variety of results are presented to assess likely behavior for different seismic hazard levels in terms of peak roof displacement, interstory drifts, residual story displacements, and floor level accelerations. 2. DESIGN CRITERIA This study is confined to an assessment of the seismic response of two nearly identical three-story tall steel buildings. As seen in Figure 1, the model buildings have regularly spaced gravity-resisting frames (continuous columns with ideal pin connections to the beams and foundation) with lateral earthquake-load resistance provided by special concentric braced frames (SCBF) or buckling restrained braced frames (BRBF) located on the perimeter of the building. These buildings were designed by others [Sabelli 2000, DASSE 2007] for a location in downtown Los Angeles, CA as commercial office buildings (Occupancy Category II). One of the model buildings was designed in conformance with the provisions of the 1997 NEHRP seismic design provisions and the others were designed in conformance with the provisions of ASCE 7-05. The importance factor and redundancy factor were assumed to be unity for all designs. Table 1 lists some of the principal attributes of the structures and the key parameters used in the seismic design.