Seismic Collapse Performance of Concentrically Steel Braced Frames

Conventional special concentrically braced frames (SCBF) have been widely used due to their efficiency in resisting lateral forces. Under severe lateral loading, braces will buckle, laterally and locally, leading to a deterioration of the strength and stiffness of the SCBF and under repeated excursions of cyclic inelastic deformation to fracture of braces. On the other hand, buckling restrained braced frames (BRBF) provide more stable hysteretic behavior. The collapse resistance of SCBF and BRBF systems is examined. A series of 2, 3, 6, 12 and 16 story tall, double story X braced frame archetypes designed using the provisions of ASCE-7/05 for seismic design category D min and D max are analyzed using the ATC-63 methodology. The paper examines modeling of steel braced frames, including brace buckling and rupture due to low cycle fatigue, as well as the application of the ATC 63 methodology. Results of representative static pushover and dynamic analyses are presented. For the assumptions in the ATC 63 methodology and the current ability to model braced frame behavior, it was found that, except for low-rise SCBF structures, a high confidence of achieving the collapse prevention limit state was provided. Reasons for the behavior predicted are presented, along with recommendations for improved design and evaluation methods.