Seismic demand on brace-intersected beams in two-story X-braced frames

Abstract

A study on the seismic demand on braced-intersected beams in two-story X-braced frames is presented to address a major concern: whether or not the beam is likely to become inelastic during the design earthquake ground motion, and if so, how such inelastic deformation affects the seismic behavior of the frames as a whole. Typical 6- and 12-story buildings using two-story X-braced frames with strong and weak braced-intersected beams were subjected to a set of 20 earthquake ground motions, and resulting seismic responses were discussed in terms of seismic strength and deformation demands on beams and the impact of inelastic deformation in weak beams on critical components in two-story X-braced frames. In addition, the 6- and 12-story buildings with inverted V-braced frames were also studied for comparison. The study concludes that brace-intersected beams in two-story X-braced frames designed with the minimum possible required strength permitted by the current design provisions would undergo significant vertical inelastic deformation when the frames experience the expected 0.02–0.04 story drift ratio response, and the inelastic deformation in the middle spans of brace-intersected beams substantially increases ductility demands on braces and beam-to-column connections. This study also finds that the use of story drift ratio as sole deformation response index fails to reveal actual seismic response of critical components in two-story X-braced frames when beams experience inelastic deformation within their own spans.