Reliability and fragility curve assessment of steel concentrically braced frames

Abstract

This article presents the results of a study in which a newly developed performancebased plastic design (PBPD) methodology was applied to steel special concentric braced frames (SCBF). In PBPD method, pre-selected target drifts and yield mechanisms are used as performance limit states. The design base shear for selected hazard level(s) is calculated by equating the work needed to take the structure monotonically up to the target drift to that required by an equivalent elastic-plastic single degree of freedom system to achieve the same state. In addition, a new lateral force distribution has been developed based on the relative distribution of maximum story shears obtained from inelastic dynamic analysis. Plastic design method is performed to detail the frame members and connections to achieve the intended yield mechanism and behaviour. Results of inelastic dynamic analyses carried out on example frames designed by PBPD method showed that the frames met all the intended performance objectives in terms of yield mechanisms and target drift levels. On the other hand, when designed by current seismic design code as SCBF, they showed very poor response due to premature brace fractures leading to unacceptably large drifts and instability. Reliability-based assessment based on FEMA 351 indicated that PBPD frames have much higher confidence levels against global collapse than those of corresponding SCBFs designed by current seismic codes. Finally, the results are proved by determining the seismic fragility curves of model frames.