Selection of Optimal Intensity Measure for Seismic Assessment of Steel Buckling Restrained Braced Frames under Near-Fault Ground Motions

Buckling restrained braces (BRBs) have a similar behavior under compression and tension loadings. Therefore, they can be applied as a favorable lateral load resisting system for structures. In the performance-based earthquake engineering (PBEE) framework, an intermediate variable called intensity measure (IM) links the seismic hazard analysis with the structural response analyses. An optimal IM has desirable features including efficiency, sufficiency and predictability. In this research, the efficiency and sufficiency of some traditional, cumulative-based, and advanced scalar IMs to predict maximum interstory drift ratio (MIDR) demand on low- to mid-rise steel structures with BRBs, under near-fault ground motion records having forward directivity, are investigated. The results show that most of the IMs considered are not sufficient with respect to source-to-site distance (R), for predicting MIDR. It is also shown that decreasing the strain hardening ratio decreases the efficiency of the IMs. In addition, IMM(λ=0.5) and Saavg are more efficient and also sufficient with respect to pulse period (Tp), for predicting MIDR demand on the low-rise steel BRB frames under near-fault ground motions, when compared with the other IMs. In the case of mid-rise structures, PGV and IMM(λ=0.33) are selected as optimal IMs. Due to the higher efficiency and sufficiency of the selected optimal IMs, the obtained fragility curves calculated using these IMs, are more reliable in comparison with the fragility curves calculated using other IMs.