Seismic performance evaluation of eccentrically braced frames with long links using FEMA P695 methodology

Abstract

Short links that primarily yield under shear are usually preferred in eccentrically braced frames (EBFs) due to their high rotation and energy dissipation capacities. Long links that yield under flexure can be used in cases where large openings are required for architectural reasons. Research conducted in the past showed that the seismic response factors recommended in ASCE7 result in designs with higher collapse probabilities than expected for EBFs with shear links. Long link behavior differs from the behavior of short links because the former is governed by flexure and subjected to significant amounts of strength and stiffness degradation. A numerical study was undertaken to evaluate the seismic response factors for EBFs with long links using FEMA P695 methodology. Twenty-four EBF archetypes were designed by considering the bay width, number of stories, the link length to bay width (e/L) ratio and column base condition as the variables. Performances of these archetypes were evaluated under maximum considered earthquake (MCE), and collapse level earthquake (CLE). The effects of degradation were studied by considering degrading and non-degrading responses separately. The results showed that strength and stiffness degradation increases the link rotation angle as much as 46 percent when compared with the non-degrading models. The recommended response factors were found to provide acceptable performance for e/L = 0.5, when 20% probability of collapse is considered under MCE level events. Remedial measures were investigated to achieve acceptable performance for collapse probability of 10% under MCE level events.