Computational Approach for Collapse Assessment of Concentrically Braced Frames in Seismic Regions

This paper proposes a computational approach for the collapse assessment of concentrically braced frames (CBFs) subjected to earthquakes. Empirical formulations for modeling the postbuckling behavior and fracture of three main steel brace shapes that are commonly used in CBFs are developed. These formulations are based on extensive calibrations of a fiber-based steel brace model with available information from a recently developed steel brace database. As part of the same computational approach, the representation of strength and stiffness deterioration associated with plastic hinging in steel columns and gusset-plate beam-to-column connections is considered. Through a case study of a 12-story Special Concentrically Braced Frame (SCBF), the influence of classical damping on the collapse capacity of CBFs is investigated. It is demonstrated that when SCBFs attain a negative stiffness during an earthquake, their collapse capacity can be significantly overestimated, if viscous damping is based on a commonly employed Rayleigh assumption with initial stiffness approximation. It is shown that sidesway collapse of CBFs should be traced based on a combination of criteria that associate large story drift ratios and the story shear resistance of a CBF at the corresponding story drift ratios. (C) 2014 American Society of Civil Engineers.