Inelastic response of ductile eccentrically braced frames

Abstract

Resultsof nonlinear analyses of 6- and 9-story regular buildings structured with moment-resisting eccentrically braced steel frames are discussed in this paper. Models were located in soft soil site conditions (Mexico City's lake-bed zone) and designed for a seismic modification factor Q = 4.0 (the maximum allowed according to the local code). Nonlinear analyses were performed in OpenSees. Beam-column elements with plasticity spread along the element length were considered for the link beam, beam segments outside of shear links, braces, and columns. A detailed model of the connection at ends of link elements with springs was also considered. Through 1756 certified laboratory coupon tests, an improved value of the actual steel yielding stress was included in order to evaluate material overstrength capacity based on local market characteristics. Despite the fact that buildings were designed in compliance with capacity design principles, undesired soft-story collapse mechanisms were developed. According to the results, the link rotation normative limit is conservative to predict actual inelastic capacities. Based on a statistical study, a proposal to evaluate link rotation capacity was developed as a function of the cross-section of the link beam. Results indicate that links do not yield at the same time or on all floors; therefore, axial demands in columns based on a capacity design philosophy might be too conservative. An improved strategy to estimate the axial design demand in columns is proposed.