Effects of Strong-Column Weak-Beam Ratios on Collapse Capacities of Tall Reinforced Concrete Moment Frame Structures

The intent of the strong-column weak-beam (SCWB) strength ratio in building codes is to reduce the likelihood of the formation of story mechanisms in reinforced concrete special moment-resisting frames subjected to seismic loading. Previous research has shown that for tall buildings the current code requirement is not sufficient to prevent these undesirable plastic mechanisms from forming and leading to collapse of the structure. Furthermore, nonlinear analyses of structures greater than four stories with strength ratios of 2:0 or greater have shown story mechanisms to still occur. It is unclear whether complete prevention of story mechanisms is possible or even necessary in tall buildings. To achieve a complete building mechanism, the required SCWB ratio would lead to dimensions that would be deemed unacceptable to project sponsors.;To determine the effects that SCWB strength ratios have on collapse mechanisms and collapse capacities of buildings, several structures with differing SCWB ratios and heights were analyzed: namely, 12-, 18- and 24-story structures each with 1:2, 1:4, 1:6, 1:8, and 2:0 SCWB ratios, for a total of 15 structures. Numerical modelling of these perimeter frame structures was done in Opensees. A nonlinear static analysis and an incremental dynamic analysis (IDA) using 30 ground motions were performed on each structure. Fragility curves were obtained using the maximum likelihood method from the results of the IDA. The probability of collapse given a maximum credible event, P(C|MCE), of each structure was subsequently obtained. Maximum beam and column end rotations occurring during the IDA were plotted to examine the types of mechanisms formed.