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 isto reduce the likelihood of the formation of story mechanisms in reinforced concrete specialmoment-resisting frames subjected to seismic loading. Previous research has shown thatfor tall buildings the current code requirement is not sucient to prevent these undesirableplastic 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 greaterhave shown story mechanisms to still occur. It is unclear whether complete prevention ofstory mechanisms is possible or even necessary in tall buildings. To achieve a completebuilding mechanism, the required SCWB ratio would lead to dimensions that would bedeemed unacceptable to project sponsors.To determine the eects that SCWB strength ratios have on collapse mechanisms andcollapse capacities of buildings, several structures with diering SCWB ratios and heightswere analyzed: namely, 12-, 18- and 24-story structures each with 1:2, 1:4, 1:6, 1:8, and 2:0SCWB ratios, for a total of 15 structures. Numerical modelling of these perimeter framestructures was done in Opensees. A nonlinear static analysis and an incremental dynamicanalysis (IDA) using 30 ground motions were performed on each structure. Fragility curveswere obtained using the maximum likelihood method from the results of the IDA. Theprobability of collapse given a maximum credible event, P(CjMCE), of each structure wassubsequently obtained. Maximum beam and column end rotations occurring during the IDAwere plotted to examine the types of mechanisms formed