UC Berkeley

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 sucient 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 eects that SCWB strength ratios have on collapse mechanisms and

collapse capacities of buildings, several structures with diering 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(CjMCE), 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.