Effects of Strong-Column Weak-Beam Ratios on Collapse Capacities of Tall Reinforced Concrete Moment Frame Structures
- Author(s): Cagurangan, Colleen
- Advisor(s): Moehle, Jack P
- et al.
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.