Nonlinear seismic modeling of reinforced concrete cores including torsion

Reinforced concrete (RC) cores are used in many residential multi-story buildings as the primary seismic force resisting system (SFRS). Due to architectural limitations, these buildings are often torsionally flexible. To assess the effect of torsion on the nonlinear seismic response of RC cores, a wide-column model (WCM) with fiber elements is used. The nonlinear warping and the nonlinear biaxial (P-Mx-My) cyclic behaviors of the WCM are validated against experimental results and exhibit excellent agreement. According to modal and linear time history analyses, the model can adequately capture the dynamic characteristics and seismic response of core structures, including torsion. The WCM is then extended to the nonlinear range to perform three-dimensional (3D) time history analyses of a typical RC building structure located in Eastern North America (ENA) that is subjected to high-frequency ground motions. Three different building configurations with increasing torsional flexibility (B = 1.7, B = 2.1 and B = 2.5, according to the current National Building Code of Canada) are studied to investigate the effect of torsion on the seismic behavior. The nonlinear envelopes of key response parameters are similar to the design envelopes obtained from the linear response spectrum analysis of a shell elements model (B = 1.7) with proper inelastic force modification factors. Aside from the story torque, the shear and moment demands remain relatively constant, regardless of the torsional flexibility value. The effective shear stiffness must be carefully selected in the WCM to avoid large questionable rotations.