Evaluation of the Torsional Response of Multistory Buildings Using Equivalent Static Eccentricity

The equivalent static eccentricities of seismic forces are usually defined by building codes with simple expressions of the static eccentricity. The inelastic torsional response of multistory buildings using nonlinear static analysis (NL-SA) with the equivalent static eccentricities is less easily predictable because the response is influenced by the extent of plastic deformations, and the eccentricity varies story by story at each step. To overcome this weakness, it is necessary to change the location of static eccentricity in each step of the NL-SA method. In this paper, a simplified procedure for the correct calculation using the NL-SA method with varying static eccentricity based on the modal response of a single-story scheme is presented. In order to estimate torsional effects on the seismic response, the associated plastic mechanism is developed in a three dimensional model of building, using an updated version of the DRAIN-3DX program. The torsional response of three different structural systems with different eccentricities and varying ground excitations is investigated. The results of the nonlinear response history analysis (NL-RHA) indicate that the torsional responses using the proposed procedure (using updated static eccentricity) and those obtained by the adaptive modal combination (AMC) method at both the flexible edge and the stiff edge are more consistent than the NL-SA with static eccentricity.