r Tabatabaei - Assistant Professor, Department of Civil Engineering, Islamic Azad University-Kerman Branch
h Saffari - Associate Professor, Department of Civil Engineering, Shahid Bahonar University, Kerman, Iran

Seismic torsion leads to increased displacement at the extremes of the torsionally asymmetric building systems and may cause suffering in the lateral load-resisting elements located at the edges, particularly in systems that are torsionally flexible. To account for the possible amplification in torsion produced by seismic response, the equivalent static eccentricities of seismic forces are usually defined by building codes with simple expressions of the static eccentricity. In this paper, a simple nonlinear static procedure for evaluating the inelastictorsional response of nominally multistory buildings using modified static eccentricity is presented. This approach reveals clearly how the double eccentric system behaves inrelationship with the natural frequencies and modes and translational or torsional motion of the modeling system. In order to estimate torsional effects on the seismic response, the associated plastic mechanism is developed in the three dimensional model of building using an updatedversion of the DRAIN-3DX program. The ability of enhanced non-linear static procedure using modified equivalent static eccentricity to estimate inelastic torsional response in an asymmetric multistory building is explored through comparisons with benchmark results obtained from a set of Nonlinear Time History Response

Seismic Response, Double Eccentricities, Plastic Mechanism, Equivalent StaticEccentricity