Torsional effects on the inelastic seismic response of structures

To evaluate inelastic torsional response of buildings due to different parameters such as unsymmetrical distribution of mass or lateral load resisting elements in the plan of the structure or inelastic behavior of resisting elements and loss of the resistance of such an element during an earthquake, a full three-dimensional nonlinear dynamic analysis is a powerful tool to evaluate such a nonlinear response.;The results of nonlinear dynamic analyses of two actual steel moment frame buildings that were damaged during the 1994 Northridge earthquake are subjected to recorded ground motions from the Northridge and Loma Prieta earthquakes. The importance of different parameters such as discontinuity of lateral resisting elements (setbacks), unsymmetrical distribution of mass or resistance in the plan of structure, intensity and frequency content of the earthquake ground motions, accidental eccentricity as prescribed by code and the effect of geometric nonlinearity (P-Delta) on the inelastic lateral-torsional response of the structures is discussed. Response parameters considered include lateral story displacement, Interstory drift index, plastic hinge rotation demand and torsional rotation of each floor.;The analysis procedures use three-dimensional nonlinear dynamic analytical models developed for the PERFORM 3-D computer program.;Study of the results for different models with different eccentricities clearly shows the effect of inelastic torsion in comparison with elastic torsion on the response of structures. The torsional rotation of floors considered as a main parameter of the torsional response of the building has an average increase of 30 to 60 percent for material nonlinearity. By adding geometric nonlinearity (P-Delta), this increases to as much as 70 to 100 percent of elastic torsional rotation. This clearly shows the inelastic torsional response of structures may be significantly underestimated by a linear dynamic analysis, especially for large value of mass or stiffness eccentricity and intensity of the ground motion.