Torsional responses under bidirectional seismic excitations: Effect of instantaneous load eccentricities

Torsional response for one-way or two-way asymmetric linear/nonlinear structural systems under uni- and bidirectional seismic excitations increases the maximum displacement demand on structural systems. In this study, we consider instantaneous load eccentricities caused by the motion of the center of mass. The eccentricities, which are time dependent, exist even for linear elastic two-way symmetric structures under seismic excitations, as the relative displacement between the center of mass and center of stiffness may not necessarily be negligible. The quantification of this second-order effect on structural response is not available in the literature. Equations are developed and some numerical results are obtained by incorporating this second-order effect for linear elastic structures under bidirectional excitations to quantify the importance of this phenomenon. The results obtained by considering more than 100 ground motion records suggest that, on average, an underestimation of seismic displacement demand can occur if this second-order effect is ignored, especially for two-way symmetrical systems. The degree of underestimation depends on the dynamic characteristics of the system and the record.