This study evaluated the inelastic torsional response due to instantaneous load eccentricities. The load eccentricities, which caused by the motion of the center of mass are time-dependent, even exist for two-way symmetric structures under seismic excitations. The eccentricities and bi-directional horizontal excitations can lead to the torsional motion. The study of the impacts of such a second-order effect (called A-Delta effect) on structural systems and, this effect in combination with the P-Delta effect on structural systems has not been reported in the literature.
This study is focused on the investigation of the structural responses under the A-Delta and/or P-Delta effects. For the assessment of inelastic seismic displacement demand and inelastic torsional response of buildings, the structure is represented using idealized one-story model and each lateral load resisting element is modeled using the Bouc-Wen hysteretic model. The governing equations of motion were developed by considering these effects and the structures that are subjected to biaxial excitations. The numerical analyses were carried out by implementing the governing equations in MATLAB. Since the ground motion is uncertain and varies from record-to-record, 123 ground motion records from 11 California seismic events were considered to take into account this record-to-record variability.
The results indicate that a slight underestimation of seismic displacement demand is observed, if the instantaneous load eccentricities are ignored, especially for two-way symmetrical systems. On the other hand, when considering both the instantaneous load eccentricities and P-Delta effect, the instantaneous load eccentricities can introduce significant changes on both lateral and torsional displacements, if the stability factor is large
