Torsion in symmetric structures due to ground-motion spatial variation

A random-vibration analysis is performed for the torsional response of symmetric structural systems subjected to spatially varying earthquake ground motion. Torsional effects are assessed by means of an equivalent eccentricity that can be expressed in terms of the system lateral displacement and rotation variances. Two case studies of systems consisting of a rigid slab supported by columns and walls are presented. A parametric analysis is conducted to evaluate the influence of local soil conditions and incoherence effects on the torsional responses of these systems. The results show that depending on the aspect ratio of the system, the soil conditions, and the times for seismic waves to travel across the base of the system, torsional effects can be significant especially for structural systems with fundamental periods close to the predominant period of the ground.