Effective eccentricity due to the effects of structural asymmetry and wave passage

The torsional vibrations in flexibly supported structures resulting from structural asymmetry and foundation rotation induced by the wave passage are studied. An idealized soil-structure system with five degrees of freedom is used for analysis. The foundation is considered to be embedded into an elastic half-space replaced by rigorous impedance functions. The earthquake excitation is assumed to be composed of obliquely incident SH-waves. The foundation-input motions arising from these traveling waves are computed by the averaging method of Iguchi. The combined torsional effects by structural asymmetry and foundation rotation are represented by means of an effective eccentricity that is consistent with the peak dynamic displacement at the flexible edge of the building. It is shown that this eccentricity may be well approximated by a direct superposition of the independent effects of structural eccentricity and foundation rotation. Selected numerical results are presented in terms of dimensionless parameters for their general application, using a set of typical earthquake motions for ensuring generality of conclusions. The significance of the torsional effects is elucidated and the adequacy of code design provisions is examined.