Coupling of torsional – translational response of buildings during earthquakes

Abstract

Coupling between the torsional and translational responses is introduced in buildings when the centre of mass and centre of resistance are not coincident. The coupling is amplified when the natural frequencies in torsion and translation are close. In this paper an attempt is made to provide guidelines for identifying situations where lateral–torsional amplification can lead to potentially severe earthquake response. The effects of different building plan configurations and arrangements of lateral load-resisting elements are first examined for idealized structures. The results show that buildings with uniformly distributed lateral resistance are especially susceptible to coupling, regardless of the plan configuration. Next, a multistorey frame building is studied for earthquake excitation, with special regard for the effect of closeness of frequencies in the presence of small eccentricity. Here, it is concluded that the common root sum of squares procedure for combining modal responses yields accurate results only when the separation of torsional and translational frequencies exceeds 20% and, also, that the occurrence of lateral–torsional coupling leads to a decrease in total base shear.