Coupled lateral-torsional behaviour of frame structures under earthquake loading

A series of parametrically defined experimental model structures has been tested under earthquake base loading using the SERC national U.K. earthquake simulator. The models have been designed with variable ratios of torsional to lateral stiffness, and with both symmetric and asymmetric mass distributions. This paper first describes the tests carried out to determine the basic dynamic model properties and the establishment of idealized analytical models which give accurate predictions of model behaviour under steady-state loading and free-vibration conditions. Secondly, a detailed discussion is made of the two highly coupled structural models having uncoupled torsional to lateral frequency ratio R_f = 1.2, commenting on the ability of the modal analysis procedures to predict accurately the maximum recorded responses. It is concluded that the theory underestimates the significance of the fundamental torsional mode of vibration in the combined structural response, and overestimates the contribution of the first lateral mode. These effects compensate each other on the side of the structure which is most severely affected by torsional response, but produce large inaccuracies on the side of the building which is commonly assumed to be affected beneficially by torsional coupling.