SEISMIC RESPONSE OF ASYMMETRIC STRUCTURES TO MULTIPLE GROUND MOTIONS

If ground motion is not uniform over a building's base, or if the building is asymmetric, torsional response occurs. This paper investigates the effect of torsional coupling induced by both multiple ground excitations and building asymmetry on earthquake forces. The system studied is an idealized single-story two-degrees-of-freedom building model with four supports. Spatially varying ground excitations at the supports are used. Thirty sets of spatially correlated ground-motion time histories are simulated for the analysis. Each set of the simulated time histories are compatible with an empirical coherency loss function of recorded motions, and they are also individually compatible with the Newmark-Hall response spectrum with 5% damping and normalized to 0.5g. The mean and standard deviations of base shear and torque corresponding to the 30 sets of simulated time histories are calculated. Results are presented in terms of nondimensional parameters, accounting for both the eccentricity and multiple-ground-motion-phase differences. The torsional coupling effect induced by both nonuniform base excitations and building asymmetry on earthquake forces is discussed. Some preliminary discussions on the adequacy of the torsional provisions of current building codes are also made.