Comparison of the Drift Spectra Generated Using Continuous and Lumped-Mass Beam Models

The drift spectrum as a new measure of earthquake induced demands in structures, was developed using a uniform-stiffness continuous beam model. Practical limitations in the procedures based on such models have caused structural engineers to pay little attention to drift spectra. In this paper, a method is proposed to estimate the seismic-induced inter-story drift demands using the modal analysis technique of lumped mass, non-uniform beam models. The proposed approach is simple and can overcome many deficiencies of the previous methods. It can take into account the very important effect of height-wise stiffness reduction on drift demands, without the numerical difficulties usually encountered in computing the drift spectra. The Drift spectra for high-rise shear buildings are computed using both continuous and lumped mass beam models. The comparisons indicate that the results of the continuous uniform-stiffness beam models in some cases are not acceptable. The effect of various parameters such as the structural system, the height-wise variation of structural stiffness, the number of stories, higher modes and damping ratios, on inter-story drift demands are investigated. Also, the importance of the structural system and height-wise variation of structural stiffness on the height-wise distribution of maximum inter-story drift demands is evaluated using a number of building models.