Inelastic seismic response analysis and design of torsionally coupled systems

Kaatsız, Kaan
Torsional coupling due to irregular placement of load resisting members and/or uneven mass distribution along a story plan is a very common phenomenon in structural systems. Unsymmetrical-plan buildings with stiffness and/or mass asymmetry behave considerably different compared to regular buildings when they are subjected to earthquake-induced forces. Modern earthquake resistant design related code provisions that employ capacity design principles aim to achieve a certain amount of ductility in the structural systems while they undergo earthquake excitation. Due to torsional coupling present in asymmetric structures, load-resisting members located at different positions along the plan attain their maximum responses at different times under ground motion excitations. This usually results in unbalanced inelastic demands on members along a story. Consequently, varying ductility demands occur at these members, which are in contrast to the code provisions that utilize a single ductility target. Therefore, following code provisions that aims for a global ductility demand among all structural members can not represent the behaviour of these types of buildings properly. The proposed thesis study aims at investigating this problem. A comprehensive parametric study on a typical single story, torsionally stiff asymmetric-plan system is conceived. The results obtained are utilized to compile "Unsymmetrical Response Spectra" and "Uniform Ductility Spectra", which are proposed as assessment and preliminary design tools for estimating the seismic performance of multi-story asymmetric structures. Furthermore, "Optimal Strength Distribution Method" is proposed for use in the design of torsionally coupled systems. This optimal strength distribution, which is determined by utilizing the Uniform Ductility Spectra, is expected to reduce the inherent ductility imbalance in asymmetric systems. The performance of the proposed method is evaluated on three different multi-degree of freedom asymmetric structures and improvements in the seismic response of these systems are evaluated in detail.