Abstract

Eccentrically braced frames (EBFs) are very effective lateral load resisting systems against earthquakes because of their hybrid behavior that includes great stiffness corresponding to the bracing actions, and considerable ductility owing to the inelastic activity of a small part of the floor beam called link. Current capacity design approach of EBFs is to confine the inelastic behavior of the frame primarily to the link such that other members remain essentially in elastic range. Limited research works are available in the literature studying the behavior of multi-storey EBFs designed according to the current Canadian seismic provisions. This thesis was aimed to study seismic performance of EBFs through nonlinear time history analysis (NLTHA) of three 4-, 8- and 14-storey chevron eccentrically braced frames. The selected EBFs were subjected to real and artificial ground motion records scaled to match the response spectrum of Vancouver. All the EBFs, designed according to current capacity design provisions, exhibited excellent seismic performance in terms of stiffness, strength, and ductility. It was observed that the inelastic link rotations of all EBFs were lower than the design limit of 0.08 rad, except for the two upper floors in 14-storey EBF where the link rotation slightly exceeded the limit. Seismic analysis also showed that maximum inelastic link shear forces exceeded the values recommended in design for many earthquake records.
Although NLTHA can precisely estimate structural seismic responses, it requires high computational demands making it impractical for engineering design offices. On the other hand, the conventional pushover analysis does not consider contributions of higher modes to the structural responses and thus it often does not provide good estimation of seismic responses for taller buildings. Capacity-Spectrum Method (CSM) and modal pushover analysis (MPA) are two simple nonlinear static methods that have been proposed and recently used for seismic performance evaluation of few lateral load-resisting systems. This research further studies the application of CSM and MPA as alternatives for rigorous NLTHA to estimate seismic performance parameters of EBFs. The three selected EBFs were analyzed using MPA and CSM and the results were compared with nonlinear seismic analysis results. It was observed that both CSM and MPA predicted the peak top displacements of EBFs with sufficient accuracy.