Seismic Response of Buildings that Utilise Unbonded Post-Tensioned Concrete Walls

Abstract

Unbonded post-tensioned precast concrete walls are a low-damage seismic resisting system that resist lateral loads by rocking at the wall base. This thesis addresses the uncertainty that wall-to-floor interaction can have on the seismic response and performance of buildings that use such wall systems. A computational model was developed and verified to accurately calculate the bi-directional lateral-load response of isolated post-tensioned concrete walls. The wall model was extended to represent a previously tested full scale, four storey building with post-tensioned concrete walls. The building model accurately captured the measured response of the test building that was subjected to increasing intensities of earthquake motion on a tri-directional shake-table. To accurately capture the seismic response of the building the model must represent the in-plane and out-of-plane floor behaviour, account for cracking of the floor, and represent the stiffness of the precast concrete floor units. Wall-to-floor interaction resulted in deformations that were concentrated in the link slab between the wall and adjacent precast floor unit of the test building. This deformation resulted in significant over-strength demands on the wall and column, which the dynamic loading further increased. During the 1995 Kobe earthquake motion the peak wall base shear from the building model was 110% greater compared to the same building model that did not account for wall-to-floor interaction or dynamic loading. A parametric study of the building model found that increasing the rib depth of the precast floor elements or the thickness of the floor resulted in significantly greater over-strength demands compared to the original building. The results of the parametric study confirmed that if the effects of wall-to-floor interaction are not considered as part of the design process, the inelastic mechanisms that develop when a building is subjected to an earthquake may be different than predicted and result in undesirable failure modes An innovative wall-to-floor connector that could eliminate the adverse effects of wall-to-floor interaction was experimentally subjected to the combination of vertical displacement and shear forces they would experience in a major earthquake. Within their design limits the connectors performed well and effectively isolated the floor from the walls vertical displacement while transferring shear force from the floor to the wall.