Seismic performance of a post-tensioned concrete masonry wall system

Abstract

This thesis describes the development of a post-tensioned concrete masonry (PCM) wall system designed using generic materials. PCM walls derive their lateral strength and self-centering behaviour from unbonded post-tensioning. During in-plane loading, a horizontal crack forms at the wall base, which minimises masonry tensile strains and associated wall damage. An extensive literature review identified numerous instances where PCM has been used in projects worldwide, and this is reflected in the growing presence of the technology in international masonry codes. However, the lack of knowledge associated with seismic behaviour has resulted in limited use of this material in seismic zones. Although recent studies have begun to address this through pseudo-static testing, there remained a clear need to investigate the dynamic performance of such walls. Pseudo-static testing of two partially grouted PCM walls demonstrated the suitability of this system for residential structures in seismic areas. A subsequent shake table test series investigated the response of rectangular walls, walls with openings and a shrinkage control joint. The series concluded with the testing of a simple square structure that investigated multiple openings and wall corners. Rocking was shown to be the predominant deformation component, with minimal residual displacements at the conclusion of testing. Damage was restricted to the lower wall corners and above and below openings. Equations provided in international masonry codes for estimating the tendon stress at the nominal strength limit state were shown to be inappropriate for in-plane walls. A new expression was developed based on test results and finite element modelling, and was shown to provide improved accuracy, permitting the complete monotonic response of PCM walls to be predicted with excellent accuracy. An investigation into the creep and shrinkage properties of PCM demonstrated that prestress losses can be significant and must be considered in design. A displacement based design method for post-tensioned masonry walls was developed and demonstrated using a design example. The widely used bracing design method for reinforced masonry structures was adapted for PCM walls and utilised in the design of New Zealand's first post-tensioned concrete masonry house.