Finite element analysis of unreinforced masonry walls subjected to inplane loads

Abstract

Masonry was considered as a highly ranked structural material in the past. It is viewed as a composite material consisting of relatively strong units and weak mortar joints. Its behaviour is made more complex by mortar joints acting as planes of weakness due to their low tensile and shear strength. This thesis work is an attempt to give a comprehensive picture of the art of modeling of unreinforced masonry, demonstrate modeling approaches for unrein forced masonry structures with nonlinear FE package to simulate their behaviour and subsequently study the various strengthening arrangements available to strengthen unreinforced masonry structures in recognized literatures and Bangladesh National Building Code (1993). Masonry can be numerically represented by three different approaches, Le. detailed micro modeling, simplified micro modeling and macro modeling. Each one of these approaches requires its own set of material parameters and its application focuses on a distinct type of problem. Micro models are believed to be the best tool available to understand the behaviour of masonry. But macro modeling approach is preferred to micro modeling approach for practice-oriented large masonry structures. The recent development of FEM and computer software package has made it possible to develop micro and macro models for brittle materials like masonry and study their behaviour. A complete model requires the elastic properties of masonry, inelastic stress-strain relations and failure criteria. A number of unreinforced masonry walls/structures were modeled with macro modeling approach and one small masonry wall was modeled with micro modeling approach by nonlinear FE package ANSYS5.6. These models were validated with experimental results and other numerical results. Reasonable good agreement was found between those results. Sensitivity analyses of unrein forced masonry structures were also carried out for the selection of correct size of mesh. The FE macro models were then used to incorporate various strengthening arrangements like inclusion of post-tensioned vertical or horizontal tie-bars with steel plate or concrete in masonry, placing of grid works at the top of unrein forced masonry structures and strengthening arrangements as per Bangladesh National Building Code . (1993). It was observed that the inclusion of post-tensioned vertical or horizontal tiebars with steel plate or concrete provides quite effective results. The use of grid work at the top of unrein forced masonry structures drastically reduces the tensile stresses, shear stresses and horizontal displacement. The use of integrated roof with unrein forced masonry structures in addition to lintel, roof band and reinforcement in masonry provides the best performance amongst all the cases analyzed. The performances of different strengthening arrangements are also analyzed with gradual increase of inplane lateral loads to determine the seismic load carrying capacity for unrein forced masonry structures. The use of lintel, roof band and inclusion of reinforcement can increase the seismic load bearing capacity of unrein forced masonry structures by three times and the use of integrated roof with unreinforced masonry in addition to lintel, roof band and reinforcement in masonry can increase the seismic load bearing capacity of unreinforced masonry structures by almost four times.