NONLINEAR BEHAVIOUR OF BRICK MASONRY SUBJECTED TO IN-PLANE LOADING

Abstract

This thesis presents a comprehensive material model which is capable of simulating the in-plane behaviour of masonry from elastic range to final failure for any angle of bed joint orientation. The final element model and computer code are developed for the analysis of masonry infilled frame and shear walls to demonstrate the suitability and accuracy of the material model. The proposed model accounts for most of the observed sources of material non-linearity in biaxial stress state. The proposed elasto-plastic model includes the generalised anisotropic quadratic failure criterion in three dimensional stress space to take into account the directional strength properties of brick masonry at failure. The magnitude of interaction between stresses like other failure theories is not constant. It is restrained in such a way that the shape of the failure surface is ellipsoidal. The accuracy of the failure criterion depends upon the interaction strength parameter. To determine this parameter stability condition must be checked. Sensitivity analysis is carried out to select the type of test result to be used for the determination of the interaction strength parameter, because a small inaccuracy in test results may change the value to a large extent such that the failure surface may become hyperboloid instead of ellipsoid. Associated flow rule is used for modelling the elasto-plastic behaviour of brick masonry. Smeared crack approach is employed to model the tensile behaviour of masonry. Maximum stress criterion is used for initiation and propagation of cracks. Tensile strain softening rule is employed to model the post cracking behaviour of masonry. Closing and reopening of cracks are allowed following the secant path. The material nonlinearity due to slip and/or cracking in the mortar joint at the interface between the frame and the infill and at the

interface between masonry wall is incorporated in the finite element model. The results of the finite element analysis of masonry frame and shear walls are compared with the experimental results to validate the accuracy and versatility of the proposed material model. A good agreement between the analytical and experimental results is observed. The model has also been used to analyse shear walls to study the influence of horizontal to vertical load ratio, effect of vertical stress level and the influence of strength properties of brick masonry on the behaviour of shear walls. It is revealed from studies that the computational model developed and implemented in the computer code is able to reproduce accurately the non-linear behaviour of masonry structures subjected to in-plane loads. A micro mechanical brick masonry model is proposed to determine strength and elastic properties of brick masonry in terms of those of its constituents. The formulae have been proposed for computing the stress distribution in the bricks and mortar joints for known stresses in masonry. So the finite element analysis at micro-level is not necessary to find out the stresses in the bricks and mortar joints in the brick masonry. Micromechanics investigations are carried out to achieve desired strength and stiffness of brick masonry in terms of strength and elastic properties of brick and mortar so that brick masonry can be analysed based on macro level approach. Thus economy in cost and time can be achieved for the analysis and design of masonry structures.