STRENGTH OF LOW RISE P BRICK MASONRY CONSTRUCTION

Abstract

Use of brick masonry for construction of low and medium rise buildings is indispensable in a developing country like India for reasons of economy, speedier construction, local availability of materials and saving on scarce materials like cement and steel. And in spite of brick masonry being a most commonly used building material, the state of background research on masonry is rather dismal so that practi^asing engineers and architects look upon this material with suspicion. As a result, even though good quality bricks are available in northemparts of the country, structural potentialities of masonry are yet to be exploited satisfactorily, especially in view of significant developments made and success achieved overseas in the last three decades. The present study is aimed at investigation of mainly the following three aspects of masonry construction: (i) Experimental and analytical determination of structural properties of constituent materials and masonry and investigation of a failure mechanism in masonry, (ii) Development of a methodology, for analysis and strength prediction of isolated masonry wall panels on the basis of control specimen results, applicable under different types of loading, edge conditions and presence of openings etc., and (iii) Development of suitable methods for exact as well as simplified three dimensional analysis of complete masonry buildings predicting their behaviour satisfactorily. (iii) The present study, however, does not pretend to carry out investigations to cover the vast number of factors which influence the strength characteristics of masonry and the behaviour of masonry structures. Emphasis, therefore, has been on the development of methodology and procedures restricting the scope of investigations up to first cracking only. In the first part of the study, suitable control specimens have been proposed to determine tensile, shear and compressive strengths as well as elastic properties of masonry more realistically which include the effect of work manship and degree of control at site. The proposed control specimen for compression test on masonry is a small square panel which facilitates evaluation of masonry properties for both directions of loadging. Empirical relations have also been proposed to predict the strength and elastic properties of masonry for local conditions. Realising that a prism model is inadequate to relate with the behaviour of a panel wall, a panel type model has been proposed to predict masonry strength and elastic properties by a simple two dimensional finite element analysis assuming a coulomb type failure criterion in the two constituents. Recognising that the finite element analysis would yield the 'exact' results, its use for study of isolated panels and complete buildings has been made economically viable by proposing a composite masonry element. Characteristics of such composite elements have been determined using analytical (iv) models or experimental properties. Stress strain character istics of the different types of masonry investigated have shown that masonry behaves practically linearly except near the final crushing and hence, a linear analysis is considered adequate for the present study. Accuracy of proposed formulations has been established by comparison with test results of a few panels reported in literature. In a limited parametric study under taken, nearly 180 different cases of panels with or without an opening have been analysed. Based upon these results, simple expressions for such solid or pierced panels have been proposed either for the estimation of critical stress for design or for strength prediction on the basis of test results of the proposed control specimens using a simple limiting tension type failure criterion. The conventional methods of analysis of masonry buildings are inadequate as they fail to take cognizance of all the complexities arising from three dimensional nature of the problem, interaction between walls and between walls and slabs, orthotropy of the material and presence of an opening in a panel etc. Taking advantage of the 'exactness' of the solution based on finite element technique, two different computer programs; 'PCMAB' which considers both flexural and membrane actions in the individual planar elements, and 'MAPWB' which ignores the flexural action altogether, have been written for computer aided analysis. The joints in the masonry construction have been idealized as rigid and pinned (v) in the two formulations respectively. The programs have been used to analyse two buildings reported in the literature and comparison as regards the accuracy and computational economy has been made. In addition, an approximate method 'AASWB1, using 'equivalent cantilever' approach, has been proposed for a simplified three dimensional analysis of masonry buildings under lateral loads. In order to facilitate idealisation of a three dimensional structure by a number of two dimensional structures so that a simplified two dimensional procedure of analysis may be adopted, an analytical approach for investigation of the problem of interaction between wall and wall in a masonry building has been proposed and the same has been adopted for studies on a cellular and a cross wall type building. Further, membrane type finite element analysis of a multistoreyed masonry building also becomes prohibitive in case of buildings having many wall panels with openings. A simplified procedure has been presented for economy in such analysis by replacing pierced wall panels by equivalent solid panels. The accuracy obtained by the above simplifications has been established by suitable examples.