STUDY OF REINFORCED HOLLOW CONCRETE BLOCK MASONRY BUILDINGS

Abstract

In today's world precast walling element are being used frequently in construction of walls. One of them is hollow concrete block masonry, which is gaining popularity amongst the engineers and architects because of its certain advantages over the other units. The use of hollow block concrete masonry was accelerated by the competitive development of block making machines. Hollow units are not only lighter and therefore more easily handled but also allowed the integration of steel reinforcing bars to resolve tensile stresses. This has opened the potential of use of hollow blocks in seismically active regions. Reinforced Hollow Concrete Block Masonry (RHCBM) has provided structural and architectural advantages in one material and is now recognized worldwide as a major contributor to the construction and building industries. In the context of India, though, it is of recent origin compared to other building materials like brick and stone, it has established for itself a unique place in the prefabricated system of building construction. Structural properties of the RHCBM are yet to be established in Indian context in a comprehensive manner, especially in view of significant developments made and success achieved in the past two decades. The present study is aiming at investigations of mainly the following aspects of RHCBM construction: . Experimental and analytical studies to assess the structural performance of RHCBM Buildings with the aim to understand and predict the response of a low rise RHCBM building. In this process a suitable material model for RHCBM buildings by carrying out tests in compression, flexural tension and shear is investigated. Tests for understanding the joint behaviour between slab and walls and to develop the moment versus rotation relationship which will help in predicting the response of the building in a more realistic manner. in • The effect of different parameters such as block strength, mortar strength, grout strength, etc. are to be studied to quantify their contribution to over strength of masonry and also to an extent that an optimum combination of these is achieved to be used in RHCBM buildings. • Development of a methodology, for analysis and strength prediction of RHCBM on the basis of the control specimen results, applicable for different types of constituent materials used in construction of RHCBM. The aim is to predict the response of building under monotonic loading and to validate the proposed material models and moment-rotation relationship for the analysis of 3-D RHCBM building. • Suggestions for a suitable design procedure of RHCBM buildings keeping in view the torsional forces generated in the event of an earthquake. The present study, however, does not pretend to carryout the 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 to develop some basic engineering properties for RHCBM and development of a suitable analytical model to cater for the wall-slab joint behaviour. Design procedure of RHCBM taking into account the effect of torsion forces due to earthquake is also presented but restricting the scope of investigation up to first cracking in the linear region. In the first part of the study, suitable control specimens have been proposed to determine shear, flexural tensile and compressive strengths as well as elastic properties of RHCBM more realistically which include the effect of different parameters e.g. blocks strength, grout strength, reinforcement and mortar. Realising that the prism model suggested by IS: 1905 (Indian code on unreinforced masonry) is not the most suitable, wallet for compression, capable of taking the contribution of vertical joint in over all strength of reinforced masonry are tested. Empirical relation for prediction of compressive strength of RHCBM and the stress- IV strain curves upto failure load in compression have been presented in this study. The simple but efficient assembly for joint shear test has been suggested for varying pre-compression and flexural tensile strength values for different grout and mortar strength have been obtained. The inadequacies in Indian masonry codes regarding reinforced masonry and their properties are critically reviewed and the comparison of test results with international literature is discussed. Wall-slab interaction testing to ascertain the moment rotation relationship of the joint is also discussed to the extent that a methodology for predicting its behaviour analytically can be formulated. Recognising that the finite element analysis would yield the 'exact' results, its use for the study of wall-slab joint behaviour and complete reinforced masonry building has been made by proposing a 2-noded 3-D general spring elements to model the joint. These elements are capable to take three translational stiffness and three rotational stiffness at each node in mutually perpendicular directions. The properties of these elements are taken from the experimental investigations and the finite element analysis using general purpose FEM package NISA have been carried out. The proposed model is validated against the experimental results of wall-slab joint test. The finite element analysis has been carried out for a five storey test building using the same approach. The results of these finite element analysis with and with out spring elements at wall-slab joints are compared by experimental investigations available in the literature for the complete building. A comprehensive design procedure for reinforced masonry buildings based on modern design provisions having forms similar to the ultimate strength provisions for reinforced concrete is suggested. This design procedure, based on shear wall design, is an improvement over the existing "no tension" design given in IS: 1905 and it takes into account the effect oftorsional forces coming into effect because of difference in centre of mass and centre of rigidity. Step by step procedure is explained with help of a design example of a four storey load bearing RHCBM building. The permissible stress values obtained from experimental investigations on RHCBM are used in this design. A computer program is also developed which is thought to be handy to compute the relative rigidities of walls, torsional moments, shear distribution etc. Finally the program is capable of computing the reinforcement required to be provided in horizontal and vertical direction in each wall at every storey of the building. In order to check the economic viability of RHCBM as against RCC construction, the reinforcement needed in RHCBM building is compared against that required in RCC building assumed to be constructed on the same plan area. Further a parametric design study using number of different building plans have been carried out to study the effect of wall opening, location of walls and wall density for RHCBM construction. VI