PERFORMANCE EVALUATION OF CONFINED MASONRY BUILDINGS UNDER CYCLIC LATERAL LOADS

Abstract

Most of the low-to-medium rise buildings in many countries, including India, are of unreinforced masonry (URM), which is known for its poor seismic performance. Moreover, the brick masonry construction in India shows a large variability as compared to those in developed countries. These contrasting differences pose challenge and call for improved technologies for masonry construction. In contrast, Confined Masonry (CM), comprising of masonry panels embraced with lightly reinforced concrete elements, is expected to have better seismic performance. However, CM potentialities are yet to be fully exploited in Indian context. Limited number of tests were performed to understand seismic behaviour of scale-down three-dimensional CM buildings, elsewhere in the world, none in India. These tests do not sufficiently cover the possible combinations of parameters such as size and location of openings, aspect ratio and slenderness of panels, axial load, boundary conditions, local material and construction practices etc. Further, little information in literature about seismic behaviour and design features of CM building, with no such attempt in India, urges to address the gap in holistic manner. Accordingly, the present work is aimed to study the behaviour of a full-scale CM building by conducting experimental investigation and numerical analysis. The information is utilized to develop design features along with its economic aspects and a comparison with already available test data of URM and RM models. The scope of work encompasses: 1. Characterization of masonry and its constituents, with reference to material available in Roorkee, Uttarakhand (State of India), subjected to compressive and tensile loads to study strength, stress-strain relationships and failure mode. 2. To study the interaction between brick and mortar of masonry prism subjected to (a) loading normal to bed joint (stack bonded prism) and (b) parallel to bed joints. 3. Experimental and numerical evaluation to study behaviour of a full-scale CM building subjected to lateral reversed cyclic displacements under quasi-static condition. 4. To develop design features for CM buildings, based on experimental results. 5. To perform economic analysis and compare the construction cost of URM, RM and CM building typology with RCC framed structure. First phase of the programme deals with the study of strength, stress-strain relationship and failure mode of burnt solid clay brick units; cement:sand (C:S) and cement:lime:sand (C:L:S) mortar of different grades and different types of masonry prisms. Series of tests were conducted on 20 solid burnt clay brick units; 24 specimens ABSTRACT iii of cubes and cylinder of mortar with four different grades; 24 brick masonry prisms and 12 brick masonry wallets to establish the properties. The strengths of masonry and its constituents obtained through tests, demonstrate substantially lower in comparison to the values that are used in the other parts of the world. The reason for this could be attributed to inferior raw material and process of manufacturing of brick units. From the experimental results it is observed that the shear modulus of masonry is of the order of 12% (C:S mortar) to 18% (C:L:S mortar) of elastic modulus of masonry, in contrast to the recommended values of 40% in EC6. Thus, the empirical relations prescribed in EC6 cannot be readily used with brick masonry, in Indian context, hence alternative empirical relations are required to be established. An empirical relationship for estimation of masonry strength from the known constituent material properties is proposed and validated for the material used in the present context. In the second phase, experimental research programme is performed on a typical full-scale single storey CM building subjected to lateral reversed cyclic displacements under quasi-static condition, at roof level. Key aspects examined are: crack pattern, lateral strength, drift, ductility, stiffness degradation, behaviour factor, and energy dissipation capacity. Due to restrained-rocking mechanism, CM suffers from flexural and sliding/diagonal shear failure at in-plane walls, while at higher displacements, crushing of masonry in compression toe takes place. Spalling of concrete cover and bending of reinforcement of tie-column resulted in strength degradation by around 13% of peak strength. The connection of tie-column – bond beam and RC slab of CM offer integral action with masonry resulting into no sliding, separation or uplift of RC slab. This action also controls the crack pattern and behaviour of CM to a considerable extent. The interface between tie-column and masonry is more vulnerable against separation for out-of-plane walls than that of in-plane walls at higher deformation. However, in-plane wall with openings are susceptible to diagonal shear cracks. A comparison of seismic performance of CM building with the previously tested URM and RM building having similar geometry and material configuration, establishes that the CM typology is far superior. The lateral strength achieved by URM and RM buildings is only 29% (44.5kN) and 38% (57.85kN) respectively, as compared to CM (152.25kN). Also the attainment of high initial stiffness in CM was attributed due to presence of confining elements. The test carried-out in the present work indicates that the CM is a promising masonry construction technique for low-to-medium rise ABSTRACT iv construction in seismic regions. However, it will be prudent to establish this fact through more number of such tests. To validate the experimental results, Stiffness Adaptation Analysis (SAA), is implemented using a commercially available software TNO-DIANA. The discretization of 3D CM building is carried out using 4-noded quadrilateral shell elements with four integration points in the plane and three integration points in thickness direction. The material properties derived from the masonry and its constituent tests are used as input to simulate the building response subjected to lateral displacements. Hordijk (HORDYK) and Thorenfeldt (THOREN) material models are used for tensile and compressive softening of masonry respectively. The result shows that SAA predicts the behaviour of CM building effectively with minimal number of input parameters resulting in computationally inexpensive technique. The applied methodology overestimate the capacity curve, by around 9%, which could be attributed to application of monotonic loading and can be implemented in the experimental study. The third phase of the programme refers to the development of design features and economic aspects of CM buildings. The design features developed for confined masonry broadly comprise checks for wall density, calculation of loads, checks for wall stability, lateral load resistance, stress verification for in-plane/out-of-plane walls, and design of confining elements. These design steps are implemented on tested CM building model to validate experimental values. Further, design features are implemented for 2, 3 and 4 storeyed CM buildings to establish the accuracy of the design steps. To establish economy in construction, cost analysis and comparison of costs of different building typologies is carried-out on 20 building samples up to 4 storeys. The buildings are designed as per Indian standards keeping uniform design parameters. The construction cost of URM, RM and CM are compared with RC building cost, as reference. As a result, CM, RM and URM buildings allow for average cost reduction of structure by 30%, 33% and 36% respectively, with reference to RCC framed buildings. Based on the above findings, the thesis has provided (a) a better understanding of masonry, its constituents and fundamental behaviour of CM construction, especially in the Indian context under quasi-static reverse cyclic loads. (b) design features for seismic resistant CM construction; and (c) demonstration of cost-effectiveness of CM for seismic regions. The above contributions may lead to more informed seismic safety, structural design, and economical aspects of CM buildings, especially in Indian perspective.