OUT-OF-PLANE SEISMIC BEHAVIOUR AND STRENGTHENING OF UNREINFORCED MASONRY WALLS

Abstract

Unreinforced masonry walls are most vulnerable in out-of-plane failure mode due to very low tensile strength of masonry. The behaviour of URM walls in out-of-plane action primarily depends on characteristics of masonry and the boundary conditions. Anisotropy of the masonry, having different strength and stiffness in the horizontal and vertical directions, makes the outof- plane behaviour more complex. In order to improve the seismic out-of-plane performance of URM walls, a number of techniques employing traditional as well as advanced materials, have evolved. Strengthening techniques using low cost material, i.e. welded wire mesh (WWM) and sustainable composites, especially those reinforced with natural fibres, i.e. basalt fibre mesh (BFM) and basalt fibre reinforced polymer (BFRP) are considered for detailed study in the present Thesis. The main aim of the thesis is to develop effective and sustainable retrofit techniques for unreinforced masonry walls and to study the behaviour of masonry wallette/wall specimens, before and after retrofits, subjected to one-way monotonic and two-way reversed cyclic out-of-plane loading. To achieve this objective, URM specimens with and without retrofits are tested and their efficacy is validated using experiments. The test specimens have been simulated using analytical modelling to further understand the performance of retrofitting techniques. First, uni-axial compression and tension tests are performed to characterize the masonry and retrofit materials, respectively, in accordance with ASTM and ACI standards. The uni-axial compression test results of the experimental study are compared with the past studies and empirical models available in literature. The uni-axial tension tests are performed for characterization of reinforcements (dry wire/fibre rovings of a mesh), and the composites (wire-/fibre-mortar/BFRP) to obtain the mechanical properties. The test results provide stressstrain curve, strength, stiffness, and failure modes to evaluate the performance of constituent reinforcement and the composites. Out-of-plane behaviour of URM and strengthened masonry (small-size masonry specimens) is studied by testing a large number of masonry wallettes in one-way bending. The test specimens are made of burnt clay bricks, typically used in masonry construction in northern India. In the presented study, unreinforced and strengthened masonry wallettes are subjected to four-point bending, i.e. two-point (line) flexural loading, in one-way out-of-plane action. The masonry wallettes are tested in horizontal position without applying any axial load. The influence of different parameters, e.g. reinforcement ratio, effect of shear span, and loading direction, i.e. bending tension parallel to bed-joints (representing the walls spanning in horizontal direction) and bending tension perpendicular to bed-joints (representing the walls spanning in the vertical direction), are investigated. The effect of grid spacing of WWM and BFM, and configuration of BFRP is also examined. The load-displacement curves and failure modes of wallettes are studied in detail. The experimental results are analyzed for flexural strength, rigidity, ductility, and energy absorption capacity. The present study also identifies the suitable non-dimensional parameters to quantify the relative amount of reinforcement provided in different systems, and the relative enhancement in the moment capacity. The experimental investigation also consists of quasi-static testing of full-size H-shape masonry walls simulating real boundary conditions in two-way bending, using airbags, to determine the reversed cyclic load-displacement behaviour and improvement in the seismic performance of the masonry walls after strengthening. The results of the experiments with detailed crack patterns, wall failure mechanisms, and test observations on cyclic loaddisplacement behaviour, are presented. The strength and stiffness degradation, ductility and energy dissipation capacity of the different walls are also compared. An analytical model is developed using a section analysis procedure, based on linear strain distribution, strain compatibility and force equilibrium, to determine the one-way out-of-plane capacity of strengthened URM wallettes. The proposed model considers crushing of masonry and rupture and debonding of composite material. The performance of the proposed analytical model is evaluated by comparing the results with the experimental tests. The section analysis using the experimentally obtained stress-strain curves of the strengthening reinforcement and masonry is also used to draw the trajectory of the load-displacement response of the strengthened wallettes. A semi-empirical tri-linear displacement-based model to simulate the out-of-plane behaviour of one-way vertically spanning URM walls subjected to uniformly distributed load is also presented. The model presented considers the effect of variation of cracking height, pre-compression load, and compressive strength of masonry. The presented analytical model is validated using experimental results and is compared with the past model. The analytical part of the study also includes predicting the two-way flexural capacity of unreinforced and strengthened masonry walls using crack/yield-line method. The lateral strength of walls is obtained using the flexural strength of strengthened masonry section estimated analytically as well as obtained directly from the one-way bending tests. To determine the lateral strength, different crack patterns based on variable slope angle, crack slope angle using brick unit geometry, and actual crack patterns, are used. The analytically predicted capacities are validated using experimental test results. A tri-linear model has also been presented for the post-cracking hysteretic envelope for the URM and strengthened walls. The model parameters are determined from the experimental results of the present study and are compared with the past studies.