SEISMIC EVALUATION AND RETROFIT OF MASONRY BUILDINGS

Abstract

Unreinforced masonry (URM) is one of the oldest and most widely used construction materials in the history of mankind. Like other parts of the world, there are large numbers of URM buildings in Indian subcontinent, most of which have not been designed for seismic loads. Recent Earthquakes have exposed the seismic vulnerability of these URM buildings. The masonry piers of URM buildings are generally strong enough to bear the compressive stresses, but weak in bending tension and in-plane shear. Further, the out-of-plane bending of the walls makes the buildings susceptible to failure during earthquakes. In order to improve the seismic performance of URM buildings, a number of techniques employing traditional as well as advanced materials, have evolved. A comprehensive review of different strengthening techniques used worldwide for in-plane and out-ofplane strengthening of URM components is presented in this Thesis. Tests are conducted according to the relevant ASTM standards for URM and strengthened specimens using ferrocement (Micro-concrete and Welded Wire Mesh) overlays. The effect of strengthening in enhancing performance of URM in ‘in-plane’ shear is studied in terms of effective modulus of rigidity, failure modes, shear strength, maximum drift capacity and pseudo ductility. Two different cases, involving uniaxial and biaxial anchorage of WWM are considered to replicate the field conditions in order to understand the behavior of splints and bandages respectively. Finite element (FE) method is used to simulate these experimental results. The analytical results are compared with the experimental findings, in terms of shear strength-drift plot and damage pattern. The similar tests are also conducted on URM and strengthened specimens to study enhancement in out-of-plane behavior. These results are also compared with analytical simulations using FE Method. An analytical study using ordinary beam theory is also performed on the strengthened specimens to obtain and force-deformation curves. A critical review of the modelling methods for masonry with a typical existing north Indian masonry school building is presented. The expected seismic performance and fragility of the considered school building is evaluated, before and after retrofit to iv Abstract quantify the enhancement in its seismic performance after retrofit using ferrocement splints and bandages. An Equivalent Frame Model of the considered building is developed to estimate the nonlinear capacity curve. The nonlinear behavior of piers and spandrels in the existing is modeled by assigning elasto-plastic hinges at pre-defined locations in the Equivalent Frame Elements. The material properties estimated from the tests are used to estimate the pier capacities in flexural (rocking), and diagonal and sliding shear. Governing failure criteria is identified based on the minimum capacity of piers. The ASCE 41 component modelling and acceptance criteria are used in the nonlinear static (pushover) analysis. The expected performance and damage probability of the existing building at anticipated DBE and MCE levels of hazard are estimated using the capacity spectrum approach of HAZUS. The performance of the building is also evaluated in terms of the maximum sustained peak ground acceleration using Displacement Modification Method (DMM) of ASCE 41. The performance of the building is found to be unsatisfactory as it is expected to collapse at MCE and a high probability of ‘extensive’ and ‘complete’ damage is anticipated even at DBE, necessitating retrofit of the building. The procedure for retrofit of the building using ferrocement splints and bandages is demonstrated through detailed drawings and photographs of on-site application. Equivalent Frame Method is extended for evaluation of seismic performance of the retrofitted building. The strength of masonry walls in different failure modes with ferrocement strips on both sides of walls is estimated using the MSJC code provisions and recent research contribution of Ghiassi et al. (2012). Nonlinear-static (pushover) analysis is performed and the capacity curves are developed to obtain the performance point and fragility functions and Damage Probability Matrix (DPM). The performance of the building is shown to enhance significantly after retrofit. The efficacy of strengthening method is also demonstrated through dynamic testing of half scale URM and retrofitted building models on Shock-Table. One traditionally constructed and the other strengthened model is tested for a series of shocks on the Shock-Table facility available at the Department of Earthquake Engineering, IIT Roorkee. The observed modes of failure of the considered models, the maximum sustained peak ground acceleration and observed displacements are presented. Equivalent Frame Method is used to simulate the response of the models subjected to Abstract v Shock-Table tests. The results show that the performance of the retrofitted model is significantly enhanced as compared to the traditional URM model and the Equivalent Frame Method is able to predict the peak displacement and damage in the URM as well as retrofitted models, with reasonable accuracy.