Static and Dynamic Behavior of Reinforced Masonry : Experimental and Analytical Investigations

Abstract

The most common form of dwellings in rural and semi-urban areas of India and other developing countries around the globe are one/two storey unreinforced masonry (URM) buildings. It is well known that such masonry buildings are most vulnerable during earthquakes. Out-of-plane flexural failures of walls are primarily responsible for collapse of URM buildings during an earthquake. The seismic performance of such buildings can be improved by reinforcing masonry walls in the horizontal and vertical directions with materials like steel, bamboo or fiber reinforced polymers (FRP). It is fairly easy to reinforce masonry in the horizontal direction by embedding the reinforcement in the bed joints of masonry construction. However, in the vertical direction, the reinforcement is generally provided in the cavities of hollow masonry units, which are grouted after placing the reinforcement. Even though the in-plane performance of masonry walls is enhanced with such a reinforcing technique, it still falls short in resisting out-of-plane lateral loads, as the vertical reinforcement is located close to neutral axis of bending. Hence, a novel technique of reinforcing masonry in the vertical direction on both the faces of the wall called containment reinforcement is proposed recently. Containment reinforcement improves ductility, energy dissipation and prevents overturning failure due to out-of-plane loading. The present study examines the role of containment reinforcement in improving out-of-plane / in-plane behavior of masonry. The research program consisted of characterizing the physical properties of the constituent materials of reinforced masonry, namely stabilized earth blocks, cement-soil-sand (1:1:6) mortar and steel and FRP reinforcement. The strength and elastic properties of masonry assemblages under compression, flexure and shear have been determined. The flexural behavior of three types of reinforced masonry assemblages namely; stretcher bond, English bond and rat-trap bond masonry beams under monotonic and reversed cyclic loading test protocols have been examined. The beams were reinforced with steel, Glass FRP (GFRP) and Carbon FRP (CFRP) materials. In the monotonic test protocol the moment-curvature relationships and ductility for each type of masonry beams were obtained. In the cyclic test protocols, the hysteretic behavior, energy dissipation and equivalent viscous damping characteristics were obtained. The shear behavior of unreinforced and reinforced masonry panels under diagonal tension (shear) was examined through monotonic and cyclic loading test protocols. A simple and cost effective device for producing horizontal to and fro motion to imitate earthquake ground motions, called shock table test facility, has been designed. The table platform is mounted on four wheels and moves on rails. The table is put into the motion through pendulum impacts. The table motion characteristics have been obtained using the parameters used to describe the earthquake ground motions like amplitude, frequency content, duration of the motion and mixed parameters. The parameters of the shock table motion have been compared with few of the recorded earthquake ground motions to evaluate the effectiveness of shock table testing protocol for examining the dynamic performance of scaled masonry building models. The performance of two half scaled containment reinforced masonry building models subjected to base motions provided through shock table and conventional shaking table was evaluated. The dynamic properties of masonry, responses and failure patterns were obtained. A non-linear finite element (FE) model was developed and calibrated using the experimental data generated in the flexural and shear testing of reinforced and unreinforced masonry beams and panels. The FE model was further used for analysis of half scale masonry building model tested on shock table and recalibrated by comparing responses of numerical model with experimentally measured responses. Furthermore, the finite element model was used to assess the performance of two storey unreinforced and containment reinforced symmetric/asymmetric masonry buildings subjected to a series of earthquake ground motions of increasing severity. The studies conducted conclude that the masonry with containment reinforcement was effective in mitigating seismic risks of masonry buildings in moderate to severe seismic regions. The provision of containment reinforcement significantly improved equivalent hysteretic damping at design displacement and offered excellent ductility to masonry elements. The existing construction practice can easily accommodate the provision of containment reinforcement with little modification to the construction sequence. The extra effort in construction does commensurate with the enhancement in the seismic performance of the masonry buildings.