Strengthening of masonry infill panels with GRP fabric for out-of-plane seismic loads

The seismic behaviour of masonry structures strengthened with fibre-reinforced polymer (FRP) materials has received very little attention experimentally and theoretically. This paper outlines an experimental investigation of the potential for FRP strengthening of unreinforced masonry (URM) infill panels. Various options for FRP strengthening of masonry infill panels are compared, and results from a series of full scale push-over and seismic tests conducted at Bristol University are presented. The test results are compared with estimates of their vulnerability produced by a simplified analytical model. This research therefore aimed at developing a methodology for modelling the behaviour of infill panels taking account of the FRP reinforcement of the infill panels. The strengthening methodology finally adopted involved the removal of excess mortar from the masonry surface using a wire brush followed by roller application of a primer. Pre-cut GRP (Glass fibre Reinforced Polymer) fabric strips were saturated with a laminating resin while lying on a horizontal surface. The GRP strips were then rolled onto the walls followed by the roller application of a resin top coat to bond the fabric to the wall. In all cases both sides of the masonry panels were treated in the same manner. It was discovered that it was important to lap the GRP fabric at least 75mm over boundaries to secure panel into the structural frame. Using this methodology the experimental tests showed that the GRP reinforcement significantly increased the lateral load capacity, the ductility and stiffness of the panels. There were also noticeable changes in crack patterns, strain distributions and the failure mechanisms of the panels. The change in the stiffness of the panels was probably the most important consequence of the GRP application as this had the effect of increasing the natural frequency of the panels, effectively “detuning” them from earthquake spectral content. In fact the strength and stiffness increases recorded were so great that engineering calculations are arguably not necessary if it is possible to ensure proper specification and quality of application for the GRP. Because the application procedure was very simple and also relatively cheap GRP strengthening of masonry infill panels is a realistic retrofitting technique. However some caution must be exercised if GRP is to be used to strengthen masonry infill panels. For example, although the panel itself is significantly stronger, this strengthened panel may act in in-plane shear with frame and generate adverse forces in frame. This may have the effect of moving a potential failure from the panels into the structural frame. Also fire resistance and other environmental factors must be considered before FRP strengthening of unreinforced masonry infill panels could be widely adopted.