Modeling of FRP-Strengthened Infill Masonry Structures

This paper presents an analytical approach to describe the behavior and mode of failure of infill masonry structures strengthened with fiber-reinforced polymer (FRP) materials. The approach is used to simulate the response of strengthened infill masonry walls under the effect of out-of-plane uniformly distributed pressure simulating wind pressure. The approach includes a finite element analysis in which bricks and mortar joints are modeled using solid elements and the interface between the bricks and the mortar is modeled using contact elements that account for out-of-plane slip as well as the opening of cracks at the interface. The FRP is modeled by beam elements and the bond between the masonry and the FRP is modeled using contact elements that simulate both the shear-slip relationship and the out-of-plane tensile-separation (or peeling) of the FRP. The proposed approach is calibrated using an experimental program completed by the authors. The experimental program included twelve unidirectional infill masonry wall specimens tested to study the behavior of various FRP materials and FRP anchorage systems. The proposed model is evaluated with respect to its ability to accurately simulate the behavior observed in the experimental program. (a) Front View (b) Profile View FIGURE 1. Test Specimens consisting of 2-node contact elements attached to the mortar surface and 2-node target elements attached to the surface of the bricks. A penalty stiffness algorithm was used in compression to restrict penetration of the bricks into the mortar. The bricks were allowed to separate from the mortar in tension according to a constant opening stiffness that was significantly less than the penalty stiffness in compression. A cut-off model was used for the behavior of the joint in tension, in which a maximum separation was defined, beyond which the contact status shifts from closed to open and the tensile stress is reduced to zero as shown in Figure 4. The shear stress-slip model is shown in Figure 5. The shear strength of the interface is a function of the cohesion of the bond between the bricks and the mortar, c, the coefficient of friction, tan(φ), and the normal compressive stress as shown in Figure 6. FRP was modeled using 2-node beam elements with a linear elastic constitutive relationship. The bond between the FRP and the masonry and the bond between the FRP and the supporting concrete caps was modeled using contact pairs consisting of 2-node contact elements attached to the FRP and 2-node target elements attached to the masonry and RC caps. Tensionseparation (mode I) debonding was modelled with a cut-off approach and shear-slip debonding (mode II) was modelled using a bilinear approximation in which after reaching a maximum value, the shear stress remains constant with slip as shown in Figures 4-5. The shear stress-slip model differs from most other models in the literature, which include the post-peak reduction in shear stress with increasing slip. The reason for the simplification in the proposed model is primarily to improve numerical stability and convergence. Since mode I debonding was observed in the experimental program to be the dominant behavior, priority was placed in simulating that non-linearity. (a) Front View (b) Profile View FIGURE 2. Experimental Test Setup FIGURE 3. Multi-linear Material Model FIGURE 4. Normal Stress Material Model FIGURE 5. Shear Stress Material Model f