Analysis and Design of Fiber Reinforced Polymer Retrofit of Reinforced Concrete Diaphragms

Abstract

In a building's lateral force resisting system (LFRS), structural elements are broadly categorized as horizontal, vertical, and foundation components. Diaphragms form the horizontal LFRS, transmitting lateral loads from earthquakes and wind events to vertical systems such as moment frames and shear walls. In reinforced concrete (RC) buildings, diaphragms often require retrofit due to renovation-driven changes, new slab penetrations, or insufficient strength, stiffness, or ductility, especially in older structures. A promising retrofit method involves externally bonded fiber-reinforced polymers (FRP), which offer reduced weight, ease of installation, and minimal disruption compared to conventional solutions like overlays or slab thickening. Despite their critical role, the behavior of as-built and FRP-strengthened diaphragms remains poorly understood, resulting in inadequate treatment within current FRP design guidelines, particularly regarding their failure modes, in-plane shear response, deformation limits, and realistic capacity predictions. Design guides like ACI 440.2R provide detailed provisions for FRP strengthening columns, beams, and shear walls but offer no diaphragm-specific guidance. As a result, practitioners often adapt shear wall and beam data to diaphragms, potentially leading to inaccuracies and even unconservative designs due to fundamental differences in behavior. This study aims to advance the understanding of RC diaphragm behavior and improve strengthening methods using externally bonded FRP through combined experimental testing and computational modeling. A finite element (FE) modeling strategy was developed and validated using experimental studies on six cantilever diaphragm specimens. The model incorporated key nonlinear behaviors governing diaphragm shear response, including concrete cracking, stiffness degradation, crushing, reinforcement yielding and rupture, and orthotropic FRP behavior. In addition, concrete-FRP bond behavior was modeled using a traction–separation law, enabling accurate simulation of FRP debonding and associated strength degradation. Furthermore, a novel approach employing specialized cohesive-zone regions was introduced to explicitly represent the ability of FRP anchors to arrest debonding. The validated FE model was then used to investigate shear interactions between concrete, reinforcement, and FRP under in-plane loading. A parametric study examined the effects of reinforcement ratio, material strength, FRP orientation, stiffness, and coverage area. The results revealed the mechanisms by which FRP enhances diaphragm shear resistance: directly, by bridging diagonal tension cracks, and indirectly, by improving the concrete constitutive behavior through enhanced tension-stiffening of diagonal compression struts, thus delaying concrete crushing and significantly increasing overall shear capacity. The numerical modeling approach was further extended by applying a density-based topology optimization framework to the FE models. This approach allowed targeted FRP retrofitting strategies to be developed by optimizing FRP layouts based on internal force flow within the diaphragm, maximizing diaphragm strength while constraining total FRP volume. To validate the topology optimized design, a large-scale, three-bay RC diaphragm (25 ft × 10 ft × 4 in.) containing openings to simulate real-world discontinuities was tested under reversed-cyclic four-point loading. Externally bonded FRP, strategically placed to address critical load-path disruptions, significantly enhanced the shear capacity, informing preliminary design approach for practical diaphragm retrofitting. The optimization method was further extended to a full building floor plan, demonstrating its potential for realistic, system-level applications as necessary. This research explored the behavior of FRP strengthened RC diaphragms and provided insights that could inform their analysis and design. Validated FE models accurately captured nonlinear shear behaviors such as cracking, debonding, and failure mechanisms that were used to identify significant interaction effects between FRP, steel reinforcement, and concrete. Simply summing the individual contributions of these shear strength components underestimated the actual diaphragm shear capacity. FRP improved the concrete's constitutive behavior under shear loading, which underscored the need to consider these effects in design. Parametric studies demonstrated that current ACI 318 shear-strength provisions tend to overestimate diaphragm capacities, particularly at higher reinforcement ratios. FRP retrofits notably enhanced concrete's compressive performance under biaxial stresses, with optimal results achieved when fibers were aligned perpendicular to expected crack paths and coverage was strategically distributed. Topology-optimized FRP layouts provided greater strength improvements than conventional designs, a finding supported by an experimental study. Measured FRP strains generally exceeded existing ACI 440.2R design limits, suggesting current guidelines are overly conservative for diaphragm strengthening.