Assessment and design procedure for the seismic retrofit of reinforced concrete beam-column joints using FRP composite materials

In this study, an analytical procedure for the evaluation of the expected performance of existing reinforced concrete (RC) beam-column joints before and after being retrofitted using fiber-reinforced polymer (FRP) composite materials is presented. Focus is given on the evaluation of the shear-strength versus deformation properties of the panel zone region either in the as-built or FRP-retrofitted configuration. Based on experimental and numerical evidence as well as on physical models representing the mechanics of the joint region, principal tensile stresses versus joint shear deformation relationships are adopted and preferred to more traditional nominal shear-strength rules, to evaluate, within a step-by-step iterative procedure, the combination of the joint shear contribution provided by the FRP composite material and that provided by the concrete core alone. The use of principal stresses allows one to directly account for the effects of variation of axial load, typically neglected in the assessment and retrofit of beam-column joints. The hierarchy of strength and sequence of events (damage mechanisms) expected within a beam-column subsystem are visualized via M-N interaction performance domains, used as a basis for a performance-based retrofit philosophy. Specific limit states or design objectives are targeted, with attention given to both strength and deformation limits. The proposed analytical procedure is validated on the results of a set of experimental tests available in the literature. With the intention to provide a simple design tool that can be easily implemented by practicing engineers, a worked example for the evaluation of the expected performance of an FRP retrofitted beam-column joint is provided and used as a basis for a parametric study to illustrate the effects of different strengthening schemes on the behavior of strengthened exterior joint panels under various axial load levels.