Strut-and-tie model for interior RC beam-column joints with substandard details retrofitted with CFRP jackets

A softened strut-and-tie model (STM) is developed for interior reinforced concrete (RC) beam-column joints without any steel hoops in the joint or intermediate longitudinal column steel reinforcement, and discontinuous bottom beam flexural steel reinforcement. The STM model is extended to identical joints retrofitted with Carbon Fiber-Reinforced Polymer (CFRP) composites. The STM model is compared to experiments of two full-scale RC beam-column interior joints, one of which was retrofitted with CFRP composites. Failure modes for the original joint included anchorage failure of bottom steel beam bars, crushing of concrete nodal zones, diagonal joint shear failure, and for the retrofitted joint CFRP laminate delamination and crushing of joint core concrete. The STM model is based on the joint reinforcement details and experimental performance, including column axial load effects and the contribution of CFRP horizontal and diagonal laminates as tension ties. STM model assumptions were verified with strain gauge measurements. The STM model was successful in estimating the ultimate shear strength of the original and retrofitted joints. Recommendations are presented for evaluating the strut width for both original and retrofitted joints that include the quality of the bond of beam steel reinforcement to concrete in the beam-column joint.