Development of a New UHPC Shear Link for Seismic Strengthening of RC Frames: Experimental Validation, Strength Models, and Finite-Element Simulation

This study presents the development and evaluation of a new seismic retrofitting technique for RC frames using a precast vertical ultra-high-performance concrete (UHPC) shear link integrated with cast-in-place UHPC connections. The proposed system aims to overcome the inherent directionality and architectural obstructions associated with conventional diagonal bracing while utilizing the superior mechanical properties of UHPC to enhance lateral resistance and ductility. Quasi-static cyclic tests were conducted to evaluate the seismic behavior of a stand-alone UHPC shear link, an as-built RC frame, and two RC frames retrofitted with the shear link at different locations. Comprehensive assessments of failure modes, hysteretic responses, stiffness degradation, curvature profiles, and strain development demonstrate that the UHPC shear link exhibits stable flexure-controlled behavior with high strength retention up to 5% drift. The retrofitted frames achieved approximately four times the initial stiffness and 2.5 times the peak strength of the as-built frame, while maintaining more than double the residual strength at large drifts. Analytical strength models captured the flexure-governed behavior and provided conservative predictions. Nonlinear finite-element models constructed in LS-DYNA accurately reproduced global load–displacement responses, localized damage mechanisms, and peak strengths with less than 5% error. The results confirm that the proposed vertical UHPC shear link offers a highly effective, space-efficient solution for seismic strengthening of existing RC frames, providing substantial improvements in strength, stiffness, and energy dissipation capacity.