Lateral load response and collapse probability of reinforced concrete shear walls retrofitted for repairability

This paper demonstrates the efficacy of a new reinforced concrete shear wall seismic retrofit method through a series of nonlinear static and incremental dynamic analyses. Unlike traditional retrofit methods, the method investigated aims to convert conventional walls into self-centering walls whose behavior is governed by rocking and flexure. The retrofit involves creating a cold joint at the foundation-wall interface, cutting some reinforcing bars to allow rocking, adding external post-tensioning to enable self-centering, and externally confining wall toes to prevent concrete crushing. The retrofit was applied to two building archetypes, each with two different shear wall designs. The four walls were retrofitted by varying retrofit parameters (portion of the vertical reinforcement bars cut, and external post-tensioning amount). Nonlinear static and nonlinear response history analyses were performed using experimentally validated, computationally efficient models that simulate walls with fiber-based beam-column elements. Incremental dynamic analysis was used to create collapse fragility functions for pre- and post-retrofit walls. The results show that the retrofit is effective when some vertical reinforcement bars are left uncut across the foundation-wall interface. The retrofit is more effective for walls with vertical reinforcement distributed across cross-section as compared to walls with reinforcement concentrated near boundary elements and for walls with structurally efficient amounts of reinforcement as compared to walls with higher amounts of reinforcement. This is attributed to the larger amount of reinforcement bars cut in walls with concentrated reinforcement layouts or heavy reinforcement amounts, leading to a larger loss of strength, recovery of which requires larger amounts of post-tensioning.