Performance Based Assessment and Retrofit of Nonductile Existing Reinforced Concrete Structures

In different high seismic regions around the world, post-earthquake reconnaissance has shown that nonductile concrete frame structures are much more susceptible to collapse than modern code-conforming frames. Therefore, for this type of structures, it is necessary to accurately model materials and members to capture the flexure, shear, and flexure-shear failure modes in members and the potential collapse of the structure. In this paper, alternative retrofit methods are evaluated for these older frame buildings using a probability-based framework, based on nonlinear dynamic cloud analysis, in order to assess the structural performance and safety at each chosen performance level. As a case study, the longitudinal frame of an existing building is modeled, including the effect of flexural-shear-axial load interaction and the longitudinal bar slip deformation component in order to be able to capture column shear and axial failures. The critical demand to capacity ratio, corresponding to the component or mechanism that leads the structure closest to the onset of limit state (e.g., near collapse), is adopted as the structural response parameter. This structural response parameter, that is equal to unity at the onset of limit state, can encompass both ductile and brittle failure mechanisms. It can also register a possible shift in the governing failure mechanism with increasing intensity. Finally, the estimates of expected life cycle cost are compared for the retrofit methods considered in this research.