Probability based comparison of retrofit methods for existing nonductile concrete frames

In different high seismic regions around the world, many nonductile existing reinforced concrete frame buildings, built without adequate seismic detailing requirements, have been damaged or collapsed after past earthquakes. These concrete frame buildings 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 nonlinear structural performance assessment methodology. As a case study, the transverse frame of an existing building is modeled, including the effect of flexural-shear-axial load interaction and the bar slip deformation component to be able to capture also column shear and axial failures. A framework for probability-based demand and capacity factor design (DCFD) seismic safety evaluation is implemented in order to evaluate the structural performance at each chosen performance level. This study shows that it is a critical issue to choose the most effective retrofit strategy based on the assessed performance of the bare frame. Moreover, the estimates of expected life cycle cost are compared for the retrofit methods considered in this research.