Performance Based Assessment And Retrofit Of Nonductile Existing Reinforced Concrete Structures Andrea Miano 1 , Halil Sezen 2 , Fatemeh Jalayer 1 , and Andrea Prota 1 1 Department of Structures for Engineering and Architecture, University of Naples “Federico II”, Via Claudio 21, 80125, Naples, Italy; {andrea.miano, fatemeh.jalayer, a.prota}@unina.it 2 Department of Civil, Environmental & Geodetic Engineering, Ohio State University Neil Avenue 2070, 43210, Columbus, Ohio, USA; sezen.1@osu.edu ABSTRACT 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. INTRODUCTION Major earthquakes that hit different countries around the world in the past years have shown the vulnerability and deficiencies of existing structures including nonductile reinforced concrete (RC) frame buildings. These nonductile concrete frame structures are much more prone to collapse with respect to the modern code-conforming frames (Sezen et al. 2003, and Liel et al. 2009). Since these buildings comprise large percentage of existing building stock, efficient assessment methods are needed to compare different retrofit methods and to predict the collapse risk of existing structures in seismic regions (Zareian and Krawinkler 2007, and Eads et al. 2013). Different conventional retrofit methods, such as concrete or steel jacketing of the columns, addition of shear walls and new methods often based on new materials, such as fiber reinforced polymers (FRP), have been proposed (Moehle 2000 and Thermou and Elnashai 2005). These methods can be applied considering the desired performance level, requirements of new seismic codes reduction in probability of collapse, optimization of cost and/or minimization of losses. In this paper, alternative retrofit methods are evaluated for older nonductile frame buildings using a nonlinear structural performance assessment methodology. Nonlinear dynamic analysis procedures can be used to perform probabilistic seismic assessment, using recorded ground motions. These procedures can be used to estimate parameters required for