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Seismic behavior of concrete frames with jacketed columns

Research on jacketed reinforced concrete (RC) columns is well established and growing. The existing studies describe the ability of jackets to rehabilitate RC columns for seismic demands. The system in the studies has proven to upgrade both strength and ductility of their unretrofitted counterparts, concentrated on repairs of deficiencies, currently in columns designed with old code provisions. These deficiencies include: low shear strength, insufficient core confinement, and short lap-splices, in particular within the plastic hinge zone. Columns with these deficiencies usually have brittle failures and cannot develop displacement ductility ( ) greater than 2. Research results on columns retrofitted with jackets have reached displacement ductility of 4 or greater levels with most jackets designs. The jacket materials used in this research are steel and fiber reinforced polymer (FRP). These two materials are widely used and accepted as retrofit for columns with deficient detailing. Steel tensile material properties, malleability and a high number of contractor who can work with this material make it a practical material to retrofit columns. FRP elastic unidirectional stress strain curve with both high moduli and low weight provides low constructability cost and high strength. These materials are externally applied in order to prevent the loss of concrete cover and allow the concrete to develop higher stresses and strains. Jacketed RC columns have a composite behavior which can be complicated to model in a finite element analysis (FEA) program accurately. Past studies have developed different numerical and physical models that attempt to capture the confining effects and shear strength of jacketed columns. To the best of our knowledge, no singular model has described all behavior of retrofitted columns with different jacket materials. The first goal of this study is to create a set of recommendations to model accurately jacketed RC columns with different detailing and jacketing materials. These recommendations provide easy access to jacketed columns strength and ductility to make design decisions. To quantify the strength and ductility, a force-deformation curve was developed. A force-deformation curve (backbone) is an accepted method to describe a column behavior. Therefore this study introduces models that describe all jacketed column behavior and create a set of rules to construct a curve of the expected behavior of jacketed columns. It was found that for jacketed columns, the potential for shear failure was eliminated and the expected failure mode is flexure. With this assumption, the lateral capacity was obtained accurately by calculating the flexural capacity of the RC column. Deformations, on the other hand, were estimated based on a statistical model using results from published papers. Jacketed columns have an improved behavior in both lateral strength and displacement ductility. These effects can change the behavior of a frame. Analyses of frames in as-built and retrofitted conditions were conducted in order to quantify the change in behavior of the frames caused by column jacketing. These frames were modeled using a lumped plasticity method. The properties of the frames were calibrated using laboratory test results of columns and published methods. This study shows the change in probability of collapse and in the collapse location for frames in as-built and jacketed condition. Overall the objective of the dissertation is to provide easy-to-implement guidelines to obtain the jacketed RC columns strength and ductility; and show the effects of the increased strength and ductility on frames behavior.
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