Seismic retrofit and design recommendations for reinforced concrete bridge columns.

Since the early 1970's the design procedures and techniques for building earthquake-resistant bridges have advanced considerably, and have been implemented in the form of design codes. However, many bridge structures in service today were built before the revised design procedures and guidelines were implemented, and their performance in the event of an earthquake is questionable. Following recent moderate earthquakes in California various retrofit techniques have been proposed to increase the earthquake resistance of existing bridge structures. An innovative method to enhance the flexural strength, ductility capacity and shear resistance of existing reinforced concrete bridge columns through externally reinforcing the column with advanced composite materials (ACM) were developed. The ACM composite straps, which were unidirectionally arranged and impregnated with epoxy resin, were wrapped around the potential plastic hinge region at specified intervals. Additional advantages would be gained by laterally prestressing the straps at the time of strengthening. The confinement and the lateral stresses induced by the straps significantly increased the shear strength and ductility capacity, and greatly improved the flexural behavior of reinforced concrete columns under simulated earthquake loading. Both experimental and analytical studies were conducted in the Structural Engineering Laboratory to demonstrate the feasibility of the proposed technique. A total of fourteen column-footing units were constructed with a 20 percent dimensional scale factor to model the typical design of many existing bridge columns. Half of these specimens were with circular cross sections and remaining specimens with rectangular cross sections. All the specimens were tested under inelastic reversed cyclic loading while simultaneously subjected to a constant axial load. The variables in the test program were the type of cross sections, the amount of ACM composite straps, different retrofit schemes and active grout pressure. Analytical models based on an energy balance approach were developed to predict the enhancement in the strength and ductility of retrofitted columns. Design examples and guidelines for seismic retrofitting were also presented.