Design of FRP jackets for upgrade of circular bridge piers

The upgrading of bridges located in seismic areas and built according to ob solete codes is becoming a priority task for highway administrations. Among the possible upgrading strategies, the use of fiber-reinforced plastic (FR P) jackets is gaining widespread acceptance. In this paper, a design equati on is proposed to determine the optimal thickness of FRP jackets, to enhanc e the ductility of existing reinforced-concrete (RC) bridge piers with circ ular cross sections. The design procedure stems from the definition of an u pgrading index, given as the ratio of the target to availability ductility at the pier base section, to be attained through FRP jacketing. The availab le ductility is that identified through the usual assessment procedures on the RC member set for upgrade, whereas the target ductility is evaluated ba sed on the expected actions on the bridge. The upgrading index is initially defined in general terms and is subsequently extended to the case of piers built in seismic regions. It results in a simple expression in terms of ea sily computable quantities, such as the ultimate strain and the peak streng th of concrete, before and after upgrading. A parametric study on old-code- designed bridge pier sections, upgraded with either glass or carbon fiber j ackets, is performed based on a fiber-section model equipped with a newly d eveloped FRP-confined concrete model. This study shows that the index, desp ite its simplicity, yields excellent predictions of the ductility increase obtained through FRP wrapping, and it is therefore used to develop a design equation. The equation allows the design of the optimal thickness of FRP j ackets in terms of the desired upgrading index, mechanical characteristics of the selected composite material, and quantities defining the initial sta te of the pier section. The design procedure has been applied to available experimental tests of a scaled bridge pier wrapped with FRP and tested to f ailure, and it has been demonstrated to be very effective.