Abstract: In the present retrofit code for the bridge, the calculation method for FRP jacketing ratio is only presented for RC bridge piers and lack of the complete ductility retrofitting design method. So, the behavior and rational seismic retrofitting plans for FRP-confined RC bridge piers has uncertainty. In order to quickly determine the economical and rational seismic retrofitting plans for reinforced concrete (RC) columns with fiber reinforced polymer (FRP) composite material, a new ductility retrofitting design method was proposed based on the target displacement ductility coefficient. In the proposed ductility retrofitting method, the relationships of macro mechanical properties of FRP strengthened bridge piers, such as displacement and ductility, was built with materials performance parameters of the ultimate compressive strain of FRP-confined concrete, the thickness of FRP jacketing, the effective ultimate tension stress and strain of FRP sheets. The proposed retrofitting design method was further used to estimate the behaviors of FRP confined RC bridge piers with rectangular and circle section, and the finite numerical simulation was applied. Among numerical analysis for the RC bridge piers, two calculation models of the ultimate compressive strain for FRP-confined concrete were selected, in which the Seible's ultimate compressive strain model can consider the complex confinement effect provided by stirrup and FRP jacketing. Based on Mander's tie reinforcements confined concrete model, the behavior of FRP retrofitted specimens was obtained, and it was compared with experimental results. The comparison results show that the proposed ductility retrofitting method can correctly estimate FRP jacketing ratio, FRP sheets thickness and the behaviors of specimens. Furthermore, the Seible's ultimate compressive strain model is verified to be applicable. The proposed ductility retrofitting design method is simple. However, it is only applicable to estimate the performance of FRP confined RC bridge piers with axial compressive ratio less than 0.2 at bending failure mode.

Key words: bridge engineering, ductility retrofitting, target displacement ductility coefficient, fiber reinforced polymer composite material, bridge pier