The 14 th World Conference on Earthquake Engineering October 12-17, 2008, Beijing, China SEISMIC PERFORMANCE NORMALIZATION OF BRIDGES USING A DAMPING-ENHANCED STRENGTHENING METHODOLOGY G.D. Chen 1 and K. R. Karim 2 1 Professor, Dept. of Civil, Architectural, and Environmental Engineering , Missouri University of Science and Technology, Missouri, USA, Email: gchen@mst.edu 2 Ph.D. Candidate, Dept. of Civil, Architectural, and Environmental Engineering, Missouri University of Science and Technology, Missouri, USA, Email: krk2q4@mst.edu ABSTRACT: In this paper, a viscoelastic layer is integrated into a fiber reinforced polymer strengthening jacket for the seismic retrofit of a three-span steel-girder bridge. The distributed damping layer is represented by a series of discrete and complex springs in the finite element model of bridge columns. Damping ensures that the bridge is operational under moderate earthquakes and strengthening mainly ensures the bridge safety under strong earthquakes. Together, they meet the multiple performance objectives under earthquakes of varying intensities. KEYWORDS: Viscoelastic damping, fiber reinforced polymer jacketing, performance-based design 1. INTRODUCTION Strengthening techniques such as fiber reinforced polymer (FRP) jacketing have been developed over the past two decades (FHWA, 1995; MCEER, 2005). These jacketing techniques can be used to effectively confine an existing reinforced concrete (RC) column to prevent it from collapsing during a strong earthquake. Strengthening alone, however, is unlikely to improve the column performance under moderate earthquakes since small column deformation does not allow the jacketing system fully engaged. Therefore, a damping-enhanced strengthening (DES) methodology was recently proposed by the first author to meet multiple performance objectives in performance- based design and retrofit of bridges (Chen et. al., 2006). The damping component is to reduce the column response under moderate earthquakes so that the operational performance objective of bridges can be met, while the strengthening component is effective for the safety performance objective. This study is aimed at developing a spring representation of the damping component in the finite element model (FEM) of bridges, applying it to the analysis of a single bent of a three-span continuous steel girder bridge, and conceptualizing a retrofitting process and design of the bridge by evaluating the normalized performance of the DES methodology against multiple performance objectives. 2. DISCRETE AND COMPLEX SPRING MODELING OF A DAMPING LAYER In the proposed methodology (Chen et al., 2006), a viscoelastic layer is sandwiched between an inner and an outer FRP sheet. The inner FRP sheet provides confinement on the columns of a bridge. The outer FRP sheet is anchored into footing/capbeam of the columns, providing an amplified constraint effect on the shear deformation in a viscoelastic layer. 2.1. Shear Strain Amplification in VE Layer To understand the effectiveness of a new constrained layer treatment in the DES methodology, consider a cantilevered column partially covered by a viscoelstic layer and subjected to a bending moment at the cantilever end. Figures 1 and 2 present a comparison of the shear stress distributions between the conventional and the new treatments. Under the end moment M 0 , the column experiences a constant moment or curvature along its height. When the constrained layer is not anchored as shown in Figure 1, the induced shear stress must be zero and change it direction at the middle height of the VE layer, and proportional to the distance from the middle height of the VE layer at other points to ensure the resultant of the distributed forces on the VE layer is zero.