Fragility curves of hollow-core bridge piers for territorial risk studies using closed-form equations

The paper presents a study aiming at obtaining fragility curves of reinforced concrete (RC) bridge piers by means of analytical equations describing the moment-curvature and force-displacement behavior. The bridge piers considered have rectangular hollow-core section, longitudinal reinforcement on all inner and outer four sides, and transverse reinforcement on the inner and outer perimeters. Closed-form equations are obtained for yield and ultimate moments and curvatures under combined axial load and uniaxial bending, which are then used to determine the force-displacement capacity of the bridge pier, including shear capacity and P-delta effect. To validate the accuracy of the developed equations, a comparison has been carried out against results obtained from fiber-section analyses, with satisfactory results. As an applicative example, the proposed approach is utilized to determine the fragility curves of a bridge pier pertaining to different damage states. The developed equations greatly facilitate multi-parametric and stochastic analyses, allowing for a rapid determination of fragility curves, with a computational efficiency that is particularly beneficial in large-scale territorial risk studies. A typical application is the risk analysis of railway and highway networks, whose most critical nodes are the piers of the common statically determined bridges.