Local versus global ductility demands in simple bridges

This paper addresses the inelastic dynamic response of simply supported bridges to ground motion in their transverse direction. The effect of the relative substructure-superstructure flexibility on the inelastic response of bridges was studied for symmetric spans. The bridges are modeled as beams with distributed mass and elasticity, simply supported at the ends by elasto-plastic springs. An analytical extension of the common generalized single degree of freedom model into the inelastic range is presented. Flexibility of the capacity-protected structural elements is shown to add significantly to the ductility demand of the energy dissipating components of bridge systems. A simple method was developed to account for these flexibilities when assessing bridge response beyond its elastic range. Numerical case studies are presented to illustrate the new analytical tool.