Parametric Optimization of Orthotropic Girders in a Cable-Supported Bridge

In the last six decades, closed-box orthotropic steel girders have been widely used in cable-supported bridges. Several parametric studies were previously carried out to reduce inherent fatigue stress problems and to generally improve bridge girder designs. However, in most cases, only one or two parameters were studied simultaneously; hence, the full potential of orthotropic girders is not achieved. In the present work, a multiscale finite-element (FE) model of a suspension bridge is established with sophisticated boundary conditions applied to a local parametric submodel of a bridge girder. With this local model an automated gradient-based parametric optimization is carried out with the goal of minimizing the weight and price of the girder. It is possible to simultaneously optimize several design variables and fulfill constraint functions on fatigue stresses, deformation, and buckling. The results show potential weight savings of 6%-14% and price savings of 9%-17%, mainly found by using thinner plates and narrower troughs. Besides the explicit savings, the results indicate the potential for applying gradient-based optimization in civil engineering designs.