Live load and long-term response of steel bridges with high skew

Abstract

High skew in bridges alters live load distribution and creates performance issues such as deck cracking, superstructure in-plane (horizontal) displacements and bearing distress. In-plane superstructure displacements, also called racking, are particularly important as they cause misalignments of superstructure and approach slabs, stress bearings and expansion joints. This study had two objectives: to evaluate live load distribution in skewed bridges and to understand how skew influences in-plane superstructure displacements. The effect of bridge design details on live load distribution and in-plane superstructure displacements was assessed through parametric studies on highly skewed bridges.A three-span, steel girder-concrete deck bridge with a 47-degree skew was load tested. Load test results were employed to validate finite element analyses that were used to understand the impact of varying skew angle, bridge features and bearing fixity on live load distribution and in-plane superstructure displacements. The ability of the American Association of Highway and Transportation Officials Load and Resistance Factor Design Bridge Design Specifications (AASHTO LRFD BDS) to accurately capture live load distribution for steel girder bridges was evaluated.Field test results, consisting of girder bending and vertical shear strains measured through strain gages strategically located on one of the exterior spans, and parametric studies showed that AASHTO LRFD BDS predicted shear and moment distribution very conservatively and closely, respectively. Seasonal temperature changes were sufficient to create in-plane (horizontal) superstructure displacements. These displacements caused the superstructure to rotate towards obtuse and acute corners for negative and positive temperature changes, respectively. Larger skew angles led to greater transverse (i.e., perpendicular to bridge longitudinal axis) displacements at girder bearings. Longitudinal (i.e., parallel to bridge longitudinal axis) displacements were consistently smaller than transverse displacements and were not affected by skew. Mixed bearing fixity arrangements, with both fixed and expansion bearings over the same pier, limited superstructure transverse in-plane displacements.