Development of Criteria for Fatigue Design of Horizontally Curved Steel Bridges

Abstract

Modern steel bridge design is moving toward the application of slender elements due to improved fabrication and construction technology. The strength and stability aspect of the liberal design philosophy has been well defined through extensive research and testing. However, fatigue concerns have not been thoroughly investigated, especially in the case of horizontally curved bridges. The out-of-plane displacements of slender webs result in secondary bending stresses at the web boundaries connections, i.e., flange and stiffeners. The so-called “web breathing” phenomenon potentially leads to fatigue crack initiation at the web boundary connections and has been studied for straight girders. Curved steel girders experience large deflection and rotations during construction and service that can intensify the web breathing effect. In addition, the curvature-induced lateral forces pushing and pulling slender curved webs develop mechanisms that can lead to critical web boundary stresses that do not typically occur in straight bridges. This study aims to investigate the web behavior of composite curved steel bridges and define design guidelines essential for the fatigue limit state. The web stress development mechanisms from construction to service were addressed through parametric finite element modeling of a horizontally curved composite bridge. Three regions of high bending, high shear, and high bend and shear locations were studied. It was observed that the slender web of the curved I-girder systems experiences much larger stress concentrations critical for fatigue compared to the equivalent straight girders. In order to define a fatigue limit, parametric finite element modeling was done through the simulation of isolated curved web panels designed based on AASHTO limits. Two cases of pure bend and combined bend and shear loading configuration were analyzed. In the case of pure bend, a fatigue design equation was developed to limit the maximum in-plane bending stress critical for web breathing. In addition, it was concluded that AASHTO considerations related to the web-to-stiffener fatigue detail category control the web-to-flange fatigue cracking due to web distortions for the normal to moderately slender webs. In the case of bend and shear loading, a fatigue design limit was derived based on the in-plane bending stress. Also, it was observed that the share of the shear stress in combined loading significantly influences the curved web panel fatigue behavior. Fatigue breathing of curved web panels becomes almost identical to the flat web panels when the in-plane shear stress is equal to bending stress.