Stiffness Behavior of Cross Frames in Steel Bridge Systems

Cross frames are critical structural elements in both straight and horizontally curved steel bridges. In order to properly size the brace for the strength and stiffness demands of the superstructure, an accurate model of the elements comprising the cross frame is required. Conventional details most commonly used for cross frames consist of single-angle members connected to form a truss system linking adjacent girders together. Most analyses of the bridges treat the cross frames as truss elements that primarily resist applied forces through the axial stiffness of the members. This paper documents the results of a research study that included full-scale laboratory tests to measure the stiffness and strength of cross frames utilizing both conventional and new details. The tests showed that analytical solutions, as well as computer models, that are routinely used to model the cross frames in analysis software can overestimate the in-plane stiffness of the brace by more than 100%. The primary reason for the discrepancy in the stiffness models was identified to be connection eccentricities that exist in cross frames comprised of single-angle members welded to a gusset plate through only one leg of the angle. Overestimating the stiffness of the braces during construction can lead to unsafe conditions, as well as errors in the geometry of the constructed bridge, resulting from underpredictions of deformations during concrete placement. Extensive parametric analyses were carried out using validated finite-element models to develop correction factors that can be applied to analytical and computer models to significantly improve the accuracy of the simplified models that are used for the cross-frame systems. The correction factors allow a designer to utilize a computationally efficient model for the cross frame while also including the reduction in stiffness that is caused by connection eccentricities.