Structural Modeling of Cross-Frame Behavior in Steel Girder Bridges

Cross-Frames and diaphragms are essential structural elements for stability during construction (and sometimes during the service life) of steel bridge systems. In horizontally curved and skew bridges, these braces can engage adjacent girders to act as a system to resist the potentially large forces and torques caused by the curved or skewed geometry. However, because of their role in laterally transmitting live load forces, they can produce fatigue cracks at their connections to the girders. This paper investigates a method for evaluating the distribution of fatigue stress ranges using finite element analysis (FEA), through user-defined subroutines in conjunction with commercial FEA software. Data from field testing under various load passes of a weighed load vehicle of a skewed steel girders bridge are used to validate the results. A case study of an intermediate cross-frame was adapted in order to show the advantage of using this stress analysis methodology, which locates the stress concentration regions where distortion-induced fatigue cracks may originate. This knowledge can then be applied in later work to assess and optimize the fatigue performance of typical bridges and connection details.