Parametric Investigation of Load-Induced Cross-Frame Fatigue Effects in Composite Steel I-Girder Bridges

Despite the historical tendency toward proportioning and spacing braces based on rules of thumb, modern specification guidance stipulates that cross-frame design be supported with a rational analysis, which is commonly aided by computer software. While cross-frame research has advanced over the last 30 years, the relatively vague nature of specification language related to rational analysis is still utilized and often misunderstood by engineers leading to uneconomical chase-your-tail design solutions—particularly related to the fatigue limit state. To improve the design specifications and economy, an extensive analytical parametric study was conducted to quantitatively investigate the sensitivity of load-induced cross-frame response in varying bridge geometries. The study included multiple three-dimensional analyses on 4,104 unique steel I-girder bridges. Among other observations, the results demonstrated that load-induced cross-frame forces are generally higher in heavily skewed systems. From these studies, two primary recommendations are proposed to improve specification guidance on fatigue design of cross-frames. First, designers can avoid overly conservative and iterative designs by utilizing staggered and/or discontinuous cross-frame configurations, thereby alleviating force demands in heavily skewed systems. Second, the study concluded that refined analyses are not warranted in straight bridges with little to no support skew since cross-frame force magnitudes are generally not significant. Geometric limits based on skew and connectivity indexes are proposed to serve as the delineation between when a refined analysis is recommended.