Engineers often use skewed steel I-girders in bridge design because they are adaptable to site constraints, help improve load distribution, are cost effective, and enhance the overall structural performance. Previous studies have focused on common finite-element modeling approaches, without consideration of cross-frame properties, boundary conditions at bearings, and superstructure–substructure interaction at integral abutments. Researchers Siang Zhou, Larry A. Fahnestock, and James M. LaFave wanted to investigate the design specifications of skewed steel I-girders and their structural response to further improve safety and efficiency..
In their recent paper published in the Journal of Bridge Engineering, “Parametric Study of Skewed Steel I-Girder Bridge Truck Live Load Response,” the researchers report on the variations in skew and bridge width under single-lane truck loads over multiple live load paths along the width of a bridge. Using two bridges in Champaign, Illinois, the team implemented a long-term field monitoring campaign to evaluate the efficiency of standard design specifications, including bridge width, stub abutments versus integral abutments, cross-frame arrangement, and the position of live loading. The researchers were able to monitor the decks during the installation in 2022 as well as test live loads over time, and assess the stress applied to the girder strong-axis bend, lateral bend, and responses near interior bridge support. Their findings will help with line girder analysis estimations and AASHTO cross-frame design requirements. Get the full results at https://doi.org/10.1061/JBENF2.BEENG-6854. The abstract is below.
Abstract
Skewed steel I-girder bridges experience complex load distribution under live load that is not thoroughly understood, while standard design practice for such bridges consists of simplifications that should be further evaluated and verified. Commonly used line girder analysis (LGA) can estimate strong-axis bending stress through the application of a live load distribution factor (LLDF) that considers the skew effect from 30° to 60°, and it accounts for skew-related lateral response by simply adding a flange lateral bending stress for skew exceeding 20°. Since LGA calculations related to skew do not account for bridge width, and because girder lateral bending response is considered in a simplified fashion, further refinement may be possible. In addition, the widely used practices of designing exterior and interior girders with the same demand and analyzing stub and integral abutment bridges in a similar way need to be further assessed. This paper evaluates the effect of bridge geometric parameters—including skew of 0°–70°, bridge width ranging from 8 to 26 m (27–84 ft), and abutment type (stub versus integral)—on skewed steel I-girder bridge response through a numerical parametric study (using field-validated models), considering live load positioning across the width of a bridge. The distribution of girder strong-axis and lateral bending stress was analyzed, with peak stress compared to LGA calculations. Exterior girders were generally observed with larger strong-axis bending stress but smaller lateral bending stress (versus interior girders) when directly loaded; estimating girder strong-axis bending stress using LGA with a controlling LLDF for all girders can be overly conservative for interior girders. The distribution of strong-axis and lateral bending stress on a skewed bridge with either stub or integral abutments was also found to be dependent on live load positioning, with peak stress closer to bridge obtuse corners (away from bridge midspan) as skew increases. The standard practice of providing a minimum distance between the bridge end and the first intermediate cross-frame was confirmed to be important to avoid lateral bending stress concentration near bridge obtuse corners. Girder response near the bridge pier was generally less significant than that along bridge spans under live loading, except for exterior girder flange lateral bending stress. Near the pier, bottom flange lateral bending stress increases with increasing skew, while interior and exterior girders behave differently under the skew effect for strong-axis bending stress.
Explore these results in depth and how they might enhance your similar bridge designs in the ASCE Library: https://doi.org/10.1061/JBENF2.BEENG-6854.
