Development of Skewed Steel I-Girder Bridge Field Monitoring Strategy through Agency Survey and Numerical Simulation

Highly-skewed steel I-girder bridges are commonly used across the US, especially in congested areas, despite complications in their analysis and design. The American Association of State Highway and Transportation Officials (AASHTO) LRFD Bridge Design Specification provides suggestions for design values of flange lateral bending stress in addition to line girder analysis when bridge skew exceeds 20° for certain cross-frame layouts. For bridges with skew exceeding 60°, a higher level of analysis is often required, with cross-frames considered primary members in design. Neither the magnitude of additional lateral stress nor the associated skew limits are particularly well understood, so more study is needed to refine and support this analysis and design approach. In addition to short-term response and load distribution, long-term bridge behavior for thermally induced stresses and deformations also needs to be more thoroughly studied. In support of an ongoing research initiative in Illinois, an agency survey was formulated and distributed across the US to understand practices used and challenges faced by state transportation agencies when designing and constructing skewed steel I-girder bridges. Findings from the responses of 23 state agencies illuminate issues, concerns, and current practice related to design, construction, and service life of those bridges. The agency survey informed selection of two bridges in Champaign, Illinois, for field monitoring, in order to provide new understanding of skew effects on bridge superstructure behavior. Three-dimensional finite-element analysis was conducted to guide field instrumentation planning, and initial measurements from the monitored bridges under traffic load confirmed good predictions compared to the planning-stage numerical study.

This paper presents the initial stages of a long-term project that employs field monitoring and three-dimensional finite-element analysis of skewed steel I-girder bridges. Findings from a survey of state transportation agencies describe issues, concerns, and current practice related to design, construction, and service life of these bridges, which can be of interest to practicing engineers and contractors. This paper illustrates the integration of data from current practice and targeted numerical simulations to establish a rigorous basis for planning a bridge superstructure field monitoring campaign. The information and experiences summarized can be helpful to both practicing engineers and researchers.