Bridge Engineering Center

About the research

The objectives of this research were to investigate the structural behavior of the U-bolt connections on an in-service steel overhead sign truss (SOST) structure, evaluate the loading through the U-bolt connections when the SOST was subjected to various wind directions, and compare the loading experienced by the in-service U-bolt connections with the capacities predicted by a 2019 Iowa Department of Transportation (DOT) study.

To achieve these objectives, an in-service SOST structure in Iowa was selected and monitored on site for 10 months. The data collected during field monitoring were processed and analyzed. Following that, a finite element model was created utilizing the commercially available software Ansys to simulate the structural response of the SOST when subjected to wind loads. The data collected during field monitoring were used to determine the loads acting on the model and to validate the model. Further, the loads transferred through the U-bolt connections when the SOST structure was subjected to the design wind loads were calculated and compared with the U-bolt capacities predicted by a 2019 Iowa DOT study.

The yield and ultimate U-bolt capacities predicted by a 2019 Iowa DOT study were compared to the Service I and Extreme I design wind loads calculated using the AASHTO LRFD Specifications for Structural Supports for Highway Signs, Luminaires, and Traffic Signals. The comparison indicates that of all of the Type A U-bolt connections, only the U-bolt types with high yield strengths have yield capacities greater than the imposed stress. The 2019 Iowa DOT study indicated that the yield capacity of a U-bolt connection is low due to the complex geometry, and stress concentrations occur when loading is low. Conversely, the ultimate capacities are greater than the Extreme I wind loads because a U-bolt connection exhibits good ductility before reaching failure.