Field testing of the Wolf Creek Curved Girder Bridge: Part II: Strain Measurements

About the report:

The Wolf Creek Bridge is a curved, multi-girder three span steel composite bridge located south of Narrows, Virginia, that was completed in 2006.  A finite element (FE) model of the bridge revealed that pier flexibility may be important in modeling the bridge.  In addition, questions have been raised as to the effectiveness of the C15x33 diaphragms in providing lateral transfer of loads between members.

This study was conducted as Phase II of a project for which the overall goal was to use field testing to obtain a better understanding of the behavior of multi-span curved girder bridges. The Phase I study was published separately (Turnage and Baber, 2009).  During Phase II, an array of 49 strain gages was installed on the superstructure of the bridge: 34 gages were installed on the four girders at the mid-point of the center span, and 15 gages were installed on the three diaphragm members located closest to mid-span. The bridge was then subjected to static and dynamic applications of a loaded dump truck for which the axle loads were quite close to those of an HS-20 truck. The static strains were measured when the truck was located at 19 different locations on the inner and outer lanes. The dynamic strains were measured under the truck crossing the bridge at normal traffic speed for the structure.

The static loading was then replicated on the FE model. The measured static strains were compared with the strains computed from the FE model. Both measured and computed strains on the girders were used to estimate distribution factors, which were compared to evaluate the effectiveness of moment transfer between girders. The measured static and dynamic strains were also compared to estimate dynamic amplification factors. Finally, measured and computed diaphragm strains were compared to evaluate the FE model’s diaphragm girder approximation.

The study found that the diaphragms transfer relatively little load from the loaded lane toward the unloaded lane but slightly more load transfers toward the outer girders than toward the inner girders.  Further, the FE model predicts slightly greater transfer of load between girders than was measured in the field, suggesting that the model overestimates the stiffness of the diaphragm to girder connection. Finally, the measured strains and strains computed using the FE model predict different neutral axis locations.

Following additional numerical studies, it was concluded that the FE model predicted the neutral axis to be higher than it should be, based upon transformed section calculations.  In addition, full composite action based upon transformed section calculations should result in a neutral axis location higher than was determined from field data measurements.  This suggests that some slip might be occurring between the girders and the haunches.

Authors