Braking Forces in Highway Bridge Substructures

Abstract

Under the Load-and-Resistance Factor Design Specification, the longitudinal braking force that must be designed for was increased when compared to a bridge designed according to the previous code, the Standard Specification. The Standard Specification required 5 percent of the live load be designed for, whereas, the LRFD Specification increased this value to 25 percent. For short to medium span bridges, this was a signification increase, as much as 400 percent in some cases. This increase in live load also increases the demand on the bridge bents and other substructure elements. This change in the code raises questions about the path the load is taking and ultimately the intensity of the load going into the supporting substructure. The primary focus of this study was to investigate the amount of shear force in the piles of a short-span bridge typical of Alabama Department of Transportation construction. This was accomplished through static pull tests and dynamic braking tests on the Macon County Road 9 bridge. The static pull tests involved positioning the load truck on individual spans and pulling on it with a tow truck that was connected with a cable and outfitted with a load cell. The load cell allowed for correspondence between bent and girder displacements and longitudinal force imparted on the bridge deck. The cable was tensioned until approximately 20 kips was achieved or the truck began to slide. The dynamic braking tests involved the load truck accelerating to approximately 12-15 mph before getting on the bridge and once entirely within the span being tested the brakes were applied and the truck came to a stop within the same span. Using the results from the field tests, an analytical model was created and calibrated to be able to obtain the forces that are not well understood. From this model, it was determined that the total braking force can be distributed among all components of the substructure and the amount of which varies depending on the location on the bridge that the vehicle was braking. Furthermore, it was established that the code provision of 25 percent of the design truck axle weight is generally not over conservative. From the field tests conducted, potentially 35 percent of the vehicle weight can be imparted into the bridge substructure components in sum.