Distribution Factors for Short-Haul Vehicular Loads on Prestressed Concrete Open Box Beam (U-Beam) Bridges

Abstract

The design and analysis of a bridge system is a complicated task. In order to simplify the design process, it is necessary to break the system into smaller, less complex, and more manageable subsystems. The AASHTO load-and-resistance factor design (LRFD) bridge design specifications provide the design methods and procedures to design the bridge components explicitly and on an individual basis. In particular, the AASHTO LRFD bridge design specifications employs the beam-line method or distribution factor method to reduce the spatial dimensionality of bridge structures into line elements. The distribution factor is used to modify the load effects determined from the beam-line analysis to account for the effects at other locations in the structure. The published vehicular design loads adopted by the AASHTO LRFD bridge design specifications are considered to be an “average design truck.” The average design truck loads exclude vehicles that are above the legal weight limits for the United States, but are regularly allowed to operate on the U.S. highway systems. These loads are short-haul vehicles such as solid waste trucks, aerial rescue fire trucks and concrete mixers. The most common type of bridge in the United States is the slab-girder bridge system. The principal function of the slab-girder bridge is to provide a roadway surface and transmit the applied loads of the roadway to the girder system supporting the roadway. This research examines the loading effects of the short-haul vehicular loads on a typical prestressed concrete open box beam (U-beam) bridge. This study consists of analytically determining the distribution factors of special vehicles such as, a concrete mixer, a solid waste truck, and an aerial fire rescue truck, and comparing the results to distribution factors recommended in the AASHTO LRFD bridge design specifications.