Abstract
The Federal Bridge Formula calculates the maximum allowable load that can legally be imposed on a bridge by any group of two or more consecutive axles on a vehicle or combination of vehicles. In the United States, trucks are typically allowed unrestricted operation and are generally considered ‘legal’ provided they meet weight guidelines of Federal Bridge Formula B. Specialized Hauling Vehicles (SHVs) with short wheelbases have had difficulty complying with the Federal Bridge Formula B. The trucking industry has in recent years introduced SHVs with closely spaced multiple axles that make it possible for these short wheelbase trucks to carry the maximum load of up to 355.8 kN (80 000 lb) and still meet the Bridge Formula. The current AASHTO (Association of State Highway and Transportation Officials) legal loads do not represent these newer axle configurations and it is therefore considered likely that these specialized vehicles may be severely overstressing some non-posted bridges. A three-year NCHRP project was initiated in 2003 to investigate recent developments in specialized truck configurations and state legal loads and to recommend revisions to the legal loads for posting as depicted in the AASHTO bridge evaluation manuals. The analytical studies completed so far have identified a new notional rating load model that will envelop the load effects of the worst possible Formula B single-unit truck configurations up to 355.8 kN (80 000 lb). The project will also recommend additional posting load models. This paper presents some preliminary findings of this ongoing research.