Recommendation of a New Truck Model for Bridge Evaluation

In the United States, trucks are typically allowed unrestricted operation and are considered legal provided they meet weight guidelines of Federal Bridge Formula B, which 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. Specialized Hauling Vehicles (SHVs) with short wheelbases have had difficulty complying with 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 (80000 lb) and still meet the bridge formula. The current AASHTO 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. This paper presents some preliminary findings of this ongoing research project. The analytical studies completed so far have identified a new notional rating load (NRL) model that will envelop the load effects of the worst possible Formula B single-unit truck configurations up to 355.8 kN. The single NRL load model with a maximum gross weight of 355.8 kN produces moments and shears that exceed the load effects for a series of three- to eight-axle single-unit trucks allowed to operate under current federal weight laws. The project will recommend additional posting load models that are representative of the newer truck configurations and suitable for inclusion in the AASHTO evaluation manuals.


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  • Accession Number: 01004102
  • Record Type: Publication
  • Files: TRIS
  • Created Date: Sep 25 2005 3:38PM