New Approach for Screening Overweight Vehicles: Allowable Ranges of Axle Configurations

Throughout the United States, state agencies are issuing an increasingly large number of permits for overweight vehicles every year. State agencies use various methods to process permit requests. The Indiana Department of Transportation employs a screening process that consists of the Tennessee Formula (allowable weight is determined and checked, for every axle group) in combination with the gross vehicle weight limit. If a permit truck satisfies these two screening criteria, it is permitted; otherwise, it is further processed with refined analyses. However, the Tennessee Formula is based on allowable stress design. As a result, cases were observed in which the load effects produced by a truck that would be permitted by the Tennessee Formula actually exceeded load effects produced by the set of rating vehicles when the load factor rating method or the load and resistance factor rating method was employed. The set of rating vehicles is considered a minimum threshold that any nonposted bridge must be able to withstand. This paper presents a different approach for evaluating requests for overweight vehicle permits. Load effects from the set of rating vehicles and typical permit vehicles were directly compared, and screening vehicles with defined ranges of axle weights and axle spacings were developed and recommended for permitting decisions. This method is ( a) safe; ( b) simple, because it can be used easily by nontechnical personnel; and ( c) practical, because a large number of actual permit trucks fall within the proposed screening vehicles.