For the purpose of the problem statement, the term "standard truck" will refer to single units trucks with no more three axles as well as semi-tractor trailer combinations with no more than five axles. Non-standard trucks will be all other truck configurations. The terms inventory stress level and operatiing stress level are defined in the AASHTO Manual for Condition Evaluation of Bridges.
Bridges in South Dakota are designed at an inventory stress level and permitted overloads are allowed up to an operating stress level. The Department uses the load factor design method and the operating stress level allowed is five thirds of the inventory stress level. Some of these non-standard vehicles are possibly exceeding the operating stress level on some bridges but are not considered to be overloaded according to the bridge weight formula. This may be detrimental to the life of some bridges. There are approximately 1200 bridges on the State trunk highway system. The four common bridge construction types include: reinforced concrete slab, steel girder, prestressed concrete girder and trusses.
It is known that non-standard vehicles exist but it is not known to what extent. To further complicate this, vehicles having more than thirteen axles are not counted by weigh-in-motion systems and traffic counters since these systems only accommodate upto thirteen axle vehicles.
A study is needed to determine the truck configuration limits (i.e. -- restriction on gross weight, restriction on number of total axles or number of axles in a group, weight of axle groups, or a modification to the bridge weight formula) that should be set so that the life of bridges in South Dakota are not prematurely shortened. To limit the gross weight of trucks or the number of axles that can be used would require legislation. Strong evidence is needed to support any legislation recommended.
Findings: Approximately 1,180 South Dakota bridges on the state and Interstate system were analyzed for approximately 200 non-standard truck configurations. A standard truck is defined as a vehicle that, if it is a single unit, contains no more than three axles, or, if a semi-trailer unit, contains no more than five axles. Typical standard trucks include the following configurations H, HS, 3S2, T3 or 3-3. All other truck configurations are considered non-standard. The allowable weight of each truck configuration using the current Bridge Gross Weight formula was compared with the analysis. It was found that the current Bridge Gross Weight formula allowed long trucks with a large number of axles to overload a significant number of bridges. Therefore, a modified Bridge Gross Weight formula is proposed. If used, the modified formula would allow the agency to select a value for a constant, based on agency policy, to set amount and percent of overloads allowed. The modified Bridge Gross Weight formula reduces the allowable gross weight for non-standard trucks with a large number of axles and high overall length so the capacity of South Dakota’s bridges is exceeded to a lessor extent than would otherwise be allowed. It increases the gross weight of vehicles with low number of axles and shorter overall length. In addition, the degree of overloading bridges is more uniform and less severe with the modified Bridge Gross Weight formula than with the original one. A procedure was developed to allow the agency to estimate the additional bridge life expected using the modified Bridge Gross Weight formula.