Behavior of Steel Bridges Under Superload Permit Vehicles

The need for more power in the United States has spurred a major power plant building program. These plants are usually located where natural gas pipelines cross the power grid, which is generally in more remote areas. To build these plants, construction companies are increasingly using the highway system to move large pieces of plant equipment to the project site. Traditional permit vehicles on highways have had limited gross vehicle weights ranging from 100,000 to 250,000 lb. Special heavyload vehicles have been developed to spread the loads both longitudinally and transversely and to allow engineers to move loads in excess of 1,000,000 lb successfully without the need for temporary support of existing structures. The different types of vehicles that are available and their effect on steel bridges are considered. Simplified methods of analysis including live load distribution, dynamic load allowance (impact), and trailer layout options are investigated. Strain gauge data are presented from an actual 1,000,000-lb permit vehicle that crossed a three-span composite steel bridge in Connecticut. Results of the gauging are compared with the estimates of live load stress from the structural analysis.