The H20-44 loading is a hypothetical cargo truck, similar to the one pictured here. The
truck has two axles spaced approximately 4 meters apart. The truck has a total weight of 180
kilonewtons (kN), with 35 applied at the front axle and 145 kN at the rear axle.
When the AASHTO truck loading is used to design a bridge, the axle weights are generally increased by a factor called the , which accounts for the effects of the moving load.
When a structural engineer designs an actual bridge, he or she must ensure that all in the structure can safely carry the generated when one AASHTO truck loading is applied to every traffic lane on the bridge . The truck loading can be positioned anywhere along the length of the bridge.
As indicated in the Design Specifications, the West Point Bridge Designer 2010 allows you to design a bridge for either of two highway loading conditions:
In the first load case, with two traffic lanes, each of the two main trusses must carry the weight of one lane. Thus the loading used in the West Point Bridge Designer 2010 consists of one H20-44 truck, with a total weight of 180 kN. To ensure that the resulting structure is , the two trucks are assumed to be facing in opposite directions. Thus the total load applied at each axle location is 90 kN.
In the second case, with a single truck centered on the roadway, each of the two main trusses must carry one-half of the total truck weight. Thus the Permit Loading used in the West Point Bridge Designer 2010 consists of 330 kN, with 165 kN applied at each axle location.
In both cases, the dynamic load allowance is 1.33.