On his website, trucking safety expert witness Lew Grill, the SAGE Corporation, offers a Bridge Formula Weights Calculator:
Federal law states that two or more consecutive axles may not exceed the weight computed by the Bridge Formula even though single axles, tandem axles, and gross vehicle weights are within legal limits. As a result, the axle group that includes the entire truck-sometimes called the “outer bridge” group-must comply with the Bridge Formula. Interior combinations of axles, such as the “tractor bridge” (axles 1, 2, and 3) and “trailer bridge” (axles 2, 3, 4, and 5) must also comply with weights computed by the Bridge Formula (see figure). More detailed information on the Bridge Formula is available at www.ops.fhwa.dot.gov/freight/size_weight.htm.