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Although the Bridge Formula applies to each combination of two or more axles, experience shows that axle combinations 1 through 3, 1 through 5, and 2 through 5 are critical and must be checked. However, 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 (Figure 3).įigure 3 shows the most common vehicle checked for compliance with weight limit requirements. As a result, the axle group that includes the entire truck-sometimes called the "outer bridge" group-must comply with the Bridge Formula. At 97 inches apart, for example, two axles may carry 38,000 pounds (Figure 2A) and three axles may carry 42,000 pounds, as shown in Figure 2B.įederal law states that any two or more consecutive axles may not exceed the weight computed by the Bridge Formula even though single axles, tandem axles, and gross weight are within legal limits. It is important to note that the single-axle weight limit replaces the Bridge Formula weight limit on axles not more than 40 inches apart, and the tandem-axle weight limit replaces the Bridge Formula weight limit for axles over 40 but not more than 96 inches apart.
Tandem axle weight distribution series#
When the Interstate System axle and gross weight limits were adopted in 1956, and amended in 1975, States were allowed to keep or "grandfather" weight limits that were higher.īridge Formula calculations yield a series of weights (Bridge Table, pages 5-6). Interstate System weight limits in some States may be higher than the figures noted above due to "grandfather" rights. The Federal tandem-axle weight limit on the Interstate System is 34,000 pounds.
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In addition to Bridge Formula weight limits, Federal law states that single axles are limited to 20,000 pounds, and axles spaced more than 40 inches and not more than 96 inches apart (tandem axles) are limited to 34,000 pounds. N = the number of axles in the group under consideration. L = the distance in feet between the outer axles of any group of two or more consecutive axles. W = the overall gross weight on any group of two or more consecutive axles to the nearest 500 pounds. This is accomplished either by spreading weight over additional axles or by increasing the distance between axles.Ĭompliance with Bridge Formula weight limits is determined by using the following formula: What Is It?Ĭongress enacted the Bridge Formula in 1975 to limit the weight-to-length ratio of a vehicle crossing a bridge. This pamphlet describes the Bridge Formula, why it was established, and how it is used. With a few exceptions noted in this pamphlet, the Bridge Formula establishes the maximum weight any set of axles on a motor vehicle may carry on the Interstate highway system. In the event of a dispute, the statute and regulation take precedence with respect to maximum allowable Federal Bridge Formula weights. This guidance paraphrases provisions of 23 U.S.C. Previous editions of this guidance pamphlet are superseded and no longer valid. The Federal Highway Administration (FHWA) revises its guidance pamphlet Federal Bridge Formula Weights (May 2015). You may need the Adobe® Reader® to view the PDFs on this page.Ĭontact Information: Operations Feedback at Department of Transportation Home > Truck Size & Weight Bridge Formula Weights