Explaining the maximum weight any combined set of axles on a truck and trailer can carry to cross a bridge on the Interstate highway system. The following formula is used for calculating acceptable weights:
W=500 [ LN/N-1 + 12N + 36]
W= the overall gross weight on any group of two or more consecutive axles the the nearest 500 pounds.
L= the distance in feet between the outer axles of any group of two or more consecutive axles.
N= the number of axles in the group that are under consideration.
Axle spacing is another consideration that must be taken into account when looking at the Federal weight compliance laws. In order to properly protect our bridges the number and the spacing of truck and trailer axles carrying the shipment must be calculated. So a bridge weight formula applies when the gross weight on 2 or more consecutive axles exceeds the limitations on the formula, except that the 2 consecutive sets of tandem axles may carry a gross weight load of 34,000 pounds each if the overall distance between the 1st and last axle is over a 36′ minimum.
There are exceptions. Federal law does include an exception to the Federal Bridge Formula and the bridge table. 2 consecutive sets of tandem axles may carry 34,000 pounds each IF the overall distance between the first axle and last axles of these tandems is 36′ or greater. For example: a 5 axle tractor semi trailer combination may carry 34,000 pounds of weight both on the tractor tandem (axles 2 and 3) and the trailer tandem (axles 4 and 5), provided the axles 2 and 5 are spaced at least 36′ from each other. Without this exception the bridge formula would allow for actual weight of only 66,000 pounds to 67,000 pounds on tandems spaced 36′ to 38′ apart. (References: U. S. Department of Transportation – Federal Highway Administration “Bridge Formula Weights“)
Note: In addition to the above formula weights Federal law says that a single axle is limited to a maximum of no greater than 20,000 pounds and that axles that are closer than 96″ from each other are limited to a maximum weight of 34,000 pounds. Also that gross vehicle weight be less than 80,000 pounds