2021 Easy – Federal Bridge Formula weights explanation.
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 Federal bridge 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.
How do I determine Bridge Formula?
THIS IS A PERFECT EXAMPLE: Bridge formula is 30 times the wheelbase* in centimeters plus 18,000 kg.
*Wheelbase in the above Bridge Formula calculation refers to the maximum axles span (center of the first axle to the center of the last axle of any axle group) in a heavy commercial vehicle.
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 feet minimum.
Exceptions to the Bridge Weight Formula:
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 feet 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 feet 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 feet to 38 feet apart. (References: U. S. Department of Transportation – Federal Highway Administration “Federal Bridge Formula Weights” – FWHA-FBFWHA-FBFF the .pdf)
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 inches from each other are limited to a maximum weight of 34,000 pounds. Also that gross vehicle weight be less than 80,000 pounds.
Federal law (23 U.S.C. 127) does include an exception to the Federal Bridge Formula and the Bridge Table—two consecutive sets of tandem axles may carry 34,000 lbs. pounds each if the overall distance between the first and last axles of the tandem axles is 36 feet or more. For example; a five-axle tractor and semitrailer combination may carry 34,000 lbs. pounds both on the tractor tandem (axles 2 and 3) and the trailer tandem (axles 4 and 5), if axles number 2 and 5 are spaced at least 36 feet apart. Without this exception the Federal Bridge Formula would allow an actual weight of only 66,000 pounds up to 67,500 pounds on tandems spaced 36 to 38 feet apart.
2020 Federal Bridge Weight Chart
|Less than 8’3″||34,000||34,000|
|More than 8’4″||38,000||42,000|