The Federal Bridge Formula was enacted by Congress in 1975 to protect bridges from excessive wear caused by heavy vehicles. This formula is especially important for commercial or owner operator truck drivers in Tennessee and other states who carry heavy loads. The idea behind the formula is to ensure that a truck and its load are spread out over a long distance to avoid over stressing bridges.
This concept is similar to a person crossing thin ice by crawling while lying down. Because the weight is spread out over the ice, no part of the ice is over stressed. On the other hand, standing up focuses all the person’s weight on to a small area which over-stresses and breaks that patch of ice.
For trucks crossing bridges, spreading out their weight is done by either using additional axles or by spacing the axles farther apart. The Federal Bridge Formula applies to any grouping of two or more axles on your rig. Here is the formula:
W = 500*{ L*N/(N-1) + 12*N + 36 }
The asterisk * indicates multiplication.
Another representation of the formula is shown here.
W is the maximum gross weight in pounds allowed on any group of two or more consecutive axles. This weight must be rounded to the nearest 500 pounds.
L is the spacing in feet between the outermost axles of any axle group under consideration.
N is the number of axles in the group under consideration.
This formula applies to all possible groupings of axles. However, the three important groups are the axles on the tractor (the tractor group), the axles directly beneath the trailer (the trailer group), and the axles of the entire tractor-trailer rig (the outer group).
Example: The Outer Group
The outer group covers the entire tractor-trailer rig. This tractor-trailer rig, has five axles, therefore N = 5.
The spacing in feet between the outermost axles is 51 feet. Therefore L = 51.
The maximum gross weight allowed for this rig is:
W = 500*{ L*N/(N-1) + 12*N + 36 }
W = 500*{51*5/(4) + 12*5 + 36 } = 79,875 which rounds to 80,000 lbs (nearest 500 pounds).
Example: The Trailer Group
There are four axles beneath the trailer. Therefore N = 4.
The spacing in feet between the outermost axles is 36 feet. Therefore L = 36.
The maximum gross weight for the trailer group is:
W = 500*{ L*N/(N-1) + 12*N + 36 }
W = 500*{36*4/(3) + 12*4 + 36 } = 66,000 lbs
Another way to do this is to use the calculator near the bottom of this page.
Looking for rig insurance, or commercial or owner operator truck insurance? Contact us at J.E.B. Insurance Services, LLC.
