Federal law provides that any two or more consecutive axles may not exceed the weight computed by the Formula even though single axles, tandem axles, and gross weight are within legal limits. In other words, the axle group that includes the entire truck—sometimes call the "outer bridge" group—must comply with the Bridge Formula. But 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 be in compliance with weights computed by the Formula (Figure 3).

The most common vehicle checked for compliance with weight limit requirements is shown in Figure 3. While 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. If these combinations are found to be satisfactory, all of the others on this type of vehicle will normally be satisfactory.

The vehicle with weights and axle dimensions as shown in Figure 4 will be used to illustrate a Bridge Formula check.

Before checking a vehicle for compliance with the Bridge Formula, its single-axle, tandem-axle, and gross weight should be checked. Here the single axle (number 1) does not exceed 20,000 lbs., tandems 2-3 and 4-5 do not exceed 34,000 lbs. each, and the gross weight does not exceed 80,000 lbs. These preliminary requirements are thus satisfied. The first Bridge Formula combination is checked as follows:

Check of 1 thru 3 (Figure 5)

Actual weight = 12,000 + 17,000 + 17,000 = 46,000 lbs.

N = 3 axles

L = 20 ft

W maximum = 51,000 lbs., which is more than the actual weight of 46,000 lbs., so the Bridge Formula requirement is satisfied.

Example—Bridge Table B

This same number (51,000 lbs.) could have been obtained from Bridge Table B as shown by reading down the left side to L = 20 and across to the right where N = 3.

Now check axles 1 thru 5 (Figure 6)

Actual weight = 12,000 + 17,000 + 17,000 + 17,000 + 17,000 = 80,000 lbs.

W maximum, from Table B for "L" of 51 feet and "N" of 5 = 80,000 lbs.

Therefore, this axle spacing is satisfactory.

Now check axles 2 thru 5 (Figure 7)

Actual weight = 17,000 + 17,000 + 17,000 + 17,000 = 68,000 lbs.

W maximum, from Table B for "L" of 35 feet and "N" of 4 = 65,500 lbs.

This is a violation because the actual weight exceeds the weight allowed by the Bridge Formula. To correct the situation, some load must be removed from the vehicle or the axle spacing (35 feet) must be increased.