Chapter 21

This section is designed to provide background and detailed information used to determine the distribution of weight on structures. It is not just the structures we are looking at but also the road itself.

Bridges on Interstate System highways are used by a wide variety of traffic. They are designed to support expected loadings. However, as trucks grew heavier in the 1950s and 1960s, something had to be done to protect bridges. The solution was to tie allowable weights to the number and spacing of axles.

Axle spacing is as important as axle weight in bridge design. A bridge is analogous to thin ice on a pond. Walking on the ice concentrates a person's weight on the small area covered by the individual's feet, and then the ice may break. Lying down, however, spreads the same weight over a much larger area, and the ice is less likely to break. Consider trucks crossing a bridge:

In Figure 1 (A), the stress on bridge members as the longer truck rolls across is much less than that caused by the short vehicle in Figure 1 (B), even though both trucks have the same total weight and individual axle weights. The weight of the longer vehicle is spread out, while the shorter vehicle has all of the weight concentrated on a small area. The Federal-Aid Highway Amendments of 1974 increased the weights allowed on the Interstate system to 20,000 pounds on a single axle, 34,000 pounds on a tandem axle, and 80,000 pounds gross weight (23 U.S.C. 127). But Congress balanced this concession to productivity by enacting the Bridge Formula {500[(LN/N-1) +12N+36]}. The result is that motor vehicles may be loaded to the maximum weight only if each group of axles on the vehicle and their spacing also satisfy the requirements of the Formula. This prevents the vehicle from overstressing bridges in the same way that a person lying down on thin ice would minimize the risk of breaking through

W = the maximum weight in pounds that can be carried on a group of two or more axles to the nearest 500 pounds.

L = the distance in feet between the outer axles of any two or more consecutive axles.

N = the number of axles being considered.

Note: When the distance in feet includes a fraction of a foot of one inch or more the next larger number of feet shall be used. This only applies to Divisible Loads.

The formula limits the weight on groups of axles in order to reduce the risk of damage to highway bridges. Allowable weight depends on the number of axles a vehicle has and the distance between those axles. However, the single-or-tandem-axle weight limits supersede the Bridge Formula limits for all axles not more than 96 inches apart.

Until 1982, Federal law set only upper limits (or ceilings) on Interstate System weight limits. A few states retained significantly lower weight limits, which eventually became barriers to long-distance truck traffic. In 1982, Federal law was amended to make Interstate Systems weights limits, including the bridge formula limits, both the maximum and the minimum weights (i.e., floors and ceilings) that states must allow on the Interstate System.

To use the formula, {W=500[(LN/N-1) + 12N + 36]}, you take the number of axles and plug them in for N and the distance measured rounded to the next foot for L. The calculation would then go in the following order:

When calculating the formula by hand you would round the measurement after adding all the other measurements together. For instance if I had the following measurements of 13’, 4’4”,14’, 4’6” they would give me a measurement of 35’ 10”

So this means that on a bridge measurement of 36 ft with 5 axles the total allowed weight is 70,500 lbs.

2 to 5 = 36’6” (37)2 to 6 = 46’6” (47)2 to 7 = 61’6” (62) = 91,000 Lbs4 to 6 = 14’4 to 7 = 29’6 to 7 = 15’

= 450 lbs per inch for tires less than 11 inches wide.= 500 lbs per inch for tires 11 inches wide or greater.