The Federal Bridge Formula limits how much weight a group of axles may carry based on how far apart they are. It exists to stop a heavy load from concentrating its mass over a short span of bridge deck: spread 80,000 lb over 51 feet and the bridge is fine; bunch it into 20 feet and it is not.
W = 500 × ( (L × N) / (N − 1) + 12N + 36 ) — where W is the maximum weight in pounds on a group of two or more axles to the nearest 500 pounds, L is the distance in feet between the outer axles of the group, and N is the number of axles being considered.
23 U.S.C. §127(a) / 23 CFR Part 658, as published in Bridge Formula Weights, FHWA-HOP-19-028, August 2019. Public domain.
The part that is easy to get wrong is not the arithmetic — it is the scope. Compliance is not "check the steer axle, the drives and the trailer tandems". It is every group of two or more consecutive axles, all N(N−1)/2 of them; a five-axle tractor-semitrailer has ten such groups. The classic failure is a rig whose every named group passes and whose interior span across axles two through five does not.
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