Bridge Formula Minimum Axle Spread
The Federal Bridge Formula solved for the minimum outer-to-outer axle spread that legally carries a target group weight: L = ((W/500) - 12 N - 36)(N-1)/N. A 5-axle group at 80,000 lb needs at least 51.2 ft; a 2-axle group at 34,000 lb needs 4 ft. If the spread comes out zero the axles already satisfy the formula bunched together. The 20,000 lb single / 34,000 lb tandem / 80,000 lb interstate caps apply independently, and over 80,000 lb needs a permit. The state DOT governs.
Formula and source
Federal Bridge Formula B solved for the spread: L = ((W / 500) - 12 N - 36)(N - 1) / N, where W is the target group weight (lb) on N consecutive axles spaced L feet outer-to-outer.
23 CFR 658.17 by section (Federal-aid Highway Act of 1956 / FHWA Bridge Formula), rearranged for the axle spread.
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