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.
Example
You enter
- Target group weight (lb) 80000
- Number of axles in the group 5
You get
- Minimum outer-to-outer spread 51 ft
Checked against published worked examples from: FHWA.
Details, formula, and sources
ScopeWith W rounded to the nearest 500 lb as FHWA's Bridge Table prints it. A 5-axle group at 80,000 lb needs 51 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 or tableFederal Bridge Formula B solved for the spread: the first whole foot L at which W = 500 (L N / (N - 1) + 12 N + 36), rounded to the nearest 500 lb, carries the target group weight on N consecutive axles spaced L feet outer-to-outer.
Source and edition23 CFR 658.17 by section (Federal-aid Highway Act of 1956 / FHWA Bridge Formula), rearranged for the axle spread.
Free accessFree at ecfr.gov.
What governsMath aid for personal verification. The ELD on the truck and the carrier tariff are the legal record. State limits may be lower than federal.
API field namestarget_weight_lb, num_axles, min_spacing_ft
- Spread inverse first whole foot at which W, rounded to the nearest 500 lb (an exact half rounds down), reaches the target; zero when the axles satisfy the formula bunched together23 CFR 658.17; FHWA-HOP-19-028 Bridge Table footnote 1
- Independent caps the 20,000 lb single / 34,000 lb tandem / 80,000 lb gross limits still apply; over 80,000 lb needs a permit23 CFR 658.17