23 CFR § 658.17 Federal Bridge Law
Federal Bridge Formula & Axle Weight Calculator
Verify semi-truck axle loads and ensure full compliance with the Federal Bridge Gross Weight Formula before rolling across DOT weigh station scales.
Semi-Truck Axle Distribution Diagram
Standard 5-Axle Tractor-Trailer Combination (Outer Wheelbase: 51 ft)
LEGAL & ROAD COMPLIANTTotal Gross: 79,000 lbs • Federal Bridge Cap: 80,000 lbs
1. Steer Axle12,000 lbsLegal: 12,000 lbs
2. Drive Tandem33,500 lbsLegal: 34,000 lbs
3. Trailer Tandem33,500 lbsLegal: 34,000 lbs
[STEER] ── 12ft ── [DRIVES] ──── 39ft ──── [TANDEMS]
Steer Axle Weight12,000 lbs
Max Legal: 12,000 lbsDrive Tandem Weight33,500 lbs
Max Legal: 34,000 lbsTrailer Tandem Weight33,500 lbs
Max Legal: 34,000 lbsFederal Bridge Gross Weight Formula (23 CFR § 658.17)
Congress enacted the Federal Bridge Formula in 1975 to prevent heavy commercial vehicles from damaging highway bridges. The formula calculates the maximum legal weight any group of two or more consecutive axles can carry based on their spacing distance ($L$) and number of axles ($N$):
W = 500 × [ (L × N) / (N - 1) + 12N + 36 ]