Federal Bridge Formula B Explained: DOT Axle Weight Limits & Spacing Chart
Complete guide to Federal Bridge Formula B (23 CFR § 658.17): axle spacing mathematics, 80,000 lbs gross cap, steer and tandem limits, inner vs outer bridge rules, and sliding trailer tandems.
- Federal Bridge Formula B prevents highway bridge fatigue by regulating weight concentration across axle spacing.
- The formula is W = 500 × [(L × N) / (N - 1) + 12N + 36], where L is spacing in feet and N is the number of axles.
- Federal statutory limits on the Interstate Highway System: Single Axle = 20,000 lbs, Tandem Axles = 34,000 lbs, Gross Vehicle Weight = 80,000 lbs.
- Outer Bridge (steer to rear trailer axle) controls gross weight; Inner Bridge (drive tandem to trailer tandem) controls internal axle group weight.
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1. What Is Federal Bridge Formula B?
The Federal Bridge Formula B (codified under Title 23, Code of Federal Regulations, Section 658.17 and 23 U.S.C. 127) is a federal mathematical standard enacted to protect American highways and bridges from premature structural fatigue caused by concentrated commercial vehicle loads.
Bridges are engineered to support maximum bending moments. While an 80,000-pound semi-truck spread across 51 feet of highway pavement disperses load safely, that exact same 80,000 pounds concentrated onto a short 20-foot dump truck would exceed bridge beam tensile ratings and trigger catastrophic fatigue cracking.
2. The Mathematical Formula Explained
Federal law computes maximum permissible gross weight on any group of two or more consecutive axles using this equation:
Where:
- W = Maximum permissible weight in pounds carried on any group of 2 or more consecutive axles.
- L = Distance in feet measured between the extreme axles of any axle group.
- N = Number of axles in the group under consideration.
3. Federal Statutory Weight Caps (Interstate Limits)
Regardless of the mathematical output of Bridge Formula B, vehicles operating on the US Interstate Highway System are subject to strict statutory maximums without special state overweight permits:
4. Master Bridge Weight Table (8 ft to 56 ft)
Below is the official Bridge Formula B weight lookup table excerpt published by the Federal Highway Administration (FHWA):
| Axle Distance (ft) | 2 Axles | 3 Axles | 4 Axles | 5 Axles | 6 Axles |
|---|---|---|---|---|---|
| 8 ft | 34,000 | 34,000 | — | — | — |
| 12 ft | 35,000 | 43,000 | 47,000 | — | — |
| 20 ft | 39,000 | 47,500 | 52,000 | 56,500 | 61,000 |
| 28 ft | 40,000 | 52,500 | 57,500 | 62,500 | 67,500 |
| 36 ft (Inner Bridge) | — | 57,500 | 68,000 | 73,500 | 79,000 |
| 44 ft | — | 60,000 | 68,000 | 73,500 | 79,000 |
| 51+ ft (Outer Bridge) | — | — | 72,500 | 80,000 (CAP) | 80,000 |
5. Inner Bridge vs. Outer Bridge Explained
Outer Bridge (Axles 1 to 5)
Measures the full wheelbase distance from the center of steer axle 1 to the center of rearmost trailer axle 5.
On a standard 53-ft tractor-trailer, this distance is typically 51 to 55 feet. Under Bridge Formula B, 51+ feet on 5 axles allows the full 80,000 lbs gross vehicle weight limit.
Inner Bridge (Axles 2 to 5)
Measures the internal distance between the tractor drive tandem (axles 2 & 3) and the trailer tandem (axles 4 & 5).
Under Bridge Formula B, an inner bridge span of 36 feet permits exactly 68,000 lbs (34,000 lbs on drives + 34,000 lbs on trailer). If an owner-operator slides the trailer tandems too far forward, the inner bridge distance shrinks, potentially making the truck overweight even if gross weight is under 80,000 lbs!
6. How to Slide Tandems to Balance Scale Weights
When pulling onto a CAT scale, weight is frequently misbalanced across axle groups. Understanding the mechanics of sliding trailer tandems allows you to balance weight legally before hitting state DOT scale houses:
- Slide Tandems Rearward (Toward the Back): Shifts payload weight forward onto the tractor drive axles.
- Slide Tandems Forward (Toward the Front): Shifts payload weight rearward onto the trailer tandem axles.
- Weight Shift Ratio: In standard 53-ft dry vans and reefers with standard 4-inch hole spacing, each hole pin shift transfers approximately 250 to 400 lbs between axle groups.
7. The California 40-Foot Kingpin Rule (KPRA)
Drivers operating in California (and several Northeastern states) face an additional regulatory restriction known as the Kingpin-to-Rear-Axle (KPRA) bridge law:
California Vehicle Code 35400 Restriction:
The distance from the trailer kingpin to the center of the rear trailer axle (or center of the tandem axle group) must NOT exceed 40 feet on two-axle semitrailers.
Even if your trailer tandems are legal under 34,000 lbs on the scale, sliding your tandems past the 40-foot mark to balance a heavy load will result in immediate citations from the California Highway Patrol (CHP) at Banning, Wheeler Ridge, or Truckee weigh stations!
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