Understanding the Federal Bridge Formula and Axle Weight Limits for Heavy Haul

Why the Bridge Formula Exists

The Federal Bridge Formula (FBF) was established to protect highway bridges from concentrated stress. A heavy load concentrated on a small number of closely-spaced axles creates much greater stress on bridge structures than the same load spread across many axles over a longer distance. The formula calculates the maximum allowable weight for any group of axles based on two variables: the number of axles and the distance between the outermost axles in the group.

Federal Legal Weight Limits (No Permit Required)

The baseline federal weight limits for commercial vehicles on the Interstate System are:

• Gross Vehicle Weight (GVW): 80,000 lbs maximum
• Single axle: 20,000 lbs maximum
• Tandem axle group (two axles within 40 inches of each other): 34,000 lbs maximum

These limits apply on the Interstate System. Some states have "grandfathered" rights allowing higher weights on specific non-interstate routes, but the Interstate limits are the federal baseline.

The Federal Bridge Formula Explained

The bridge formula calculates the maximum allowable weight (W) for any group of consecutive axles:

W = 500 × [(LN / (N-1)) + 12N + 36]

Where:

• W = Maximum weight in pounds for the axle group
• L = Distance in feet between the outer axles of the group
• N = Number of axles in the group

The practical implication: the more axles you have and the more spread out they are, the more weight you can legally carry. This is why heavy-haul trailers use multi-axle configurations — spreading the load across more axles over a longer distance allows heavier loads without exceeding bridge formula limits.

Practical Examples for Heavy Equipment Transport

Here's how the bridge formula affects common heavy-haul scenarios:

• Standard 5-axle semi (3 axles on tractor, 2 on trailer): With typical axle spacing, the bridge formula allows approximately 80,000 lbs GVW — matching the federal GVW limit.
• 7-axle lowboy (3 tractor + 4 trailer): Additional trailer axles spread the load, allowing approximately 105,000–115,000 lbs GVW under the bridge formula, though an overweight permit is still required above 80,000 lbs.
• 13-axle heavy-haul configuration: Used for very heavy loads (200,000+ lbs), multi-axle trailers with extended axle spacing can accommodate extreme weights while staying within bridge formula limits on individual axle groups.

When Overweight Permits Are Required

Any load exceeding 80,000 lbs GVW requires an overweight permit, regardless of whether it complies with the bridge formula. Overweight permits are issued by each state's DOT and specify:

• The maximum allowable GVW for the permitted move
• Required axle configuration and spacing
• Specific route (some bridges may be excluded)
• Travel time restrictions (daylight only, no weekends, etc.)
• Escort vehicle requirements

For loads exceeding standard permit thresholds (often 150,000–200,000 lbs GVW, depending on the state), superload permits are required. Superloads require engineering review of specific bridges along the route and may require police escorts.

State Variations in Bridge Law

While the federal bridge formula applies on the Interstate System, states have authority over non-interstate routes and may apply different formulas or restrictions:

• California: Uses a more restrictive state bridge formula that differs from the federal formula. A load compliant with federal law may still require a California permit.
• Texas: Has its own weight limit structure with specific provisions for certain axle configurations.
• Northern states: Spring weight restrictions (typically February–May) reduce allowable axle weights on state and county roads during the spring thaw, when roadbeds are most vulnerable to damage.

How IronHaul Manages Bridge Formula Compliance

IronHaul's dispatch team reviews bridge formula compliance as part of standard load planning for every heavy-haul move. For loads approaching or exceeding 80,000 lbs, we:

• Calculate the required axle configuration to comply with the bridge formula
• Select the appropriate trailer (number of axles, axle spacing) for the load
• Obtain overweight permits specifying the approved axle configuration
• Review bridge-specific weight limits along the route for very heavy loads
• Coordinate superload engineering reviews when required

Need to move a heavy load and not sure about permit requirements? IronHaul Logistics handles all aspects of heavy-haul compliance, from axle configuration planning to multi-state permit acquisition. Call (208) 495-2260, email [email protected], or request a free quote online.