Bridge Law and Axle Weight Limits: What Heavy Equipment Shippers Need to Know

What Is the Federal Bridge Formula?

The Federal Bridge Gross Weight Formula (commonly called "Bridge Formula B" or simply "bridge law") is a federal regulation that limits the maximum weight a commercial vehicle can carry based on the number of axles and the distance between them. Its purpose is to protect highway bridges from the concentrated stress caused by heavy, short-wheelbase vehicles — a 100,000-lb load spread across 10 axles over 60 feet does far less damage to a bridge than the same weight concentrated on 3 axles over 15 feet.

The formula is: W = 500 × [(LN / N-1) + 12N + 36], where W is the maximum allowable weight in pounds for a group of axles, L is the distance in feet between the outer axles of the group, and N is the number of axles in the group. In practice, carriers and permit agencies use lookup tables rather than calculating this formula manually for every configuration.

The Three Core Federal Weight Limits

Federal law establishes three primary weight benchmarks for the Interstate Highway System:

• Gross Vehicle Weight (GVW): Maximum 80,000 lbs for a standard 5-axle combination (tractor + 2-axle trailer). Exceeding this requires an overweight permit.
• Single Axle Weight: Maximum 20,000 lbs per single axle (axles spaced 40 inches or less apart).
• Tandem Axle Weight: Maximum 34,000 lbs for two consecutive axles spaced between 40 and 96 inches apart.

A vehicle can be within the 80,000-lb GVW limit and still be overweight if an individual axle group exceeds its bridge formula limit. This is why trailer configuration — specifically the number of axles and their spacing — matters so much for heavy equipment moves.

How Bridge Law Affects Heavy Equipment Transport

For equipment shippers, bridge law has direct practical implications:

Adding Axles to Distribute Weight

When a piece of equipment weighs more than a standard 3-axle lowboy can legally carry, the carrier adds axles. A 3-axle RGN might legally carry 42,000 lbs of payload. Adding a 4th axle (making it a 4-axle RGN) increases the legal payload to approximately 60,000–65,000 lbs. A 5-axle configuration can handle 80,000+ lbs. Each additional axle adds cost — both the trailer cost and the permit cost — but it's the only legal way to move heavy equipment on public roads.

The 36-Foot Tandem Exception

Federal law includes a specific exception: two consecutive sets of tandem axles (4 axles total) can each carry 34,000 lbs (68,000 lbs combined) if the distance between the first and last axle of the two tandems is at least 36 feet. This "36-foot tandem exception" is commonly used in standard 5-axle tractor-trailer configurations and is why the 80,000-lb GVW limit works mathematically for standard trucks.

Short-Wheelbase Equipment

Compact, heavy equipment — like a large compactor or a short-wheelbase crane — concentrates weight over a short axle span. Even if the machine weighs "only" 60,000 lbs, its weight distribution may violate bridge formula limits on a standard 3-axle trailer. The solution is either a longer trailer (to spread the load over more axle distance) or additional axles.

State Variations

While federal bridge law applies to the Interstate Highway System, states can impose stricter limits on non-interstate routes. Some states also have "grandfather" provisions that allow higher weights on specific highways that predate federal standards. When your load crosses multiple states, the most restrictive state's rules govern the permit requirements for that portion of the route.

States with historically stricter bridge law enforcement include Michigan (which has some of the strictest axle weight limits in the country, particularly for spring thaw periods) and California (which enforces bridge formula compliance rigorously at weigh stations). States with more flexible rules for certain configurations include Texas and Florida.

Practical Takeaways for Equipment Shippers

• Always provide accurate weight AND dimensions when requesting a quote — weight alone isn't enough to determine the right trailer configuration.
• Ask your carrier how many axles the trailer will have and what the permitted GVW is for your specific load.
• If your equipment is borderline on weight, ask whether adding an axle would reduce permit costs or eliminate the need for a superload permit.
• For multi-state moves, confirm that the carrier's permit covers the bridge formula requirements for every state on the route, not just the gross weight.

IronHaul Logistics handles the bridge law and permit complexity so you don't have to. Our team knows how to configure trailers and route loads to minimize permit costs while staying fully legal. Call (208) 495-2260, email [email protected], or use our online quote form to get a quote for your next heavy equipment move.