Bridge Formula & Weight Limits for Heavy Haul

Why Weight Limits Exist — and Why the Number Isn't Simple

Most people who have dealt with trucking freight know that 80,000 pounds is the standard federal gross vehicle weight limit on US Interstate highways. What far fewer people understand is that 80,000 lbs is a ceiling, not a guarantee. A truck can weigh less than 80,000 lbs and still be in violation of federal weight law if its weight is distributed incorrectly across its axles — specifically, if the load is concentrated on too few axles that are spaced too closely together.

This is the domain of the federal Bridge Formula, codified in federal law under 23 U.S.C. § 127 and enforced through the Federal Highway Administration (FHWA). The Bridge Formula sets the maximum permissible weight for any group of two or more axles on a vehicle or vehicle combination, based on the distance between those axles. Understanding this formula is essential for anyone shipping heavy equipment, because it governs not only whether a standard flatbed load is legal, but also how heavy haul carriers configure multi-axle lowboy trailers to legally transport loads that would otherwise require overweight permits.

R&RM LLC has been navigating bridge formula calculations for overweight load transport since 2011. This guide explains the formula in plain language and covers what it means for shippers dealing with heavy equipment moves.

The Federal Bridge Formula: How It Works

The Formula Itself

The federal Bridge Formula calculates the maximum allowable weight (W, in pounds) for a group of axles based on the number of axles in the group (N) and the distance between the first and last axle in the group (L, in feet). The formula is:

The output tells you the maximum weight that a given axle group can legally carry based on those two inputs. The critical insight is that spreading weight across more axles that are spaced farther apart increases the permissible weight. Concentrating the same weight on fewer closely-spaced axles reduces the permissible weight — and may violate the formula even if gross vehicle weight is below 80,000 lbs.

A Practical Example

Consider a tandem-axle rear group (N=2) with the two axles spaced 52 inches (4.33 feet) apart — the standard tandem axle spacing on most commercial trailers. Plugging that into the formula:

W = 500 × [ (4.33 × 2 / 2-1) + 12×2 + 36 ] = 500 × [ 8.66 + 24 + 36 ] = 500 × 68.66 = 34,330 lbs

But federal law applies a separate, absolute tandem axle limit of 34,000 lbs for tandems with axles 40–96 inches apart — so the tandem limit is the binding constraint at standard spacing. Now consider a tri-axle rear group (N=3) with the first and last axles 9 feet apart:

W = 500 × [ (9 × 3 / 3-1) + 12×3 + 36 ] = 500 × [ 13.5 + 36 + 36 ] = 500 × 85.5 = 42,750 lbs

Adding a third axle and spacing them out increases the permissible group weight by more than 8,000 lbs. This is exactly the logic heavy haul carriers use when adding axles to a lowboy trailer to legally carry a heavier load — each additional axle, properly spaced, increases the amount of weight the group can carry within the Bridge Formula limit.

Federal Axle Weight Limits

In addition to the Bridge Formula, federal law specifies absolute maximum weights for individual axle types on Interstate highways:

• Single axle: 20,000 lbs maximum
• Tandem axle group (axles 40–96 inches apart): 34,000 lbs maximum
• Gross vehicle weight: 80,000 lbs maximum

These absolute limits coexist with the Bridge Formula — a load must comply with both the formula and the absolute axle limits. The 80,000-lb gross vehicle weight limit applies on Interstate highways but does not preempt stricter state limits on state highways. Many state-maintained roads — particularly county and municipal roads — have posted weight limits significantly below the federal Interstate standard.

When a load exceeds any of these limits — a single axle over 20,000 lbs, a tandem over 34,000 lbs, gross vehicle weight over 80,000 lbs, or a Bridge Formula violation on any axle group — the carrier must obtain an overweight permit from each state the load transits. The permit specifies the legal route, the exact configuration of the vehicle, and the travel conditions that apply.

How Trailer Configuration Affects Compliance

Adding Axles to Stay Within Limits

For heavy haul carriers, the Bridge Formula is not just a regulation to comply with — it is a tool for load planning. When a carrier needs to transport a 100,000-lb piece of equipment, the first question is whether the load can be legally transported within the Bridge Formula on a standard permit, or whether an overweight permit is required. Adding axles to the trailer and spreading them farther apart can often bring a borderline load into compliance without requiring an overweight permit at all.

A standard 3-axle lowboy (with a single rear axle and tandem jeep axle) has limited capacity to carry heavy loads within the Bridge Formula. Adding a fourth, fifth, or sixth axle to the trailer deck — and spacing those axles appropriately — allows the trailer to carry substantially more weight while keeping each axle group within its formula limit. This is why you see 10-axle, 12-axle, and 13-axle lowboy trailer configurations on heavy haul moves: each additional axle increases the legal carrying capacity of the trailer combination, within the Bridge Formula framework.

There is a practical limit to how many axles help, however. Adding axles costs money in rental and logistics. Beyond a certain point — typically around 200,000 to 250,000 lbs for standard multi-axle trailers — axle addition alone cannot keep per-axle weights within formula limits, and an overweight permit becomes unavoidable. At that point, the overweight permit fee is simply a cost of the move, and the permit specifies which route the load must use based on the bridge ratings along that route.

Jeep Axles and Stinger Steerable Axles

Heavy haul trailers can also add jeep axles — auxiliary axle units inserted between the tractor fifth wheel and the front of the trailer — and stinger steerable axles at the rear of the trailer. Jeep axles distribute some of the trailer's weight onto the tractor side of the fifth wheel, reducing the weight on the trailer's rear axle group. Stinger axles extend behind the rear of the trailer's deck, increasing the axle base of the rear group and allowing more weight per the Bridge Formula. Both configurations are commonly used by experienced heavy haul carriers to maximize legal carrying capacity without overweight permits.

The tradeoff is maneuverability. Adding steerable rear axles improves turning radius on a long multi-axle trailer. Jeep axles change the tractor-trailer articulation point. An experienced driver who regularly operates these configurations handles these characteristics routinely — but a carrier unfamiliar with multi-axle lowboy configurations may decline loads that a specialist can legally carry without special permits.

State Weight Limits vs. Federal Limits

Where State Law Differs

The federal Bridge Formula and the 80,000-lb GVW limit apply on the National Highway System (NHS), which includes Interstate highways. States may set their own weight limits on state and local roads, provided those limits do not exceed federal standards on NHS routes. In practice, this creates a patchwork of weight limits:

• Interstates: Federal limits apply (20,000/34,000/80,000 lb and Bridge Formula)
• State primary routes: State limits, which may equal or be stricter than federal limits
• County and local roads: Often lower posted limits — 10-ton, 15-ton, 20-ton — that reflect bridge capacity on older infrastructure
• Posted bridge limits: Individual structures may be posted below the surrounding road's limit, requiring a permit review or alternate routing

Some states have historically allowed higher single-axle and tandem limits than the federal standard — particularly states with their own trucking industries that pre-dated federal standardization. The Federal Surface Transportation Assistance Act of 1982 froze these "grandfather" allowances in place, which is why some states appear to allow more than the federal standard on some routes: they are operating under that historical grandfathering, not a modern exception.

Spring Weight Restrictions

Every northern and mountain state imposes seasonal weight restrictions in spring, when thawing ground weakens road subgrades and bridge approaches. During spring restriction periods — which typically run from late February through April or May depending on the state and the year's weather — state and county roads are posted at reduced weight limits, sometimes as low as 50% of normal summer limits. These restrictions apply even to loads that have valid overweight permits for summer operation.

Spring weight restrictions are a major planning factor for heavy haul moves in states like Minnesota, Wisconsin, Iowa, Michigan, Ohio, Pennsylvania, and Montana. A 90,000-lb excavator that can be permitted over a specific county road in July may require an alternate route — or may simply be unmovable on that road — during the spring restriction period. Seasonal planning for heavy haul requires knowing not just the summer permit rules, but also when and how severely spring restrictions apply on your specific route.

Overweight Permits: When They Are Required

What Triggers an Overweight Permit

An overweight permit is required whenever a truck-trailer combination exceeds any of the following on the applicable road:

• 80,000 lbs gross vehicle weight on an Interstate highway (or lower limit on state/local roads)
• 20,000 lbs on a single axle
• 34,000 lbs on a tandem axle group
• The weight calculated by the Bridge Formula for any axle group at its actual axle spacing

A load that exceeds only the gross vehicle weight limit but whose per-axle weights are within formula limits needs a gross weight permit. A load whose per-axle weights exceed formula limits even if gross vehicle weight is below 80,000 lbs needs an axle weight permit. A load that exceeds both needs both covered under the permit application. States issue permits based on the combination of violations, and the permit fee schedule often scales with how far the load exceeds legal limits in each category.

Permit Processing and Route Review

When a carrier applies for an overweight permit, the state DOT reviews the application to determine whether the load's axle weights are compatible with the bridges on the proposed route. This is not a rubber stamp — a bridge built in 1940 to a load rating of 20 tons may not be able to carry a 40-ton single-axle load safely, and the permit reviewer will flag that bridge and either deny the permit for that route or require a detour. The more severely the load exceeds formula limits, the more careful the bridge review.

For extreme overweight loads — sometimes called superloads, typically loads over 150,000 to 200,000 lbs gross — some states require the carrier to submit engineering calculations certifying the load's effect on specific bridges along the route. This adds time and cost to the permit process but is required to protect infrastructure that was never designed to carry that level of concentrated load. See our oversize permit guide for a detailed walkthrough of the permit application process by state.

What This Means for Shippers

Why Your Carrier's Trailer Configuration Matters

As a shipper, you may not need to understand the Bridge Formula in detail — but you should understand that the trailer your carrier uses directly affects whether your load can be moved legally under a standard permit or requires a more expensive and time-consuming overweight permit. An experienced heavy haul carrier evaluates your load's weight and dimensions, calculates the optimal axle configuration to stay within formula limits, and in many cases can avoid the overweight permit entirely through proper trailer selection and axle configuration.

Carriers who do not specialize in heavy haul may default to requiring an overweight permit for any load over 80,000 lbs — not because the permit is always necessary, but because they lack the multi-axle trailer equipment and the formula expertise to configure the load otherwise. This can add cost and days to your transit timeline. When evaluating a carrier, ask specifically how they plan to configure the trailer for your load's weight and dimensions and whether they expect to need an overweight permit.

How to Work Efficiently Within the Rules

A few practices help shippers work efficiently within Bridge Formula constraints:

• Provide accurate weights early. Operating weight, transport weight, and fuel load all affect permit requirements. Give your carrier the precise transport weight as early as possible so they can plan the trailer configuration correctly.
• Know what can be removed. Buckets, counterweights, attachments, and implements can often be removed and transported separately, reducing the main load's weight enough to shift it out of overweight territory. Discuss disassembly options with your carrier before assuming the full machine weight must be permitted.
• Plan for state-specific timing. Overweight permits in most states process in two to five business days. Complex superload permits can take two to four weeks. If your move has an overweight component, start the permit process before you need to move.
• Confirm the last-mile route. Interstate bridges are engineered to modern standards. County roads serving a mine, port, or construction site may not be. Confirm that the final approach to your delivery point can legally carry the loaded trailer combination before booking the move.

How R&RM LLC Manages Bridge Formula Compliance

R&RM LLC has been coordinating overweight load transport across all 48 continental states since 2011. Every load we quote begins with a weight and dimension assessment, a trailer configuration recommendation, and a determination of which states will require overweight permits and which can be covered under a standard oversize permit with a properly configured multi-axle trailer.

For loads that require overweight permits, we submit permit applications simultaneously to all states in the corridor, track processing status, and manage any back-and-forth with state DOT bridge reviewers. We do not ask shippers to navigate state permit portals — that is our job, and it is something we do on every move that requires it. Our dispatchers are familiar with the requirements and typical processing timelines at each state DOT permit office in the continental US.

When a load is borderline — where adding an axle or spacing axles differently can bring the load into formula compliance without a permit — we work through the configuration options before concluding that a permit is required. This approach can save shippers permit fees and transit time on loads that fall just above the standard limit.

To discuss your specific load's weight, dimensions, and routing, call us at (404) 987-6225 or request a quote online. R&RM LLC — R&RM LLC, based in Cumming, Georgia — handles the entire process from permit application through delivery.

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