Bridge Formula
The Federal Bridge Formula (also called Bridge Formula B) is a federal regulation that limits the weight a commercial vehicle can carry based on the number of axles and the distance between them, to prevent damage to bridges and road infrastructure.
In Depth
The Bridge Formula applies to all axle combinations, not just individual axles. It sets a maximum weight based on the formula W = 500[(LN / (N-1)) + 12N + 36], where W is the maximum weight, L is the distance between the outer axles, and N is the number of axles. This prevents trucks from concentrating too much weight in a small road footprint.
In practical terms, the Bridge Formula affects long-distance hauls where axle spacing is calculated from the steer axle to the last trailer axle. Vehicles with shorter axle spacing can carry less gross weight. This is why some permit loads (oversize/overweight) require specific vehicle configurations with widely spaced axles.
Violating the Bridge Formula results in fines proportional to the excess weight and potential liability for bridge damage. Most ELD and scale ticket systems now flag Bridge Formula violations automatically.
Usage Example
Example: 'Even though my gross was 79,200 lbs, the bridge formula required I reduce weight because my axle spacing was under 51 feet.'
Related Terms
Axle Weight
Axle weight is the total load transmitted to the road through a single axle or tandem axle group. Federal law limits single axles to 20,000 lbs and tandem axles to 34,000 lbs, and states may impose stricter limits.
Gross Vehicle Weight Rating (GVWR)
GVWR (Gross Vehicle Weight Rating) is the maximum safe operating weight of a single vehicle as specified by the manufacturer, including the vehicle itself and its maximum cargo load. For combination vehicles (truck + trailer), the combined GVWR is called the Gross Combination Weight Rating (GCWR).
Oversize/Overweight Permit
An oversize or overweight (OS/OW) permit is a state-issued authorization that allows a vehicle to operate above standard size or weight limits. Permits are required when loads exceed 8.5 feet wide, 13.5 feet tall, 53 feet long, or 80,000 lbs gross.
Weigh Station
A weigh station is a roadside facility operated by state departments of transportation where commercial vehicles are required to stop for weight and safety inspection. They verify that trucks comply with federal and state axle weight limits and safety regulations.
Frequently Asked Questions
How does the Bridge Formula affect my load weight?
Trucks with shorter axle spacing must carry less gross weight. A 48-foot trailer combination allows less gross weight than a 53-foot trailer configuration.