Living With the “Bridge”

Date: 01/22/2002
Item: Selecting The Right Trailer To Meet The Federal Bridge Law

A 1944 algebraic equation still governs truck weight and length. Here’s how to use the “bridge formula.”

Living by the “bridge formula” is a way of life for truckers today. It’s the law on the Interstates in every state with two exceptions: grandfather provisions that date back to 1956, or for states that stayed at the old federal maximum of 73,280 pounds gross after the national Interstate weight limit was raised to 80,000 pounds in 1974.

Truck operators should understand the federal bridge formula – known as Formula B or Bridge Table B – because it is the basis for determining what a vehicle can legally weigh, and how long it must be.

Formula B involves both axle weights and axle spacing. There’s an “inner bridge” and “outer bridge” that must be met – bridge in this case meaning the distance between various axles.

Original bridge formulas go back more than 50 years and started in the West. The idea was to protect marginal bridges by requiring a vehicle’s weight be spread out over axles spaced far apart. That way the full weight of the vehicle isn’t concentrated on a short section of a bridge.

Many truckers in western states today use long wheel base trucks and tractors to get maximum “bridge.” And where multiple unit, over-80,000-pound combinations are allowed, bridge laws determine how much can be carried, based on number of axles and distance between them.

By comparison, many states in the East and New England have heavy axle limits and no restrictive axle spacing. An example is Pennsylvania, which on non-federal highways still will allow 73,280 pounds on a four-axle straight truck 40 feet long. This vehicle, with a normal 189-inch wheelbase, would be limited to 52,500 pounds when complying with the federal Formula B.

It’s an accepted engineering principle that concentrated weight is what breaks bridges and pavement if they aren’t designed for heavy loads. So the U.S. Bureau of Public Roads (now the Federal Highway Administration) adopted the bridge formula concept for federal roads in 1944 and, except for modification in 1974, it’s been with us ever since.