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Determining Bridge Weight LThe next time you drive over a bridge, keep a lookout for a weight limit sign. You may not give these signs much thought, but many bridges have weight limit designations, usually posted before you reach the bridge itself.

This article will take a look at when these designations came about, why they're there, and how these weight limits are determined.

When Did Bridge Weight Limits Come About?

Bridge weight limits date back more than 100 years. State governments enacted the first truck weight limits in 1913. These restrictions were adopted by four states, ranging from 18,000 pounds (Maine) to 28,000 pounds (Massachusetts). At the time, the primary concern was to protect gravel roads from damage caused by large vehicles.

By 1933, every state had some form of regulation, and the Federal-Aid Highway Act of 1956 gave rise to the Interstate Highway System and set a federal truck weight regulation of 73,280 pounds.

It wasn't until 1975 that we saw the Federal-Aid Highway Act Amendments of 1974 take shape, which established the bridge formula as law, limiting gross weight to 80,000 pounds, though today's limits depend on various other factors.

Why Do Bridges Have Weight Limits?

Bridges are designed to bear a certain amount of weight, and that load-bearing capacity deteriorates with age and use. Commonly, a deteriorating beam can reduce the bridge's structural integrity, limiting the size of the vehicles that can cross it safely.

In other words, a bridge simply can't support the weight of every vehicle that could attempt to cross it, and the weight limit signs indicate the limit of the structure's support.

A professional engineer recommends a weight restriction for the bridge. The bridge owner becomes responsible for posting the limits on the bridge itself and at the nearest intersection to allow large vehicles to determine whether they are safe to proceed.

What Determines Bridge Weight Limits?

Bridge weight limits are determined by applying the Bridge Formula, which was codified in 1975:

W = 500 x [(LN)/(N-1) + 12N + 36]

Where:

  • W = the overall gross weight
  • L = the distance in feet between the vehicle's outer axles
  • N = the number of axles

This formula is used to determine bridge weight limits, but federal law also dictates the following limitations:

  • Single axles are limited to 20,000 pounds
  • Axels spaced between 40 and 96 inches are apart are limited to 34,000 pounds
  • Gross vehicle weight is limited to 80,000 pounds

The reasoning is simple: short, heavy trucks can introduce greater structural strain than trucks that distribute their weight across a longer distance. Therefore, bridge weight limit signs will typically provide a series of limits based on vehicle size and axel count.

Safety and Integrity

Federally-enacted bridge weight restrictions protect the structural integrity of America's bridges, as well as protecting the safety of truckers and other motorists. Signs indicating weight limits can be found before reaching the bridge itself and usually at the advance interchange to allow drivers to adjust driving routes when necessary.