More, heavier vehicles take toll on U.S. roads

Bigger, heavier loads
Berndt, who recently studied rural Minnesota truck traffic, said that mostly farmers who used to drive small trucks are now using bigger and heavier tractor trailers.

America is gaining a great economic advantage by using more and heavier trucks, said Walton. But the next step is to take just part of the cost savings and translate that into fixing roads and bridges, he said.

Boyce said his trucking group supports an increase in gasoline tax as long as it is targeted to fixing roads.

“We’ve gone too long really making just incremental improvements to the interstate program. We haven’t kept up with maintenance,” he said.

The problem boils down to basic engineering. When engineers design bridges and roads there are two factors to balance: load, the force weighing on the structure, and resistance, the ability to withstand that force.

What’s happening is that loads are increasing while time, weather and fatigue weakens resistance.

Bridge problem is pure weight
Heavier loads cause different problems for bridges and roadways. For bridges the problem is pure weight.

Bridges are designed to withstand up to twice as much as the anticipated weight loads, said W. Gene Corley, a forensic engineer with the Skokie, Ill.-based engineering firm CTL Group.

But when a bridges can’t handle the weight, states put up load limits, restrict the size of trucks using them.

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A 2006 Department of Transportation Inspector General study checked out 43 bridges in Massachusetts, New York and Texas and found that at least 12 of them allowed vehicles to cross that were heavier than the bridge’s maximum weight limit. And 11 of those did not have the required posting signs.

The collapsed Minnesota bridge had no weight limits but was categorized as structurally deficient, one of over 73,000 U.S. bridges with that designation last year. The federal government usually places bridges that have weight limits on the functionally obsolete list, which includes another more than 80,000 bridges.

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