More than a half-million overweight trucks are allowed onto the nation’s roads and bridges – an increasingly routine practice that some officials say is putting dangerous wear and tear on an already groaning infrastructure.

In interviews with The Associated Press, some experts warned that the practice of issuing state permits that allow trucks to exceed the usual weight limits can weaken steel and concrete, something that investigators say may have contributed to the Minneapolis bridge collapse Aug. 1 that killed 13 people.

“We talk about this all the time and the fear that we have is that we’re going to have the same sort of disaster here that happened in Minnesota,” said Don Lee, executive director of the Texas Conference of Urban Counties.

In 2000, Milwaukee’s Hoan Bridge collapsed when steel girders cracked. Several factors were blamed for the collapse, including a significant number of heavy trucks, some over the normal weight limit, that routinely traveled over the bridge.

The weight limit for nearly all interstate highways is 40 tons. According to a government study, one 40-ton truck does as much damage to the road as 9,600 cars.

But permits frequently allow vehicles to exceed that amount by two tons in Texas and sometimes as much as 85 tons in Nevada. Some states grant one-time permits that allow trucks to be considerably heavier.

Around the country, many transportation officials dismiss such fears as overblown and say roads and bridges are safe, though some express concern that not enough is being spent to repair the damage done by extra-heavy trucks.

As for why they issue overweight-load permits, many state officials said they have no choice – they are simply carrying out the laws as passed by the legislature. Critics of those laws say they are often written to benefit powerful local industries, such as logging in the West, or oil and gas in Texas.

In the vast majority of cases, a single truck can safely pass over a sound bridge, even if the rig is way over the posted weight limit. But the cumulative effect of stress on the steel and concrete can eventually prove deadly.

Engineers liken the effect of heavy trucks on a bridge to bending a paper clip: It can bend again and again without breaking, but eventually it will snap.

Darrin Roth, director of highway operations at the American Trucking Association, said it is not fair to put all the blame on trucks because permit loads are a tiny proportion of total traffic.

States allowed more than 500,000 overweight trucks to traverse the nation’s bridges and highways at will in the past year, according to an AP review of figures in all 50 states. Those permits were good for an entire year. While 10 states do not issue yearlong permits, all states hand out shorter-term permits good for a few days, weeks or months. Those add up to more than 1.8 million permits not included in the AP’s count.

Eric Lockwood, who routinely carries 42 tons of hot oil all over Texas and has a state-issued overweight-load permit, said he doesn’t worry much about bridges and weight limits.

“From what I understand, the way those bridges are engineered and built – even the ones that do have a weight limit on them – you can grossly exceed that weight limit without having a problem,” Lockwood said, stopping to rest in New Braunfels after an early morning run from Houston. “That’s what I’ve heard. I don’t know what the truth is.”