MINNEAPOLIS – 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, according to the Associated Press. And some experts warn 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.

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, the AP report said.

Many state officials say the have no choice but to issue overweight-load permits because they are carrying out the laws passed by the legislature. In the vast majority of cases, a single truck can safely pass over a sound bridge, even if the rig is over the posted weight limit. But the cumulative effect of stress on the steel and concrete can eventually prove deadly.

Many states charge fees ranging from $12 to $1,000 for overweight-load permits, depending on the weight of the load. In theory, those fees are supposed to offset the damage done to the highways. Texas, for example, granted nearly 39,000 such permits in the past year, generating $7.5 million, most of which was divided among the state’s 254 counties for road maintenance.

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.

California is more cautious with its overweight permits. Truckers in California, where about 23,000 single-trip permits are issued annually, must request permission to travel on a specified route for each trip. California transportation officials said they perform an extensive review to ensure the load can safely travel on the requested highways without damaging pavement and bridges. Often, truckers are required to reduce their loads.

But in Colorado, where almost 21,000 permits are issued annually, truckers are given a map with their overweight permits showing how much weight bridges around the state can handle. Drivers there operate on the honor system, and officials say they have no way of knowing if drivers are taking bridges appropriate for their loads.

The AP report further stated that a recent federal finding that 18 percent of the nation’s bridges either do not have weight limits posted or incorrectly calculated the weight limits that are posted. Also, a recent federal study classified 26 percent of the nation’s bridges as either structurally deficient or functionally obsolete.

In the year before the Minneapolis disaster, the cause of which is still under investigation, the state Transportation Department granted permits for 48 overweight loads, including construction cranes and supplies weighing as much as 72 1/2 tons. The bridge had been categorized as structurally deficient, one of over 73,000 U.S. bridges with that designation last year.