Six county bridges in North Dakota have been closed after a state inspection that started after last month's bridge collapse in Minneapolis.

The total could rise as hundreds more bridges are checked, but state transportation officials say the closures are not out of the ordinary. Fifteen other bridges have been shut down since last October as the result of regular inspections, said Grant Levi, deputy director for engineering with the state Department of Transportation.

North Dakota has slightly more than 5,000 bridges. Transportation officials in late August began inspecting the 751 bridges deemed structurally deficient, after the Interstate 35W bridge collapse in Minneapolis that killed 13 people and injured dozens.

About 40 percent of North Dakota's structurally deficient bridges have now been inspected. Two bridges in Cass County have been closed, along with one bridge apiece in Richland, Emmons, Oliver and Traill counties. Traill County has closed 10 bridges in all over the past year.

John Wright, office manager for the county's highway department, said that is only a small percentage of the county's hundreds of bridges and all but one of the structures are in rural areas where the closure affects only a few people.

"A lot of (the closed bridges) are small wooden structures built in the 1940s and '50s that are just approaching the end of their life span," Wright said.

One bridge closure - over the Red River - "will be some inconvenience" to Traill County residents, Wright said.

North Dakota and Minnesota officials are inspecting 36 Red River bridges in all. Late last month, a bridge over the river near Drayton was closed for about four days so officials could repair a crack under its deck.

Ten two-person inspection teams are in the field as part of the interim inspection of structurally deficient bridges directed by Gov. John Hoeven. Levi said the department expects to finish the inspections by the end of October.

The department has set aside $200,000 in its budget for the project.

Counties normally are charged $25 per bridge for inspections, but the fee is not being assessed for the interim inspections.