A crack in one of four steel girders supporting Interstate 190 over Silver Street in Rapid City forced state officials to temporarily close the southbound passing lane on the bridge Friday.

Kevin Goeden, state bridge design program manager, said the bridge is still safe to use and is not at risk of failing. Installation of steel plates, a splice that will span the crack and reinforce the girder, will not affect users.

"This is a very unusual event for us," Goeden said in a telephone interview from the state Department of Transportation in Pierre. "While rare here, they do occur once in a while around the country. … It's definitely a serious crack. That's why we moved traffic away from that girder and shifted it onto the girders that are sound."

The fracture was not related to an impact, like the tall truck strike that forced recent repairs on the Interstate 90 bridge at La Crosse Street. Goeden said the Silver Street bridge girder crack indicated weld flaws and failure of construction material in the area. The structure dates to 1958-59.

He said DOT workers found the fracture while looking over the structure for upcoming road work. He said the crack was not visible when the bridge passed a "detailed top to bottom" inspection in recent weeks as part of a regular national bridge standards review.

"Obviously, we want to repair it as quickly as we can and look at it in more detail to make sure there are not more issues with that same span," he said.

The driving lane will remain open during the repair.