With part of the northbound structure of the Missouri 291 bridge over the Missouri River closed indefinitely due to “significant deterioration,” officials are weighing long-term options on the 70-year-old structure, including outright replacement.

Inspectors first noticed signs of rust damage during a biennial inspection in May, said Stephen Collyott, district bridge engineer for the Missouri Department of Transportation. Then during an in-depth followup in late August they noted the damage had gotten much worse. On Thursday, MoDOT closed the right northbound lane.

MoDOT made significant repairs on the bridge next to LaBenite Park in Sugar Creek in 2015, closing the northbound structure for several weeks and diverting traffic to the southbound structure, with one lane of traffic in each direction.

That project cost about $1 million, Collyott said, and the damage this time wasn't present in 2015 and would take about that much in repairs.

MoDOT estimates the bridge carries about 11,000 vehicles per day across the Missouri River.

“We haven't really come up with our plan with how we want to approach the structure,” Collyott said. “We were looking at possibly replacing it in 2027, but now do we move that up? Do we fix it again? Our engineers in Jefferson City are really looking at what to do.”

Specifically, Collyott said, the damage is pack rust around the gusset plates, which connects and beams and girders in truss structures. Sometimes water gets trapped in such structures, and rusted steel expands, which can cause rivets to expand.

“It's just going to happen more and more with the age of the bridge,” Collyott said.

Collyott said bridges receive biennial inspections unless they are considered in poor condition, at which point inspections are annual.

The damage is worse on the east side of the structure, underneath the right lane, due in part to the winter, he said.

“When we snow plow most of the time we push to that side,” Collyott said.

MoDOT considered a weight limit for the northbound lanes at perhaps 30 tons, Collyott said, but with a quarry and business parks nearby on each side, that would be far from ideal, and engineers figured the highway could handle northbound traffic to one lane over a short span.

The northbound structure of the bridge was built in 1949 and was the only bridge until a separate structure for southbound lanes went up in 1996. Collyott said the southbound structure showed no problems during inspection.