Curving, sloping span eyed for Highway 100 Extension

There's a reason the Iowa Department of Transportation is outsourcing the design of the planned Highway 100 Extension bridge to a Chicago engineering firm.

The 1,700-foot-long bridge will cross the river in a curving arc, so sharp that motorists will be able to see about six of the piers the bridge rests on by looking off to the side.

Its roadway surface will be banked from 2 percent to 5.3 percent to compensate for the tendency of the arc to pull vehicles to the outside.

“It takes a specialist,” said Ron Meyer, a DOT transportation engineer specializing in bridges and structures. “It would have taken us a lot of extra time - maybe not twice as much, but it's much more complicated.”

The bridge design contract is expected to be final within two months, Meyer said. After it's awarded, Meyer expects the design work to take about a year. Funding for construction has not been placed in the state's five-year highway plan.

“We're probably going to be out well ahead of the construction,” Meyer said. “We will be in a position where if other additional funds show up, it will be ready to go.”

The last new bridge built over the Cedar River in Linn County was the Edgewood Road bridge, which opened 40 years ago, in 1970. It was temporarily closed during the massive June 2008 flood, forcing traffic to reroute over the I-380 bridge.

“Where we might not have thought it (the

Highway 100 extension bridge) was that important before June 11, 2008, it obviously is,” said Adam Lindenlaub, administrator of the Corridor Metropolitan Planning Organization.

The Edgewood Road bridge also has shown signs of age. It was closed from Dec. 11 to Dec. 30, 2002, after fatigue cracks showed up in two long main steel beams. Some lanes remained closed until Jan. 13.

The new bridge will be designed to handle higher flood levels that are now considered possible after the June 2008 flood.

Meyer said the bridge will have a clearance of 23 feet, 6 inches, mainly to accommodate the Iowa Northern Railway.

Final cost estimates on the Highway 100 bridge have not been released, but they will likely dwarf the $1.29 million cost of the Edgewood Road Bridge.

The DOT also is seeking proposals this summer for the design of the initial 7-mile stretch of Highway 100. Like the bridge, it is not currently funded in the five-year highway plan.

The first phase of the Highway 100 extension will go west from the current end of Highway 100 at Edgewood Road to Covington Road. In the second and final phase, it will go south to link up to Highway 30.