Building the U.S. 20 bridge

Since the mid-1960s, planners sought to find a way to expand U.S. 20 into a four-lane highway across Iowa in order to drive economic development in northern Iowa and provide more convenient transportation options. Efforts to plan this expansion ran into several hurdles. Most notably, the efforts ran into significant environmental hurdles at the Iowa River greenbelt. This area is a small remaining example of old-growth woodland that has continued to flourish.

A major feature of the greenbelt is a site where bald eagles roost for the winter. Planners and residents united in their desire to avoid disturbing that site. Also, three separate species of extremely rare freshwater mussels reside within the Iowa River. Any effort to construct a bridge over the river needed to happen without harming the freshwater mussels. The rare northern monkshood plant grows in the greenbelt. Because the river is a popular recreation spot, any bridge built would need to be inconspicuous enough to preserve the scenery while also managing to avoid spilling chemicals into the river or eroding the river’s banks. With the area’s popularity as a recreational location, local citizens wanted planners to leave the waterway open for that use as well.1

As a solution, planners, and engineers adopted a European construction technique known as launching. This technique involved constructing the bridge sections on the east side of the Iowa River, and gradually pushing it across to the west. This prevented the need for construction equipment to operate over the river. While some project teams in the United States had utilized the launching approach before, those project teams normally launched the bridge from both sides and linked them in the middle. In the case of the U.S. 20 bridge, that was not possible because the bald eagles’ roosting site was on the west side. In order to avoid disturbing the bald eagles, planners stretched beyond anything previously launched, creating the largest free cantilever yet attempted at the time and the longest single span of launched bridge yet at 1,510 feet.2

The bridge team put into place several important environmental measures. To protect the eagles, the bridge team planned no construction activity on the west side between Nov. 1 and April 15, the time of the eagles’ roosting. The builders prepared to shut down at any moment if the sounds of construction disturbed the eagles. The team also built an earthen berm to serve as a sound and visual barrier between traffic and the eagles’ roosting site. To preserve the river, engineers designed a containment system to prevent spills. The team designed and built the bridge with only two supports, one on each bank, as a further measure to protect the waterway from intrusion. For building material, the team used weathering steel, removing the need to paint and reducing future maintenance overall. At all times, a biologist remained onsite to ensure that the impact of the project did not present a threat to the eagle, mussels or environment.3

On Aug. 22, 2003, the bridge opened to traffic, providing improved transportation to residents and businesses in northern Iowa.


Notes

1. Iowa Department of Transportation, “The Highway: U.S. 20,” Iowa Department of Transportation, http://www.iowadot.gov/iowariverbridge/1d_highway.htm.

2. David M. Rogowski, Norm McDonald, and Ron Meyer, “The Iowa River Bridge on Relocated U.S. 20: A Launched Steel I-Girder Structure,” http://www.iowadot.gov/iowariverbridge/Downloads/article.pdf.

3. Dena Gray-Fisher, “One-of-a-Kind Bridge Project Protects National Bird,” Public Roads (September/October 2001): 40-41.

4. Gray-Fisher, 41-42.


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