The BRIDGE

The US 20 bridge is a 460-meter (1510-foot) steel I-girder structure, consisting of five 92-meter (302-foot) spans supported on four concrete piers and two end abutments. Two 20-meter (66-foot) precast concrete spans connect the bridge to the roadway approaches.

• The two central piers are supported on 8-foot diameter concrete drill shafts extending some 25 to 28 meters (82 to 91 feet) to bedrock.
  
  
• The two outer piers and two end abutments are supported by 100-ton piles driven to bedrock.

The deck of the bridge carries traffic approximately 42 meters (137 feet) above the Iowa River.

To minimize the impact of construction on the floor of the Greenbelt valley, engineers used a construction technique called Launching. This technique has been used widely in Europe with concrete box structures. It is not commonly used for the erection of steel I-girder bridges, and it has never been used on a project of this magnitude in the United States.

View the Construction Sequence__________________________________
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• Construction of bridge piers began at the bottom of the valley and progressed upwards. This allowed construction to be accomplished largely within the designed footprint of the bridge superstructure.
  
  
• None of the bridge piers were located in the waterway.
  
  
• Sections of the superstructure were partially constructed in a pit located above the valley at the east end of the site, behind the east abutment. This "launching pit" was dug in-line with what eventually became the approaching roadway.
  
  
• As bridge sections were built in the launching pit, they were attached to other sections, forming a line of sections. The lines were pushed out (launched) from the edge of the valley and onto the bridge piers. A launching skid at the front of each line reduced tilt and helped keep the lines on target. Temporary bearings placed on the bridge piers assisted with the process of rolling the lines all the way across the valley.
  
  
• Bridge sections were temporarily attached to the bridge piers after each launching. When the launching process was completed, the launch skids were removed and the full-length superstructure was permanently attached to the piers.