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
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
the Construction Sequence__________________________________
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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.
on the photo above to view the