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|Federal Highway Administration > Publications > Public Roads > Vol. 65 · No. 2 > One-of-a-Kind Bridge Project Protects National Bird|
One-of-a-Kind Bridge Project Protects National Bird
by Dena M. Gray-Fisher
The Iowa Department of Transportation (DOT) and its construction and design partners are taking extreme measures to protect the environment and our national symbol, the bald eagle, during the relocation of a 40-mile (64-kilometer) segment of U.S. Route 20.
Some may say the U.S. 20 project has had more than its share of environmental issues to address. Located in Hardin and Grundy counties in north central Iowa, the corridor is characterized as a rural agricultural area punctuated by several small communities and containing an environmentally sensitive greenbelt area in the Iowa River valley.
The Iowa River Greenbelt is a rare, remaining fragment of old-growth woodland that has survived the age of agriculture. The region features a steep river valley rich with traces of prehistoric cultures and remnants of 19th century farmsteads. It is also a roosting and wintering area for bald eagles, and it is a home for the rare northern monkshood plant and three endangered or threatened species of freshwater mussels.
Residents of the communities along the greenbelt value this precious natural resource and strongly support its preservation. Finding a way to build a modern, safe, and efficient highway facility through such an area, while minimizing construction impacts and highway intrusion at the Iowa River crossing in Hardin County, proved to be very challenging.
This project addressed some major issues of environmental impact, including:
The Iowa River bridge segment of the 40-mile highway relocation project was let in 2000, and site preparation work began in late July 2000. The total construction process will take less than three years; the bridge is scheduled to open to traffic in November 2002. Site cleanup and replanting will continue until July 2003.
The cost for design and construction of the bridge is approximately $20 million. Of this amount, 15 to 20 percent of the costs can be directly attributed to design and detailing considerations added to accommodate the environmental concerns and site accessibility challenges.
The team making this project possible includes the Iowa DOT, which is responsible for overall project management; the Federal Highway Administration (FHWA), serving as technical consultant and providing financial support; HNTB Corp. of Kansas City, Mo., serving as the design and construction consultant; Jensen Construction of Des Moines, Iowa, the general contractor; A-Seeding of Colfax, Iowa, a subcontractor providing the specialized erosion-control services; and several other subcontractors affiliated with Jensen's parent company, The Rassmussen Group.
Norm McDonald, director of Iowa DOT's Office of Bridges and Structures, has been working on the project, in one capacity or another, for years. He said, "I started on the project while I was the department's chief structural engineer. The department had been looking for volunteers to work on the project, and I spoke right up thinking this would be a very interesting project. It has been that and more. The best part may be the fact that my office is only an hour away from the project site so I've been able to visit the site frequently to monitor its progress."
In all, nearly 40 Iowa DOT staff members have been involved in some aspect of the U.S. 20 bridge project. Planners, designers, environmental specialists, contracts and specifications staff, materials technicians, and construction personnel all have played an important role in the project.
Since taking office a few years ago, FHWA Iowa Division Administrator Bobby Blackmon has been involved in the project's design and permitting processes. FHWA Division Bridge Engineer Curtis Monk has also been a valuable contributor to the project. Monk's geotechnical expertise helped resolve several complicated engineering issues associated with the bridge pilings and drilled shafts.
The U.S. 20 bridge over the Iowa River is a 1,510-foot (460-meter) weathering steel I-girder structure, consisting of five 302-foot (92-meter) spans supported on four concrete piers and two end abutments. Two 66-foot (20-meter) precast concrete spans connect the bridge to the roadway approaches.
The two central piers are supported on 8-foot- (2.4-meter-) diameter concrete drilled shafts extending 82 to 91 feet (25 to 28 meters) to bedrock. The two outer piers and two end abutments are supported by 100-ton (890-kilonewton) piles driven to bedrock. To protect the Iowa River, none of the bridge piers is located in the waterway.
The bridge deck will eventually carry traffic approximately 137 feet (42 meters) above the Iowa River. Each deck structure consists of a nine-inch (230-millimeter) concrete slab with a 1.5-inch (38-millimeter) low-slump concrete wearing course, supported by a system of four 136-inch- (3,450-millimeter-) deep I-girders spaced at 142-inch (3,600-millimeter) centers. The deck structures are supported on cast-in-place reinforced-concrete substructure units consisting of two column bents ranging in height from 66 to 125 feet (20 to 38 meters) and founded on the drilled shafts or driven H-piles.
David Rogowski, project engineer and construction consultant with HNTB Corp. said, "I have been involved with this project for the last five years, and as you can imagine, I am very excited about seeing this one finally built. It is amazing to see numbers generated with computer models come to life during the actual construction of the bridge."
Atypical Bridge-Building Technique
To minimize the impact of construction on the floor of the greenbelt valley, an atypical bridge-construction technique called "launching" is being used on this project. Although this technique has been widely used in Europe with concrete box structures, it is not commonly used for the erection of steel I-girder bridges. It has never been used on a project of this magnitude in the United States.
"This bridge will be a record span for a launched set of highway girders in both the category of longest total launched steel bridge and longest free cantilever," said Rogowski.
The only other project in the United States known to have used a similar construction technique involved a railroad bridge in Redland, Okla. The Redland bridge has a longer main span (350 feet or 107 meters); however, it was launched from both ends of the bridge. (The U.S. 20 bridge is being launched from one end of the bridge.) The maximum free cantilever was 250 feet (76 meters), which was the span length of the approach spans. The total bridge length was 2,100 feet (640 meters); therefore, each launched half was 1,055 feet (322 meters).
The following principles and methods of the launching process were applied to the U.S. 20 project.
The contractor began by assembling the first two steel spans of the eastbound bridge superstructure in a "launching pit" located above the river valley and behind the east abutment. The pit was dug in-line with what will eventually be the roadway approach.
A temporary "launching skid" or steel nose was then attached to the front of the first span. During launching, the girders deflect downward to such a degree that they are actually at a level below the top of the pier and would hit the side of the pier without the skid. The launching skid is tapered, allowing the skid to land on top of the pier and guide the girders upward into place.
The connected spans were then launched from the edge of the valley onto the first pier and out toward the second by means of hydraulic rams and Dwidag bars and temporary 275-ton (2,446 kilonewton) roller bearings. The bearings are placed at each girder centerline under the steel bridge spans as they are constructed in the pit and on each of the bridge piers. They serve an important function in the process of rolling the lines the entire distance across the valley.
After reaching the second pier and achieving a major milestone in the project, the launching procedure was temporarily halted as the next spans were constructed in the pit and attached to the first two. Each new section that is added to the line serves as a counterweight for the cantilevered portion.
This process of constructing a span, launching it, adding a new section, and launching will continue until all five spans of the bridge are in position.
Then, the contractor will remove the launching skid from the eastbound lane and attach it to the first span of the westbound lane. The entire process will be repeated until all spans of the westbound lane are in position. Then the full-length, steel superstructure will be permanently attached to the piers.
The environmental concerns associated with the bridge site posed many construction challenges for both the designers and the contractor.
Construction Project Manager Dan Timmons of Jensen Construction said this project has generated a range of emotions. "First, there was anxiety over all of the unknowns associated with use of the new launching method. Then, there was a sense of relief as major milestones were achieved. Finally, great pride in all of our accomplishments."
"Jim Rasmussen, the president of The Rasmussen Group and owner of Jensen Construction, has spent 40 years building this company so that it could become a self-sufficient organization. This project has allowed us to exploit that expertise," said Timmons. Besides Jensen's bridge construction crew, various subsidiary companies of The Rasmussen Group have been involved in the project, including companies who made the steel, hauled materials and equipment, fabricated the roller bearings, and provided the ready-mix cement.
Building Iowa's Future
Iowans have had a long history of meeting extraordinary challenges through the development and use of innovative approaches in the field of highway transportation. The U.S. 20 project is just another example of the ingenuity and tenacity that it takes to provide highway users with the benefits of an excellent transportation system, while respecting the ecological, historical, and archaeological treasures of the state.
Dena M. Gray-Fisher is the director of Iowa DOT's Office of Media and Marketing Services. She has held the position since 1996. She is also the secretary-treasurer of the American Association of State Highway and Transportation Officials (AASHTO) Administrative Subcommittee on Public Affairs.
For more information on the technical aspects of this project, contact Norm McDonald, director of Iowa DOT's Office of Bridges and Structures, at (515) 239-1206.
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