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Highway Quality Compendium
by Bob Templeton
Joining environmental sensitivity with innovation, the Iowa Department of Transportation (IDOT) met the many challenges of constructing the new U.S. 20 Iowa River Bridge in Hardin County by using a unique construction method known as incremental launching. This innovation has been honored with the National Partnership for Highway Quality's (NPHQ) 2005 National Achievement Award. NPHQ brings together state, federal and highway industry leaders to encourage the use of quality practices that will improve safety and service for highway users.
The new bridge is located on U.S. 20 in the Iowa River Greenbelt, a rare remaining piece of old-growth woodland. The site is home to roosting bald eagles and endangered species of freshwater mussels, as well as historical Native American artifacts. While building the bridge was integral to the state's effort to expand U.S. 20 to four lanes, the challenges were considerable. Construction equipment was prohibited from crossing or entering the river, and all construction activity had to be completed within a small footprint surrounding the bridge. With these stringent restrictions, IDOT and its project partners, HNTB Corp. and Jensen Construction, quickly ruled out using conventional erection methods for the 1,630-ft bridge. Employing the incremental launched-girder technique instead eliminated the need for the temporary erection towers and piece-by-piece in-place erection of structural steel required by conventional methods.
Using the launched-girder method, the superstructure of the bridge was erected on one side of the waterway and then rolled across the river into its final position. The technique has been used for years in Europe to erect concrete-box structures and also had been used for a smaller steel-box girder railroad bridge in the U.S. However, it had never been used to launch a long-span I-girder bridge made up of nearly 10 million lb of structural steel.
Construction of the bridge began in August 2000. The bridge's 10 302-ft spans were launched after completion of steel erection in a specially excavated 15-ft-deep, 600-ft-long launching pit behind the bridge's east abutment. A temporary launching nose was attached to the front of the first span, and large hydraulic rams pushed the structural steel into place on a system of guided roller bearings. This sequence was repeated for the other spans. The entire steel bridge system, including all diaphragms, lateral bracing and drain pipes, was successfully launched at a pace of approximately 1 ft per minute.
The bridge opened to traffic in August 2003, shaving 15 miles and more than 30 minutes off the commute between I-35 and Waterloo, Iowa, while still preserving the natural resources of the area. Its slender, low-profile structure was designed to blend in with the area's natural surroundings. The use of weathering steel, meanwhile, will reduce the need for future painting and maintenance.
The innovative project's success has led to launched steel girder bridge projects in other states, such as a bridge constructed near Moorefield, W.Va. The Federal Highway Administration's Accelerated Construction Technology Transfer program also has identified incremental launching as an option for quick and safe bridge construction.
Bob Templeton is the executive director of the National Partnership for Highway Quality.
Reprinted from Road & Bridges, February 2006.
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