The Iowa Department of Transportation is taking accelerated bridge construction (ABC) to the next level with the Little Silver Creek Bridge on Iowa 92, west of Treynor in Pottawattamie County. The bridge replacement work currently underway and a webcam is set up so you can follow the entire construction process, from fabrication of the bridge components all the way through demolition and replacement of the existing structure.
This will be Iowa DOT’s second ABC bridge project built using the modular construction method. Our first project, a 2010 bridge project on U.S. 6 over Keg Creek, also in Pottawattamie County, was a nationally recognized success. “The Keg Creek project demonstrated the potential of accelerated modular construction in Iowa, but it also identified several opportunities for improvement,” said Curtis Carter, lead design engineer for the Little Silver Creek project. “We wanted to simplify the process even further, and adapt the concept to a more challenging bridge site.”
The hallmark of an accelerated bridge construction project is the shortened road closure time, reducing the disruption to travelers. For the traditional bridge construction process, most of the road closure time is due to on-site construction. With the modular construction technique, the bridge is divided into several large pieces, or modules, that are prebuilt before the road is closed. Since much of the bridge can be prefabricated off site, the amount of time the road needs to be closed is dramatically reduced.
When you accelerate the schedule of a project while maintaining the same high standards of quality, it is expected that project costs might increase. This is an unfortunate reality that often holds true for accelerated bridge construction projects. However indirect costs such as improved safety and less road closure time for travelers can outweigh the increased direct costs. The Federal Highway Administration has set up grant programs to promote the use of innovative and accelerated construction nationwide, and this project has been awarded $1 million in incentive funding as a part of FHWA’s Accelerated Innovation Deployment Program.
While the existing bridge, built in 1953, remains safe to cross, it is nearing the end of its service life and has been classified as structurally deficient. While it may sound daunting structurally deficient simply means one or more elements of the bridge have defects that need to be addressed. The replacement structure will be safer, stronger and wider, to better serve the highway’s current and future traffic needs.
Bridges generally consist of two structural systems, referred to as the superstructure, which is the part that you drive on, and the substructure, which is the foundation system. The superstructure configuration allows for nearly 90 percent of the bridge superstructure to be constructed off site and in advance. The bridge substructure was also designed to save on-site construction time, including an assortment of prefabricated and simplified components that can be easily connected into place when it comes time for actual bridge assembly. The superstructure modules and other prefabricated components will be stored at the fabrication yard until the contractor is ready to transport them to the bridge site.
The contractor will be watching for a window of nice weather in October to schedule the most exciting part of construction. After all preparations are made, the contractor will close the road, demolish the existing bridge, and assemble the components of the new structure into final position. When the pieces are set, the modules will be connected together using a special blend of concrete referred to as “ultra-high performance concrete.” Think of this material as superglue for bridges. It is incredibly strong and only takes a small amount to get everything permanently locked into place.
If this had been a conventional bridge replacement project, this site would require about six months of full road closure for construction. Using accelerated construction methods, full road closure time will be cut to only 21 days. That’s an almost 90 percent reduction.
Why this bridge?
This bridge supports a considerable amount of traffic to and from the Omaha/Council Bluffs metro, and a lengthy detour will be required when the bridge is closed for replacement. Because of the high inconvenience and increased user cost in gas, time, and vehicle wear and tear resulting from bridge closure, this project was deemed to be a good candidate for accelerated construction. This project was also identified as an ideal opportunity to apply the lessons learned from the Keg Creek project on a similar, but more challenging project. Compared with Keg Creek, the Little Silver Creek project includes longer bridge spans, steeper and more variable road slopes, and a more complicated channel crossing. Carter says, “The Little Silver Creek site is representative of a wider range of bridge sites statewide. When we combine our previous experience with the new learning outcomes from this project, we will have a better understanding of how to best apply these construction techniques in the future.” Increasing the Iowa DOT’s ability to utilize ABC construction helps fulfill our mission of getting our customers there safely, efficiently, and conveniently.