Scheduled for construction in fall 2015, the project consists of replacement of the bridge on Iowa 92 over Little Silver Creek, in Pottawattamie County. The existing 150’ x 28’ continuous concrete beam bridge was constructed in 1953 and is currently classified as structurally deficient. The proposed bridge replacement is intended to increase the structural capacity of the bridge, improve roadway conditions, and enhance safety by providing a wider roadway.
The replacement structure will be a three-span, 234’ x 44’ modular rolled steel beam bridge. Concept selection for this structure was based on consideration of several factors, including: (1) minimized impact to highway users through implementation of an accelerated construction schedule; (2) demonstration of innovative design, construction, and material technologies; and (3) evolution and improvement of the modular decked beam method of accelerated bridge construction, as first deployed in Iowa during the U.S. 6 over Keg Creek Strategic Highway Resource Program, SHRP2-Project R04, demonstration project.
The superstructure of the Iowa 92 over Little Silver Creek replacement bridge will consist of 18 prefabricated, decked beam modules. Each module will consist of a nominal 7’ strip of concrete deck cast on a pair of rolled steel beams. The modules will be fabricated off-site and will be hauled to the project area for final assembly and connection under an accelerated construction schedule.
The bridge substructure will incorporate simplified details that allow for rapid construction. Steel bearing piling will be employed to directly support structural concrete caps at the piers and abutments. The structural concrete caps, which may be precast or cast-in-place at the contractor’s option, will serve as the seats for the prefabricated superstructure modules.
After construction of the substructure, the prefabricated superstructure modules will be lifted into place and adjusted into position. The modules will be connected to each other using narrow longitudinal joints filled with ultra-high performance concrete and the superstructure system will be connected to the substructure using high performance concrete transverse closure pours.
The proposed combination of accelerated construction techniques will considerably reduce the duration that highway users are impacted. Under normal, non accelerated conditions, a typical bridge replacement project at this site would likely require approximately six months of full road closure. However, the accelerated construction technologies incorporated into this project will allow the highway impact duration to be limited to a total of 61 days, of which only 21 days will require full road closure (two-way traffic will be maintained with construction zone traffic control for the remainder of the 61 day construction schedule). The total impact duration (61 days) and the road closure duration (21 days) represent anticipated schedule savings on the order of 66 percent and 88 percent, respectively, compared to conventional, non accelerated construction.
Whereas the U.S. 6 over Keg Creek project was intended to serve as proof-of-concept for this method of accelerated bridge construction, the Iowa 92 over Little Silver Creek project is intended to evaluate a broadened application of this technology. For this project, a deliberately more challenging site was selected. A primary objective of this project is to provide the department with valuable data on the suitability and cost-effectiveness of these construction techniques, as applicable to a range of geometric scenarios representative of many typical bridges across the state. Iowa’s proof-of-concept deployment was on a square (non skewed), relatively level road alignment, with generally agreeable site access. In contrast, the Little Silver Creek project includes 20° skew, considerable vertical curvature of the road alignment, longer bridge spans, and less agreeable site access. When combined with the learning outcomes from the U.S. 6 over Keg Creek project, successful completion of the Iowa 92 over Little Silver Creek project will provide Iowa DOT with reliable data on how to optimize use of these technologies to best serve the traveling public in our state.