In July, the Michigan Department of Transportation (MDOT) began a $4.9 million project to construct an innovative new bridge over the Prairie River (east of Centreville in St. Joseph county). The project is expected to be completed in November 2016.
The new 104-ft (32-m) M-86 bridge will be one of only two bridges in the state to feature the innovative concrete bulb T-beam design. Traditional side-by-side box-beam bridges are economical and can be constructed rapidly and easy; but the steel reinforcement in these structures is susceptible to corrosion from moisture and chlorides in salt used for deicing roadways in freezing weather. The corrosion leads to concrete cracking and deterioration, and the freeze-thaw cycles experienced in northern climates accelerate this process. As a result, many of these types of bridges in Michigan are nearing the end of their service lives.
The new design utilizes prestressed decked bulb T-beams, which have a wider upper flange than traditional I-beams that give them a T-shaped cross section. These upper flanges form the deck of the bridge, so construction is expedited and traffic disruption is minimized, and the T-shaped cross section provides open space at the bottom of the bridge for periodic inspection and maintenance.
The new M-86 bridge also will be the first MDOT bridge to use corrosion-resistant carbon fiber composite cable (CFCC) reinforcement instead of steel cable in the bulb T-beams. Noncorrosive and lightweight carbon fiber strands make the CFCC comparable in strength to steel cable, and it provides many benefits, such as high tensile strength that will extend the service life of the bridge by 25 years or more.
By pairing the concrete bulb T-beam design with the CFCC, the beams in the new bridge are expected to help reduce the concrete cracking, deterioration, and corrosion typically experienced by traditional steel-reinforced bridges. By implementing these innovative designs, the new bridge beams could last more than 100 years and require less maintenance, leading to significant cost savings for Michigan taxpayers.
These concrete beams will be monitored to compare their longevity to beams with traditional reinforcement that comprise other new bridges elsewhere in the state.
Part of this project also includes repairing and relocating the existing M-86 camelback pony truss bridge over Prairie River to a new location over the Dowagiac River. The aging structure was originally built in 1923 on Telegraph Road over the Rouge River in Detroit and was moved to its Prairie River location in 1938 under the Public Works Administration program.
This historic truss bridge is the last remaining bridge of its type on the state trunkline system in Michigan, and demonstrates the portability of a truss bridge. The restoration includes replacing portions of the rusted steel and then painting the entire truss. To preserve the historic elements of the bridge, more than 1,800 rivets are required to reassemble the truss elements. For more information, visit MDOT’s Web site.