The new High Road Bridge over Long Run Creek in Lockport Township, Ill., is a unique structure disguised as an ordinary bridge tucked away in the scenic Des Plaines River Valley.
The old bridge was built in 1935 and had outlived its useful service life, having been categorized as both structurally deficient and functionally obsolete. To reduce future maintenance costs, its replacement contains the first permanent highway installation of the hybrid-composite beam (HCB), a technology developed by HC Bridge Co. that provides at least 100 years of service life with minimal or no maintenance.
The beams are composed of three main subcomponents: a fiber-reinforced plastic (FRP) shell, compression reinforcement, and tension reinforcement. The compression reinforcement consists of self-consolidating concrete that is pumped into an arch conduit within the beam's shell; the tension reinforcement consists of high-strength steel prestressing strands that run along the bottom flange of the shell.
The shell protects the beam from salt corrosion and provides added structural capacity. The result is a bridge-framing member that's simple to design, manufacture, and construct — as well as one that's more lightweight and cost-competitive with conventional bridge-building materials.LIGHTER AND COST-COMPETITIVE
In its pure form, fiber-reinforced plastic is as strong as steel and 10 times more flexible. The benefits of its light weight and corrosion-resistance are well-known, but until now bridges incorporating the material were generally four to 10 times more expensive than bridges built of conventional concrete and steel. Like other building materials, the material needed to find an efficient form.
It wasn't until 1879 that Francois Hennebique developed concrete structures with steel tension reinforcement; until then, concrete was only effective as a building material in compression. In 1907, Eugene Freyssinet designed the first concrete bridges to employ pre-stressed steel reinforcement. Each advance in structural engineering sought to employ a more efficient use of building materials, generally by combining them. Innovations have always been predicated on simple fundamental principals of structural behavior and willingness by the designer to deviate from traditional form.
Ninety percent lighter than pre-stressed concrete beams, the hybrid-composite beams offer considerable savings over traditional materials with respect to shipping and erection costs. These advantages make the HCB cost-competitive with conventional materials. All six girders for the two-lane, 58-foot-long bridge were shipped on one truck. Because of the combination of plastic, concrete, and steel, fewer quantities of each material are required, resulting in a smaller carbon footprint during manufacturing.
The beams can also be made continuous over several supports with simplistic splicing technology using the same materials and manufacturing processes.
Laboratory testing of a prototype beam at the Advanced Engineered Wood Composites Center at the University of Maine showed that the load-carrying capacity of the beam is almost twice the code-specified limit.
To satisfy American Association of State Highway and Transportation Officials deflection requirements, the concrete deck for the bridge is made to act compositely with the HCBs, not only reducing live load deflections but also providing significantly increased strength: There's enough redundancy in the bridge that if its deck were to completely deteriorate, the bridge itself would still have sufficient capacity to carry the factored design load of 176,000 pounds.
The project was designed by the Chicago office of Teng & Associates Inc. in conjunction with the Lockport Township Highway Department and the Illinois DOT. All three parties worked closely to secure a $250,000 grant through the Innovative Bridge Research and Deployment program from the Federal Highway Administration.
The final cost of the project was $2.1 million, approximately 5% below the engineer's cost estimate.
— Hillman is a senior associate for Teng & Associates Inc. and president of HC Bridge Co. LLC.