FHWA Resource Center
Plastic bridge in Huron County may be path to the future
NORTH FAIRFIELD, Ohio - Bridges don't get much smaller than one carrying Ridge Road over a Huron River tributary in Huron County's Fairfield Township.
By DAVID PATCH
BLADE STAFF WRITER
"It's only 17 feet long. Even at 20 miles an hour, it's like half a second to cross it," said Douglas Nims, an associate professor of civil engineering at the University of Toledo.
But Mr. Nims and several of his students were there to observe when the Huron County Engineer's Office installed a replacement last month, and they will be back in the future to take readings from an array of sensors built into the structure.
That's because the new Ridge Road bridge is not made of steel or concrete or even wood.
It's plastic, reinforced with fiber glass. The only metal on it is the guardrails. Inside are 16 gauges to measure the strain placed on the bridge by the loads that cross it, and eight that measure deflection - the degree to which it bounces or is pushed around by traffic.
A Huron County truck tests the new bridge.
"Per square foot, it's probably the most instrumented bridge in the world," said Mr. Nims, whose ongoing bridge research includes studying the massive Veterans' Glass City Skyway on I-280 in Toledo, a structure more than 4,500 times as long as the Huron County structure - not to mention hundreds of feet higher and four lanes wider.
The professor's interest in the Ridge bridge shows that small bridges matter too. Historically, small spans on rural roads tend to crumble, not under heavy traffic pounding, but rather from the effects of heat, cold, rain, and winter weather, plus the corrosive effect of salt used to combat the latter.
"Salt eats the deck. What could [it] eat on a plastic bridge? What could rust?" said Joseph Kovach, the Huron County engineer, who secured a $155,000 Federal Highway Administration grant to install and study the plastic structure, hand-built at a Kansas factory and trucked to Ridge Road.
Nonetheless, the effects of cyclical heating and cooling, and even sunlight exposure, on plastic need to be studied before the material can be used on a broader basis for structures like bridges, Mr. Kovach said. By testing it on a lightly used rural road, he said, any problems that arise won't disrupt much traffic.
Crews fit a section of the structure into place.
With a materials price of $73,000, the bridge cost somewhat more than a traditional concrete or steel-beam structure of the same size, Mr. Kovach said. But it is expected to last considerably longer than traditional bridges, and the modular design and light weight allowed it to be installed in just a few days, including time needed to ramp the roadway to it, he said.
Speed of installation and portability, Mr. Kovach and Mr. Nims said, could make modular plastic bridges ideal for military engineers building bridges in combat zones. The panels also have tested successfully as barrier material to protect soldiers' foxholes from explosive ordnance, the professor said.
And once such bridges' components are made in regular production, rather than custom-built by hand, their price should decrease, both men said.
Before building the bridge panels, Kansas Structural Composites of Russell, Kan., made a small sample panel that was stressed until destruction at a University of Cincinnati laboratory, Mr. Nims said. The maximum load it could withstand proved to be 10 times stronger than expected, he said.
Once the bridge was built, county officials maneuvered two dump trucks weighing a combined 240,000 pounds onto it, and the sensors inside the panels measured "very small" shifting from the load - much smaller than normal bridge-design maximums. Under normal traffic, vehicle weights won't come anywhere close to that test, Mr. Nims said.
Yet for all its strength, the plastic used to build the bridge panels isn't solid. Instead, it's two slabs with a honeycomblike structure in between.
The inside is only about 15 percent solid, Mr. Nims said, but that's all that is needed to give the bridge its strength. Anything more would just be additional weight that the structure would have to support, he said.
Mr. Kovach said the bridge doesn't even sound like a conventional structure.
"When you step on it, it sounds like hollow Styrofoam," the county engineer said.
The bridge design is similar to one Kansas Structural Composites built in the 1990s near its plant and studies. Mr. Nims said he knows of one other plastic bridge in Ohio, in Hamilton County, but isn't familiar with its construction or research.
Considering the Ridge bridge's small size, the professor doubts it will attract too many visitors, though he was surprised when a man showed up during the pre-opening testing Sept. 26 hoping to be the first person to drive across it.
"He was very disappointed to find out we'd been driving trucks back and forth across it all day," Mr. Nims said.
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