A creek-ing noise

Bridges Article February 18, 2002
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There’s an easy way to recycle plastic in Sugar Grove,
Va. Hop on your bike and cross over a bridge spanning Dicky Creek. When you get
to the other side, cycle back.

One of the longest fiber-reinforced polymer (FRP) composite
bridges was dedicated recently—fueling further development of these
“plastic” structures. And, yes, cars and trucks can run back and
forth, too.

The 38-ft, two-lane, clear-span bridge is supported by eight
36-in.-deep composite beams and meets U.S. standards (AASHTO HS-20) for full
tractor-trailer traffic.

The project was the culmination of an Innovative Bridge
Research and Construction program sponsored by the Federal Highway
Administration. The major participants were VDOT headquarters personnel and
VDOT District 9 engineers and construction crew, the Virginia Transportation
Research Board, two engineering departments at Virginia Tech University and
Strongwell Corp., Bristol, Va., which manufactured the beams.

Between 1995-1998, Strongwell researched a 36-in. double web
beam with carbon fiber and top and bottom flanges for extra stiffness, then
conducted testing with Virginia Tech.

“The beams are like rectangular tubes with wings on
top and bottom,” Glenn Barefoot, corporate marketing manager for
Strongwell, told ROADS & BRIDGES. “This is a double-web beam, and the
reason we went that way was for structural capacity and for stability. One of
the things you have to deal with steel plate girders is you have to put in
diaphragms or brackets every 8 ft to keep them from rolling or buckling during
loading. By having a double web you eliminate that buckling effect.”

During testing, the fiber-reinforced polymer composites were
over seven times the strength required in the design, and each beam was around
6.3 to 6.6 million on the modulus for elasticity.

“The Achilles heel for FRP is stiffness,” said
Barefoot. “They are a great product in terms of weight and corrosion
resistance. In cold weather they actually get stronger.”

Adding polymer carbon gives the beam more flexibility, but
it also can loosen one’s grip on the budget. The product costs about 13
times more than steel.

“There’s a cost value problem,” said
Barefoot. “Steel is a great, great product and it’s very, very
cheap. Same way with concrete.”

However, steel and concrete need constant attention.
Maintenance dollars are needed for both. On the other hand, FRPs jibe with the
environment and are more prone to corrosion.

“Federal Highway is looking at some ways of reducing
the high costs of maintaining the transportation system,” said Barefoot,
who sees FRPs used for spans 50 ft or less.

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