Sep 6 2005
Some very smart bridges are telling engineers just exactly how they're holding up.
They've reported on the stresses and strains of construction. They've quantified the ability of high performance steel to move metro traffic. And they've advanced the state of the art in bridge engineering.
Engineers from Iowa State University's Bridge Engineering Center have helped the Iowa Department of Transportation monitor and evaluate three bridges as part of the Interstate 235 reconstruction project in Des Moines.
"This work that we're doing is going to help build more durable bridges," said Terry Wipf, the director of the Bridge Engineering Center and the Pitt-Des Moines Professor in Civil Engineering at Iowa State. "They'll last longer and they'll save the state money."
Here's how Iowa State engineers are helping the state build better bridges over I-235:
Fiber optic sensors
Engineers have installed fiber optic strain gauges in the East 12th Street bridge and the pedestrian bridges at 40th and 44th streets. The gauges work by transmitting light through glass fibers. Every strain on the fibers changes the light that's reflected back. A black box records the changes and therefore the strains. The system can take up to 250 measurements every second. That strain history is recorded by computers and can tell engineers how a bridge is doing.
It's new technology, said Justin Doornink, a doctoral student from Sioux Center, Iowa, who's specializing in the gauges. Iowa State engineers are researching its uses and applications. And they're helping the transportation department put the technology to work.
High performance steel
The East 12th Street bridge is the first -- and still the only -- Iowa bridge made with high performance steel. The steel is designed to be stronger and tougher. That means it takes longer for cracks to develop. And once cracks develop, they spread much slower.
Brent Phares, the associate director of the Bridge Engineering Center and an adjunct assistant professor of civil, construction and environmental engineering at Iowa State, said researchers have built fiber optic strain gauges into the bridge to continuously measure how the bridge and its new steel perform over time.
The Federal Highway Administration supported the monitoring research with a $155,000 grant from the Innovative Bridge Research and Construction program.
Pedestrian bridges
To overcome difficulties constructing the first pedestrian bridge over I-235 at East Sixth Street, the Iowa Department of Transportation asked Iowa State engineers to install strain gauges on the project's other two pedestrian bridges. Those bridges over 40th and 44th streets are scheduled to be completed this fall.
Mike LaViolette, a bridge research specialist with the Bridge Engineering Center, said the gauges were installed on the hangers supporting the bridges' concrete walking surfaces. The gauges measured the forces on the bridges when crews installed the walking surfaces. Iowa State engineers also installed accelerometers on the bridges as another way to measure the forces during construction. The instruments helped crews prevent cracking by keeping the forces within the bridge's design capabilities.
The Iowa Department of Transportation supported the monitoring project with about $50,000.
Ahmad Abu-Hawash, the chief structural engineer in the transportation department's Office of Bridges and Structures, said the department called on Iowa State engineers for help with the I-235 bridges because they have specialized expertise.
"Any time you use new design methods or innovative materials, you want to do some research to evaluate performance," Abu-Hawash said. "That's why they're helpful."
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