In Depth: How to Make Bridges Safer

Aug. 3, 2007 — Just 13 weeks before the tragic collapse of the Interstate 35 Bridge in Minneapolis, inspectors from the Minnesota Department of Transportation examined the structure. According to the U.S. Department of Transportation, "No imminent dangers were observed."

But the 1967 steel arch deck truss bridge, which rated 4 on the DOT's condition scale (0 = shut down, 9 = perfect), was far from safe. On August 1, the overpass buckled and plunged 64 feet into the Mississippi River, killing at least five people and injuring 60.

For the most part, bridge inspectors do their job with their eyes, scanning literally the nuts and bolts of the structure as well as welded connections for defects.

But this method is growing as antiquated as the bridges themselves, which average between 50 and 60 years old. And even if the best eyes could see every external flaw, it's humanly impossible to detect the extent of corrosion, cracking or fatigue hidden inside.

The Federal Highway Administration conducted a study in the late 1990s looking at the reliability of visual bridge construction, according to Brent Phares, associate director of the Bridge Engineering Center at Iowa State University.

"Those inspections were not very good or reliable. They frequently missed cracks. That points to the potential need for better tech that is easy to use and makes better assessments than we can do visually," Phares said.

Watch as a heavily traveled, potentially unsafe bridge near Washington, D.C., is brought down to be replaced by a new, safer structure. Discovery News' James Williams was there.

In light of that, researchers around the country are developing high-performance concrete, advanced sensors and even robotic surveyors to make the nearly 600,000 bridges in the United States safer and inspections more efficient and accurate.

Not Your Father's Concrete

Since the time of the Romans, engineers have used concrete to build their bridges. In more recent times, they have incorporated steel. But concrete crumbles and steel corrodes.

Researchers at Iowa State University's Bridge Engineering Center are investigating ways to use more durable materials for better bridges. For the first time in the United States, the team constructed a bridge using an ultra-high performance concrete.