Are bridge inspections adequate?

It's a question that's been on a lot of commuters' minds over the last two weeks as they drive over area bridges: Why didn't engineers know that the Interstate 35 west bridge in Minneapolis was so close to disaster?

The primary methods of testing bridge safety are decidedly low-tech. For the most part, inspectors stand on beams or in bucket trucks and look around carefully. They spray red dye to locate cracks and drag a heavy chain across the roadway listening for hollow sounds that suggest a danger below. When simple calipers will not work to measure the thickness of aging steel, they get a bit more high-tech and use an ultrasonic gauge.

But there's debate about whether more sophisticated technology would have made a difference in the deadly Aug. 1 bridge collapse.

Bridge inspectors say their methods are thorough and based on decades of experi ence.

"The inspection process is really a combination of low-tech methodology combined with some high-tech when it makes sense," said John Carney, a project manager for HNTB, a construction firm that has a bridge inspection contract with the Massachusetts Highway Department. Many technologies are too large and cumbersome for practical use on a ladder or in a bucket truck, he said.

On the other hand, those who make high-tech alternatives say any method based on people doing periodic inspections is flawed -- and that something like ground-penetrating radar, which can be used from the back of a moving vehicle, is more economical and reliable than dragging a chain across a road.

And technologies are being developed that will provide a real-time heads up when a catastrophe is looming, leaving time to stop traffic and evacuate the bridge.

But the low-tech inspection process in Minneapolis was sophisticated enough to predict trouble. Engineers in Minnesota had been complaining for nearly a decade that the bridge had extensive corrosion and cracking, with certain components operating "beyond tolerable limits."

Whether they made their concerns loudly enough to get the attention of politicians and open the state's coffers likely will be debated for years.

Reports last week suggest that the repair work going on at the time of the bridge's collapse may also have contributed to the disaster. The US Secretary of Transportation warned states last week to beware of placing too much extra weight on bridges during construction projects.

Bridges like the I-35W crossing were designed at a time when less traffic was expected, and when there were fewer, lighter trucks jamming their lanes. That extra weight over the years is what causes a lot of the damage that gives bridge engineers sleepless nights.

Regardless of the inspection technology, there's no way to guarantee that other aging bridges will not collapse before cash-strapped state and local governments can repair or replace them. One consolation: Many modern bridges -- such as the Leonard P. Zakim Bunker Hill Bridge -- are designed with intrinsic redundancies, so that a crucial element of the bridge can fail and still leave enough time to evacuate before a collapse.

And bridge health monitoring specialists say the potential is there for the next generation of bridges to include technologies that will provide continuous status reports -- warning if a catastrophe is imminent -- rather than waiting for the semiannual inspections required by federal law, said Masoud Sanayei, an engineering professor at Tufts University who specializes in bridge health monitoring.

"We have the knowledge today where we can embed sensors into the construction, such as accelerometers, tilt meters, and strain gauges, and hook them up to real-time monitors to see if the bridge is behaving the way we think it should," he said. "These newer technologies are just emerging, but they're worth investing in."

The key to avoiding future disasters, Sanayei said, lies in tax dollars.

"This is not an engineering problem; it's a policy problem," he said. "We have the way, but it costs money. Are we, as a nation, willing to invest in our infrastructure?"

Inspection process

The federal government requires that bridges be inspected every two years. Inspectors evaluate four main areas of a bridge: its deck condition, superstructure, supporting substructure, and the flow of channel items below the bridge. Each main element is given a rating between zero and nine.

The main inspection process is largely visual. Inspectors use ladders, cranes, bucket trucks, barges, and climbing gear to get access to the various components of the structure, and conduct a visual analysis for such things as stress, cracks, fatigue, and deterioration.

They also spray dye to reveal hidden cracks, and drag heavy chains across the roadway to listen for evidence of hollow areas. Calipers or ultrasonic thickness gauges, which utilize sound waves, are used to measure the remaining thickness of the metal.

The data is then compared with the original bridge plans -- when available -- to determine an overall rating on the current soundness of the structure.