I-35W bridge showcases ‘smart bridge’ technology

The collapse left a lasting impression on the rebuilt structure, prompting designers to include “smart bridge” technology.

Danielle Nordine

Although the Interstate 35W bridge collapsed more than three years ago, the event is anything but a distant memory for Twin Cities residents. Victims are still waiting to see whether they’ll be compensated for their turmoil, and the beams from the old bridge still lay scattered along the Mississippi on the West Bank.

The collapse left a lasting impression on the rebuilt structure, prompting designers to include “smart bridge” technology, a wide array of equipment in the current bridge that can sense everything from steel corrosion to the amount of strain on the bridge.

The collapse on Aug. 1, 2007, killed 13 people and injured 145. A year-long investigation by the National Transportation Safety Board determined that the causes of the bridge’s collapse
included support plates that were too thin and added weight on the bridge from construction equipment
before the collapse.

When the bridge was rebuilt, designers included features such as an anti-icing system, security sensors and technology that measures the bridge’s reaction to traffic or weather conditions.

The sensors measure how the concrete expands or contracts due to weather and use and sends a constant stream of data to a computer nearby, Alan Phipps, the project’s design manager, said.

“If there were a problem with the bridge, the signs would show up long ahead of time with this technology,” said Phipps, who is also the senior vice president and director of operations for FIGG Engineering Group, the company that designed the new 35W bridge. “You can see if there’s something strange in the data, and then you can go do a visual inspection.”

The measurements are available to the Minnesota Department of Transportation and the University of Minnesota via fiber optic cables, Phipps said.

Members of the University’s civil engineering department monitor a constant stream of information that comes in about the bridge’s condition and report back to MnDOT periodically.

Originally, much of the technology was used for monitoring the bridge during the rebuilding process, especially over the winter, Kevin Western, the deputy project manager from MnDOT, said.

However, officials quickly realized the technology could be used after the bridge was completed to make sure it was working properly.

They decided to add the extra technology and give data access to MnDOT and the University to continue to monitor, said Western, also the state bridge design engineer.

So far, the technology has shown that the bridge is behaving as expected and as designed, Western said.

The current bridge was built to last at least 100 years, and Phipps said the technology installed on the structure was included partially to restore the public’s faith in the bridge.

“When we were taking a look at the design for the bridge, re-establishing public confidence in infrastructure was an important goal,” he said. “We included the technology as another layer of comfort we could propose, reassuring people this was the safest bridge there.”

Rebuilding the bridge in the shadow of the collapse was exhausting, but it also created a strong, hard-working team, Phipps said.

“Everyone worked tirelessly, because everybody had the same goal and the same attitude,” he said. “Everyone wanted to make this a quality project and get it back up and running as quickly as possible.”

The current bridge was designed and built in 11 months, a project that would normally take more than two years, Phipps said.

Western said the rebuilding process was “probably the greatest challenge of my career,” and said while it was emotionally painful, he was proud of the bridge’s quality and the team’s speed.

Across the United States, almost 25 percent of the 603,245 public bridges are still structurally deficient or functionally obsolete, according to the Federal Highway Administration.

That doesn’t mean the bridges are unsafe to drive on, Western said, but it does mean they require changes to the layout or physical components to continue to handle strains like traffic and weather.

The previous I-35W was classified as structurally deficient.

“The biggest issue is investment in our infrastructure,” Western said. “It is important for us to keep communicating that there are needs as we go forward.”

Still, Phipps said he doesn’t see this technology becoming standard in future bridges, at least not new ones.

“The new structures aren’t the ones you need to worry about,” he said. “Installing this on older bridges is probably a better application of the sensors. You need to distribute your resources and spend your money where it does the most good.”