Mackinac Bridge to be test site for stress sensors

EAST LANSING  - Thousands of people will walk across the Mackinac Bridge Monday as part of the annual Labor Day bridge walk.

Later this month, MSU prof Nizar Lajnef will hit the bridge for a step forward of his own. He'll be lowered beneath the iconic, 58-year-old suspension bridge that connects Michigan's two peninsulas to place sensors under its deck.

The purpose: Monitor stress on the bridge's aging structure.

"We don't have to wait until it's broken and then try to fix it," said Lajnef, who teaches environmental and civil engineering at Michigan State University.  "We're trying to monitor its health over time."

The project is a test, but if it works, stress sensors could become commonplace on structures around the country, from the Mackinac Bridge to the Hoover Dam.

Lajnef and an MSU team have been working on the sensor project for seven years with funding from the Federal Highway Administration. A recent grant of $1.5 million to MSU, Washington University at St. Louis and the University of Nevada-Reno will put sensor prototypes in place at several spots around the country.

The 5-mile-long Mackinac Bridge is the third-longest suspension bridge in the world. About 3.9 million vehicles crossed it in 2015.

Weather and traffic loads cause cracks in roads and bridges that weaken them over time. A typical roadway in the United States lasts five to 10 years; a bridge is 50 to 70 years.

"People have been trying for a long time to design methods to assess the condition of bridges," Lajnef said. The current standard: periodic visual inspections.

Lajnef's team created a sensor that charges itself from the energy of movement across the bridge, so it requires little in the way of maintenance. He projects the production cost to be less than $1 apiece.

The sensors wirelessly transmit data about the structure's response to traffic. After a baseline is created, any variance could indicate a problem.

"When the stresses are outside the normal range, the sensor will tell you so that you can fix it before it is too late, before the components break," he said.

Lajnef expects to install the bridge sensors around Sept. 14. Initially, six sensors will be put in place. If they perform as expected, more will be added in the spring. The bridge could eventually contain hundreds of sensors.

Lajnef said he expects the test sensors on the Mackinac Bridge to do their job. He just hopes he can do his.

"I think it will be fun," he said. "I'm afraid I will get vertigo or something when the lower me over the bridge to put them on."

Contact Kathleen Lavey at 377-1251 or klavey@lsj.com. Follow her on Twitter @kathleenlavey.