Engineering: Sensing Weakness

It was one of the most shocking disasters in Minnesota history: On Aug. 1, 2007, at the height of the Minneapolis rush hour, the I-35 bridge over the Mississippi River suddenly gave way, sending dozens of cars and trucks plummeting 60 feet into the water below. Thirteen people were killed and 145 injured in the disaster, which sparked a national debate about saving crumbling infrastructure.

“The average age of bridges in the United States is 42 years, and roughly 25 percent of bridges are either structurally deficient or structurally obsolete,” says Branko Glišić, an associate professor of civil and environmental engineering. “There is a huge demand for improvement.” Government reports have identified an estimated $1.7 trillion backlog in surface-transportation repairs — and state and federal governments don’t even have enough money to adequately inspect bridges to determine which are most at risk.

Glišić and Yueh-Lin (Lynn) Loo, the Theodora D. ’78 and William H. Walton III ’74 Professor in Engineering, hope to eliminate the need for costly and highly subjective visual inspections by developing a plastic sensor that can continuously monitor bridges for failure. Currently, the Federal Highway Administration requires that all bridges be inspected every two years, a wasteful expenditure of resources when bridges are not at risk. Remote sensing is a better alternative, but the metal sensors currently in use break down over time and are effective only when placed exactly where breakage would occur.

“Frequently, we don’t know a priori where that is,” says Glišić. “In the case of the Minneapolis bridge, it was the gusset plates that failed, which was an area of 5 square meters. If you don’t have the sensor in that location, you cannot really monitor that site.” Glišić and Loo are working on what they hope will be a more reliable and accurate sensor made out of conducting polymers — flexible plastic that can carry an electrical charge. 

The polymer has a surprising quality, a graduate student in Loo’s lab, Melda Sezen, has discovered. “Usually when you stretch the material, it will increase the resistance, which is how you measure strain,” explains Glišić. But Sezen found that when the polymer is treated with acid, the opposite occurs: “The more you stretch it, the more resistance decreases,” Glišić says. Sensors made from a combination of treated and untreated polymers could enable inspectors using simple calculations to tease out which changes in a bridge were due to a weakening of the bridge’s material and which were due to temperature fluctuations or other usual processes. Drawing that distinction would give inspectors a better idea of what repairs were needed.

Loo’s lab is still testing the polymers; once research on the material itself concludes, Glišić’s lab will work on implementing sensors in civil infrastructure.

 Glišić is working separately with electrical engineering professors Naveen Verma, Sigurd Wagner, and James Sturm to develop sensing sheets that can detect strain over larger areas. In April, the team installed a prototype on Streicker Bridge on the Princeton campus. If all goes well, the two projects will be combined into a sophisticated sensing system that could provide a wealth of data at relatively low cost.