MINNEAPOLIS -- As vehicles travel across the I-35W bridge over the Mississippi River, they create vibrations, which show up as little blue waves on a real time moving graph on a Brock Hedegaard's laptop at the University of Minnesota.
"We can look at temperatures on the bridge, we can look at traffic loading, we can look at dynamic behavior of the bridge, to know if it's vibrating correctly," Hedegaard, a graduate student in the Department of Civil Engineering.
He's spent years monitoring the readings from diagnostic tools built into the span when it was constructed in 2008. In fact, there are 500 sensors inside the bridge transmitting data.
The devices are designed to determine if the $238 million dollar investment is living up to the designers' expectations.
"It's telling us, effectively, how the bridge is behaving," Hedegaard explained.
"But the engineers have to be able to recognize whether that behavior is bad or if it's a normal bridge behavior."
As it turns out Hedegaard's "patient" is behaving well and in great overall health. It's holding up to traffic loads, as it should, considering it was designed to hold the extra weight of a light rail line that was never placed on it.
The instruments bear out that concrete is constantly stretching and shrinking, but that's what it was predicted to do because of temperature extremes. There are even temperature fluctuations within the bridge, that affect the shape of it.
"The top grows faster than the bottom because of the temperature increase, and so it will move up and down as much as an inch a day," Carol Shield, a professor of Civil Engineering at the U of M, told KARE.
Those changes in the bridge's shape, or "deformations," go undetected for the most part by the human eye. But any measurements outside the norm can affect alert the engineers to problems that could affect the functionality and life expectancy of the bridge.
"And it's the thermal - the environmental loading - that tends to put more strain and stress on the bridge than the traffic does," Shield remarked.
Now that her study has confirmed the bridge is performing as expected, the next phase is to design a method for monitoring the movements inside those critical bridge bearings, which allow bridges to roll with the punches Minnesota weather delivers on daily basis.
Those bearings are visually inspected in person at least once every two years by MnDOT technicians, but the sensors could tip off inspectors to potential problems much earlier in the process.
"There really is no bridge monitoring system available, so this is really cutting edge technology that we're trying to develop," Shields said.