Technology keeps a close eye on 35W bridge

Bridge sensor
A cluster of three of the many sensors installed in the wire mesh of the new 35W bridge that measure and send data to civil engineering researchers.
Photo Courtesy of MnDOT

The new 35W bridge is not even a year old but already researchers have data telling them how the 1,200-foot long span is doing.

Catherine French, a University of Minnesota civil engineering professor who's monitoring the bridge, said its performance so far matches predictions.

The $234 million structure was built in less than a year to replace the old 35W span, which collapsed on August 1, 2007.

The ten-lane 35W bridge is actually three bridges -- two five-lane wide spans, one going north, one going south, and a third span as one of the exit ramps. The bridge was built to handle more traffic than is currently using it.

It is also wide enough and strong enough to accommodate future transit service including light rail. Engineers assumed the bridge would expand in length with warmer weather and French said, so far, the numbers are on target.

"The expected change in length is on the order of eight to ten inches over the course of that free span portion of the bridge," French said.

One way to take the pulse of the new 35W bridge is visual inspection, part of the Minnesota Department of Transportation plan.

MnDot officials say the bridge will be inspected in September for the first time since it opened and will probably be on a two-year inspection cycle after that.

Catherine French
University of Minnesota civil engineering professor Catherine French and several of her colleagues monitor and decipher the readings that are coming from the 500 sensors placed in the new 35W bridge across the Mississippi river in Minneapolis.
MPR Photo/Dan Olson

Catherine French's approach is different.

She and civil engineering colleagues at the U of M, working with MnDOT and federal officials, have data streamed onto their computers.

The data are readings from 500 sensors in the bridge. The sensors are connected by miles of wire and go to a nearby server allowing scientists to review the readings from virtually anywhere.

French said the sensors look like barbells or handweights. A wire running through the sensor tells the researchers what's happening.

Think of all those wires, French said, as guitar strings being strummed.

"When the bridge elongates that wire gets longer and tightens," French said. "There's actually a little sensor or activator that plucks that string electronically, that causes that string to vibrate and so, when that string gets longer or tighter, we can relate its change in vibration to how much the bridge has grown or shortened."

Analyzing the data
Carol Shield (left), a civil engineering professor at the U of M, and Brock Hedegaard, a graduate student, look at bridge data sent from the sensors.
Photo Courtesy of Catherine French

The sensors are also sending numbers measuring what French calls, deflection.

That's especially noticeable on a day when the top side of the bridge is warmed by the sun, the bottom is in the shade and the warming and the cooling cause the bridge to change shape.

The numbers make sense, but only if they have a baseline for comparison, French said.

For example, how much weight can the bridge hold?

Installing sensors
Workers install one of the many sensors installed inside the 35W bridge that sends information to researchers so that they can guage stress and other factors on the bridge.
Photo Courtesy of MnDOT

To help create a baseline measure to answer that question MnDOT, the night before the span was opened to traffic, filled trucks with hundreds of thousands of pounds of sand. French said the trucks were parked at particular points so researchers could measure what the weight did to the structure.

"We did these truck tests with fully loaded sand trucks and it was barely even readable within the data because of the robust nature and the amount of load the bridge was designed for," she said.

Engineers say the bridge is designed to last more than 100 years.

The sensors can warn us if the new 35W bridge might fail and collapse, but there isn't an alarm that would set off bells and whistles or lower gates preventing drivers from going on the bridge.

What the information from the sensors will do, French said, is tell how the bridge is aging over time.

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They will also tell how corrosion from water and chemicals, for example, is affecting the reinforcing steel bars in the concrete and how the concrete surface is holding up to traffic wear.

Those values, French said, will be watched for signs of trouble.

One unknown about the new 35W bridge is how long will the sensors last. French said the sensors may have a life expectancy of about 20 years.

After that, the bridge has doors that allow inspectors to go inside the 1,200-foot long span.

"If there was a desire in the future to add sensors or replace those you could at least do it through the box sections of the bridge," French said.

Catherine French said only a few other bridges around the world have used sensors to measure their health, and in those cases many used fewer than the 500 sensors monitoring the new 35W bridge.

Federal officials appear to be counting on bridge sensor research to help monitor the nation's nearly 600,000 bridges, with more than a one-fourth of them rated structurally deficient or obsolete.

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