McCormick Magazine

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Better Bridges

McCormick’s infrastructure Technology institute helps the CTA monitor the structural health of its bridges

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Imag eof Dan Marron and David Corr of the Infrastructure Technology InstituteUnder the 100-year-old “L” tracks that cross over Devon Avenue in Chicago, a little silver box is hard at work. As hundreds of Red and Purple Line trains rumble overhead each day, the box collects measurements from an array of sensors on the viaduct. They measure the strain on the bridge’s retrofitted shoring—the steel supports installed more than a year ago to help deteriorating concrete columns carry the load. Each day the box uploads the data to a secure project web-site at the Infrastructure Technology Institute, and from there engineers from the institute and the Chicago Transit Authority can monitor the structural health of the bridge.

The project is a perfect example of the sort of work ITI was created to carry out. The institute, supported by a grant from the US Department of Transportation, employs a team of research engineers and faculty members to develop strategies and tools to protect and improve the condition, capacity, and performance of the nation’s highway, railroad, and mass-transit systems. State-of-the-art technology like that used at the Devon Avenue bridge helps the CTA determine which bridges need to be replaced and when. It also provides huge data sets on long-term bridge health that will help engineers everywhere make better choices about which ones to replace or repair.

The project began when CTA president Richard L. Rodriguez saw an ITI newsletter and arranged a meeting to ask how the two organizations might work together. The partnership seemed a perfect match: The CTA has hundreds of century-old bridges that need monitoring. ITI engineers have years of experience installing sensors to monitor bridges around the country remotely, providing data to help make decisions about structural integrity.

“Over its 18-year history, ITI has deployed remote sensing systems at more than 80 sites around the country,” says Joseph Schofer, director of ITI as well as associate dean for faculty affairs and professor of civil and environmental engineering at McCormick. “We’re equally pleased to contribute something of value to our hometown transit system.”

“I don’t know of anybody else who has continuously monitored bridge data over that span of years and years,” says Dan Marron, ITI’s chief research engineer. The engineers work as a team, from planning and installation to data analysis, and have made some critical discoveries. For instance, they were able to alert one state’s department of transportation when their sensors detected a crack in the main gear of the drivetrain of a drawbridge.

In late 2009, as the CTA installed the steel shoring under the Devon Avenue bridge, ITI engineers installed 10 vibrating-wire string gauges in the shoring’s concrete pads to measure long-term loading. When the bridge load starts to transfer from the concrete columns to the shoring, the sensors pick it up. A few months later the engineers added seven resistive-strain gauges that measure how much the steel beams stretch or compress each time a train passes. The tiny sensors—each an inch or two square—were spot-welded to the steel columns and wired to the data logger in the silver box. A computer processes the signals from the sensors, filters out the noise, and stores the signals until their daily transmission to the ITI servers at Northwestern. There they immediately become part of the project’s website, displayed as easy-to-read graphs that show the load for each train that crosses the bridge.

“We’re doing a load test every 10 minutes every day for a year,” says ITI research engineer David Kosnik. “We’re seeing real-world response rather than one from a computer model.”
ITI’s monitoring shows that the steel shoring, which is 7-1/2 feet tall, compresses about 305-millionths of an inch per train—a tenth of the diameter of a human hair. “That means the original bridge is carrying most of the load.” Marron says. “If we used the sensors available five years ago, we wouldn’t be able to detect strain this small.”

“The device that Northwestern developed is a valuable tool that supplements our own inspection of the aging concrete bridges along the Red and Purple Lines,” says CTA President Richard L. Rodriguez. “By virtue of their conservative design, the bridges have withstood time and the elements. Until these bridges can be replaced, CTA has erected shoring, and Northwestern’s device supplies another layer of data for our analysis when determining if adjustments need to be made. The device at Devon has shown that the shoring is installed properly and provides a rigid path between the bridge deck and the foundation.”Image of a bridge monitor

David Corr, clinical associate professor of civil engineering and an ITI research engineer, is working with Pablo Durango Cohen, associate professor of civil and environmental engineering, to analyze the sensor data and determine the overall health of bridges, taking into account the different materials (steel, concrete, etc.) that make up the structures. Durango Cohen has created statistical methods to determine the condition of pavement, and he and Corr are creating methods to determine the condition of more complex structures like bridges. “We don’t just collect the data,” Corr says. “We use it to develop quantitative tools that can help make decisions about what needs replacing and repairing.”

The CTA project also serves as a way to get undergraduates excited about civil engineering. Students have helped ITI engineers on nearly every aspect of the project, including creating CAD drawings, building and testing the sensor systems, installing them on the bridge, and examining the data streams.

“This partnership benefits everyone,” Schofer says. “The CTA gets high-quality, real-time data to assess the integrity of its infrastructure. Our research engineers and faculty get a unique data stream of research. And students get hands-on experience with civil infrastructure systems.”

Since the bridge is just three miles from the Northwestern campus, the ITI team would like to make it a platform for trying out new bridge-sensing technologies such as acoustic testing, whereby engineers locate the source of a crack or defect by detecting sounds within the concrete. “We’d like to figure out if there is a simpler way—perhaps using only one or two strain gauges—to implement a system like this on more CTA bridges around the city,” Marron says.

Ultimately, the project exemplifies what the Infrastructure Technology Institute aims to do: use remote structural-health monitoring to better manage public infrastructure. “Everybody knows there’s not enough money to replace every aging bridge,” Marron says. “Using sensors like these is another tool that allows you to make better choices about which ones to replace or repair.”

Emily Ayshford