After the collapse of the Interstate 35W bridge in Minneapolis in 2007, which killed 13 people and injured 144, congressional interest in fixing the nation’s bridges escalated. But efforts have not matched concerns: Since the tragedy, the addition of billions of extra federal dollars specifically for the repair and replacement of deficient and obsolete bridges has failed to come to fruition.
In the meantime, several states have begun using structural monitoring – a combination of sensors, wireless communication and cloud computing -- to watch the structural integrity of bridges and potentially reduce infrastructure repair costs, enhance safety, and reduce load postings and detours.
Today, 73 percent of U.S. road traffic and 90 percent of truck traffic travels over state-owned bridges. The American Association of State Highway and Transportation Officials estimates that nearly one in four of those bridges needs repairs, and it would cost $140 billion to repair all of them. The average age of America’s bridges is 43 years.
Traditionally, visual inspections by technicians have formed the basis of U.S. spending on bridge infrastructure -- but it’s an imperfect system.
“Visual inspection is subjective and highly variable,” said Peter Vanderzee, CEO of Alpharetta, Ga.'s Lifespan Technologies, which uses special sensors to monitor bridges for stress. “It is rare that visual inspection findings are challenged, even if they are unusual or questionable, simply because the process is expensive and time consuming, and not required by federal regulation.”
But structural monitoring utilizing highly accurate sensing devices can enable objective, precise and timely performance data on the condition of bridges, said Vanderzee. Using technology-based monitoring, a state can install a suite of sensors on a bridge or other structure to monitor its structural integrity either once or on a continual basis. Sensor placement is determined by engineers, and data from the structure is collected and sent to a secure data center for analysis.
“Based on work we have done, we estimate 30 to 40 percent of bridges evaluated using advanced condition assessment technologies are in better to much better condition than presumed based on visual inspection,” he said.
Sensor Tech in South Carolina
That means some bridges designated for replacement, rehabilitation or restriction may not need such alterations after all. That could translate to substantial savings for U.S. taxpayers. The South Carolina Department of Transportation (SC DOT) has been using sensors from Lifespan Technologies to monitor the structural behavior of several bridges in the state for more than five years.
“One bridge was scheduled to be replaced, but replacement was some time off,” said Lee Floyd, South Carolina’s state bridge maintenance engineer. “We were concerned about the need for constant monitoring, and we felt this technology would help relieve us of some of that.”
Utilizing the sensors, data from the bridge is now sent to SC DOT in real-time, saving the state significant inspection costs. The bridge also has a weight restriction, and the sensors help ensure the restriction is not exceeded on a regular basis.
“I can sign in from anywhere, anytime and get real-time information,” said Floyd. “I can also set thresholds, so if a certain sensor goes to a certain level, I’ll get an email or a text message.”
A few years after the department deployed the technology on one particular bridge, Floyd said he began to see sensors spike after midnight. Some investigation revealed that logging trucks were going over the bridge at night in order to avoid the transport police -- who were then deployed to the bridge and ticketed several overweight trucks.
South Carolina also deployed sensors on another bridge in the state that was scheduled to be replaced. When replacement plans ran up against opposition given the bridge’s proximity to a historic plantation, sensors were installed to allow SC DOT to continuously monitor the bridge's condition to ensure safety until the replacement can be completed.
“The technology doesn’t just help us in avoiding or adjusting restrictions, or avoiding replacement,” said Floyd. “There is a benefit also in postponing replacement because that allows you to better utilize the cash flow of your funds to other bridges. So it really is a combination of all three of those things.”
SC DOT also installed sensors on the Arthur Ravenel Jr. Bridge, a cable-stayed bridge over the Cooper River in South Carolina that connects downtown Charleston to Mount Pleasant. The bridge has a main span of 1,546 feet, the third longest among cable-stayed bridges in the Western Hemisphere. Sensors will allow SC DOT to measure the day-to-day performance of the bridge, as well as its performance during high-wind events (the span is designed to endure wind gusts in excess of 300 mph).
Overall, Floyd said he would like to eventually expand use of the technology in South Carolina.
“There’s a lot of viability in the technology,” he said. “You still have to do your hands-on and visual inspections, but I see this as a complementary tool to help us make better decisions.”
Colorado's Yearlong Bridge Sensor Pilot
In April, the Colorado Department of Transportation (CDOT) installed sensors under the Williams Canyon Bridge on U.S. 24, allowing CDOT to monitor the structure’s movements from its Denver headquarters. It is the first bridge in Colorado to utilize the technology.
“We’re in the testing phase at this point, but it’s important to find out how well it works in order to better manage our bridge infrastructure,” said CDOT staff bridge manager Josh Laipply. “If it proves successful, we’ll install these sensors on other structures throughout the state, allowing us to monitor a variety of characteristics, such as load capacity and bridge movement, which will enhance everything that we’re already doing as part of our inspection program.”
Parsons engineering firm is assisting CDOT with the installation of the system under a $150,000 contract.
“We selected a load restricted bridge that was relatively complex for this pilot project,” said Mark Nord, CDOT bridge asset management engineer, who adds that they plan to monitor the bridge for a year to see if the technology adds value for the state.
As of press time, an initial load test had been completed, but the analysis of the results was not yet available.
“Long-term monitoring will allow us to see what loads cross the bridge, how the structure behaves under live loads, and how the bridge is performing,” he said. “The load test will also help us validate the rating. If we get an improvement in rating, that means there are less restrictions on permitted loads that can cross it. And if the bridge is performing like we expect or even better than we expect, then it may delay replacement or change the nature of a rehabilitation project. The idea is to see if it can help us save money and use the funds we have more strategically.”
Nord said if the technology works well, it may be expanded in the state.
“I don’t picture it on every bridge we own,” he said. “It would probably be targeted primarily to bridges that we have questions about.”
To date, technology-based structural monitoring has also been utilized by transportation departments in Pennsylvania, Massachusetts and New York, as well as the Canadian Pacific Railroad, according to Vanderzee.