It’s the busiest interchange in a city where nearly everybody drives. More than 200,000 vehicles a day cross the flyover where the Dolphin Expressway intersects the Palmetto Expressway in Miami.
All those cars, buses and semi-tractor trailers hurtling over the concrete and asphalt take their toll on the roads and bridges. So do South Florida‘s extreme heat and torrential pounding rains.
Over time, the elements can cause bridges to shrink and even become deformed. For years, those charged with inspecting them have relied only on what their eyes could see as they try to maintain the roadways and ensure the safety of motorists.
That’s not good enough anymore. Engineers need to know what’s going on inside those bridges before the signs show up on the outside.
Alfred Lurigados ’96 knows this. As director of engineering and deputy executive director at the Miami-Dade Expressway Authority (MDX), he, together with the Florida Department of Transportation (FDOT), manages several major transportation projects in South Florida, including the makeover now happening at the Dolphin Expressway and Palmetto Interchange, which features the first multi-section bridges in South Florida.
So when engineers at Florida International University approached MDX last year with the idea of installing sensors to monitor the health of one of the four segmental bridges, Lurigados was intrigued. The network of sensors, placed at critical points, can deliver data about how the bridge behaves under heavy traffic, high temperatures and other conditions.
The sensors can also identify serious problems – hidden cracks, erosion – before they’re apparent to the human eye. That can mean the difference between a terrible disaster and a minor retrofit.
MDX co-funded the project with the FDOT. So on a Friday morning in January, Miami motorists driving eastbound on the Dolphin Expressway to the Palmetto Expressway North began traveling over one of the first “smart bridges” in South Florida and one of just a handful in the state of Florida.
It’s part of a trend around the country where billions of local, state and federal government dollars – with the help of civic planners and researchers – are being spent to build roads and bridges that are more intelligent. The latest advances in sensors, wireless communications and computers speaking to one another via the Internet power this smart infrastructure.
“This is the first time that anybody’s ever done this with segmental bridges in Florida. We will have data come out of bridges,” said Lurigados, who earned his bachelor’s degree in civil and environmental engineering. “It is, in essence, a smart bridge.”
Unlike traditional bridges that are made of very large sections, segmental bridges are built in short sections, one piece at a time. The bridges are made of concrete that’s either cast in place at its final location – as was the case with the flyover in Miami – or precast and transported to the final location for placement.
Late last year, Nakin Suksawang, a professor of civil and environmental engineering and a researcher with the Lehman Center for Transportation Research at FIU’s College of Engineering & Computing, along with two graduate and two undergraduate students, installed 96 stainless steel embedded sensors in critical locations over six segments of the bridge prior to the concrete being poured.
“As the concrete shrinks or creeps it will push in on the sensors here on the end to compress it,” explained Brandon Mintz ’97, a Ph.D. candidate in civil and environmental engineering at FIU, who helped install the sensors. “And it will get conducted through the cable and it will measure the change in strain in the concrete itself.”
The segmental bridge uses large segments of concrete that are placed together and compressed. Unlike steel, concrete suffers long-term changes, including creeping and shrinkage. Creep is the slow movement or permanent deformity of the concrete that happens as a result of prolonged exposure to high levels of stress, particularly heat. Even pre-stressed concrete shrinks over time, which can lead to cracking.
“It’s like a sponge. As a sponge loses water it tends to shrink, and it’s the same thing with concrete,” Suksawang explained. “Some of the pre-stressing force in the bridge is lost and the bridge could potentially sag. History has shown us that improperly designed bridges have ended in disastrous collapse.”
While construction codes try to predict this type of behavior, Suksawang said, a lot of code is written based on small-scale testing using cylinders often only six or 12 inches in length. Questions remain over how well such testing can tell a bridge’s future, particularly given the variation in properties of concrete.
“Making concrete is like cooking,” Suksawang explained. “The variation of these properties makes it harder for engineering to come up with a formula that can precisely predict behavior.”
A research study of this kind has never been conducted in South Florida, an area known for high temperatures.
“This data that will come out of these sensors would definitely help us and future design and construction engineers learn from what’s happening over time with these bridges,” explained Lurigados.
The project also allowed FIU students to use the newest in civil engineering technology. The students prepared and installed all the sensors themselves at the casting yard under Suksawang’s supervision. The students also traveled to the bridge construction site to install and change batteries in the bridge girders. The system, including the sensors and data logger, was initially powered by batteries and is now powered by electricity inside the bridge.
“It was good to learn how to install these instruments in actual structures,” Mintz said. “I think the sensors can make the bridges safer by supplying data so that decisions can be made about maintenance and how to spend money on different aspects of the structure itself over time.”
A recent report found that more than 11 percent of the 600,000 bridges in the United States are structurally deficient, and in 10 years one in four bridges will be 65 years or older. Trying to identify structural problems in a bridge through visual inspection is a practice plagued by inconsistency and expense.
“The best thing to do is to actually put instrumentation in the bridge and monitor it,” said Suksawang. He believes the data collected will help engineers design better bridges and assist them in the evaluation of existing bridges. The term engineers use is structural health monitoring.
“It’s like we’re monitoring the pulse of the bridge,” he said. “The system provides early warning and information that could prevent catastrophic failure.” ♦