UW, WISDOT teamwork gives bridge-building a lift
Bridges like the Golden Gate in San Francisco or the Brooklyn Bridge in New York never fail to thrill us with their elegance and the ingenuity of their design. They also make the squat, concrete spans we drive over every day seem pretty mundane.
But looks can be deceiving. Several of Wisconsin’s highway bridges are in fact far from ordinary, thanks to a partnership between the University of Wisconsin-Madison and the Wisconsin Department of Transportation (WisDOT). With major grants from the Federal Highway Administration (FHWA), the team has designed and built four innovative experimental bridges on the state’s roadways during the past eight years, with a fifth project now wrapping up in St. Croix County in northwest Wisconsin.
Although the five projects differ in their details—two replaced the steel inside bridge decks with a high-tech plastic, for example, while two others took advantage of pre-cast concrete parts—they all share a common goal.
“As the Federal Highway Administration says, ‘Do it quickly, do it well, do it better than we have in the past,’” says Finn Hubbard, who oversaw several of the projects as WisDOT’s former state bridge engineer. “Both the steel-free decks and pre-cast deck panels are examples of that: They were very quick and easy to erect. And, in theory, they should be very long-lasting. That’s important, too.”
Just how important became tragically clear in August 2007, when the collapse of the I-35W bridge in Minneapolis ignited concerns nationwide about bridge safety and durability. Since then, reviews of the country’s highway bridges have uncovered hundreds of aging spans in need of fixing and maintenance. And while Wisconsin has fewer problem bridges than many other states, its sheer number of spans—some 13,600—cries out for longer-lasting structures and faster methods for replacement and repair.
“Closing down a highway to one lane or sending traffic on a detour route for a month can cost millions of dollars to the trucking industry and the public,” says Oliva, who led the UW-Madison team with fellow professors Jeff Russell and Larry Bank. “So now this has become a focus nationally: Doing construction faster to reduce the disruption.”
Addressing these problems was exactly the goal of the Federal Highway Administration’s Innovative Bridge Research and Construction program, which launched in 1998 to support research by teams of university and state engineers, including the UW-Madison/WisDOT group. But the program contained a major twist. In addition to original ideas and laboratory studies, teams would need to produce a real highway bridge that would be exposed to the normal wear-and-tear of traffic and weather. What’s more, they’d need to do this following the state’s standard construction schedules and using its regular construction contractors.
It soon became clear that to get the projects done, UW-Madison and state engineers would have to span the natural differences between them and work together more closely than ever before.
“Researchers at universities always want to try these cutting-edge things, and they tend to talk a lot of theory and equations, whereas state engineers have a completely different agenda,” says Bank. “So this program allowed us—with the support of some really visionary engineers at the DOT—to start to break through those barriers.”
In their first two projects, the team addressed one of the biggest threats to Wisconsin’s bridges: the winter climate. “Our weather here is really tough on all materials, but it’s especially tough on concrete,” says Oliva. Not only do cracks widen with freezing and thawing, he explains, but when road salt-laden water seeps into these cracks, the steel reinforcing bars within end up rusting, expanding and cracking the concrete even more.
The solution came from Bank, who is an expert in fiber-reinforced polymer (FRP), also known as fiberglass. Found in everything from golf clubs to airplanes, this lightweight, strong and rust-proof material seemed the perfect alternative to steel reinforcement. But the group was by no means going to accept this out of hand. Instead, the professors and their graduate students carried out a series of rigorous laboratory tests and analyses (and they did for all the projects), to prove the material would work in a real bridge—and that the bridge would be safe.
“Being engineers, we’re conservative, and being structural engineers, we’re ultraconservative,” says Hubbard, now principal bridge engineer with the consulting firm HNTB in Madison. “So we don’t (build) anything that the public is going to be on or around unless we really know it’s going to be safe.”
FRP did pass muster, and now the concrete decks of two bridges along Highway 151 near Fond du Lac and Waupun contain FRP instead of steel. Because FRP doesn’t rust, the researchers expect the decks could last 20 to 30 percent longer, or up to 100 years. Still, while this potential led Popular Science to name the innovation one of its “Best of What’s New” in 2005, whether it ends up being met, only time can tell. The state is now monitoring the bridges to see how they perform.
In the meantime, the team has also been exploring the use of pre-cast concrete parts as an alternative to concrete that’s cast on-site. Because of all the steps involved—building the molds, pouring the concrete and allowing the slabs to harden—conventional construction can snarl traffic for two to three weeks, says Oliva. In contrast, the group’s third project culminated with contractors assembling a bridge deck from pre-cast panels on busy Interstate 90 in just one day.
Pre-cast concrete is also of higher quality, Oliva adds, because it sets inside a climate-controlled factory rather than outside in the sun, wind and rain. For the team’s final project, located about 30 miles east of the Twin Cities, the WisDOT is now building a bridge abutment, or support, from pre-cast parts for the very first time. Total assembly time in the field is estimated at just two days.
Promising as all this sounds, there are still some hurdles to overcome before these innovations see wider use, says Scot Becker, WisDOT chief development engineer. Cost is definitely a drawback in some cases, as is the bridge building industry’s relative inexperience with the new materials and design techniques. And because they put such a premium on safety, structural engineers themselves can be slow to change, adds Hubbard.
Still, he and Becker agree the collaboration has definitely made an impact. “We’ve talked about some of the challenges,” Becker says. “But we’re moving in the next 10 years into some new practices, and that’s one major result of these types of projects. We’re very happy with them.”
“It has really been an eye-opener for us, because we’ve done things the same way for so long,” adds Hubbard. “But I tell you, the world keeps moving on. The computer is a great example: It keeps changing. And guess, what? The bridge world does, too.”
On the other side, Oliva also couldn’t be happier with the way things turned out.
“For me, the fun thing is that we actually got to build bridges,” he says. “To do all the research and then have it just stuck up on a shelf always kills me. But to develop the project, see the bridge built and then test it afterwards, that’s the fun part—to see research put to use.”