Designing a Bridge That Lasts
By Grant Martin
America’s infrastructure is decaying. That fact was made painfully clear in 2005, when Hurricane Katrina overwhelmed the levees in New Orleans, and again in 2007, when a major Interstate bridge in Minneapolis, Minnesota, collapsed into the Mississippi River, killing 13 people and injuring 145 others. Unlike the levee breaches in New Orleans, there were no extraordinary conditions on that August day—just normal, everyday traffic flow.
The cause of the I-35 tragedy was eventually traced to under-designed gusset plates. These plates, which connect truss members, were only half as thick as they should have been, based on the codes and specifications in use when the bridge was built in the 1960s. But if a 40-year-old bridge in a large metropolitan area could suddenly fail, what does that say about the hundreds of thousands of other bridges in the United States that are the same age and older?
According to the Report Card for America’s Infrastructure, a report issued every three years by the American Society of Civil Engineers (ASCE), more than one in five bridges in the United States are structurally obsolete or deficient. In Alabama, the numbers are even worse: 30 percent of the state’s bridges received those low marks.
|MULTIMEDIA: The Art and Science of Bridge Building (audio slideshow)|
Making the Grade
This is unsettling news in a country where millions of cars, trucks, and pedestrians cross multiple bridges every day. But UAB engineers say that while the report raises serious concerns, it’s not a cause for panic. “When you see that more than one in five bridges are structurally deficient or obsolete, that’s terrible,” says Fouad H. Fouad, Ph.D., chair of the Department of Civil, Construction, and Environmental Engineering. “But when a bridge is deemed structurally deficient, that doesn’t mean it is going to fall down the next day. It means some elements are badly deteriorated and need attention.”
In some cases, “structurally deficient” bridges need maintenance or repair, but many of the “deficiencies” cited in the ASCE report are actually a reflection on how the bridge is being used. “The terminology that engineers have established for bridge rating can be misleading to the general public,” says Christopher Waldron, Ph.D., an expert in structural engineering who worked on the I-35 replacement bridge in Minnesota before coming to UAB in 2008. “A lot of bridges are rated structurally deficient because they are carrying more traffic than they were designed to carry. It doesn’t mean they can’t safely carry that traffic; it just means that they’re exceeding their original design intent.”
After a bridge is rated structurally deficient, engineers determine how much weight the bridge can safely carry. If that number is less than the legal limit, signs are posted, and the bridge is marked on trucking maps, but it remains open to traffic. “So even though the language may be alarming to the general public, the engineers who rate those bridges have procedures in place to handle any deficiencies,” says Waldron.
Even though the language in recent news-making reports may not have been intended for the general public, interest in the state of American infrastructure has increased since the I-35 collapse. “That was a wake-up call,” says Fouad. “These reports have been coming out for 20 years, but now everyone in the country is looking at them very seriously.”
The I-35 collapse is especially alarming considering that the span opened in 1967, making it a decade younger than the oldest parts of the Interstate Highway System. “Most of our highway bridges were built when the Interstate system was under construction in the ’60s and ’70s,” says Waldron. “All of those bridges were built at roughly the same time, and they are going to deteriorate at the same time as well.”
With a system of inspections in place, a few bridges needing maintenance or repair would not be dangerous, says Waldron. But the 600,000 bridges in need of work across the country tell a different story. The ASCE report estimates that it would cost $9.4 billion per year for 20 years to eliminate all bridge deficiencies in the United States. “Who is going to pay it?” asks Fouad. “Repairing or maintaining existing infrastructure is not sexy or exciting, so politicians don’t want to put money into it. The ever-increasing traffic and the underfunding for needed maintenance means the problems become more and more exaggerated.”
Some hope may be on the horizon. President Barack Obama has proposed a wide-ranging investment in infrastructure as a way to jump-start the struggling U.S. economy. Even with that help, however, design and construction will have to be faster, cheaper, and smarter, says Fouad. “We need to be looking at new materials and serious enhancements and improvements from every angle.”
Onward and Outward
Throughout American history, bridges have been sources of both pride and controversy. In fact, many of today's great American cities would never have thrived without the innovations in bridge design that connected them to a growing country.
In the 19th century, the westward expansion of the United States was made possible in large part by the development of steel, which allowed previously impassable spans to be bridged for the multiplying railroads (see “Onward and Outward”). Technical improvements in construction materials also played an integral part in the development of the Interstate Highway System, Fouad says; the vast, nation-spanning network could not have been built without the advent of prestressed concrete. Developed in Europe, this high-strength material began to catch on in the United States in the late 1940s. “You can see the advances in materials from cast-iron to structure steel to high-strength steel, then to reinforced concrete and prestressed concrete,” says Fouad. “All of these advancements led to longer spans and stronger bridges.”
With prestressed-concrete highway bridges nearing the ends of their life spans across the country, however, Fouad says that engineers need to be looking for the next innovations in materials and construction methods. At the School of Engineering, “we are working closely with the Department of Transportation to develop high-strength concrete,” says Fouad. “Regular concrete can handle 3,000 or 4,000 pounds per square inch (PSI) of pressure. We are developing 15,000 PSI concrete.” Stronger concrete would make for stronger bridges; in addition, these bridges could be constructed more quickly and more cheaply than their predecessors, Fouad notes.
Still, no matter how innovative the materials at their disposal, bridge engineers must also take into account human factors, says Fouad. “One of the biggest considerations in building a bridge is the disruption of traffic during construction,” he says. UAB engineers are also working on a solution to that problem. Fouad is the principal investigator in a study of an accelerated bridge construction method in which the main components of a span are built in a factory and transported to the site, where they are pieced together.
Once a bridge is built, what can be done to keep it in working order for decades to come? Fouad says the primary concern is not the weight placed on the bridge at any one time, but rather the intermittent strain of passing traffic. “If a well-designed bridge is stressed to its designed limits, that is really no problem,” he says. “It can carry the load for a very long time. But the on-and-off effect of loading and unloading as vehicles pass over—with the vibrations and induced fatigue—has a massive impact on the structure over time.”
Ahead of Its Time
UAB faculty and students recently designed an innovative pedestrian bridge for the nearby city of Vestavia Hills. In fact, it was so innovative that no contractor would take the project.
Fouad is working with the Alabama Department of Transportation to study a related problem: the fatigue of cantilevered highway signs. “Every time a truck passes beneath one of those signs, the sign moves up and down from the truck-induced wind,” he says. “And that wind also causes fatigue.” Signs can be engineered to withstand storm gusts fairly easily, but Fouad says the monotonous turbulence created by passing vehicles—coupled with everyday breezes—is a more difficult design challenge. These seemingly minor forces are known to be the main cause of damaging fatigue to the signs, but the extent of the wind effect is still unclear. UAB scientists have placed instruments on a highway sign in Huntsville to measure the movement created under a variety of conditions. Once the data have been collected, it will be entered into a computer program that will simulate the fatigue those movements cause over several years.
This small-scale project may eventually lead to new sensors that could be built into bridges to precisely measure stress and fatigue levels, giving inspectors a better way to pinpoint the structures most in need of repair. “Monitoring is now very important,” Fouad says. “In order to have safer bridges in the future, we have to be able to measure their health. We want to be able to go to a structure like the I-35 bridge and find out when it has begun responding differently to the traffic load.”
In addition to averting a catastrophe, such a monitoring system would reduce the need for costly repairs and extend bridge life spans. “You can’t design a bridge to last forever without maintenance,” says Waldron, “but with the new materials that have come along in the past 40 years, plus improved monitoring, maybe we can push up the next great service interval. Maybe instead of looking at major repairs 50 years down the road, it will be 75 or 100 years.
“Eventually, though, any bridge will require maintenance, and it will require an investment on society’s part to maintain it. We’re hoping that we’ll continue to be inspired to do that, and we hope it doesn’t take more catastrophes to keep the inspiration alive.”