Nebraska civil engineers discuss the nature of bridge collapses
Calendar Icon Aug 09, 2007 Person Bust Icon By Carole Wilbeck | Engineering RSS
In the aftermath of the Aug. 1 collapse of the Interstate 35W bridge in Minneapolis, the public has realized what Atorod Azizinamini frequently tells his students: Bridges are nearly invisible--until they're no longer standing.
When an incident like the one in Minneapolis occurs, he said, the public becomes attuned to what can go wrong when infrastructure fails. In response, Azizinamini, a professor in the University of Nebraska-Lincoln Department of Civil Engineering, organized "The Layperson's Guide to Bridge Engineering," a seminar to clear up misconceptions about bridge safety and to explain what might have caused the Minneapolis bridge to collapse.
About 45 people attended the event, held Aug. 8 in the Nebraska Union auditorium. Panelists included Azizinamini, Maher Tadros and Andrzej Nowak, all UNL bridge experts; and Sam Fallaha, assistant state bridge engineer with the Nebraska Department of Roads.
It's unusual for a bridge to collapse in heavy traffic like it did in Minneapolis, Azizinamini said. More common causes are natural disasters or scouring, which occurs when water erodes the bridge's foundation.
"That may have played some role in the Minneapolis bridge," he said. A number of bridge collapses actually occur during construction, when it becomes apparent that the design is flawed or materials are deficient.
"If something bad wants to happen, it's during construction," he said. "I tell my students that all the time."
Other common causes of bridge failure are improper maintenance, overloaded semi trucks, collisions, fire, vandalism and terrorism.
Some types of bridges are more vulnerable than others. The I-35W bridge was a steel truss. If just one of the beams weakens, the entire bridge becomes unstable, Nowak said. Parts of this 43-year-old bridge were exposed to repeated tension, so fatigue could've been a factor, he said.
Nebraska has only a few steel truss bridges, most of which span over the Missouri River. Most bridges in Nebraska are the concrete slab style. "They're fairly safe," Nowak said. "If something wants to go bad, you usually have warning."
The media have reported that inspectors knew as early as 1990 that the I-35W bridge was structurally deficient, which means the condition of its deck, superstructure or substructure was rated as poor. An estimated 27 percent of the nation's bridges are structurally deficient.
However, Nowak said, that label isn't as severe as it seems.
A structurally deficient bridge isn't necessarily in danger of collapsing, he said. Bridge safety ratings are based on the inspection of individual elements. However, many bridges have safety features that work together to strengthen the structure, so just because one part is weak doesn't mean the whole bridge is, he said.
Fallaha, an engineer for the state roads department, said the good news for Nebraskans is that 94 percent of the state's bridges are structurally sound and adequate for their purpose, which is higher than the national average.
Tadros said Nebraska's structurally deficient bridges are typically located on back roads.
"They're not going to fall," Tadros said. "It simply means they can't handle loads as heavy as we'd like."
He said semi-truck drivers should avoid these bridges because the structures weren't designed to handle the weight of a loaded trailer. However, people who drive passenger cars and trucks shouldn't panic.
"If the bridge is open to traffic, by all means, don't worry about it," Tadros said.
Furthermore, every bridge in Nebraska is inspected every two years.
Nowak said when engineers design a bridge, they use a complex set of calculations to plan for worst-case scenarios. Designers over-estimate the load the bridge will be expected to handle, and underestimate the strength of the materials.
The hard part, Nowak said, is deciding how safe is safe enough.
"We can't over-design because we can't afford it (the monetary cost). But we can't under-design because we can't afford the cost (of human lives)," he said.
The ability to control the risk of a bridge collapse is in the hands of many. Nowak identified the most important risk-management practices:
- using better methods to predict loads;
- ticketing semi-drivers who carry illegal overloads on their trucks, which causes roadway fatigue;
- using durable materials;
- having more reliable structural systems;
- performing bridge inspections regularly; and
- doing preventative maintenance before a bridge is damaged beyond repair.
Azizinamini said there could be more accidents like the one in Minneapolis if the United States doesn't allocate more funding for bridge maintenance and replacement. The consequences of letting maintenance languish could be catastrophic in heavily populated areas, he said.
That will require significant financial resources. The cost of repairing the Queens Expressway in Brooklyn, N.Y., for example, is estimated at $800 million, he said.
"Most problems we may have in the future will be due to lack of funding, not lack of knowledge," Azizinamini said.
UNL HOME TO BRIDGE INNOVATIONS
University of Nebraska-Lincoln researchers have found a niche in creating prefabricated bridge components. Often made of precast, prestressed concrete, the pieces can be made offsite ahead of time and moved into place later.
That means bridge construction is faster, and drivers notice fewer traffic disruptions. The process also is less dangerous for construction crews, said Maher Tadros, UNL civil engineering professor.
Don't be fooled by the "pre-fab" name: These parts extremely durable and are designed to last even longer than traditional materials.
"These products are far superior to any product in existence," Tadros said.
Here are some of the innovations in bridge design that originated at UNL and have been used across the United States:
NU I-Girder - This system allows bridges to have long spans and shallow structural depth.
Nebraska Inverted Tee - Used for short- to medium-span bridges, the Nebraska Inverted Tee is a precast concrete beam. It is 20 percent lighter than a regular I-beam but just as strong.
NUDECK - A patented system, NUDECK is a precast, prestressed concrete panel for bridge decks. The deck can be built offsite and installed later, which speeds up construction time.
Ravenna, Neb. is home to the first tied-arch bridge in the U.S. that has post-tensioned steel tubes filled with concrete. Click here to learn more. http://www.nuengr.unl.edu/ENonline/Spring06/19.shtml