By Eric M. Weiss
Washington Post Staff Writer
Sunday, August 5, 2007
The 40-year-old bridge that collapsed in Minneapolis last week was built during an era when designers were confident they knew enough about bridge strength and weight loads that they could build bridges lighter and cheaper.
But a number of bridge collapses have taught engineers painful lessons about the frailty of bridges and the punishment they take from heavy trucks, strong tides and even the errant barge that slams into bridge supports, according to engineers, bridge builders and academics.
The challenge, they say, is that many of the nation's 594,709 bridges were built during the 1950s and 1960s, an era when designers didn't fully understand the effects of metal fatigue or other challenges. Now, many of those bridges are facing increased scrutiny.
"Maybe we out-thought ourselves for a little while," said Mal Kerley, Virginia's chief engineer, referring to postwar bridge-building when the interstate highway system was created. "What has happened over time is that we learned things."
In 1883, the Brooklyn Bridge opened to horse-and-buggy traffic. Now it carries more than 144,000 vehicles a day. Because engineers on the Brooklyn Bridge and other spans did not have sophisticated methods to accurately calculate loads, they made their best guess -- and then multiplied. The result was over-engineered, overbuilt structures that will probably outlast their younger brethren.
"The smaller the amount of knowledge you have, the bigger the factor of safety you use," said Joseph Yura, emeritus professor of civil engineering at the University of Texas. "In those days, you would do the calculations, and the chief engineer would just up everything by one size."
Today, bridge designers are again taking a more conservative approach. Spans are built with stronger materials, redundant support systems and super-strength steel that can withstand the pounding of thousands of vehicles a day.
But after World War II, with steel prices as high as the nation's confidence, engineers thought they could "minimize materials and maximize stresses," Kerley said.
The result was structures such as the Frederick Douglass Bridge over the Anacostia River. Built in 1950, the Douglass Bridge sits on only two main horizontal steel beams bolstered by vertical supports. The main beams were so thin that engineers at the time attached numerous vertical "stiffeners" to keep the beams from bending.
The stiffeners required a lot of manual welding, but "steel was more expensive and labor was less expensive" at the time, said Kathleen Penney, the District Department of Transportation's deputy chief engineer and director of a $27 million refurbishing of the bridge.
If the Douglass Bridge were built today, she said, the steel used would be 25 percent thicker, there would be more than two main beams in case one failed, and there would be far fewer welds and connections to minimize the number of corrosion and fatigue points.
"Collectively, there was more confidence in using less material," Penney said. "Since that time, we've had a lot of experiences that pushed us back to more conservative approaches."
One of those key experiences took place Dec. 15, 1967, during the beginning of the afternoon rush hour, when the 39-year-old Silver Bridge -- which connected Point Pleasant, W.Va., with Kanauga, Ohio -- collapsed, killing 46 people.
Within one minute, according to the National Transportation Safety Board report, the 700-foot center span, two 38-foot side spans and both towers collapsed into the Ohio River.
The investigation found that a crack caused by metal fatigue in a single piece brought down the entire bridge. Metal fatigue occurs when steel is flexed -- by the weight of 80,000-pound trucks, for example -- over and over until it becomes brittle and fails. It is the same principle as bending a paper clip back and forth: At first it bends, but eventually it breaks.
"That was the first time we as engineers really became aware of the elements of fatigue and fracture," said Sue Lane, an engineer and manager of codes and standards with the American Society of Civil Engineers. Today, she said, bridges are built with fewer joints, stronger and more flexible steel and greater redundancy.
In June 1983, a 100-foot section of Interstate 95 collapsed into the Mianus River in Greenwich, Conn., killing three people. The NTSB found that metal fatigue, corrosion and a lack of rigorous inspections contributed to the failure of one of the bridge's two main supports, which then brought down the rest of the structure. The bridge was built in the mid-1950s.
Today, bridges are built with multiple structural elements, so the failure of one piece doesn't lead to a total collapse.
"Redundancy is all about sharing," Lane said. "If four of us are moving a piano and one of us falls, maybe three will be able to hold it up. But not if there are only two carrying it."
The collapses led to changes in how bridges were built and inspected. Today, bridges that are "fracture critical" -- meaning that they have at least one piece that, if it gave way, would lead to a complete collapse -- are subject to more intensive inspections. These include bridges with only two main girders, such as the Douglass Bridge, and most truss bridges, including the Interstate 35W bridge in Minneapolis, which relied on two main 988-foot structural elements.
Kerley said that although many of the nation's postwar bridges might not be as sturdy as the Brooklyn Bridge or as technologically advanced as the new Woodrow Wilson Bridge over the Potomac, the safety record of America's spans is still impressive.
He pointed to the old Woodrow Wilson Bridge, a six-lane span built in 1961. It was designed for 75,000 vehicles a day but was carrying more than 225,000 a day before it was replaced a year ago.
It was built in an era when they tried to maximize stresses," Kerley said. "It really took a beating for what it was designed for, but look what it did."
Since it would be impossible to immediately replace all of the nation's bridges built in the 1950s and 1960s, the key is to stay on top of any problems, Kerley said. Today, inspectors use ultrasound and X-ray technology to detect cracks, corrosion and other defects.
"It means that you have to inspect those bridges and make sure all the members are intact and there is no corrosion," Kerley said.
Even with today's technology, though, there are still lessons to be learned.
"We have learned from history and unfortunately, one of the ways we do learn from history is by learning from our failures," Lane said. "Regrettably, they come along with a loss of life, and that happened this week. The best we can do is pray for the families and try to learn why this has happened."