MINNEAPOLIS – A house of cards is how some engineers describe the steel-truss system used on the Interstate 35W bridge that collapsed into the Mississippi River, saying that almost any piece of the complex design that failed would have brought down the entire span.
Experts said Thursday that it was too early to speculate about the specific causes of Wednesday’s bridge failure, but most previous failures of bridges over rivers have been for a handful of known reasons: weak foundations, corrosion, metal fatigue, ship collisions and errors in the original design.
The bridge in Minneapolis, just south of downtown, was being closely watched by transportation officials after studies warned about its deteriorating condition. Six years ago, significant corrosion and cracking were found under the roadway, but a top expert in metal fatigue concluded that they did not pose a safety hazard.
The Interstate 35W collapse also occurred during a construction program to replace the roadway, another red flag to many experts who theorized that vibrations or unintended disruptions to the truss structure under the roadway could have weakened the bridge so much that it could not bear the load of rush-hour traffic Wednesday evening.
“We thought we had done all we could,” Dan Dorgan, an engineer for the Minnesota Department of Transportation, said Thursday. “Obviously, something went terribly wrong.”
Thousands of steel-truss bridges were built in the 1950s and 1960s interstate boom, because they can support large traffic loads with minimal amounts of steel. Engineers stopped designing them long ago, because they require so much labor to build, require high maintenance and were eclipsed by newer technologies.
The I-35W bridge was entirely supported by two main trusses, composed of many small pieces of steel bolted or welded together like a child’s erector set. Though it is possible to design a steel-truss bridge with redundancy, the I-35W bridge was supported only by two main trusses that were both essential to its integrity.
“A truss-arch bridge is like a chain – if you try to take out one link, you lose the whole system,” said Abolhassan Astaneh-Asl, an expert in steel-truss bridges at the University of California, Berkeley. “They are very vulnerable to instability.”
Astaneh-Asl compared a steel-truss system to a house of cards, which will collapse quickly if one card is pulled out.
The risk was demonstrated in 1967, when a steel-truss bridge collapsed over the Ohio River, killing 46 people, said Kent Harries, a civil engineer at the University of Pittsburgh. Not long afterward, an association of highway engineers banned any new bridges that could collapse if a single element failed, so-called non-redundant structures.
The I-35W bridge was completed in 1967.
The bridge collapse was caught on a surveillance video. It shows the bridge’s 458-foot main span dropping straight down and within seconds the west and east spans buckling and collapsing.
“The real notable thing is the speed and completeness of the collapse,” Harries said.
A number of bridges collapse each year in the United States – though seldom as dramatically as occurred Wednesday.
Of the roughly half-million bridges that cross water, about 1,500 collapsed between 1966 and 2005, according to Jean-Louis Briaud, a Texas A&M University civil engineer and expert in bridge failure. About 900 collapses occurred when rivers scoured the foundations of bridge support piers, he said. A few dozen deaths each year occur when bridges collapse.
Briaud would not rule out scour in the I-35W collapse, even though the piers rest on the river banks and the main span covers the entire width of the river. Scour still could have occurred on the banks and caused the piers to slowly tilt over time, he said.
Divers pressed their search Thursday for victims of the catastrophic collapse as dozens of distraught families awaited word on missing relatives.
At least four people were confirmed dead Wednesday and 79 were reported injured when the bridge suddenly gave way. As bumper-to-bumper rush-hour traffic crawled over it, a nearly 500-foot span of the truss bridge crashed into the river about 64 feet below, bringing dozens of vehicles down with it.
Police said the death toll was certain to rise with the recovery of bodies trapped in vehicles under tons of concrete or in the murky river. The recovery effort proceeded slowly, impaired by strong currents, stiff winds and perilous wreckage, including chunks of concrete, mangled steel beams and a jumble of cables and electrical wires.