When bridges collapse suddenly, flaws in design or construction, or inadequate inspection and maintenance, are usually to blame. In the case of the Morandi Bridge in Genoa, Italy, which stood for half a century before falling apart Tuesday in a tangled heap of concrete and steel, both design and maintenance may be at fault, experts said.
The bridge, named for its designer, Riccardo Morandi, is of the cable-stayed type, in which sections of roadway are cantilevered from towers like diving boards and supported by stays. This type of design, more common now than it was 50 years ago, is often used when the gap to be spanned is too long for the structure to be supported at both ends, but not so long that a suspension bridge would be a better solution.
The stays are critical elements of this kind of design. Without them, the cantilevered sections would have to be so massive as to make the bridge too costly or otherwise impractical. And more than anything else, the stays are what made the Morandi bridge unusual: They are constructed of concrete with steel rods, called tendons, inside them, which enables the concrete to handle the pull of the roadway. More modern cable-stayed bridges use steel cables instead.
“This type of stay or cable is not very common,” said Bassem Andrawes, a professor of civil engineering at the University of Illinois. “You would rarely see them anywhere.”
What may be even more important in regard to the collapse is that the Genoa bridge only had two stays — one on each side — for each cantilevered roadway section. Most recent bridges use far more stays: The new replacement for the Tappan Zee Bridge across the Hudson River north of New York City, for example, has 24 cables for each section, 12 on each side.
Dr. Andrawes and others said it is too early to speculate on the cause of the disaster, which killed at least 38 people, and basic facts of the collapse are still unclear — whether it began with one of the stays, for example, or on a cantilevered section, or in the tower itself.
But Italian investigators will no doubt look at both the makeup of the cable stays and their number. If one stay failed, that could have caused a shifting of loads that led to a cascade of rapid failures of other structural elements of the span. A bridge with multiple stays would be better able to withstand the loss of one of them.
“We design structures to be as redundant as possible,” Dr. Andrawes said. Speaking of the Italian bridge, he added, “Obviously if something was to happen with one of these cables, the bridge will collapse.”
One area of much research in modern bridge design is how to stop cascading failures somewhere along the chain by designing robust elements that can handle the excessive loads that might result from the failure of some part of the structure.
Design or construction flaws can remain undetected for years, until a disaster occurs. The collapse of an interstate highway bridge in Minneapolis that killed 13 people in 2007, for example, was attributed to undersized gusset plates: half-inch thick sheets of steel that connected girders and other structural elements of the bridge. The flawed plates had been part of the bridge since it was built four decades earlier. But additional weight on the bridge — including an extra layer of concrete added to the surface in the intervening years — might have finally caused one plate to tear apart, setting off the collapse.
In Genoa, there was apparently no lack of concern about the bridge design and potential flaws that could lead to disaster, especially because traffic loads were far greater than when the structure opened in 1967. A professor of structural engineering at the University of Genoa, Antonio Brencich, pointed out some of the bridge’s problems two years ago and suggested that it would eventually have to be torn down and replaced.
For years, the private company that operated the bridge made efforts to inspect, monitor and maintain it, but these efforts were apparently insufficient to avert a disaster, experts said.
Guy Nordenson, a structural engineer who heads his own firm, said it appeared that much infrastructure in Italy — as elsewhere, including the United States — was in poor shape, especially compared with other elements of transportation networks.
“If you think about airplanes, if you did not maintain an airplane, then you’d start to have problem,” Mr. Nordenson said. “There are differences in standards that we hold infrastructure to as opposed to other engineered things like automobiles or planes.”
Compounding the problem, he said, is that the Genoa bridge was part of a privatized highway system. Although the company that operated it, Autostrade per l’Italia, said it spent a billion euros in the last five years on maintaining and upgrading its highways, that might not have been enough in this particular case.
“If you privatize infrastructure, then you’ve brought in the profit motive,” Mr. Nordenson said. “You start to have a conflict of interest between the good of the public and the good of the company.”
There are other reasons why bridges collapse besides design and maintenance. Earthquakes can shake structures apart, and many bridges have been reinforced or rebuilt after quakes, with the $6 billion replacement for the eastern span of the San Francisco-Oakland Bay Bridge, which opened in 2013, the most expensive example.
Collisions with ships or vehicles occasionally bring down bridges. In 1980, in Tampa Bay, Fla., a tanker struck the Sunshine Skyway bridge, causing part of the span to collapse and killing 35 people. Another Morandi cable-stayed bridge, on Lake Maracaibo, Venezuela, partially collapsed in 1964 after being hit by a tanker. Five people drowned.
Floodwaters can also undermine bridge foundations, leading to collapse. In 1987, 10 people were killed when a bridge over a creek on the New York State Thruway collapsed after raging waters in the creek scoured away the soil around one of its supports.
There was a violent storm at the time of the Genoa collapse, but there is no suggestion yet that it played a role in the disaster.