The Christchurch, New Zealand quakes are evidence that quakes are brutal and require careful thought regarding design.
The following feature from China Daily, shows what is being done to ensure the stability of a bridge in the United States.
How to make a bridge to sway with limited damage when the earth moves
The engineers of the new San Francisco-Oakland Bay Bridge have planned for the long term.
At intervals inside the $6 billion span’s box girders are anchor blocks, called deadmen.
They are meant to be used perhaps in the next century, when the concrete girders start to sag.
By running cables from deadman to deadman and tightening them, workers will be able to restorethe girders to their original alignment.
The deadmen are one sign that the new eastern span of the Bay Bridge is meant to last at least 150years after its expected opening in 2013. (The existing eastern bridge will then be torn down.)
At some point, the new span may have to survive a major earthquake, like the one that destroyedmuch of San Francisco in 1906 or the one that partly severed the Bay Bridge in 1989.
Keeping the bridge intact in an earthquake is the engineers’ chief goal. So they are designingflexible structures in which any potential damage would be limited to specific elements.
“The flexibility of the system is such that it basically rides the earthquake,” said its lead designer,Marwan Nader, a vice president at the engineering firm T. Y. Lin International.
Another potential approach, making the bridge structures large enough, and rigid enough, to resistmovement, was rejected.
The new design includes a 160-meter-tall suspension bridge tower made up of four steel shafts thatshould sway in a major quake, up to about 1.5 meters at the top. But the brunt of the force wouldbe absorbed by connecting plates between the shafts, called shear links.
The bridge’s concrete piers are designed to sway as well, limiting damage to areas with extra steelreinforcing. And at joints along the entire span there are 18-meter sliding steel tubes, called hingepipe beams, with sacrificial sections of weaker steel that should help spare the rest of the structureas it moves in a quake.
“At the seismic displacement that we anticipate, there will be damage,” Mr. Nader said. “But thedamage is repairable and the bridge can be serviceable.”
It was the Loma Prieta quake of 1989 that made this 3.5-kilometer replacement span necessary.The 6.9-magnitude quake caused part of the existing span to collapse, killing a motorist and closingthe bridge for a month. That quake caused movement far greater than the 1930s-vintage bridgehad been designed to handle. Most experts believe a stronger quake could cause a total collapse ofthe span.
Unlike more conventional suspension bridges, in which parallel cables are slung over towers andanchored at both ends in rock or concrete, the 624-meter suspension bridge has only a singletower and a single cable that is anchored to the road deck itself, looping from end to end and back. (With a conventional design it would have been extremely difficult to create an anchorage on thebridge’s eastern end, in the middle of the bay.)
The new bridge is the longest self-anchored suspension bridge in the world, with one side of thespan longer than the other.
In a self-anchored design, the road deck has to be built first. “You have a kind of chicken-and-eggsituation,” Mr. Nader said. “You need the deck to carry the compression so that the cable anchorsinto it, but the deck can’t carry itself until the cable is there to carry it. So you have to build atemporary system,” which needs to be seismically secure as well.
The single tower created design problems. It’s “just like a pole,” he said. “If you have a pole and thepole starts shaking, all the damage will occur at the bottom.”
The solution was to split the tower into four shafts and tie them together with the shear links.
The links are of a special grade of steel that deforms more easily. Their placement at points alongthe length of the tower affects how the shafts will move in a quake.
Mr. Nader said the shear links about two-thirds of the way up would be most damaged in a majorearthquake. But the tower would still be structurally sound, he said, and the links would not have tobe replaced immediately.
It’s like a fender bender, he said. “Your car is perfectly drivable, and it’s designed that way, with abumper that can take the shock.
“So you basically stop, just to make sure,” he went on. “You see everything’s O.K., and you cancome in anytime you want to repair your bumper.”
The New York Times