Hugh Pickens writes writes "Henry Fountain writes that the new skyway of the San Francisco-Oakland Bay Bridge, being built at a cost of over $6 Billion, is designed to last at least 150 years after its expected opening in 2013 and that means the new span will at some point have to survive a major earthquake like the one that destroyed much of San Francisco in 1906 or the one that partly severed the Bay Bridge in 1989. To meet that goal, engineers are planning for the largest motions expected to occur within 1,500 years and have gone with the flow: designing flexible structures in which any potential damage would be limited to specific elements. “We wanted to make this bridge flexible so that when the earthquake comes in, the flexibility of the system is such that it basically rides the earthquake,” says lead designer Marwan Nader. The design includes a 525-foot-tall suspension bridge tower that should sway up to about five feet at the top in a major earthquake. The single tower is split into into four shafts tied together with shear links constructed out of a special grade of steel that deforms more easily than other grades that are placed at specific points along the length of the tower. Nader already knows which shear links will be most damaged in a major earthquake — those that are about two-thirds of the way up the tower. “Based on where you place the shear links, you can tune the dynamic response of your tower,” says Dr. Seible, of the University of California, San Diego. The shear links, measuring 6.6 to 9.8 ft. long and 3.3 ft. deep, stiffen the tower such that it will not sway excessively in a major earthquake but more importantly, the links are like 'structural fuses' that are made to dissipate energy in a seismic event, like what happens after a fender bender. “Your car is perfectly drivable, and it’s designed that way, with a bumper that can take the shock. So you basically stop, just to make sure,” says Nader. “You see everything’s OK and you can come in anytime you want to repair your bumper.”"
If it happens once, it's a bug. If it happens twice, it's a feature.
If it happens more than twice, it's a design philosophy.