At intervals inside the elevated roadway’s box girders — which have the
closed-in feel of a submarine, if a submarine were made of concrete —
are anchor blocks, called deadmen, cast into the structure. They are
meant to be used decades from now, perhaps in the next century, when in
their old age the concrete girders will start to sag. By running cables
from deadman to deadman and tightening them, workers will be able to
restore the girders to their original alignment.
The deadmen are one sign that the new eastern span
of the Bay Bridge, which includes the skyway and a unique suspension
bridge, is meant to last at least 150 years after its expected opening
in 2013. (The existing eastern bridge, which is still in use, will then
be torn down.)
But to make it to the 22nd century, the new span may 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.
With two faults nearby that are capable of producing such large quakes,
survival is no simple matter.
Say what you want about the project — and as the construction timeline
has lengthened past a decade and costs have soared over $6 billion,
plenty has been said — keeping the bridge intact in an earthquake has
always been the engineers’ chief goal.
And to meet that goal, they are going 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,” said its lead designer, Marwan Nader, a vice president
at the engineering firm T. Y. Lin International.
That contrasts with another potential approach: making the bridge
structures large enough, and rigid enough, to resist movement. “Massive
and stiff structures would look absolutely ugly and be very, very
expensive,” said Frieder Seible, dean of the Jacobs School of
Engineering at the University of California, San Diego, who tested many
elements of the bridge design.
That design includes a 525-foot-tall suspension bridge tower made up of
four steel shafts that should sway in a major earthquake, up to about
five feet at the top. But the brunt of the force would be 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 steel reinforcing. And at joints along the
entire span there are 60-foot sliding steel tubes, called hinge pipe
beams, with sacrificial sections of weaker steel that should help spare
the rest of the structure as it moves in a quake.
“At the seismic displacement that we anticipate, there will be damage,”
Mr. Nader said. “But the damage is repairable and the bridge can be
serviceable with no problems.”
Emergency vehicles and personnel, at the least, should be able to use
the bridge within hours of a major earthquake, after crews inspect the
structure and make temporary fixes, like placing steel plates over
certain joints. Given that the Bay Area’s two major airports would be
expected to be out of service after such a disaster, this bridge and the
Benicia-Martinez Bridge, another seismically secure span about 20 miles
to the northeast, would be “lifeline” structures to bring assistance to
the stricken region from an Air Force base inland, said Bart Ney, a
spokesman for the California Department of Transportation.
It was an earthquake that made this replacement span, which runs for 2.2
miles between Oakland and Yerba Buena Island in the middle of San
Francisco Bay, necessary. The Loma Prieta quake of 1989, the first to
occur along the San Andreas fault zone since the 1906 disaster, caused
part of the existing steel-truss span to collapse,
killing a motorist. The bridge was closed for a month. That quake, with
a magnitude of 6.9, caused strong shaking that lasted about 15 seconds
and movement far greater than the 1930s-vintage bridge had been designed
to handle.
Πηγή: New York Times
No comments:
Post a Comment