If earthquakes are darts, the California dartboard was hit 18 times
in the past 20 years with impacts of magnitude 6 or greater. UCSD
researchers who analyze the effects of earthquakes on buildings and
bridges are about to create their own experimental darts at a massive “shake
table” under construction near campus. The Large High Performance
Outdoor Shake Table, to be completed by October 2004, will replicate
the motions of actual earthquakes.
“
It’s really important to simply watch one of these shake-table
experiments and see with your own eyes what happens,” says
Frieder Seible, dean of the UCSD Jacobs School of Engineering.
The table is being constructed with a $5.9 million grant from the
National
Science Foundation at the Camp Elliot field station eight miles
east of the campus near I-15. Structures as high as 20 stories,
and weighing up to 2,200 tons, will be built
on the table and jolted by two powerful hydraulic arms.
“
Earthquakes cause building failures that start in connections,
joints and other components, but you can’t properly downscale
all of those connections when you scale down buildings,” says
Seible. “This
is why full-scale tests are so important.”
The table will
also be used to test bridge abutments, embankments and foundations
in collaboration with the California Department
of Transportation. However, the first shake-table experiment at
Camp
Elliot may involve a collection of casks—the ones in which
the U.S. Department of Energy is planning to store spent nuclear
fuel at Yucca Mountain, Ariz. The
D.O.E. has tested the casks in laboratory simulations but, says
Seible, “They
need a full-scale test to see if their models are correct.” The
casks, which vary from 5 to 15 feet in diameter and weigh between
25 and 150 tons, are made of steel and lead. Some of the smaller
ones will be trucked
to the
Camp Elliot site for testing.
Three years ago, at the existing
shake table on the UCSD campus, Seible watched a two-story wood-frame
house go through a simulation
of the Northridge quake.
It survived, although its contents were
thrown around. “I’ve
been preaching for years about safety precautions we should take
here in California,” says Seible. “Guess what I did
after that test? I went home and tied my water heater and bookcases
to
the wall.” The odds are relatively low that Seible’s
house will be hit, but Californians can be sure that buildings
and bridges are likely to be near the bull’s-eye of the next
big one. |
