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April
11, 2007 EARTHSHAKING
IMAGES But this was no
ordinary earthquake. In a
groundbreaking series of tests, engineering researchers from UC San
Diego’s
Jacobs School of Engineering jarred a full-size 275-ton building
erected on a
shake table, duplicating ground motions recorded during the January 17,
1994
Northridge earthquake in Los Angeles, California. To record the impact
on the building, the structure
was fitted with some 600 sensors and filmed as the shake table
simulated the
earthquake, yielding a flood of data including stress, strain, and
acceleration
-- so much information that engineers were having a hard time making
sense of
it all. That’s
where visualization experts from the San
Diego Supercomputer Center (SDSC) at UC San Diego came in. “By
recreating the shake table experiment in movies
in a virtual environment based on the observed data, this lets
engineers explore
all the way from viewing the ‘big picture’ of the
entire building from a
360-degree viewpoint to zooming in close to see what happened to a
specific
support,” said SDSC visualization scientist Amit Chourasia.
“Integrating these
disparate data elements into a visual model can lead to critical new
insights.” Added
José Restrepo, a professor of structural
engineering at UCSD, “These visualizations give us an
intuitive way to see how
the building behaves in our shake table experiments -- this tool will
be very
valuable in helping us understand the tests in ways we can’t
from traditional
approaches, and also in sharing this research with other engineers and
the
public.” The costliest quake
in A paper by Chourasia
describing the project was published
in a special graphics issue of ACM
Crossroads, the
student journal of the Association for
Computing Machinery. In addition to
helping engineers understand the
earthquake’s impact on the building, the visualizations can
also give
researchers a tool to do “what if” virtual
experiments. “We found
that the recorded motion aligns very well
with the movie we created,” said Steve Cutchin, director of
Visualization
Services at SDSC. “This is important because knowing the
model is accurate
means it can be used to take simulated earthquake data and predict the
sensor
values -- you can ask, ‘What if a larger 7 point earthquake
hits?’ and simulate
how the building will shake in response.” To make the
visualizations more useful and provide
a rich visual context, the researchers wanted to incorporate
recognizable
elements from the surroundings, which meant integrating features from
the
actual video footage recorded during the test. “Our goal was
to have fidelity
not only in rendering quality but also in visual realism,”
said Chourasia. In
addition, the integrated video would let the researchers validate this
virtual
reconstruction visually. Once Chourasia and
his colleagues had developed the
building model and animated the deformation caused by the shaking, they
worked
to align the virtual camera and lighting with the real world video
camera so
that the scene would match in the recorded footage and the virtual
version. “However,
when we tried to composite the actual
video footage, we found that the instruments had sampled the data at 50
Hz but
the video was recorded at 29.97 Hz.,” Chourasia explained.
“And there wasn’t
any timing synchronization between the building sensors and
camera.” This posed
a serious hurdle for compositing. “After
viewing the video footage, we noticed that
the recording also contained audio data, because the moving building
and shake
table make noise, and this proved to be the key.” By
“listening to the
building” and analyzing the audio and sensor signals, the
researchers were able
to synchronize the video and instrument data for the visualization. In the future, the
visualization researchers plan
to develop lighting models for more realistic rendering and to find
automated
ways to match the real and virtual cameras. They are also distilling
lessons
learned from this study into requirements for a visualization workbench
for
analysis of the dissimilar types of data that come from structural and
seismic
experiments. In addition to
providing visualization services for
the shake table experiments, SDSC is also home to the NEESit Services
Center
(NEESit) which is developing and maintaining a state-of-the-art grid to
meet
the cyberinfrastructure needs of the earthquake engineering community,
including the UCSD facility and 14 other collaborating experimental
sites
across the nation. This
research was supported by the National Science Foundation.
http://ucsdnews.ucsd.edu/newsrel/supercomputer/04-07Earthquake.asp
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