It takes just seconds for tall buildings to collapse during
powerful earthquakes. Knowing precisely what's happening in those seconds can
help engineers design buildings that are less prone to sustaining that kind of
damage.
But the nature of collapse is not well understood. It hasn't
been well-studied experimentally because testing full-scale buildings on shake
tables is a massive, expensive and risky undertaking.
That's why researchers at the University at Buffalo and
Japan's Kyoto University teamed up recently to try an innovative
"hybrid" approach to testing that may provide a safer, far less expensive
way to learn about how and why full-scale buildings collapse.
"One of the key issues in earthquake engineering is how
much damage structures can sustain before collapsing so people can safely
evacuate," explains principal investigator Gilberto Mosqueda, Ph.D., UB
assistant professor of civil, structural and environmental engineering.
"We don't really know the answer because testing buildings to collapse is
so difficult. With this hybrid approach, it appears that we have a safe,
economic way to test realistic buildings at large scales to collapse."
The UB/Kyoto team's positive results could enable engineers
to significantly improve their understanding of the mechanisms leading to
collapse without the limitations of cost, reduced scale and simplified models
necessary for shake table testing in the U.S.
In the unusual "slow motion earthquake" test
conducted in late July, UB and Kyoto engineers successfully used the hybrid
approach (Video
here) to mimic a landmark, full-scale experiment conducted in 2007 on the
E-Defense shake table at the Miki City, Japan, facility. In that test (2007 test video), a
four-story steel building was subjected to a simulation of ground motions that
occurred during the 1995 Kobe
earthquake.
But instead of using a full-scale steel building, this time,
the researchers developed a hybrid representation of that test by combining
experimental techniques carried out in earthquake engineering labs in Buffalo and Kyoto
with numerical simulations conducted over the Internet.
The landmark data from the E-Defense test was used to verify
the effectiveness of the hybrid approach. Only the parts of the buildings that
were expected to initiate collapse were tested experimentally.
"If this had been a real building, it would have
toppled over," says Mosqueda.
That presents a real problem in a laboratory.
"You can't allow a structure to collapse completely on
a shake table," he said. "You need to have support mechanisms in
place, like scaffolds, to catch the falling structure."
The building in the original full scale test weighed more
than 200 tons. That kind of weight puts shake tables under enormous stress,
Mosqueda explains. It not only forces them to operate at full capacity, there
is the additional potential for the heavy structure to crash down on the
equipment.
"But in this case, we simulated the load with
high-performance hydraulic actuators so the specimen overall was actually
pretty light," explains Mosqueda. "We completely did away with the
hazard of having tons of weight overhead that could come crashing down. Here,
we just shut off the hydraulics and the load disappeared."
It took the U.S.
and Japanese researchers, who were communicating over the Internet, about two
hours to subject the hybrid model to the powerful ground motions that
represented approximately the first five seconds of the 1995 Kobe quake.
According to Mosqueda, the hybrid test paves the way for
additional experiments that will allow researchers to more precisely learn
about the nature of structural collapse.
"We want to know, for example, what is the probability
that a building will collapse in the next expected earthquake," says
Mosqueda. "First, we need to develop this capability to understand and
simulate how they collapse. Then we can determine how to improve new
construction or retrofit existing buildings so that they are less likely to
collapse."
The experimental part of the test involved a half-scale,
nine-foot-tall structure in UB's Structural Engineering and Earthquake
Simulation Laboratory (SEESL), while a second experimental component was
located at Kyoto University. Together, the two
experimental substructures represented the first one-and-a-half stories, while
numerical simulations represented the rest of the building.
Mosqueda explains that while reduced-scale models were used
in this preliminary test to evaluate the method, the capacity exists at UB and
other laboratories to apply this approach to full-scale buildings.
Mosqueda's colleagues on the test include Maria
Cortes-Delgado, a doctoral student in the UB Department of Civil, Structural
and Environmental Engineering, Tao Wang, Ph.D., of the Institute of Engineering
Mechanics in Beijing, and Andres Jacobson, a doctoral student, and Masayoshi
Nakashima, Ph.D., a professor at Kyoto University
These "distributed hybrid tests," were made
possible by UB, its international collaborators at Kyoto University and the
Institute of Engineering Mechanics in Beijing, and the National Science
Foundation's George E. Brown Jr. Network for Earthquake Engineering Simulation
(NEES) Facility, a nationwide earthquake-engineering "collaboratory"
of which UB is a key node.
The project is the result of a prestigious $400,000 Faculty
Early Career Development Award Mosqueda received from the NSF to develop a
hybrid simulation platform for seismic-performance evaluation of structures
that collapse.
Original article
University at Buffalo
