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Earthquakes: Seismic test data that could save lives where earthquakes strike
By Christopher Jones
Minutes before dawn’s
first light on December 26, a 6.7 magnitude earthquake struck
the southeastern Iranian city of Bam, severing power lines and
transforming nearly every one of that ancient city’s structures
into rubble. Lost too were most of the city’s historic quarter,
26,000 lives, and the city’s 2,000-year-old mud-brick citadel—a
world heritage site and the largest mud-brick structure in the
world.
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“The
major objective of our work as earthquake engineers is to
preserve life,” says Professor Mosalam. “We do
this by providing ideas for new construction and offering
ways to strengthen existing construction.”
LENNY GONZALEZ PHOTO |
Despite the fact that several major fault lines crisscross Iran,
few buildings there are built to withstand earthquakes. In the
last half-century, 11 earthquakes have killed 100,000 people in
Iran. It’s just one country among many that is woefully
unprepared for the wrath nature wields when the earth shakes.
In much of the world, as in Iran, houses are built of mud-brick.
It’s cheap and keeps homes cool in the summer and warm in
the winter. But it buckles and collapses in even a moderate temblor.
“Mud-brick is the oldest construction material around, and
in many situations it works well,” says Berkeley civil and
environmental engineering professor Khalid Mosalam, “but
not in an earthquake. Earthquakes move the ground in all directions,
especially horizontally, leading to huge shearing forces. As soon
as the bricks bend, they crumble. They can’t carry the tension
of bending and shearing forces.”
In 1999, Mosalam traveled to northwest Turkey as part of a Pacific
Earthquake Engineering Research Center (PEER) team visiting the
site where a 7.4 magnitude earthquake decimated a densely populated
city, leaving more than 17,000 people dead, 300,000 people homeless,
and 350,000 buildings damaged or destroyed. Walking through a
blighted urban area, he stopped to talk with an elderly man standing
in front of what had been his home. The man showed Mosalam the
small window through which he’d escaped, and then revealed
that his family had been trapped inside.
Recalling how many times he’d heard similar tales in other
areas devastated by earthquakes, Mosalam shakes his head. “Visiting
damaged sites for reconnaissance work is an invaluable learning
experience because this is the real laboratory. But it’s
an extremely hard task. When people see us, they think we’re
there to fix their homes. We go to collect data and to learn from
the damage we see,” says Mosalam, who has visited earthquake
sites around the world for the last 12 years. “We don’t
have immediate answers for them. It takes a very long time to
see results from our work. You have to do an awful lot of number
crunching to turn this research into information that can be used
in new engineering designs.”
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Packing
some 95,000 pounds, this specimen is one of the largest and
bulkiest ever tested on a shake table. Once hoisted onto the
table, the team added a full-scale interior masonry wall,
80 sensors, and 150 strain gauges to measure the stresses
and forces endured during months of testing.
LENNY GONZALEZ PHOTO |
Mosalam is spearheading a critical research effort at Berkeley
funded by the National Science Foundation that could save lives
and preserve masonry buildings during earthquakes. With his current
focus on masonry—from 800-year-old stone mosques in the
Middle East to new brick buildings in San Francisco—he is
evaluating how masonry behaves under simulated earthquake conditions.
An Egyptian native, Mosalam speaks of the awe he feels for the
ancient structures that dominate the landscape of his childhood.
“The oldest existing structures in the world are the great
Egyptian pyramids,” he says. “The ancient practice
of building with masonry is one of the most beautiful building
techniques. Yet masonry is one of the least understood materials,
and that lack of understanding can lead to devastation, as it
did in Bam.”
To pick apart the resilience and the pitfalls of masonry, Mosalam
and CEE graduate students Alidad Hashemi and Tarek Elkhoraibi
have spent the better part of a year designing a series of earthquake
simulation tests. Using state-of-the-art equipment at UC Berkeley’s
Richmond Field Station (part of the nationwide Network for Earthquake
Engineering Simulation), the team developed two distinct tests
to understand how a five-story building performs when the ground
begins to rumble.
In preparation for their tests—designed to run over a seven-month
period this spring—the team built two near-identical specimens.
The first was a single-story structure, meant to represent the
first of five stories, with masonry walls much like American buildings
from the early 1900s and low-cost apartments in Third World countries.
This specimen has six reinforced concrete columns that form three
frames, including a vertical masonry wall down the middle. The
second is composed of two separate frames, one of which has an
unreinforced masonry infill wall. Built at 75 percent scale, these
are among the largest specimens ever tested on a shake table.
To simulate the realities of a lived-in multi-story building,
the team stacked some 57,000 pounds of lead weights—simulating
the weight of people, furniture, and other building contents—atop
the first structure. They’ll learn from the hundreds of
sensors and gauges meticulously placed inside and out what happens
as the structures endure the stresses of many different kinds
of simulated earthquakes. The sensors will measure the forces,
accelerations, deformations, and strains that occur during the
tests.
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Tarek
Elkhoraibi exhibits the hydraulic actuators he’s using
to run a series of pseudodynamic seismic tests this spring.
LENNY GONZALEZ PHOTO |
One of Mosalam’s primary goals in this project is to measure
how and why ‘hybrid’ structures—concrete and
masonry in this case—break down during an earthquake. “Engineers
often think of masonry as one of the nonstructural components
of a building,” Mosalam says. “But building with masonry
stiffens a structure, which attracts more forces that may damage
it.”
Mimicking nature’s
wrath
The first specimen was put to the test on the field station’s
20' x 20' shake table—the largest and most powerful of its
kind in the country. Really a huge slab of reinforced concrete,
the shake table sits above an airtight pit containing 12 industrial-grade
hydraulic actuators. Similar to giant jacks, the near-10-foot-long
actuators use an oil-powered piston chamber to apply thousands
of pounds of force to the table. The actuators, which are programmed
to produce various motions on horizontal and vertical planes,
simulate the unique forces and accelerations of earthquakes.
Prior to initializing the series of tests, the pit is pressurized
to balance the total weight of the table and structure against
the difference in air pressure in the pit and the ambient air
above. Picture a massive concrete slab plus its experimental specimen
balancing on a balloon. When the test begins, the specimen is
put to the test, shaking violently for about 30 seconds, a sequence
that is repeated over and over at increasingly strong magnitudes.
In the fine art of simulated building destruction, another test
yields even more refined data than the shake table yields. While
a shake table test is fast and explosive, the pseudo-dynamic test
is a more controlled, computer-intensive process.
In this test, several 11' x 9' reinforced concrete blocks are
anchored in the middle of the simulation lab to form a wall where
actuators are mounted and supported to form a reaction wall. On
one side, a set of hydraulic actuators is bolted into the reaction
wall. Each customized, industrial-sized actuator can apply up
to 220,000 pounds of push-and-pull, lateral force against the
structure, up to 1,000 times during one test—mimicking the
grains of tectonic plates rubbing together to create shocks. On
the other end, the actuators are bolted into the test structure.
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With
the help of five engineering students last summer, Alidad
Hashemi prepared and embedded more than 200 sensors from which
he’ll extract seismic information from this year’s
shake table experiments.
LENNY GONZALEZ PHOTO |
The team used actual earthquake data to program the simulation,
in this case the 1940 El Centro and 1994 Northridge earthquakes,
which exhibited very different types of forces, frequencies, and
periods. To simulate a 30-second earthquake, the software program
divides the data into timed steps. At any individual step, the
actuators can perform the corresponding stroke—each at different
lengths and weight loads—to mimic the dynamics of the earthquake.
They do this in controlled time frames rather than in real time.
Using customized computational tools, Mosalam and his team can
simulate behaviors in the structure, but with the pseudo-dynamic
test, they have the advantage of controlling the time frames,
slowing down the motion, even calling a temporary halt to take
a read on the emerging data or simply to mark cracks as they appear.
“It gives us an enormous advantage,” says Mosalam.
“With the shake table, you tell it to move in a recorded
motion, it takes 30-40 seconds, and it’s done,” he
says. “Here we have much more control.”
Transparent coating
binds bricks together
Among the most critical factors in evaluating how a building gets
through a quake is its ductility—the ability of materials
to deform without losing too much strength. Mosalam hopes these
sophisticated tests will provide information about how existing
walls might be reinforced to ensure that they will undergo more
deformation, or gradual rather than sudden disintegration, should
an earthquake hit.
To that end, Mosalam and his students have been testing a material
that could handily manage such a feat. Last spring, they ran a
series of shear tests on small structures they’d ‘painted’
with a fiberglass reinforced polymer (FRP) coating composed of
fibers and resins. These mini-shake tests confirmed that the affordable,
easy-to-apply transparent coating significantly minimized building
damage.
“FRP can be an economical way of strengthening new or deteriorating
structures by applying a thin layer of polymer material, followed
by an epoxy that binds and transfers stress between fibers,”
says Mosalam. “Because masonry walls crumble so quickly
under earthquake forces, FRP can be an extremely effective and
economical retrofitting technique, working the way straw, once
routinely added to ancient mud-brick as a binder, functioned.”
In this year’s tests, the team will apply the FRP after
the walls are damaged, then recreate the tests to see how well
it strengthens them.
The impact of this type of research in countries like Iran,
which is surrounded by tectonically active zones and experiences
earthquakes on all sides, could be enormous, according to Alidad
Hashemi, a native of Iran. He recalled the day in 1990 when the
7.3 magnitude Manjil-Rudbar earthquake struck in the mountainous
Gilan Province in northern Iran, killing an estimated 40,000 to
50,000 people. At his apartment in Tehran that day almost 150
miles away from the epicenter, he remembers the water in the pool
jumping violently and a nearby wall cracking. Should an earthquake
strike closer to densely populated Tehran, he fears the damage
and death toll could be considerably worse.
“The devastation in Bam a few months ago shows just how
much work there is to be done,” says Hashemi. “Tehran
and its surrounding cities are a huge concern. It’s a place
where 20 million people live, close to one-third the population
of Iran. I know of studies now that show terrible results if there
is a strong earthquake. The casualties are estimated at more than
700,000, with up to 70 percent of the city’s buildings destroyed.
The city’s infrastructure would be devastated.”
In fact, in light of Bam’s recent tragedy—now considered
one of the deadliest natural disasters of modern times—Iranian
authorities are considering what it would take to move the capital,
which sits above a major seismic fault, from Tehran to a safer
location, perhaps Isfahan in the center of the country.
“But,” says Mosalam, “there are engineering
solutions that are not nearly as drastic, that are realistic,
and really would take very little to accomplish. If you have vulnerable
structures, you have to find a way to mitigate their collapse.
Tying the roof to the walls in intelligent and suitable ways can
do it. When you look at Istanbul, which has seen so many earthquakes,
buildings are still standing. As earthquake engineers, it’s
our hope that we can identify potential hot spots where strong
earthquakes are likely to hit, study those areas, and introduce
building strengthening techniques.
“We must keep learning about the structures that survive
earthquakes. An earthquake of 6.7 magnitude like Bam’s shouldn’t
have killed 26,000 people,” adds Mosalam. “I wouldn’t
say the toll could be brought to zero, but the loss of lives could
be much lower. There are so many areas vulnerable to earthquakes—India,
Japan, Afghanistan, Turkey, China, and of course here in California.
It’s why it’s so important to look closely at what
happened in Bam.”
Christopher Jones, a science
writer at the Lawrence Berkeley National Lab, was formerly technology
editor at Wired News and senior writer at Webmonkey.
See related story on Berkeley architecture professor Gary
Black's method for reinforcing buildings at http://www.berkeley.edu/news/media/releases/2004/01/28_black.shtml
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