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Testing, testing: new dorms' seismic foundation
system passes muster
By Jessica M. Scully
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| Berkeley architects,
engineers, and administrators teamed up last fall to test
the seismic strength of foundation piers at a campus construction
site. Using an innovative dynamic testing device, above, engineers
collected and analyzed data that will lead to more effective
retrofitting and lower construction costs. Nick Sitar photo |
A series of seismic tests on drilled piers, conducted in a campus
parking lot, could help existing Berkeley structures ride out
quakes more effectively while cutting costs for constructing earthquake-safe
buildings in the future.
The Berkeley Seismic Review Committee (SRC) launched the project
last fall as part of its continuing effort to lower the cost and
improve the effectiveness of seismic design on the Berkeley campus.
Funded by Berkeley's Capital Projects, the office that oversees
campus construction, with participation by the Pacific Earthquake
Engineering Center (PEER), the tests assessed the performance
of drilled cast-in-place piers, not in a lab, but at an actual
campus construction site.
"While drilled cast-in-place piers are a long-established
part of constructing earthquake-safe building foundations,"
says Michael Ordonia, project manager with Capital Projects, "the
twist here is that rather than relying on scientific guesses as
to the capacity of the piers, these piers were actually tested
in situ to measure their ultimate capacity."
Supervised by structural engineering firm Rutherford and Chekene,
the tests took place at the Underhill Parking Lot on Bowditch
and College, adjacent to dormitory Units 1 and 2. The team used
the parking lot, nestled between the highrise dorms where over
the next four years additional construction will take place, to
test new technology that could be put to use quickly, and right
next door.
Site engineers divided the parking lot into two test areas, drilling
three holes into the ground at each test area, then pouring in
concrete, which cured for two weeks. Then they ran a series of
static and dynamic tests on the piers.
The static tests measured the steady force the piers could withstand
by pulling them out of the ground over a few hours; the dynamic
tests were performed using a relatively new dynamic test device,
a Fundex Pile Load Test (PLT), recently introduced in California
by American Piledriving Inc. This device mimics axial seismic
loading by dropping a heavy weight onto a pier for a duration
of about 0.1 seconds. Test results came as a pleasant surprise,
according to PEER director Jack Moehle. "The piers were three
times stronger than was previously thought," he says.
Foundation piers transfer the force of an earthquake from a building
to the soil, and do this at significant depths. But determining
the size and quantity of piers necessary to safely accomplish
this transfer depends on the unique properties of the soil on
which a building is constructed.
Unlike pre-made piers, whose strength can be evaluated each time
they are driven into the ground, direct tests of cast-in-place
piers are rare because of the way they are constructed. "That
means," says Moehle, "that you don't have any direct
measure of the strength or capacity of the cast-in-place piers."
To compensate for this, traditional design formulas are conservative,
which often results in using more and larger diameter piers than
are necessary.
"The dynamic nature of the PLT tests will allow us to use
better values for seismic design of foundation elements on the
Berkeley campus and, hopefully, we can also influence future design
to take better advantage of the good dynamic response of foundation
elements," says civil engineering professor Nick Sitar, outgoing
SRC chair. "Also, the PLT test is relatively simple and quick,
which means we should be able to use it on a more routine basis
on future projects."
The project will create significant savings for the University,
Ordonia says. Capital Projects staff estimate that after spending
roughly $100,000 to conduct the tests, the campus will garner
a net savings of up to $400,000 in reduced construction costs.
Test results have already been incorporated into the residential
hall building project adjacent to the test site. And because the
soils under the campus are fairly uniform, the research could
be used for upcoming construction projects as well, says Ordonia.
Test data will also help retrofit existing buildings. "If
there's some structural element that has to be rebuilt as part
of the building, now we have an additional tool with which to
analyze the structural system," says Ordonia. "What's
more, the project offered a unique opportunity for architects,
structural engineers, geotechnical engineers, earthquake engineers,
and University administrators to work together using performance-based
engineering techniques."
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