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January 30 2006
IGPP geodesists Yehuda Bock & Fan Yang apply 50 Hz GPS to outdoor shake table tests of a 7-story building at Camp Elliott
Figure 1: 7-story building at UCSDs NEES Outdoor Shake Table at Camp Elliott instrumented with seven 50 Hz GPS.
SOPAC Director and Research Geodesist Yehuda Bock and graduate
student Fan Yang have recently participated in tests at the Large
High-Performance Outdoor Shake Table at University of California San
Diego, funded by the National Science Foundation under the
NEES program.
During the tests, which took place between mid-November 2005 and mid-January 2006,
they were able to supplement the myriad
of monitoring sensors blanketing a to-scale 7-story building with 7
real-time 50Hz GPS displacement measurements, a novel new application
of GPS technology. The overall objective of this research program is
to verify the seismic response of reinforced concrete wall systems
designed for lateral forces that are significantly smaller than those
currently specified in building codes in United States.
The first phase of experimental program investigated the response of the web
cantilever wall configuration to different levels of excitation. This
included low amplitude 0.5-25 Hz band-clipped white noise tests, a low
intensity earthquake, two medium intensity earthquakes that were somewhat
above the site response spectra for the period of the building for 50%
probability of exceedance event and a large intensity earthquake whose
spectral acceleration in the period range of interest was above the site
response spectra for 10% probability of exceedance in 50 years. The low
intensity earthquake record is the vnuy longitudinal component from the
1971 San Fernando earthquake. The two medium intensity records are the
vnuy transverse component record from 1971 San Fernando earthquake and
the whox longitudinal component from the Northridge 1994 earthquake. The
large intensity record is the Sylmar Olive View Med 360o component record
from the 1994 Northridge earthquake.
Figure 2: Yehuda Bock and Marios Panagiotou on a man lift installing the GPS antenna on a cantilever at the 3rd floor level of the 7-story building.
The role of Bock and Yang was to deploy as part of the shake table tests
high-rate real-time precise GPS technology, previously developed for
regional-scale seismic monitoring and local volcano monitoring. They
deployed seven 50 Hz Navcom GPS receivers and Dorne Margolin antennas
with chokerings: 3 on the roof of the 7-story building, 2 cantilevered
on the 5th and 3rd floors, one on the shake table itself, and one as
a reference just off the shake table. They computed instantaneous 50
Hz displacements using the Geodetics, Inc. RTD-Net software for all
tests, and compared these results with accelerometer data, and with the
induced earthquake motions. Their results demonstrate consistent mm-level
(one-sigma) accuracy for the measured displacements and the usefulness
of very high rate GPS displacement measurements for seismic monitoring
of structures. Results of their research will be presented at the SSA
Centennial Annual Meeting in San Francisco in April, in collaboration
with colleagues Prof. Jose Restrepo, Prof. Joel Conte, and graduate
student Marios Panagiotou at the Department of Structural Engineering
at UCSDs Jacobs School of Engineering.
Figure 3: The shake table induces accelerations in the east-west direction using a powerful hydraulic system. The above two plots show the 50 Hz east component displacements of the GPS antennas on the shake table and on the roof of the 7-story building. The vertical scale is in meters relative to the reference antenna off the shake table. The horizontal scale is time (mm/ss.ss format). The induced accelerations are based on records from the 1994 Mw 6.7 Northridge earthquake. Unlike accelerometer measurements, high rate GPS directly measures displacements, which is highly desirable for structural monitoring.
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