Estimation of System Damping at the Lotung Site by Application of System Identification.
Estimation of System Damping at the Lotung Site by
Application of System Identification.
(5969 K)
Glaser, S. D.; Leeds, A. L.
NIST GCR 96-700; 195 p. August 1996.
Sponsor:
National Science Foundation, Washington, DC
Available from:
National Technical Information Service
Order number: PB96-214697
Keywords:
system identification; earthquakes; system damping;
seismic design; clay; sand
Abstract:
Possibly the best set of data for earthquake excitation
of soils exists for the test site operated by the Taiwan
Power Company in conjunction with the Electric Power
Research Institute (EPRI) at Lotung, Taiwan. At this
site, two locations are instrumented with
three-component accelerometers at depths of 47 m, 17 m,
11 m, 6 m, and at the surface. One array is in the
free-field, while the other is adjacent to a one-quarter
scale nuclear containment vessel. The site is also well
instrumented with piezometers at various depths and
locations. The simplified soil profile consists of 30 m
to 35 m of silty sand and sandy silt with some gravel,
overlaying a thick clay and silt deposit. The water
table is within half a meter of the ground surface.
This area is seismically active, and strong shaking
generated by many earthquakes exhibiting a wide range of
magnitudes have been recorded since 1986. For this
study, the modal frequencies and damping ratios were
calculated for events 3,4,7,8,9,10,12 and 16 with local
magnitudes ranging from 4.5 to 7.0. The modal
frequencies and damping ratios calculated are examined
for the effect of local energy intensity and soil
structure interaction. Modal frequencies are seen to
decrease with increasing intensity once a certain
threshold of acceleration/intensity is reached. This
result is consistent with the data obtained by other
authors using different techniques. For the 0-6 m
interval the decrease in frequency with event energy is
less pronounced under a model containment structure than
in the free field. This soil-structure effect is
increasingly diminished with depth and absent by the
17-47 m interval. Calculated damping values demonstrate
an expected increase with input seismic energy. For the
0-6 m and 6-11 m intervals the damping values are higher
under the model structure than in the free field. This
distinction is completely missing in the 17-47 m
results. The transition to non-linear behavior, while
less pronounced with increasing depth, consistently
occurs above a peak acceleration of 0.05 g or Arias
Intensity of 100 m/sec. The results clearly indicate a
degree of non-linear response over the intervals
studied. Evidence of a decrease in specific interval
fundamental frequency and an accompanying general trend
of increased damping with higher seismic energy are
clear. Comparison of the results of this study with
previous work considered with the inherent superiority
of parametric modeling for transient and/or
non-stationary time series such as earthquakes indicate
that system identification is a more robust method for
identifying fundamental frequencies and damping values
for layers of earth materials when borehole information
is available.
Building and Fire Research Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899