Transcript MillikanDemo

Millikan Library Demonstration
Earthquake Engineering Research Laboratory
2005 Preview
November 10, 2005
Case Bradford
[email protected]
http://ce.caltech.edu/case/
Overview
• Dynamic characteristics of Millikan library
– Forced and ambient vibrations
• Changes in these characteristics
– Weather patterns
– Earthquake behavior
• Structural Health Monitoring
– Use changes in properties of the building to infer damage
• Wave propagation / radiation damping
– Applications to seismology
Robert A Millikan Memorial Library
Millikan Library, one of the worlds most heavily
researched and instrumented buildings, has
been studied extensively since its construction
in the 1960s. Very stiff 9-story reinforced
concrete building with one basement.
A synchronized vibration generator (“shaker”) is
installed on the roof for forced vibration tests.
USGS/Caltech Dense Instrumentation Network
installed in 1998 -- 36 instruments, three
horizontal accelerometers on each floor (from
basement to roof) and three vertical
accelerometers in the basement.
California Integrated Seismic Network (CISN)
station MIK installed in 2001. Three-component
accelerometer on 9th floor.
Earthquake
Movies
Data from dense array.
24 Hours of Building Motions
24 hours of Millikan data from
9th floor station MIK -Friday afternoon through
Saturday afternoon
Note the daily drift (instrument
error) and the marked
quiet period when the AC
systems are turned off,
daily, from 1:00-5:00AM
Also note that there appears
to be something
interesting happening
between 2:00PM and
4:00PM
Previous Demonstration…
Here’s a quick summary of
a demonstration from
last year.
Forced EW shaking
with intermediate
weights
For more information on
forced vibration testing
please refer to my
website,
ce.caltech.edu/~case
Forced EW shaking
with full weights
“People Power” - NS shaking
People Power
Large excitations
from the first five
volunteers, and even
larger when the
entire group helped!
The decay of the
response with time
when we stopped
‘shaking’ can be used
to approximate the
damping of the
building.
What’s a Fourier Transform?
What’s a Fourier Transform?
So, take an arbitrary section of
Millikan data…
…and take a FT to determine the
frequency content, this represents
the many sine curves you’d have
to sum to obtain the original data.
These two ways of looking at data
contain the same information you can switch back and forth
between them if you need timedomain versus frequency domain
information.
FFT of 24 hours
Natural Frequencies
EW1
NS1
T1
T1
EW2
EW3
NS2
From one day of ambient
data, a Fourier
Transform shows the
fundamental
frequencies of the
library.
The characteristics of the
building (including
these fundamental
frequencies) change
on several time scales.
Modeshape movies: EW
Resonance at 1.1Hz
Modeshape movies: NS
Resonance at 1.7Hz
Modeshape movies: Torsion
Resonance at 2.4Hz
Historical Behavior of Millikan Library
Fundamental EW and NS
modes of Millikan Library
since construction.
Crosses indicate frequencies
from forced vibration tests.
Circles indicate estimates from
recorded earthquakes.
[ Earthquake Abbreviations: LC: Lytle Creek, SF: San Fernando, WN: Whittier
Narrows, SM: Santa Monica, NR: Northridge, BH: Beverly Hills, BB: Big Bear ]
(Clinton, 2004)
Dashed lines represent the
observed natural
frequencies of the library,
and the shaded region is
the likely variance from
such factors as weather
conditions, weight
configuration of the shaker
used for forced vibration
tests, and experimental
error.
Dynamic Properties of Millikan Library vary on different
timescales:
Clinton, 2004
Days
Years
Weeks
Spectrogram (Color Intensity Represents Variations in Amplitude)
Spectrogram (Normalized by Peak Amplitude)
Forced Vibration Testing
Big Bear Earthquake
M5.4 D=119 22Feb2003
Santa Ana Windstorms
Forced Vibration Testing
Santa Ana Windstorms
Big Bear Earthquake
M5.4 D=119 22Feb2003
Weather Conditions
Clinton, 2004
Heavy winds temporarily decrease all fundamental frequencies by 2-4%. The building
recovers immediately after the wind event.
Rainfall causes an increase in the EW and Torsional fundamental frequencies of 3-5%,
the NS mode is less strongly affected by rain. Rain effects persist for 1-2 weeks,
gradually reverting to pre-rain levels.
When using the natural frequency of the building we have to account for these nonstructural changes. A change of 5% in natural frequency corresponds to a 10%
change in stiffness. Care must be taken when applying Structural Health Monitoring
techniques that infer damage from changes in natural frequency.
Long Distance Studies
J. Favela, 2004
Long Distance Studies
• Finite element
modeling of the
Pasadena area -excitation from our
forced vibration tests
J. Favela, 2004
Review
• Vibrational characteristics of Millikan library
– Mode shapes and natural frequencies
– Forced vibration behavior
• Changes in these characteristics
– Weather patterns: Rain and wind cause large changes in
stiffness
– Earthquakes
• Structural Health Monitoring
– Traditional SHM techniques use changes in natural frequency to
infer changes in stiffness, which is assumed to be from structural
damage. We are working to refine these techniques.
• Wave propagation / radiation damping