Millikan Library DemonstrationEarthquake Engineering Research Laboratory

Case Bradfordcase@ecf.caltech.eduhttp://ce.caltech.edu/case/

2005 PreviewNovember 10, 2005

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 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

24 Hours of Building Motions

Here’s a quick summary of a demonstration from last year.

For more information on forced vibration testing please refer to my website, ce.caltech.edu/~case

“People Power” - NS shaking

Forced EW shaking with full weights

Forced EW shaking with intermediate weights

Previous Demonstration…

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?

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 time-domain versus frequency domain information.

What’s a Fourier Transform?

FFT of 24 hours

Natural Frequencies

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.

EW1 T1 EW2

T1NS1 NS2

EW3

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.

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.

[ Earthquake Abbreviations: LC: Lytle Creek, SF: San Fernando, WN: Whittier Narrows, SM: Santa Monica, NR: Northridge, BH: Beverly Hills, BB: Big Bear ] (Clinton, 2004)

Dynamic Properties of Millikan Library vary on different timescales:

Weeks

Days

Years

Clinton, 2004

Spectrogram (Color Intensity Represents Variations in Amplitude)

Spectrogram (Normalized by Peak Amplitude)

Santa Ana Windstorms

Big Bear Earthquake

M5.4 =119 22Feb2003

Forced Vibration Testing

Santa Ana Windstorms

Big Bear Earthquake

M5.4 =119 22Feb2003Forced Vibration Testing

Weather Conditions

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 non-structural 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.

Clinton, 2004

Long Distance Studies

J. Favela, 2004

Long Distance Studies

J. Favela, 2004

• Finite element modeling of the Pasadena area -- excitation from our forced vibration tests

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