ANALYSIS OF AMBIENT VIBRATION DATA

COLLECTED FROM TWO B UILDINGS

Caitlin Collins, Tufts University

NEES REU Site: University of California, Los Angeles

Faculty Mentor: Bob Nigbor

August 13, 2010

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ABSTRACT

In the summer of 2010, NEES@UCLA, Center for Embedded Network Sensing (CENS), and

High School Summer Research Program (HSSRP) interns conducted ambient vibration testing

on the Engineering 1 building at University of California at Los Angeles. During the same

summer, NEES@UCLA staff traveled to Istanbul, Turkey, and ran more extensive ambient

vibration tests on the ETA- e data

from both sets of tests and discovered that both buildings experienced lower amplitudes of

vibration on the side of the building that was anchored to the hill. ETA-B Blok also displayed

torsional vibration about the anchored corner of the building, as well as having a separate mode

for the top three floors of the building, which were not anchored and were slightly narrower

along the strong direction of the building. Interns also observed the phenomenon of frequency

drift, which correlated well with local air temperature.

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TABLE OF CONTENTS

Abstract ii

Table of Contents iii

Figures v

1.0 Introduction 1

1.1 Background 1

1.2 Purpose 1

1.3 Scope 2

2.0 Methods and Materials 2

2.1 Equipment 2

2.2 Buildings 3

2.2.1 Engineering 1 3

2.2.2 ETA-B Blok 3

2.3 Tests 3

2.3.1 Backbone test 3

2.3.2 Single-Floor test 4

3.0 Data and Results 5

3.1 Engineering 1 Results 5

3.1.1 Frequencies 5

3.1.2 Mode shapes 5

3.2 ETA-B Blok Results 5

3.2.1 Frequencies 5

3.2.2 Mode Shapes 6

3.2.3 Additional Tests 6

3.2.4 Frequency Drift 7

4.0 Discussion and Conclusion 7

4.1 Analysis of Engineering 1 7

4.1.1 General analysis 7

4.1.2 ARTeMIS analysis 8

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4.2 Analysis of ETA-B Blok 8

4.2.1 General analysis 8

4.2.3 Frequency drift 8

5.0 Acknowledgements 8

List of References

Appendix

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FIGURES

Figure 1: Typical floor plan of ETA-B Blok 4

Figure 2: Sensor positions for Engineering 1 roof test 4

Figure 3: Graph of the PSD of data from Engineering 1 5

Figure 4: Mode shape for the 2nd

order E-W mode of ETA-B Blok at 6.96Hz 5

Figure 5: PSD Graph of data from ETA-B Blok 5

Figure 6: Mode shape for the E-W mode of ETA-B Blok at 3.12Hz 6

Figure 7: Mode shape for the 2nd order E-W mode of ETA-B Blok at 6.96Hz 6

Figure 8: Graph of frequency drift and local air temperature from 7/21 to 7/24 7

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1.0 INTRODUCTION

1.1 Background

Structural monitoring has a multitude of uses in the field of civil engineering. Sensors may be

placed on a building or other structure to detect levels of acceleration, displacement, strain,

temperature, and other indicators of building health. Sensor networks can also measure a

such as interstory drift,

acceleration, and component stress and strain. NEES@UCLA (The George E. Brown, Jr.

Network for Engineering Earthquake Simulation at the University of California at Los Angeles)

uses a mobile laboratory which includes a sensor network and data acquisition equipment to

perform field tests that investigate structural response to earthquake aftershocks (Wallace,

Lemnitzer, and Salamanca, 2010), or harmonic excitation using mobile shaking devices (Yu, et

al., 2008).

Test data from extensively instrumented buildings can be very valuable for the design and

modeling of buildings, as well as for future research (Yu, et al., 2008). However, such data is

hard to come by, as these large-scale field tests are rarely conducted. The NEES@UCLA

laboratory provides a set of instrumentation that is rare in its large scope and many possibilities

for application (Skolnik, 2008). Additionally, their large field shakers make it possible to

conduct tests at much larger amplitudes of vibration, a kind of test that is rarely conducted with

so much accompanying instrumentation. (Yu, et al., 2008)

Although the NEES@UCLA mobile lab is often associated with its mobile shakers, it also allows

researchers to conduct ambient vibration testing, a much less destructive type of test that

s due to random activity such as wind and foot traffic

(Gentile and Gallino, 2007). However, ambient vibration testing requires careful measurements

because the vibrations being measured are very small (acceleration measurements are commonly

in thousandths of a g) and can easily be overshadowed by vibrating machinery or other sources

of non-random vibrations. Ambient vibration testing can yield valuable knowledge of a building

in terms of its natural movement and can help to calibrate building models that can then be used

h as an earthquake.

1.2 Purpose

This paper describes the ambient vibration testing of two buildings: the Engineering 1 building

at University of California, Los Angeles (UCLA); and the ETA-

University in Istanbul, Turkey.

Testing Engineering 1 was mostly an activity for the REU interns to gain experience conducting

ambient vibration testing, and as a dry run to prepare the instrumentation for the upcoming trip to

Turkey. Engineering 1 has a shape similar to that of ETA-B Blok, so by testing Engineering 1

using the same equipment intended for use in Turkey, NEES@UCLA staff members were able to

troubleshoot any problems before the equipment was sent to Turkey.

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Running ambient vibration testing on ETA-B Blok is an excellent chance for NEES@UCLA to

gather data on a building before and after a seismic retrofit. Also, because the building was

completely gutted for the retrofit, there is no machinery running that would interfere with

ambient vibration data.

1.3 Scope

This paper focuses mainly on ambient testing of both the Engineering 1 building at UCLA and

the ETA-B Blok building in Turkey, with more emphasis on the excellent data from ETA-B

Blok. Frequency drift due to temperature is mentioned in this paper, but is an unexpected

finding that will be the subject of future research. The data discussed in this paper is acceleration

data from triaxial accelerometers, and was analyzed using MATLAB software from Mathworks

and ARTeMIS software from Structural Vibrations Solutions.

Ambient vibration testing was conducted on two buildings, the Engineering 1 building at UCLA,

and the ETA-

Engineering 1 were conducted on July 15, 2010, and the tests on ETA-B Blok were conducted

from July 20th to July 24

th, 2010.

2.0 METHODS AND MATERIALS

2.1 Equipment

The same equipment was used for every test. The sensors were Kinematics ES-T triaxial

accelerometers, Kinematics 24-bit Q330 digitizers converted the analog signal from the

accelerometers to digital counts, which were passed to a laptop computer that recorded the data

at a rate of 200 samples per second. -4g down to a few

micro-g in amplitude, 0 to 80 Hz in frequency. The data was collected in a program called

Rockhound, which parceled the data into one-hour datafiles, each with 25 channels: one for the

timestamp and 24 for z, y, and x-axis acceleration for each of the 8 accelerometers.

For the tests run in Engineering 1, the accelerometers were not bolted to the floor, although

cables were taped to the floor whenever there was a tripping hazard. For the tests done in

Turkey, hot glue was used to attach the sensors to the building, and caution tape was placed

around the sensors to protect them from tampering. All sensors, with the exception of those on

the roof of ETA-B Blok, were oriented with the positive y-axis pointing north and the positive x-

axis pointing east. Because there was no roof access for ETA-B Blok, the roof sensors were

inverted on the ceiling of the top floor, with the x and y axes switched so that the positive y

direction was east and the positive x direction was north (See Sensors 1 and 2 on pages 1 and 3

of the Appendix).

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2.2 Buildings

2.2.1 Engineering 1

The Engineering 1 building at UCLA is a 3-story heavily reinforced concrete building. The

north face built into the side of a hill and a freight

elevator on the south side. Engineering 1 is currently used for office space and labs, as well as

machine shops that operate on the first floor. There is a large amount of running machinery in

the building.

2.2.2 ETA-B Blok At the time of testing, ETA-B Blok was undergoing a seismic retrofit. All nonstructural

components (partition walls, ceilings, plumbing, etc.) had been removed, and the columns had

been scraped down and had holes drilled through them to accommodate extra rebar for the

retrofit. Building traffic was limited to construction activity, which was concentrated mostly in

the lower levels of the building. No permanent machinery was running in the building, but

construction workers were using a jackhammer and a sledgehammer to remove and break up

large pieces of concrete from the structure.

ETA-B Blok is also a rectangular building, and is built into the side of a hill. The top two stories

are completely aboveground, with the rest of the building anchored into the hill on the western

face. The basement level is completely underground.

2.3 Tests

2.3.1 Backbone Test

Sensors for the backbone test were placed in a vertical line over the full height of the building.

One sensor was placed on each floor at column location A (Figure 1) in ETA-B Blok, and in the

southeast stairwell of Engineering 1 (Figure 2). Appendix pages 1 and 2 contain all of the sensor

information for the backbone tests for ETA-B Blok and Engineering 1 respectively.

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Figure 1: Typical floor plan of ETA-B Blok. Letters in boxes denote possible sensor positions.

2.3.2 Single-Floor Test

The second test done on both buildings was the instrumentation of a single floor using all eight

accelerometers. In ETA-B Blok, the test was done on the 4th floor, and in Engineering 1 the test

was done on the roof. Sensors were placed at all perimeter locations (4B-4J) on ETA-B Blok,

and at comparable locations on the roof of Engineering 1 (see Figures 1 and 2). Appendix pages

3 and 4 contain sensor information for the 4th floor and roof tests.

Figure 2: Sensor positions for Engineering 1 roof test. Red dots mark the sensor positions.

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