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The IRIS Damage Assessment Methodology 3

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T i m e

S p e c t r a l I n t e n

s i t y

Authors:

Hiroshi Tanaka

Michaela Höllrigl-Binder

Helga AllmerHelmut Wenzel

3 The IRIS Damage AssessmentMethodology

MotivationDamage quantification is a major desire of the SHM community. Methodologies to

introduce a quantity for actual condition of a structure into the assessment process are

desired.

Main Results

The idea that the condition of a structure is represented in the character of its dynamic

response is fully accepted by the SHM community. The VCLIFE methodology quantifies

condition analysing input from monitoring.



3 The IRIS Damage Assessment Methodology

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T i m e

S p e c t r a l I n t e n s i t y

3-1 Introduction

Damage detection in civil engineering has long been concentrated on the change ofstiffness with increasing damage. This indicator, however, has been proven to be by far

not sensitive enough to satisfy the practical requirements. Our experience is that a more

sensitive indicator could be observed when the measured frequencies of higher order are

carefully examined. While the lower fundamental frequencies appear not much affected

by the stiffness changes, the higher frequencies can indicate much earlier signs of it. It

was found that actually an energy transfer is happening from lower to higher frequency

ranges with increasing damage. F.3-1 is an example showing this phenomenon. The re-

duction of eigenfrequencies and the transfer of energy are both observed in this graph.

This phenomenon has never been described in literature before except it was briefly

mentioned in an earlier article by VCE after the first observation of this kind. Meanwhileit is also identified that the increase of damping goes along with a drop of spectral peaks

caused by increasing damage. It is obvious that this is not the modal damping and it

needs to be described by a system behaviour parameter. The physical concept, the math-

ematical modelling and a clear simulation of the phenomena are not available yet.

3-2 Structural Non-Linearity and Energy

TransferCascading energy transfer in a dynamical system could be caused by the develop-

ment of non-linear characteristics of the structural response caused by various reasons.

Suppose there is a development of non-linear mechanisms in both damping and stiffness

of the structure, whose dynamic behaviour is simply expressed by an SDOF model, associ-

F.3-1Spectral development over time for a sound structure (left)

and during a damage test at MPA Stuttgart (right)



The Energy Cascade in Turbulence 3-3

ated with the progress of a structural damage. It can be typically represented in the equa-

tion of motion by modifying both damping and stiffness terms as follows:

+ + + − =

ε ε 2 1[1 ( )] [1 ( )] 0mz c z z k z z E.3-1

where ε 1 and ε

2 are the linear or non-linear correction terms introduced corresponding

to the development of structural damage and z (t ) represents the dynamic response of the

structure in general. Note that it is unlikely to have any change in the inertia term. E.3-1

can be rewritten as

+ + = − = ε ε 1 2( ) ( ) ( , )mz cz kz k z z c z z F z z E.3-2

where F( , z z ) is generally a non-linear function of z and/or z , such as

2 3

1 2( , )F z z C z C z = + E.3-3

for example. It implies that if z is given as a vibration with frequency ω , F ( , z z ) is generally

a function of fluctuations with the frequencies expressed by the multiples of ω . For exam-

ple, substitution of z = Asinω t to E.3-3 results in

2 3 3

1 2( , ) (1 cos2 ) (3cos cos3 )2 4

C A C AF z z t t t

ω ω ω ω = − + + E.3-4

which in turn will result in the dynamic response including functions of twice, thrice high-

er frequencies of the original for this case. The same process will be repeated as time al-

lows and, as a result, a part of the system’s dynamic energy will be gradually distributed

to higher and higher frequency ranges. Where would this process end? For the case of

damage-caused non-linearity, the high frequency energy components dissipate as heat

or noise and, if not, the destruction or rupture of the structure would play a role. Even if it

does not reach the destruction point, the mechanism of structural response will change

largely when damage progressed that far.

3-3 The Energy Cascade in Turbulence

Energy cascading, such as the one described above, can be associated with various

types of non-linear physical phenomena. It is typically observed in the dynamics of tur-

bulent fluid flow. In fact, the case of fully developed turbulence, the energy cascading

process is one of the most central issues.

An English physicist L. F. Richardson (1881–1953) conceived turbulence as an assem-

bly of eddies of different sizes, where eddies could be hypothetically visualized as indi-

vidual vortices of some measurable diameter. He then had the idea that large eddies are




prone to break into smaller eddies, which break up into even smaller eddies and so on as

spelled out by the famous parody as follows:

"Big whirls have little whirlsThat feed on their velocity,

And little whirls have lesser whirls

And so on to viscosity." by Richardson (1922)

In each break-up process, the larger eddy transfers its dynamic energy to the smaller

ones without dissipating it, meaning that the energy transfer process in turbulence is in-

viscid, or, in this process, the role of viscous forces is negligible in comparison to the iner-

tia forces. However, energy has to be eventually dissipated somewhere, at much smaller

length scales, or higher frequencies, which is a viscous process. Fluid viscosity has an im-

portant role only at those small scales. This process can be mathematically considered as follows: the Navier-Stokes equation

is a non-linear equation because of an inherent non-linearity of fluid inertia as seen in the

equation below:

1 1i j

j j j j

u u p uu

t x x x x ρ µ

∂ ∂ ∂ ∂ ∂+ + =

∂ ∂ ∂ ∂ ∂ E.3-5

Out of the three terms involved in the equation, the pressure gradient can be ignored

in the present discussion. In the energy dissipation process described above, the iner-tia term becomes less important in comparison to the viscous force in higher frequency

ranges, and hence the whole equation becomes almost linear for this case, whereas at

much larger length scales, or in the lower frequency ranges, the inertia term becomes pre-

dominant and hence the equation becomes highly non-linear, and this is where cascading

takes place. Note therefore that the non-linear characteristics of the equation, in particular

of the inertia force for this case, are deeply associated with the energy cascading phenom-

ena explained by Richardson.

3-4 Non-Linear Damping

The relationship of energy cascading with non-linearity of dynamical systems is there-

fore evident in these two different phenomena. A very interesting aspect of this point is

that the detection of energy cascading could be potentially utilized as a tool for the struc-

tural health monitoring. As mentioned earlier, the traditional idea of knowledge-based

structural health monitoring is by identifying the reduction of stiffness, which has been

proved to be far less sensitive than desired for practical purposes. In contrast to that, by

finding the transfer of dynamic energy to higher frequencies through spectral analysis of

the ambient vibration survey, it may be possible to detect the damage development in

a structure at its earlier stage. Any extent of structural damage can of course change the



Data Analysis 3-5

local structural damping or energy dissipation and stiffness. As a consequence, the global

dynamic properties of the structure, i. e. the eigenfrequencies, mode shapes and modal

damping would be all somewhat influenced.

It needs to be kept in mind that structural non-linearity is attributed, however, notonly to developing damages. Field experience indicates that the magnitude of modal

damping is often amplitude dependent. Increase of damping, when the vibration ampli-

tude is significant, is due to energy consumption at increased friction at bearings, bending

action of piers, behaviour of the bridge outfitting and also the structure-vehicle interac-

tion [Wenzel, 2009].

Admittedly the present method would also detect the developing structural non-lin-

earity due to large motion. However, if there is a development of structural damage as its

consequence, the non-linear characteristics will remain with the structure after the large

amplitude motion disappeared and should be thus detected.

3-5 Data Analysis

For identifying the energy cascading phenomena, the following data analysis can be

applied.

3-5-1 Data Preparation

Acceleration signals ai (t ) are measured for a period of t i ≤ t ≤ t i + T , where i = 1,2,…,n ,with the sampling frequency and measuring period of typically f

s = 500 Hz and T = 5 min,

respectively. n is the number of files.

3-5-2 Analysis

Calculation of the acceleration spectra Gi (f ) by a conventional FFT routine for the fre-

quency range of 0 ≤ f ≤ f M

is required first. f M

= f S/2 is the folding frequency. The normal-

ized spectral density functions are then calculated as

2

2

( )( ) where ( )i

i i i

f i

G f F f G f f σ

σ = = ∆∑ E.3-6

Normalization of spectral density makes sense since our interest is only in the change

of energy distribution patterns and not in actual magnitudes of the spectral density,

which depends on the total dynamic energy supplied by excitation and is always expect-

ed to change during the ambient vibration survey. It is also useful to calculate the fraction

of dynamic energy corresponding to less than any particular frequency level (f ) as follows:

0

( ) ( )f

i i

k

E f F k k =

= ∆∑ E.3-7




E i (f ) is the spectral distribution function which is expected to more clearly reveal the

fraction of energy transferred to different frequency ranges, resulting in the change of its

pattern.

3-5-3 Presentation

Visual presentation of F i (f ) and E

i (f ) against time (i ) and frequency (f ) would indicate

the transfer of energy to higher frequencies by the change in spectral pattern, where 1 ≤

i ≤ n and 0 ≤ f ≤ 250 Hz.

3-5-4 Reading of Spectral Patterns

When the distribution function E i (f ) is examined, it should be noted that the energy

cascading caused by structural non-linearity as discussed in this document is only a partialtransfer of energy through the free vibration process of the structure, and, as it is evident

from the discussion in section 3-2 above, not all the dynamic energy is transferable to high

frequency ranges. Some of the energy should remain with the lower vibration modes.

Another important point is that, while the vibration survey is carried out, there may be

various dynamic excitations or disturbances from the external environment acting on the

structure. There will be, as a result, new dynamic energy supplied to the system and it will

augment the energy fraction at corresponding frequencies. The characteristics of these

excitations are often not readily identifiable. However, when there are any predominant

excitation frequencies, there may be distinct spectral peaks observed at those particular

frequencies. If the excitation is more broad-band noise, a part of this energy will be ab-sorbed at eigenfrequencies and corresponding spectral peaks will show up as additional

spikes in the figures.

The change of pattern in E i (f ) is, therefore, not expected to be monotonous. Hence,

what needs to be observed is a general tendency of the energy shift, which will be hope-

fully indicated by a gradual change of the coloured pattern.

The shift of pattern can be quantified by locating the centroid of the area under the

spectral distribution E i (f ) as follows:

( ) / ( )i i

f f

r fE f f E f f = ∆ ∆∑ ∑ E.3-8

Again, also in terms of the centroid, its shift is unlikely to be monotonous. What should

be observed is the general tendency of its change.



Example: Overpass S101 in Reibersdorf (2008) 3-6

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1 s t l o w e r i n g

1 s t c u t o f t h e p i e r

2 n d

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2 n d

l o w e r i n g

3 r d

l o w e r i n g

L i f

t i n g

C u

t t i n g c o n c r e t e

E n

d o f e x p o s i n g

2 n d

c a b l e c u t

3 r d

c a b l e c u t

R e

v e a l i n g o f

4 t h

s t r a n d

5 w i r e s i n t e r s e c t e d

S h

i f t i n l a s e r s i g n a l

11/12/2008 12/12/2008 13/12/2008

3-6 Example: Overpass S101 in Reibersdorf

(2008)An example given below is based on the measurement of the dynamic bridge re-

sponse carried out in December 2008 for the highway overpass S101 in Reibersdorf, Up-

per Austria. Prior to the demolition, progressive damages were deliberately exerted on

the structure to observe their effects on dynamic characteristics [Wenzel et al., 2009].

Sampling frequency of the acceleration record was 500 Hz.

F.3-2 and F.3-3 depict the progressive change of the normalized acceleration spec-

trum F i (f ) and the distributed spectrum E

i (f ) for the frequency range of 0 ≤ f ≤ 25 Hz from

the measured results. There were a number of physical operations applied to the bridge

during a three-day measurement period and some of them are specified beside the fig-ures. Some of these operations can be clearly identified from the pattern of F

i (f ) and E

i (f )

functions. When the concrete pier or slab is being cut, presumably the severing operation

produced a large extent of high frequency noise and, consequently, a large fraction of

the total dynamic energy appears in a much higher frequency range, which appears as

a substantial spike in E i (f ). In the afternoon of the third day, for example, there was “a

vibrating roller working next to the bridge, causing clearly noticeable vibrations on the

bridge”, the measurement report states. This noise may be also contributing to the above

mentioned spikes.

It is clear by observing E i (f ) patterns that, with the progressive damage artificially ex-

erted on the bridge, more and more fractions of dynamic energy were redistributed tothe higher frequency ranges, which is indicated by the shift of dark blue and yellow band

F.3-2Normalized acceleration spectra F i (f )




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L o c a t i o n o

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2 n d


2 n d

l o w e r i n g

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l o w e r i n g

L i f t i n g

C u t t i n g c o n c r e t e

E n d o f e x p o s i n g

2 n d

c a b l e c u t

3 r d

c a b l e c u t

R e v e a l i n g o f

4 t h

s t r a n d

5 w i r e s i n t e r s e c t e

d

S h i f t i n l a s e r s i g n

a l

11/12/2008 12/12/2008 13/12/2008

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2 n d


2 n d

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L i f t i n g



2 n d

c a b l e c u t

3 r d

c a b l e c u t


4 t h

s t r a n d

5 w i r e s i n t e r s e c t e

d

S h i f t i n l a s e r s i g n a l

11/12/2008 12/12/2008 13/12/2008

towards right, meaning the higher percentage fraction is in the high frequency side of

the figure. F.3-4 is the same E i (f ) function shown for a much wider frequency range up to

250 Hz. It is clear that almost stepwise energy shift took place after the lifting of the dam-aged pier at the beginning of the second day.

The same tendency is presented even more clearly by calculating the shift of the cen-

troid r of the area under E i (f ) as shown in F.3-5 By disregarding the spikes due to vari-

ous kinds of noise, the general tendency is clear – it is shifting towards higher frequency

ranges with time.

F.3-5

F.3-3 F.3-4

Normalized acceleration spectra F i (f )

Spectral distribution functions E i (f ) Spectral distribution functions E

i (f )






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1 s t c u t o f t h e p i e

r

2 n d


2 n d

l o w e r i n g

3 r d

l o w e r i n g

L i f t i n g



2 n d

c a b l e c u t

3 r d

c a b l e c u t


4 t h

s t r a n d

5 w i r e s i n t e r s e c t e d

S h i f t i n l a s e r s i g n a l

11/12/2008 12/12/2008 13/12/2008

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1 s t

l o w e r i n g

1 s t


2 n d


2 n d

l o w e r i n g

3 r d

l o w e r i n g

C u


E n


2 n d

c a b l e c u t

3 r d

c a b l e c u t

R e

v e a l i n g o f

4 t h

s t r a n d

5 w

i r e s i n t e r s e c t e d

S h


11/12/2008 12/ 12/ 2008 13/12/2008

L i f

t i n g

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1 s t

l o w e r i n g

1 s t


2 n d


2 n d

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C u


E n


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c a b l e c u t

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c a b l e c u t

R e

v e a l i n g o f

4 t h

s t r a n d

5 w

i r e s i n t e r s e c t e d

S h


11/12/2008 12/12/2008 13/12/2008

L i f

t i n g

to be associated with the cutting of concrete slab and steel tendons. It is interesting to

observe that no change of the first and second eigenfrequencies was observed during

this operation. The third tendon was severed on the third day and apparent re-settling of

the structure was stated in the measurement report. Further transfer of dynamic energyis obvious in F.3-3, though again this operation had no visible effects on lower eigenfre-

F.3-7

F.3-8

Normalized acceleration spectra F i (f ) on the pier

Spectral distribution functions E i (f ) on the pier



Other Sample Cases 3-7

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quencies. Another set of spectral presentations, F.3-7 to F.3-8, results from the accelera-

tion record obtained right above the damaged pier.

The general tendency, namely the reduction of eigenfrequencies and energy transfer

towards high frequency ranges, is the same as seen in the other results but it can be ob-served even more clearly with this set of data. What is clearly different from the other sets

of data is that conspicuous spectral peaks are found in the frequency range of 8 ≤ f ≤ 13 Hz.

Explanation of these peaks is not immediately given.

3-7 Other Sample Cases

The proposed spectral analysis method for damage identification seems to be suc-

cessful at least in the case of the S101 overpass. Admittedly, however, it was rather anideal case where the scheduled damage was successively applied to the structure and

F.3-9

F.3-10

Europabrücke: based on midday records

Europabrücke: based on midnight records




week01

34

32

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week05 week09 week13 week17 week21 week01

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week05 week09 week13 week17 week21

the measurement was carried out in a protected environment without being disturbed

by on-going traffic, for example. Being encouraged by this case, nevertheless, the method

has been further tried out for some other bridges. This is a brief summary of sample cases.

3-7-1 Europabrücke (2005)

The Europabrücke, opened in 1963, is one of the main alpine north-south routes for

urban and freight traffic and currently stressed by over 30 000 motor vehicles per day. Due

to the requirement to assess the prevailing vibration intensities with regard to possible

fatigue damage, a permanent measuring system has been installed since 2003. The super-

structure is a steel box girder of variable height along the span with an orthotropic deck.

The bridge is 657 m long and consists of six spans of different length, carrying six lanes,

three for each direction, over a total width of 25 m. Existing records of vibration measure-

ment are quite extensive. Attached F.3-9 and F.3-10 represent the analysed results fromMay to October of the records in 2005, at middays and midnights, respectively. They basi-

cally show a healthy, stable condition of the structure, no indication of serious structural

non-linearity being detected.

The statistical evaluation of this structure gives a significant change of the pattern for

80 % of energy after the 4th week of observation, but the whole plot shows a fluctuation

that might be due to different traffic. F.3-11 are the boxplots of midday and midnight

results.

It has to be kept in mind, however, that the existence of structural non-linearity, both

or either in stiffness and damping, is not necessarily 100 % equivalent to the state where

the structure is damaged in any ways. There may be a case where micro-cracks are devel-oping, for example, but the overall structural behaviour does not show any sign of non-

linearity.

F.3-11Europabrücke: midday (left) and midnight (right) – boxplot of 80 % of energy



Other Sample Cases 3-7

* * *

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1 2 / 0 5 / 2 0 0 0

1 3 / 0 5 / 2 0 0 2

0 8 / 0 9 / 2 0 0 4

1 3 / 0 9 / 2 0 0 6

2 5 / 0 5 / 2 0 0 9

1 2 / 0 5 / 2 0 0 0

1 3 / 0 5 / 2 0 0 2

* * * 0 8 / 0 9 / 2 0 0 4

1 3 / 0 9 / 2 0 0 6

2 5 / 0 5 / 2 0 0 9

20

10

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40

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0.80

0.90

1.00

0.60

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0.50

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0.00

1 2 / 0 5 / 2 0 0 0

1 3 / 0 5 / 2 0 0 2

* * * 0 8 / 0 9 / 2 0 0 4

1 3 / 0 9 / 2 0 0 6

2 5 / 0 5 / 2 0 0 9

* part without excitation

16.8

8.5

0.2

25.1

33.4

41.7

Hz 50.0

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40

0

3-7-2 Melk B3A (2000–2009)

An example of a structure deteriorating over a period of nine years is demonstrated

here. Disregarding some irregularities, the gradual change of the spectral pattern (F.3-12) clearly indicates that more and more dynamic energy is transferred towards higher

frequency ranges.

One important matter in looking at the change in spectral patterns is that comparison

must be made between the cases of the same structure under similar physical conditions.

For example, all measurements of this particular bridge were carried out under its service

conditions, namely open to the traffic load excitation. Since the traffic loads tend to en-

hance the bridge vibration in a certain limited range of frequency, the resulted spectral

pattern is different from that obtained under more random ambient excitations such as

micro-tremors or wind. It tends to shift the spectra towards a lower frequency range com-

F.3-12

F.3-13

Melk B3A

Melk B3A: without traffic




2000

32

33

31

30

29

28

2002 2004 2006 2009 2000

30

35

25

20

15

2002 2004 2006 2009

2 3 / 0 5 / 2 0 0 1

2 0 / 0 4 / 2 0 0 9

2 1 / 1 0 / 2 0 0 8

1 7 / 0 6 / 2 0 0 3

1 8 / 0 3 / 2 0 0 4

0 9 / 0 9 / 2 0 0 4

2 2 / 0 3 / 2 0 0 5

1 0 / 1 1 / 2 0 0 5

2 7 / 0 4 / 2 0 0 6

3 0 / 1 0 / 2 0 0 6

2 3 / 0 4 / 2 0 0 7

2 3 / 1 0 / 2 0 0 7

2 4 / 0 4 / 2 0 0 8

Bridge removal

New pavement

New bridge

New configuration

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0.00

pared to the time without traffic. In order to avoid this effect, an effort was made to extract

some data obtained when the bridge was freely vibrating without traffic loads. The result-

ing spectral pattern without traffic excitation is shown in F.3-13.

Concerning the statistical evaluation of this bridge, it depends on which value is used.

For this bridge the results of the location of the centroid of the energy distribution and the

value of 80 % of energy are presented to show that there can be a difference: significant

changes for this structure can be observed with the analysis of variance (ANOVA) for the

location of the centroid from the 5th observation period (that is the year 2006) and for the

F.3-14Melk B3A: boxplot of the centroid (left) and boxplot of 80 % of the energy (right)

F.3-15Flughafen






1 1 / 2 0 0

5

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1 0 / 2 0 0

6

1 0 / 2 0 0

7

1 0 / 2 0 0

8

0 4 / 2 0 0

6

0 4 / 2 0 0

7

0 4 / 2 0 0

8

0 4 / 2 0 0

9

1 1 / 2 0 0

5

180

200

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100

1 0 / 2 0 0

6

1 0 / 2 0 0

7

1 0 / 2 0 0

8

0 4 / 2 0 0

6

0 4 / 2 0 0

7

0 4 / 2 0 0

8

0 4 / 2 0 0

9

Removal of bridge

Change of parking configuration

Milling of the ramp

50

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1.00

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0.20

0.40

0.10

0.00

1 0

/ 1 1 / 2 0 0 5

2 7

/ 0 4 / 2 0 0 6

3 0

/ 1 0 / 2 0 0 6

2 3

/ 0 4 / 2 0 0 7

2 3

/ 1 0 / 2 0 0 7

2 1

/ 1 0 / 2 0 0 8

2 0

/ 0 4 / 2 0 0 9

2 4

/ 0 4 / 2 0 0 8

Removal of bridge

Change of parking configuration

Milling of the ramp

10

0

20

30

40

Hz 50

1.00

0.70

0.90

0.30

0.50

0.60

0.80

0.20

0.40

0.10

0.00

1 0

/ 1 1 / 2 0 0 5

2 7

/ 0 4 / 2 0 0 6

3 0

/ 1 0 / 2 0 0 6

2 3

/ 0 4 / 2 0 0 7

2 3

/ 1 0 / 2 0 0 7

2 1

/ 1 0 / 2 0 0 8

2 0

/ 0 4 / 2 0 0 9

2 4

/ 0 4 / 2 0 0 8

F.3-17Flughafen: E i (f ) the last 98 files

F.3-16 and F.3-17 compare the normalized spectra F i (f ) and the cumulative distribu-

tion E i (f ) of both cases. It is obvious that (B) has much more information than (A). Transfer

of energy to a higher frequency range that took place between various events is clearly

better recognized by the results of (B). The high spectral peaks started appearing in thehigher frequency range after milling of the ramp started in April 2007, indicating a signifi-

F.3-18Flughafen: boxplot of the centroid (left) and

boxplot of 80 % of the energy (right)



Concluding Remarks 3-8

cant change of spectral pattern. In terms of the cumulative spectral energy E i (f ), it is more

clearly recognized by case (B) rather than (A), since presumably more and more energy

is transferred to the frequency range beyond 50 Hz. Note, however, that high frequency

noise is also effectively cut off for the case of (A) due to low sampling frequency and itsometimes makes it easier to look at the colour pattern since the spikes caused by opera-

tional noise are reduced.

The statistical analysis gives significant changes for this structure with the analysis of

variance (ANOVA) for the location of the centroid from the 3rd observation period (that is

the year 2006) and for the value for 80 % of energy from the 4th observation period (that is

the year 2004). F.3-18 shows the boxplots of these results.

3-8 Concluding Remarks The proposed spectral method (VCLIFE) was applied to the results of on-site measure-

ments at several different bridges. The presented results here further emphasize a pos-

sibility of effectively detecting the development of structural damages by looking at the

change of the spectral pattern due to the shift of dynamic energy towards a higher fre-

quency range. In relation to this observation, it should be noted that the results are more

informative when the sampling frequency is high enough, generally speaking.

This energy shift seems to be quite characteristic to the structures with developing

damages. It is considered now that a combination of the ambient vibration survey and the

proposed spectral analysis can be an effective tool, which is applicable as a simple struc-tural health monitoring tool. To this end, it would be ideal if a criterion for the extent of

structural damage corresponding to any indicator of the energy shift can be established.

Locating the centroid of the area under E i (f ) curves is one possibility but its practicability

would require further discussion.

For the future measurements, it is advisable to have a sampling frequency of 500 Hz.

For identifying the high frequency shift of dynamic energy, it is desirable to minimize the

effects of extraneous disturbances, particularly the traffic load. Ideally, if the spectrum of

excitation force can be identified even approximately, its contribution towards the out-

put spectra can be estimated but this is not the case most of the time. Minimization of

noise effects could be achieved by taking a long enough record so that undesirable noise,

including the traffic load, can be regarded more or less an evenly distributed excitation.

Taking several consecutive files, each 330 seconds long, would suffice. Ideally, the free vi-

bration record of the structure should be observed over a certain period of time. It should

be also mentioned that, in any measurements involving multi-locations on the structure,

it is desirable to keep the reference point at the same location throughout the campaign.




References

Furtner, P., 2009. Flughafen Wien Schwechat Vorfahrt Ost Terminal 2 – Objekt 102, Dynamis-che Charakteristik der Bauwerke, Periodische Nachmessung und Interpretation der Ergeb-

nisse. Report 09/1042, April 2009.

Wenzel, H., Veit-Egerer, R., Widmann, M. and Jaornik, P., 2009. WP3 Demonstration Re-

port . Deliverable D11.1, October 2009.

Wenzel, H., 2009. Health Monitoring of Bridges. Wiley.

Iris Chapter03

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