Depar t m en t of
Civil &
St r u ct u r al
En gin eer in g.
Forced vibration modal testing of
‘International Bridge’
at Wayne, New Jersey, 21-23 July 2010
Prof. James Brownjohn
Dr Ki-Young Koo
Dr Chris Middleton
The University of Sheffield
Department of Civil and Structural Engineering
Vibration Engineering Section
Depar t m en t of
Civil &
St r u ct u r al
En gin eer in g.
Planned test grids
Depar t m en t of
Civil &
St r u ct u r al
En gin eer in g.
Test grids for AVT and FVT using 19 sensors
Single swipe with 19 sensors
21
20
19
18
17
16
15
14
13
11
10
9
8
Span 1
7
6
5
Span 2
28
18
48
17
27
26
16
47
58
25
15
46
57
68
24
14
45
56
67
78
44
23
13
55
66
77
88
43
22
12
54
65
76
87
98
42
53
21
11
64
75
86
97
52
41
63
108
74
85
96
51
107
62
73
84
128
118
95
106
61
72
83
127
117
94
105
71
126
116
82
93
104
125
115
81
92
103
124
114
91
102
123
113
101
122
112
121
111
Span 2: using 6 swipes with 2×16+triax by shaker
4
3
2
Depar t m en t of
Civil &
St r u ct u r al
En gin eer in g.
The workers ….
Chris on data acquisition, and Ki on the cherry picker
Depar t m en t of
Civil &
St r u ct u r al
En gin eer in g.
The equipment ….
QA 750 servo accelerometers, APS400 shaker
Depar t m en t of
Civil &
St r u ct u r al
En gin eer in g.
Single ‘swipe’ ambient test on
walkway -during afternoon and 90+F
Depar t m en t of
Civil &
St r u ct u r al
En gin eer in g.
Displacement signals from double
integration of acceleration.
Are we seeing real coupling of spans?
C:\DOCS_TP\Proposals\Drexel\05-Testing\AMBIENT_SWIPES\displacement\fvt_20100721_31_dsa_scaled_0p1dis_mm_r
2.5
2
1.5
1
mm
0.5
0
-0.5
-1
-1.5
-2
-2.5
308
310
312
314
seconds
316
318
320
TP61
TP62
TP63
TP64
TP65
TP66
TP67
TP68
TP51
TP52
TP53
TP54
TP55
TP56
TP58
TP57
TP51z
Depar t m en t of
Civil &
St r u ct u r al
En gin eer in g.
Mode shapes obtained using NExT/ERA:
(30 minute data set)
Clear evidence of coupling here
mode: 1 f=3.75Hz zeta=0.71%
mode: 2 f=4.3Hz zeta=2.3%
mode: 3 f=5.27Hz zeta=0.94%
mode: 4 f=5.96Hz zeta=0.66%
mode: 5 f=9.5Hz zeta=0.8%
mode: 6 f=10.2Hz zeta=0.61%
mode: 7 f=11.7Hz zeta=2.3%
mode: 8 f=12.4Hz zeta=1.5%
mode: 9 f=14.2Hz zeta=1.1%
mode: 10 f=14.9Hz zeta=0.4%
mode: 11 f=15.1Hz zeta=0.91%
Depar t m en t of
Civil &
St r u ct u r al
En gin eer in g.
C:\DOCS_TP\Proposals\Drexel\05-Testing\AMBIENT\ambient_20100721_5_DSA_scaled
TP5
TP6
TP7
TP8
TP9
TP15
TP16
TP17
TP18
TP19
2
1
m/sec2
Single vehicle
free decay and
small damping
and frequency
variation
3
0
-1
-2
-3
1590
1600
1610
damping vs amplitude; average damping: 0.6332%
1620
1630
1640
seconds
1650
1660
1670
1680
1690
frequency vs amplitude; average frequency: 3.6824Hz
3.71
1
3.7
f /Hz
 /%
0.8
0.6
0.4
3.68
3.67
0.2
0
0
3.69
0.1
0.2
0.3
0.4
ch 20 (m/sec 2)
damping vs frequency
0.5
3.66
0
0.1
0.2
0.3
0.4
ch 20 (m/sec 2)
log-dec damping vs time
0.5
Depar t m en t of
Civil &
St r u ct u r al
En gin eer in g.
Force vibration test/analysis methods
~between 11PM and 5AM
• Excitation modes
–
–
–
–
Random (15 minutes)
Chirp (15 minutes)
Swept sine –for reference sensors only, during moves
Shaker shutdown –for reference sensors only, during moves
• Analysis methods
–
–
–
–
–
NExT/ERA on random excitation (MODAL)
GRFP on chirp excitation (MODAL)
OMAX on chirp excitation (MACEC)
Circle-fit on swept sine (MODAL)
Log-dec free decay on shaker shutdown (MODAL)
, f, 
, f, , m
, f, , m
f, , m
f, 
Depar t m en t of
Civil &
St r u ct u r al
En gin eer in g.
5-swipe output only modal analysis using
NExT/ERA (with 15 minute random excitation)
mode: 1 f=3.07Hz zeta=1.4%
mode: 2 f=3.67Hz zeta=0.66%
mode: 3 f=4.34Hz zeta=3.3%
mode: 4 f=5.14Hz zeta=0.66%
mode: 5 f=5.17Hz zeta=0.59%
mode: 6 f=9.42Hz zeta=0.76%
mode: 7 f=11.6Hz zeta=1.4%
mode: 8 f=12Hz zeta=1.8%
mode: 9 f=12.2Hz zeta=1.3%
Depar t m en t of
Civil &
St r u ct u r al
En gin eer in g.
Cross-section reveals increasing transverse
mode order
mode: 1 f=3.07Hz zeta=1.4%
mode: 2 f=3.67Hz zeta=0.66%
mode: 3 f=4.34Hz zeta=3.3%
mode: 4 f=5.14Hz zeta=0.66%
mode: 5 f=5.17Hz zeta=0.59%
mode: 6 f=9.42Hz zeta=0.76%
half-sine
half-sine
mode: 7 f=11.6Hz zeta=1.4%
mode: 8 f=12Hz zeta=1.8%
mode: 9 f=12.2Hz zeta=1.3%
full-sine
Depar t m en t of
Civil &
St r u ct u r al
En gin eer in g.
Chirp excitation & response
–Shaker induced response
drowned by traffic effects
–on last swipe (4AM?), almost
zero traffic
20
TP51z f/Hz
20
TP61 f/Hz
20
TP62 f/Hz
20
10
10
10
10
20
TP65 f/Hz
20
TP66 f/Hz
20
TP67 f/Hz
20
10
10
10
10
20
TP52 f/Hz
20
TP53 f/Hz
20
TP54 f/Hz
20
10
10
10
10
TP57 f/Hz
20
20
20
TP51x f/Hz
20
10
10
10
10
0
10
20
seconds
TP58 f/Hz
30
0
10
20
seconds
30
0
TP51z f/Hz TP63 f/Hz
20
10
TP105 f/Hz TP68 f/Hz
20
10
TP92 f/Hz TP55 f/Hz
20
10
20
TP101 TP64
f/Hz f/Hz
20
10
10
20
TP106 TP51
f/Hz f/Hz
20
10
10
20
TP93 f/Hz
TP56 f/Hz
20
10
10
TP97 f/Hz TP51y f/Hz
20
20
10
520 530
540 550
10
20
30
0
10
20
seconds
seconds
seconds
TP98 f/Hz
TP51z f/Hz
20
10
10
520
30
530 540
550
0
10
20
30
seconds
seconds
Depar t m en t of
Civil &
St r u ct u r al
En gin eer in g.
Good quality FRF; used for Global RFP
curve fitting
1.2
-3
x 10Modulus H2 (H1=red): ch20 vs ch1
4
1
x 10
-4
Real
: ch20 vs ch1
x 10
2
-2
0
-4
-2
-6
0.2
-4
-8
0
-6
-10
0.8
-4
Nyquist
: ch20 vs ch1
Coh
: ch20 vs ch1
0.6
0.4
Phase
: ch20 vs ch1
0
0
x 10
-3
Imag
: ch20 vs ch1
1
-0.2
0.95
-0.4
0.9
-0.6
0.85
-0.8
0.8
-1
0.75
-50
-100
-150
-200
5
10
15
Frequency /Hz
20
-1.2
5
10
15
Frequency /Hz
20
0.7
5
10
15
Frequency /Hz
20
Depar t m en t of
Civil &
St r u ct u r al
En gin eer in g.
Global rational fraction curve
fitting for two modes …
Depar t m en t of
Civil &
St r u ct u r al
En gin eer in g.
mode: 1 f=3.71Hz, zeta=0.92%
Modes 1,2 from GRFP
mode: 2 f=5.15Hz, zeta=0.56%
mode: 1 f=5.22Hz, zeta=0.42%
Depar t m en t of
Civil &
St r u ct u r al
En gin eer in g.
…. And using MACEC/OMAX
20
Z
10
0
120
100
80
60
40
40
20
20
0
Y
X
0
20
Z
10
0
-10
120
100
80
60
40
40
20
20
0
0
X
Depar t m en t of
Civil &
St r u ct u r al
En gin eer in g.
How did we do?
Ambient
mode Walkway Random
AVT
AVT
f /Hz  /% f /Hz  /%
0?
3.07 1.4
1
3.75 0.71 3.67 0.66
2
4.3 2.3 4.34 3.3
3
5.27 0.94 5.17 0.59
4
5.96 0.66
5
9.5 0.8 9.42 0.76
6
10.2 0.6
7
11.7 2.3 11.6 1.4
8
12.4 1.5 12.2 1.3
9
14.2 1.1
10
14.9 0.4
11
15.1 0.91 15
0.77
12
Forced
GRFP
Shaker
Circle fit
OMAX
shutdown
1
1
f /Hz /% f /Hz  /% m /t f /Hz  /% m /t f /Hz  /% m /t
3.69 0.61 3.69
0.63 81
5.22 0.63 5.22
0.57 273 5.22 0.61 300 5.20
9.46 0.58 9.49
0.52 354 9.48 0.53 2075
12.2 1.2
3.70 0.51 94
11.7
12.2
1.64 285
0.96 130 12.16 1.26 102
15
15.8
0.51 266 15
0.33 843
0.51 1237
3.68
0.83 71
0.96 192
Depar t m en t of
Civil &
St r u ct u r al
En gin eer in g.
Lessons
• Expected to be able to do two spans: took much longer to move
between swipes! Could have done one span per night
• We could get good quality FRFs with light ‘automobile’ traffic
• OMAX proved very effective
• Walkway swipe was very valuable –dynamic link between spans, no
evidence of amplitude dependence
• Orthotropic nature of deck revealed by set of swipes
• Movement at piers was surprising, shame we couldn’t study
horizontal movement
• Step-sine testing was tedious and not good value
• Shaker testing did give OK FRFs
Depar t m en t of
Civil &
St r u ct u r al
En gin eer in g.
What we would do in future
• Think seriously about the logistics of moving accelerometers
• Probably foregoing the luxury of full realisation of mode shapes & use
reduced cross-deck measurements
• Pay more attention to the bearings, which in any case are more
accessible
• Estimate modal mass, frequency and damping from shaker chirp
excitation
• Use either shaker shutdown or passage of heavy vehicle to obtain
reliable frequency and damping estimates.