Civil Engineering Reference
In-Depth Information
Channel: 74
Channel: 75
2
0.83
0.82
1.5
0.81
0.8
0.79
0.78
1
0.5
Sun12AM
Sun12AM
Sun12AM
Sun12AM
Sun12AM
Sun12AM
Fig. 6.7
The time variation of the first vertical mode of the main span (
green and blue lines
) and the RMS values of the recorded ambient vibration
(red-line)
data
a
b
8
10
9
7
8
6
7
5
6
5
4
4
3
3
2
2
1
1
Sun12AM
Sun12AM
Sun12AM
Sun12AM
Sun12AM
Sun12AM
TIME
TIME
Fig. 6.8
The average amplification factor for: (
a
) the first vertical mode, (
b
) the first torsional mode of the mainspan of the IMSNC bridge
A recent work at The University of Nevada at Reno studied the effect of traffic on a bridge during a seismic event [
15
].
It was found that there is a significant improvement on the bridge performance during small amplitude motions due to the
presence of vehicle loads. This may be due to increased damping in the vehicle-structure system. Further study to investigate
any relationship in changes in damping and modal frequency with traffic load is warranted.
Such fluctuation in the RMS values does not only cause shift in modal frequency, but also causes amplification in the
modal response. Figure
6.8
shows the time variation of the amplification of first vertical mode of the bridge. This
amplification,
C
0
, varies for each modal frequency and is calculated for 2 min of moving windows. The amplification is
calculated as a constant value for each moving window. The value of the constant is typically determined by taking the
average of the standard FASs over a selected frequency band (
f
i
and
f
n
) as given in (
6.1
)
P
f
f ¼fi
X
ð
f
Þ
Y
ð
f
Þj
j
jj
C
0
¼
(6.1)
P
f
f ¼fi
X
2
ð
f
Þ
where
X(f)
and
Y(f)
are the Fourier amplitude spectrum of reference state and current state, respectively. The amplification is
calculated over a narrow frequency band,
f
i
and
f
n
, which contains the modal response for which the amplification is
calculated for. Derivation of constant amplification,
C
0
, is based on least-square and more information about it can be found
in [
16
]. The reference state is taken as the state on Wednesday August 17th at 12:10 a.m. where the constant average
amplification is calculated as 1 for the first vertical mode frequency as shown in Fig.
6.8
. Amplification of the first vertical
mode fluctuates between 1 and 10.2 with respect to reference state. The traffic load on the bridge amplifies the response of
the first vertical mode seven and ten times during daytime weekdays and daytime weekends, respectively. The first torsional
mode of the structure amplifies 5.5 and 4 times in daytime weekdays and weekends, respectively.
