Civil Engineering Reference
In-Depth Information
6.5 Comparison of Temperature to Modal Frequency
A plot of the changes in temperature over the period of 17 days is shown in Fig.
6.9
. The temperature varies between 9.4
C
and 25.7
C, with maximum during the day and minimum at night as expected; however the maximum and minimum values
are not consistent from a day to day basis, as was seen with the RMS and modal frequencies. Therefore it is not possible to
conclude that the temperature effect is directly affecting the change in modal frequency. Further analysis over a longer
period of time is will be performed to have a better understanding about the relation between temperature and modal
properties of the IMSNC bridge.
6.6 Summary and Conclusions
This paper presented a study on the monitored change in fundamental frequency for the IMSNC in Vancouver BC.
Examining 17 days of data, the fundamental frequency was shown to vary between 0.75 and 0.82 Hz in a roughly sinusoidal
pattern with the highest frequency occurring at night and the lowest occurring during the day. The change in frequency was
compared to changes in monitored temperature and in RMS acceleration of the vertical sensors at midspan of the bridge.
From the results, it was found that:
• The fundamental frequency of the bridge (vertical mainspan) varied between 0.75 and 0.82 Hz in a daily pattern, with
maximum (approximately) at midnight and minimum (approximately) at noon. Similarly, the first torsional mode of the
midspan of the bridge varied between 1.15 and 1.175 Hz with maximum at midnight and minimum at noon.
• The RMS amplitude of the vertical acceleration at midspan varied between 0.22 g at midnight and 2.48 g at noon, which
corresponds well with general traffic on the bridge having the highest levels during the day with reduced traffic at night.
Further, upon examination of a weekly pattern, it is seen that the fluctuation is regular during the 5 weekdays, with less
amplitude on Saturday, and the least amplitude on Sunday.
• Temperature is seen to vary normally with peaks during the day and lows at night. The variation over the 17 day period is
between 9.4
C and 25.7
C.
• 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.
Channel: 74
Channel: 75
X: 7.351e+005
Y: 25.75
24
22
20
0.825
0.82
0.815
18
0.81
0.805
0.8
0.795
0.79
16
0.785
0.78
14
12
X: 7.351e+005
Y: 9.403
10
Sun12AM
Sun12AM
Sun12AM
Fig. 6.9
The time variation of the first vertical mode of the main span
(green and blue lines)
and the measured temperature values
