Ambient Vibration Test

CHAPTER 5
ESTIMATION OF NATURAL FREQUENCIES OF BRIDGE
5.1 GENERAL
This chapter present a discussion of the findings obtained from the methodology that were presented in the chapter 3. Firstly the result for the ambient vibration test using Fourier spectrum analysis are outlined. This is followed by the presentation of the result of the analytical methods. The result from the two analysis techniques are discussed.
5.2 ESTIMATION OF NATURAL FREQUENCIES: EXPERIMENTAL METHOD
Ambient vibration test is performed on the highway bridges and Fourier amplitude spectrums draw for the time histories of vibration for all five bridges. Frequencies for different mode are selected from the frequency-amplitude spectrum. The spectrum indicates the locus of the frequencies of the same type. The Fourier spectrum produced three resonant frequencies with at least three times repeatability in the

Ambient vibration test (AVT) (Hz) IS Code (Hz)
1 3.44 2.72
2 3.20 2.38
3 3.74 2.38
4 3.22 3.59
5 4.28 2.23

It is observed from the Table-5.4 frequencies obtained for longitudinal direction of bridge by IS code method is less than the frequencies obtained through the AVT (ambient vibration Test). It difference is varies from for 20% to 47% for bridge (S.No. 1,2,3,5) but bridge 4 estimated natural frequencies is more than the AVT it is due to the over estimation of stiffness of the structure. In IS code there is no provision for the estimation of natural frequencies of pre-stressed structures, so the estimation is done without considering the effect of the pre-stressing.
Table 5.5: Comparison of Frequencies of Bridge
S.No. Bridge Ambient vibration test Frequency Multi-degree Model Frequency Mode 1 Mode 2 Mode 3 Mode1 Mode 2 Mode 3
1 Bridge 1 3.44 5.48 7.95 2.72 3.42 8.77
2 Bridge 2 3.20 5.80 8.78 2.38 5.76 18.53
3 Bridge 3 3.74 4.6 5.74 2.38 4.48 10.43
4 Bridge 5 4.28 5.48 5.84 2.23 3.67 10.43
Frequency: