Effect in transverse direction

THE EFFECT OF DIFFERENT SCOUR DEPTHS ON DYNAMIC PROPERTIES OF THE BRIDGE

7.5.2 Effect in transverse direction

Figures 7.21 to 7.26 show the simulated response of the Finite Element model in transverse direction for ground motion recorded in the east end of the bridge for the earthquake of 2008 for different scour depth. Peaks of acceleration have been found same for all scour depth. The frequency distribution in each case is as same as that of ground motion. Therefore, scour depth has. no effect in response along transverse .direction in this study.

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0,4

~% ^0,3

u .~

C.

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«

0,1

"" o ¹⁰

TIJre(sec)

(a)

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I I Ii. I"

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0.0

~ Q1 10

Frequency(Hz)

(b)

Figure 7.21: Response of bridge deck along transverse direction obtained from FEM due to the ground motion at west end of the bridge of the earthquake 2008 considering

Om scour depth, (a) Acceleration, (b) FFT.

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a

~ 0

v ] .10

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TIJre(sec)

(a)

OA

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0.1

0,0

50 0.1

Frequency{Hz)

(b)

10

Figure 7.22: Response of bridge deck along transverse direction obtained from FEM due to the ground motion at west end of the bridge of the earthquake 2008 considering

3 m scour depth, (a) Acceleration, (b) FFT.

10

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...,

0 10 20 30 40 50

Time(sec) Frequency(Hz)

(a) (b)

Figure 7.23: Response of bridge deck along transverse direction obtained from FEM due to the ground motion at west end of the bridge of the earthquake 2008 considering

6 m scour depth, (a) Acceleration, (b) FFT.

0.'

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Frequency(Hz) 0.1

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20 30 40

Time(sec) 10

40 30

~

20

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c 10 0

~•

~ ^-10

-20

-30 -40 0

(a) (b)

Figure 7.24: Response of bridge deck along transverse direction obtained from FEM due to the ground motion at west end of the bridge of the earthquake 2008 considering

9 m scour depth, (a) Acceleration, (b) FFT.

40

40

o ^{10 20 30 40}

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0.'

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00

50 0.1

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~+

10 Time (sec)

(a)

Frequency (Hz)

(b)

Figure 7.25: Response of bridge deck along transverse direction obtained from FEM due to the ground motion at west end of the bridge ofthe earthquake 2008 considering

12 m scour depth, (a) Acceleration, (b) FFT.

30

c:r 20

',1e_c ¹⁰

i !

^-10

-20

-30 -<0

o ^{10 20 30}

Time(sec)

(a)

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% ^0.3

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~E 0.2

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0.1

0.0

40 so 0.1

Frequency(Hz)

(b)

Figure 7.26: Response of bridge deck along transverse direction obtained from FEM due to the ground motion at west end ofthe bridge of the earthquake 2008 considering

IS m scour depth, (a) Acceleration, (b) FFT.

7.5.3 Effect inup-down direction:

Acceleration peaks along up-down direction of the simulated response of the FEM to the earthquake 2008 occurs near I Hz, 2 Hz, 5 Hz and 8 Hz for each scour depth as shown in Figures 7.27 t07.32. The components of 5 Hz and 8 Hz decrease with the increase of scour depth, whereas components of I Hz and 2 Hz increases. This entails that, energy transmission shifts to the lower frequencies with the increase of scour depth.

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10

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c-u

o

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.9!m

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B ^-5

-10

-15

0 10 20 30 40 so

Time{sec)

(a)

Frequency(Hz)

(b)

Figure 7.27: Response of bridge deck along up-down direction obtained from FEM due to the ground motion at west end of the bridge ofthe earthquake 2008 considering

Om scour depth, (a) Acceleration, (b) FFT.

10

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i

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01

50 20 40

10 15

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-15

o

-10

I

c o

I

Time(sec) Frequency(Hz)

(a) (b)

Figure 7.28: Response of bridge deck along up-down direction obtained from FEM due to the ground motion at west end of the bridge ofthe earthquake 2008 considering

3 m scour depth, (a) Acceleration, (b) FFT.

15 0.40

10 0.35

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0.05

_15 0,00

10 20 30 40 50 0.1 10

Time(sec) Frequency(Hz)

(a) (b)

Figure 7.29: Response of bridge deck along up-down direction obtained from FEM due to the gro\ffid motion at west end of the bridge of the earthquake 2008 considering

6 m scour depth, (a) Acceleration, (b) FFT.

10 Frequency(Hz)

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-10

-15

0 10 20 30 40 50

Time{sec)

(a) (b)

Figure 7.30: Response of bridge deck along up-down direction obtained from FEM due to the ground motion at west end of the bridge of the earthquake 2008 considering

9 m scour depth, (a) Acceleration, (b) FFT.

1 10 Frequency (Hz)

(b)

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20 20

Time (sec)

(a)

10 -15

o

15

10

-10

Figure 7.31: Response of bridge deck along up-down direction obtained from FEM due to the ground motion at west end of the bridge of the earthquake 2008 considering

12 m scour depth, (a) Acceleration, (b) FFT.

15 0.7

10 0.6

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c %

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-10 0.1

-15 00

0 10 20 30 50 0.1 10

Time(sec) Frequency(Hz)

(a) ^(b)

Figure 7.32: Response of bridge deck along up-down direction obtained from FEM due to the ground motion at west end of the bridge of the earthquake 2008 considering

15 m scour depth, (a) Acceleration, (b) FFT.

7.6 Summary

The Jamuna Multipurpose Bridge is a 4.8 KIn long bridge, which contains seven individual modules. The river bed level varies throughout the length. Since, the pile- cap stiffness significantly varies with the change of free length of piles; support conditions of the piers of the seven modules are different although the super structure properties are the same. This study includes response of the Module 2 only, as this module is the equipped with accelerometer sensors. Scour depth is zero beneath Module 2. The deviation in the dynamic behaviors of the other modules due to the difference in scour depth has been observed in this study.

Significant increase in the modal periods corresponding to soil-structure interaction has been observed due to increase in scour depth. Peaks of acceleration of the responses in longitudinal and up-down direction are reduced due to increase in scour depth. Increase of scour depth minimizes the response of bridge module to the earthquake of 2008. Therefore, it can be said that, the responses of other modules of the bridge to the earthquake 2008 are less than the recorded response in second module; since the scour depth of this module is zero.lncrease in scour depth makes the supports less stiff. That leads the energy of excitation shift to lower frequencies due to increase in scour depth.