19mm Dia x 720mm Fibreglass Tube Natural Frequency 3

[paulmech01]

I need to calculate the natural frequency of a 19mm Dia x 1.6wall x 720mm long Fibreglass Tube. I am trying to calculate the critical speed of of this member as it is developing vibrations at around 80km/h (wind speed). I would to see if the critical speed is close to this 80km/h.
If it is I will need to see what can be done to design out this problem, ie increase thickness, stiffness, length etc dampening etc...

 

[electricpete]

I just took another look at the coefficients in Den Hartog and Steve had the correct answer for the 3rd mode. I had jumped to the 4th mode. The 5 modes listed for this geometry (clamped on one end and free on the other) are:
a1=3.52
a2=22.0
a3=61.7 (I had used 121)
a4=121
a5=200

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