Eigen frequencies of a multiple span cantilever beam 1

[vishkan]

Hello everyone,
I am very new to this forum. I am involved in a dynamic stuctural anylisis project.
A chimney stucture made of 4m diameter steel cylinder (thich t =10mm)and around 200m tall. Structure is supported in 3 locations around 50m spans.
Should i have to consider wind load to calculate eigen-frequency?
How to cal culate the first natuaral frequency? Should i use the difflection method to find the curvature then calculate the formula to natural frequency.
I am using a FE program, when i use the two different model one using beam elements and other using surface elements, unexpectabbly beam elementsmodel gives lover values than surface. I am not sure where i make mistakes. I have no real expereince. Can you please give anyone give me advice, how to approach. I need to get same results in both model...

i would like to confirm with the analytical freqency values.
Can you please advice me solve this.

Thank you in advance
i add here my small sketch of structure.

 

[GregLocock]

wind loads don't matter for mode shapes and frequencies. The first natural frequency of that structure is not easy to estimate by eye, as the mode shape is rather irregular. It will be of the order of a pinned pinned beam 50m long. However I have no doubt that in your class you have been given sufficient clues to solve it analytically. I'm guessing Rayleigh-Ritz. You may even find the solution in RD Blevins excellent book.

Once you have a good analytical solution debugging your FEA should be easy.





Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[boyze]

this might be of help

Have Fun!

James A. Pike

http://www.xl4sim.com

http://www.erieztechnologies.com

 

[vishkan]

Thank you very much Mr Locock, your idea already helped me to calculate analitically.. i got almost similar value for my first mode frequency..from FEM and equation..
Thank you Mr Pike.. it seems really more helpful for my future work too.. thanks for the link.. you guys really helping people like me to start this structural engineering..
Greets
Vishva