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How to find natural frequency of pole?

How to find natural frequency of pole?

How to find natural frequency of pole?

(OP)
Hi

I need to find the natural frequency of a tall, hollow steel round pole.  basically it is a hollow steel pipe, 16" diameter.

Does someone know the equation for this?

Thanks

RE: How to find natural frequency of pole?

Roark's Formulas for Stress and Strain gives various formulae for a "uniform bar...vibrating along its longitudinal axis, upper end fixed, lower end free".

RE: How to find natural frequency of pole?

The fundamental frequency of a beam is (1/(2*pi))*sqrt(K/M).  Working this out for a cantilever with a uniform load (w)(selfweight) and a load at the end of the cantilever (P), the equation works out to  f = 3.13 / sqrt(((P+0.236*w*L)*L^3)/(3*E*I)).  P in kips, w in kips/inch, L in inches,

RE: How to find natural frequency of pole?

I'm glad you can answer this without knowing the fixity of the system.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: How to find natural frequency of pole?

Light Pole, Flag Pole, these both seem to be cantilevered examples.  Usually anything that is not fixed at the bottom and free at the top is not called a pole.  

Cheers

RE: How to find natural frequency of pole?

(OP)
Thanks for the replies.  Actually it is a foul pole at a baseball stadium.  It is indeed fixed at the base, and free at the top.

I am glad to see an equation.  I used Staad to output the frequency, but not having enough experience with this, I wanted some manual equations to compare with.  It is not in seismic zone, and is to find the gust factor for wind loading, to determine if it is considered a flexible structure, with the higher gust factor.

It might be overkill for a foul pole, but the stadium is municipally owned, and they are a good customer of ours, so I don't mind the extra effort.

Thanks for the Rourk's tip, I have a copy (as all engineers should) but didn't think to look in there.  

RE: How to find natural frequency of pole?

be very careful about the additional compliance introduced by the foundation.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: How to find natural frequency of pole?

(OP)
Thanks for the input.  I was already thinking about what type of foundation would be best.  I was thinking that a drilled pier would probably give me the most stable base.  I have loose sandy soils here.  I am going to get with the geotechnical engineer for the job and see what he thinks as well.

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