How to find natural frequency of pole?

[structuresguy]

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

 

[dbuzz]

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".

 

[nad1019]

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,

 

[GregLocock]

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.

 

[nad1019]

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

 

[structuresguy]

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.

 

[GregLocock]

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.

 

[structuresguy]

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.