how to calculate the natural frequency of poles
how to calculate the natural frequency of poles
(OP)
I try to find the wind load on a pole with ASCE-7.
I assume the pole is flexible structures so I need to calculate the Gust Effect Factor G. so I have to know the natural frequency of the pole.
In the code, the fundamental period of structures is calculated as Ct h^x in seismic section. but if I use this formula, the pole is rigid structure (f > 1 HZ).
I am wondering if I used the right equation or should I just calcualte the frequency using flexural rigidty (EI) of a cantilever beam (the pole).
any suggestions are welcome.
I assume the pole is flexible structures so I need to calculate the Gust Effect Factor G. so I have to know the natural frequency of the pole.
In the code, the fundamental period of structures is calculated as Ct h^x in seismic section. but if I use this formula, the pole is rigid structure (f > 1 HZ).
I am wondering if I used the right equation or should I just calcualte the frequency using flexural rigidty (EI) of a cantilever beam (the pole).
any suggestions are welcome.






RE: how to calculate the natural frequency of poles
RE: how to calculate the natural frequency of poles
If you want to do this by hand procedure is as follow :-
Here I have considered that there is mass lumped at the tip of the pole.
1) Calculate the deflection of pole with horizontal unit force at the tip of the pole.
2) Inverse the value which you have got, this is stiffness of the pole K.
3) You have mass lumped at the tip of the pole. Let's call it as M.
4) Circular frquency of vibration is given by
OMEGA = sqrt(k/m)
5) Frequency in Hz is given by
N = OMEGA / [2*pi()]
If there is no mass attached at the tip of the pole, and pole is self standing then frequency of pole is given by
OMEGA = [3.66/L^2] x sqrt[E x I / M]
L = Height of pole
E = Modulus of elasticity
I = Moment of inertia
M = Mass per unit length
Note - In above equation it is assumed that pole is considered as of uniform shape
Hope this will answer your querry
RE: how to calculate the natural frequency of poles
3.66/in^2 * sqrt(lb/in^2 * in^4 / (lb/in))
=3.66/in^2 * sqrt (in^3) ?
RE: how to calculate the natural frequency of poles
RE: how to calculate the natural frequency of poles
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: how to calculate the natural frequency of poles
Cheers
Greg Locock
I rarely exceed 1.79 x 10^12 furlongs per fortnight
RE: how to calculate the natural frequency of poles
Why is that wrong?
RE: how to calculate the natural frequency of poles
What's wrong with it?
RE: how to calculate the natural frequency of poles
Formula 3-3 of DG#11 is for beams supported at both ends. For a cantilevered beam, the attached example solves for the first natural frequency for a cantilever with a uniform weight and a concentrated load at the end.
RE: how to calculate the natural frequency of poles
Is DG #11 wrong or I am reading something wrong?
RE: how to calculate the natural frequency of poles
You're right. DG #11 says the natural frequency can be approximated using Eq 3-3 to 3-5. The difference between the formula I posted vs. DG #11 is 12%, so it's a good approximation. See the attached example.
RE: how to calculate the natural frequency of poles
Cheers
Greg Locock
I rarely exceed 1.79 x 10^12 furlongs per fortnight
RE: how to calculate the natural frequency of poles
RE: how to calculate the natural frequency of poles
n = [0.56/L^2] x sqrt[E x I / M]