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Lateral Stiffness - Building frequency

Lateral Stiffness - Building frequency

Lateral Stiffness - Building frequency

(OP)
Hi,

I'm trying to get an idea of a buildings frequency, i've got a 70m high tower with a concentrated mass at the top.

Is there a rule of thumb when first considering its sway sensitivity?

Can i use f=1/2pi * sqrt k/M ? How do i go about determining it's lateral stiffness?

RE: Lateral Stiffness - Building frequency

Lateral stiffness, based on Hooke's law, is the force divided by deflection (F = kx, or k = F/x). For a concentrated mass on top of a tower, k = 12EI/h^3.

Brian C Potter, PE
http://simplesupports.wordpress.com

RE: Lateral Stiffness - Building frequency

(OP)
Thanks for the response, and nice blog!

RE: Lateral Stiffness - Building frequency

(OP)
Out of interest, what is an acceptable frequency? I'm getting around 0.4Hz

RE: Lateral Stiffness - Building frequency

The frequency is a determinant of the wind and seismic forces, so in part your lateral force resistance will determine what's acceptable. Otherwise you'll have to examine whatever code you're using for frequency/deflection requirements for towers.

Brian C Potter, PE
http://simplesupports.wordpress.com

RE: Lateral Stiffness - Building frequency

Structure Magazine has a article on Building periods that you may find helpful: http://www.structuremag.org/Archives/2008-6/C-CS-B... . Note that different approximate periods (inverse of frequency) are used for wind and seismic. ASCE 7-10 has some equations for both. The equations depends upon what type of lateral force resisting system is used. These could be used as a starting point. What percentage of the total building mass is concentrated at the top?

RE: Lateral Stiffness - Building frequency

(OP)
Thanks for the information,

It's an airport control tower, so its basically a 70m stairwell and then 3 cabs at the top.

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