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[TowerTalk] Lateral Forces on Tower Base?

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Subject: [TowerTalk] Lateral Forces on Tower Base?
From: ni6w@yagistress.minden.nv.us (Kurt Andress)
Date: Wed, 05 Aug 1998 08:40:01 -0700


T A RUSSELL wrote:

> OK, if you dispute that the lateral forces are NOT zero (or near zero)
> at the BASE of a tower,  would you kindly explain the origin and
> magnitude of these lateral forces you claim to exist?
>
> When there is NO  WIND, I cannot fathom any lateral forces at the base
> of a guyed tower.

I'm with you Tom, the thing might stand up on its own without the guys in
NO WIND.

>
>
> When there is a wind, the only lateral forces I see would be due to the
> (small) wind force against the bottom few sections, below the first set
> of guy wires, and these are shared between the ground and first set of
> guys, not to mention that the wind near ground level is highly abated
> by trees, buildings, and the earth itself.  (The wind load of Rohn 25
> tower is approximately 2 sq. ft. per section.)

This is also correct and agrees with what I've seen in the models.

The other source of lateral forces at the base is due to bending moments
caused by wire stretch. Imagine the whole tower leaned over several inches.
This creates a host of bending moments along the length of the tower and
the one at the base is usually one of the highest ones (exception: Big
stacks of antennas out the top can cause largest moments to occur in the
top of the tower). This only occurs when the base is rigidly mounted. To
react the base moment, the soil has to push back one direction at the top
of the foundation  and the opposite direction at the bottom. Hence more
lateral forces to add to the one you mentioned.
If the guy wire didn't stretch, the moments would only be caused by the
windload on the tower sections and the antennas, quite a bit less.
If the tower was free to rotate at the base, there wouldn't be any lateral
suppport requirements to counteract the base moment, only the 1/2 of the
side load on the lower span of tower.
Hope this helps!

73, Kurt

--
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