[TowerTalk] Parting comments re: Tower Forces

[T A RUSSELL]

On Thu, 06 Aug 1998 02:25:33 -0700 Kurt Andress
<ni6w@yagistress.minden.nv.us> writes:
>
>Forgot to say a few things that may help a few fellows,
>For guyed towers,
>My analyses have found that when the wind blows in line with a set of
>guys, the guy line loads are maximized and the tower bending loads are
>minimized.

	The MAXIMUM load on guy wires occurs when the
	wind blows parallel to a tower face, 30 degrees off
	line from a guy wire (assuming 3 way guying),
	NOT  IN  LINE.      de  N4KG

>When the wind blows exactly between two sets of guys, the guy loads 
>are
>at their lowest value, and the tower bending loads are at their 
>highest
>value. This is caused by geometry completely!

	When the wind bisects two guys spaced 120 degrees,
	the load splits between the guys and is EXACTLY	
	the SAME as when the wind blows IN  LINE with
	a guy wire.  Consider  horizontal guys for simplicity,
	spaced 120 degrees.   For the bisecting case, the load
	is carried by the guy tension times SIN 30 degrees (=0.5)
	With TWO  guys, each carries 1/2 the load, equal to
	the load carried by a single guy when the wind is in line.
	(Kurt, have you ever had a Mechanical Engineering course
	in STATICS? The above erroneous claims lead me to 
	believe your knowledge of static force resolution is lacking.) 
	de  Tom  N4KG  (BSEE, MSEE, and yes, I studied Statics)
	
>Since, there are apparently many who can't understand how wires 
>stretch
>etc., there are many more comments to come regarding how it doesn't
>matter what direction the wind blows. You guys get to sort this thing
>out! Believe it or not!
>
>Oh yeah, why are all of the Rohn tower footings deeper than they are
>wide?
>If the tower loads were pure compressive (I.E. pushing only down), 
>they
>would have spec'd  very shallow footings that were 1' deep and 3'
>square.
>Since, they spec deep footings, compared to the vertical footprint, 
>one
>might assume that they think that some kind of lateral loads are
>present, and need to be counteracted by the obvious depth of their
>footing.

	The primary purpose of a base is to carry the VERTICAL
	load of the tower, preventing it from sinking into the 
	ground when the ground is "soft" and preventing the 
	ground from pushing up on the tower and over tensioning
	the guys when the ground is frozen.  The depth below
	ground is to get below the level where freezing will
	cause problems.  Lateral forces are minimal.  Consider
	a 100 ft tower, with a 1 foot off-center tilt   The lateral 
	force at the base of the tower will be 1/100 of the vertical
	force.  Lesser "tilt" will have even lower lateral force.
	de   Tom   N4KG

>What do they know? Obviously precious littlte!
>When in doubt, do what the tower manufacfturer says!
>Sorry I suggested that guy wires might not have infinite stiffness!
>CUL,  Kurt
>
>--
>YagiStress - The Ultimate Software for Yagi Mechanical Design
>Visit http://www.freeyellow.com/members3/yagistress
>
>

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