[TowerTalk] Adding guys to self supporting towers

[Jim Lux]

On 8/13/13 6:29 PM, Steve Maki wrote:
> On 8/13/2013 10:09 AM, Jim Lux wrote:
>
>> On 8/13/13 6:47 AM, Jim Thomson wrote:
>>> ##  The stress on the cross bracing is actually reduced when guys are
>>> added to a
>>> free standing tower, not an issue.
>
>> How so?
>>
>> In a no wind static case, the guys add down force, some of which will
>> appear on the diagonal cross braces.  How much depends on the relative
>> compression stiffness of the brace vs the verticals.
>>
>> In the wind loaded case, the guys transform some of the horizontal
>> bending load into down force.  That could be a wash in terms of loads
>> but it's hard to tell, depending again on the relative stiffnesses. E.g.
>> downwind side has less load due to bending, but more load due to guy
>> induced downforce, but the guy downforce is distributed among all
>> three/four vertical legs.
>
> I'm having a hard time with this. It seems to me that in the guyed
> no-wind state, the added force to diagonals from guy tension will be
> almost nonexistent as long as the guys are tensioned equally. Isn't all
> of the additional vertical force, well, vertical?

No.. think about two angled elements forming a triangle, and loading the 
apex of the triangle.  Both elements are in compression.

Same thing if you had 3 elements, one vertical, and two angled on either 
side.  The forces will distribute according to the compressive stiffness.

The whole stiffness thing is a real challenge..

Think of this.. say you have a 10 foot length of piano wire which can 
support 1000 lbs.  You also have a 10 foot length of nylon rope that can 
support 1000 lbs before it breaks.

Two nylon ropes in parallel support 2000 lb.
Two piano wires support 2000 lb
A piano wire and a rope in parallel support just over 1000 lb.  The 
piano wire is a lot stiffer, so it takes all the load




>
> I thought that stress in the diagonals is the result of flexing of the
> legs, which MUST be most pronounced in the unguyed windy condition???
>
> -Steve K8LX
>
>
>
>
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