[TowerTalk] Adding guys to self supporting towers

[Steve Maki]

On 8/14/2013 12:27 AM, Jim Lux 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.

Sure.

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

Sure.

> 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

Again easy to see.

My gut feeling on this is mostly the observation that a self supporter 
with roughly the same loading capacity as a guyed tower is built with 
MUCH more material, which tells me that horizontal wind force dwarfs 
dead weight in terms of member stress and requires heroic methods to 
withstand. These heroic assemblies if not actually helped by them, 
should be able to laugh at a few puny wire restraints.

I'll have to look up some actual structural's I have. We used to do a 
fair number of beef-ups. On self supporters it almost always involved 
beefing up diagonals and horizontals, but the failure was wind force 
related. Of course the SA never separates out what we would like it to 
for education, i.e. the contribution of dead weight vs wind force.

-Steve K8LX