[TowerTalk] Adding guys to self supporting towers

[Jim Lux]

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.

looking at a BX manual (the only one I have handy).. the diagonal braces 
have maximum allowable compressive loads around 1000-1200 lbs (sections 
1-4) or twice that (sections 5-8).  The verticals have max loads around 
5000-6000 lb (sec 1-2), gradually increasing, up to around 20,000 lbs 
for section 8.

the loads analysis shows vertical loads in the 19000 lb range for the 
bottom (BX-8) and 2400 lb for the top (BX-1).  That's not a huge margin 
for the bottom section.  The diagonal brace loads are a few hundred 
pounds, which is roughly 25-30% of the allowable.  The question I would 
have is whether increasing the down load (some of which transfers to the 
diagonal braces.. Rohn uses cos(angle) for the factor) would change things.

The other question is (particularly in the case of BX, which has a very 
low allowed antenna area vs the tower area) whether the guying is being 
done not to "help" the existing design and usage, but to allow putting 
more antenna on the tower.  Or, is it to handle "extreme events" (in 
which case the guys could actually be slack in normal use with normal 
winds).

I think the big take home is that while something like Rohn 25 is fairly 
straightforward to analyze, being of constant cross section and 
construction, any sort of tapered self supporting tower gets a lot more 
complex.  A cursory glance at the BX analysis shows that the ratio of 
"expected load" vs "allowable load" varies a lot up and down the tower 
(mostly because there are standard sized parts used in various places). 
  Add in the non-uniform wind speed over the height of the tower, and I 
think any sort of casual analysis is doomed to failure.