[TowerTalk] Adding guys to self supporting towers

[Steve Maki]

Richard was talking about self supporting towers, not self supporting 
crank up towers.

Yes, the lift cable is a definite factor in any consideration of adding 
guys to a crank up.

But I've never seen an analysis of a simple self supporter which 
indicates that adding *reasonable* guys does anything other than 
increase straight on wind load capacity.

Twisting torque for long boom scenarios must be considered independently.

-Steve K8LX

On 8/12/2013 5:58 PM, Grant Saviers wrote:

> I can't follow your analysis, so here are some numbers for an HDX589
> from my PE analysis:
>
> EIA-222-F 85 mph 3 second gust 71 mph fastest mile no ice
>
> leg compression 41538#  base connection moment 80330 ft-lbs  lift cable
> force 6332 lbs.  Big numbers compared to your calcs.
>
> A guy at 45 deg will add 1lb of downforce for every 1.414 lbs of
> tension.  So, just the pretension of 1/4 EHS would add about 1500 # with
> 3 guys.
>
> My casual review of the PE analysis convinced me that the engineering of
> self supporting towers has thin safety margins, around 1.3 to 1.6.  Like
> many structures, the weight vs load vs strength is a complex set of
> tradeoffs.   An optimized design would have all safety factors for all
> components the same, minimizing weight and cost.  The tradeoff for a
> tower maker is making multiple towers from the same components, thus any
> particular model isn't optimized.  Would a guy on the fixed section
> help?  On the 589, it appears that the third section has the least
> safety factor, it's likely every tower model is different.
>
> That said, I would bet on the hoist cables as the failure point if guys
> were added.  The guy pretension alone adds 24% load to the hoist cable.
> There are many posted anecdotes of cable failures, whether they are poor
> maintenance or something else, and seem to outnumber leg failures by a
> significant ratio.  Others have noted the as designed cable load is
> higher than what is usually accepted practice, certainly way above "man
> rated".
>
> Grant KZ1W

> On 8/12/2013 2:06 PM, Richard Karlquist wrote:

>> The topic of adding guys to self supporting towers has been
>> discussed a number of times on this reflector, but I am
>> still not clear on exactly why it supposedly won't work.
>> If I have figured crrectly, the worse case compressive load
>> on a leg at the bottom of a trianguler tower
>> is equal to the product of the horizontal windloading
>> at the top times the factor h/[w*sqrt(3)] where h is
>> the height and w is the width of a face.
>> For example, a 100 ft windload at the top of a 50 foot
>> tower with 18 inch face will induce 1925 lbs of
>> compressive force.  If a the tower is guyed at the top
>> with the guys at 45 degree angles, the compressive
>> force is simply equal to the horizontal wind load, or 100 lbs.
>> Much less than the unguyed 1925 lbs.
>>
>> It seems to me the guyed tower is much stronger and could
>> handle a considerably larger wind load based on this
>> simple analysis.
>>
>> The only way I can see that this wouldn't work is if a
>> larger antenna resulted in torque loads that the tower
>> couldn't handle.  For example, the HDBX series, well known
>> for its poor torque strength, would be a poor candidate
>> for guying.  OTOH, a light weight tower with a large
>> face width might be able to take a lot of torque.  To
>> facilitate this, you might want to build the tower with
>> less taper than it typically has in the self supporting
>> configuration, or maybe no taper.   All of this depends
>> on wind area and boom length.
>>
>> Comments?
>>
>
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