[TowerTalk] Guying an Aluminum Tower

[Jim Lux]

At 09:05 AM 11/21/2006, Doug Renwick wrote:
>The topic about guying a self supporting tower has been
>discussed here before.  There seems to be an unlimited
>supply of misinformation on this topic.  FACT...self
>supporting towers can be, have been and will continue to be
>guyed!  This is supported by experience and by tower
>manufacturer's recommendations.  So let's get rid of this
>flawed opinion.

I think more correct statements might be:

a) some manufacturers provide for guying of some self supporting 
towers, with revised loads
b) A casual, "let's just throw some guys on this tower" approach will 
probably work, but may have failure modes that are unexpected, and 
the actual failure loads might be different (higher, lower, who knows).

Lots of people put up towers, don't follow the manufacturer's 
recommendations, grossly exceed the design limits, and have no 
problems.  Essentially, they're just playing the odds, and that's 
fine.  If the consequences of a failure are something you can accept, 
have at it.

Heck, even the engineered limit is based on probability.  The wind 
speed rating for the county is not a "guaranteed to never be 
exceeded" limit, but a "really unlikely to be exceeded" speed. It's 
sort of like designing for the 100 yr rainstorm or flood...in any 
given year, there's a 1% chance that the limit will be exceeded, so, 
over 20 years, there's a 82% chance that the structure will survive.

There's two sorts of numbers you'll see.. a "serviceability limit" - 
a load that can reasonably be expected to occur sometime in the 
design life, albeit infrequently; and a "ultimate failure limit" - a 
load that is unusual and represents a point at which "it's ok if it 
fails".  These are sometimes given in years, e.g. 20 years for 
serviceability, 1000 years for ultimate failure.

In the ham world, the dictum: "If the antenna didn't fall down last 
year, it wasn't big enough" implies a serviceability number on the 
order of 1 year.  Your spouse's dictum: if that thing falls down and 
breaks the house, you're history, implies a design period of 50 years or so.

There's also a "safety factor" in the material properties used for 
the design (i.e. you might design using a failure stress for the 
steel of 20,000 psi, which is a lot less than it is likely to fail 
at.. this accounts for unforeseen voids or defects in the steel)

If you're in a situation where you need any sort of "knowledge" about 
the structure, then you need to do the calculations, etc.


>Doug/VA5DX
>
>Subject: Re: [TowerTalk] Guying an Aluminum Tower
>
>This is like a trapeze artist wondering if he should work
>without a net.
>This idea is flawed from the get-go. A SELF supporting tower
>is just that.  It is not designed to be guyed and should
>not. If you get a warm fuzy feeling about guying the tower,
>that it is ok since you are overloading it, you should
>re-think that.
>
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