[TowerTalk] Antenna spacing question

jimlux jimlux at earthlink.net
Wed Jan 27 07:03:28 PST 2010

TexasRF at aol.com wrote:
> Hi Jim  and Pete, measuring the deflection of a mast is not the data  that 
> matters in wind survival. With a given diameter and wall thickness, the  
> deflection will be about the same for all steel alloys.
> What is needed is the stress that will be applied so it can be compared  
> with specifications of the material used. Measuring the stress is not a home  
> project but we have plenty of computer software available to accurately 
> compute  the stress.

An interesting point.

If you do the horizontal test, and load it to your design load, and it 
doesn't "fail", then, hey, you know that your design load is less than 
the failure load.  You just don't know by how much.

And, as pointed out, the deflection is more dependent on the modulus 
(stiffness) of the steel, and they're all about the same.

If you want to test empirically, you can either do a "proof test", where 
you load it with something MORE than you expect (say, twice) and verify 
no failure, or you can load an identical unit to failure, and then 
you'll know how strong it is.

The challenge with the load to failure is "how do you know the test 
article is *identical* to the real thing?", so you have to either do 
many tests (to get some statistics on variability) OR you make the 
failure test a preparation fora proof test (e.g. if your design load was 
100 lb, and you failed at 300 lb, then you can feel relatively confident 
that your proof test at 200lb for the real item is reasonable.

For one-offs that aren't close to the edge in performance, proof tests 
are easier.

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