[TowerTalk] Aluminum Tower Mast

[Roger (K8RI)]

>
> 6063 is cheaper than 6061 so is the alloy of choice for electrical
> conduit (sch 40)&  irrigation pipe (sch 10 if it has gasketed
> couplers).  Gas/water pipe uses NPT (national pipe thread) tapered
> threads for sealing, conduit uses NPST (Nat pipe straight threads) which
> have no taper, thus allowing more adjustment in joints.
>
> 6061 and 7075 are the common structural aluminum grades, usually in T6
> or T651 temper and are the ones that should be used for a mast.  I had a
> TH7DX at 15' above top of rohn 25, 2" od x 0.375 wall 6061 T651 and it
> was up for 15 years and 3 hurricanes w/o a problem.  I used 6061 to save
> weight because the tower was hinged at the base.  Weights: 94lb for 2" x
> .25 wall 4140 vs 46lb for 6061 x .375 wall vs 70lb for rohn 45 x 10'.
> So hoist of a 20' x 2" x .25 wall steel mast is more load than rohn45
> for a gin pole.
>
> A problem with aluminum not mentioned before is there is no defined
> stress threshold to avoid fatigue, it just keeps going down with stress
> cycling.  Alloy steel has a plateau in the fatigue curve which if not
> exceeded, the structure will never have a fatigue failure  (about
> 35ksi).  But it takes 10 million cycles or so to get into trouble with
> aluminum, so that may not be a concern for the typical ham mast, but
> some airplanes did fall out of the sky until this was understood.
>    
And that was due more to construction methods used in the Dehavalin 
Comet as well as the much more severe expansion and contraction with 
pressurization. We had no experience with such extreme pressurization 
cycles prior to that time as well. Remember those things are built like 
beer cans and depend a great deal on shape for strength.  Remember the 
737 convertable that landed in Hawii? How'd you have liked a window seat 
in that one? <:-)) I can see an open cockpit in an old Stearman, but a 
737?  The failures in the Comet came from holes for window fittings 
which created what are called "stress risers"
IIRC the Hawiian convertable was due to some ignored cracks from the 
corner of a door.

People fail to realize just how frail that skin is.  I once observed a 
guy stand his 5 year old kid on the horizontal stab of an almost 
priceless, one of a kind, antique .  Yes the skin of the airliners is 
much stronger compared to smaller planes,  but even the door on mine is 
structural.

73

Roger (K8RI)
> The Jorgensen handbook/catalog has a wealth of information, pdf at
> http://www.emjmetals.com/pdf/bluebook-k.pdf
>
> Grant
> KZ1W
>
> knormoyle at surfnetusa.com wrote:
>    
>> [Dennis] "From day one, we used an aluminum 2" OD THICK WALL ALUMINUM CONDUIT Mast."
>>
>> I was curious about aluminum conduit. A quick search says that modern aluminum conduit is 6063 temper T-1
>> (our typical 6061 use is T6)
>>
>> Wikipedia says ( http://en.wikipedia.org/wiki/6063_aluminium_alloy )6063 T-1 has a yield strength of only 9kpsi
>>
>> Regardless of the wall thickness, I would think the low yield strength would make many other options better for equivalent weight.
>>
>> also, conduit is specified as I.D. was this 1.5" I.D. ->  1.9" O.D. conduit? or 2" I.D. ->  2.375" O.D. ?
>>
>> ??
>> -kevin
>>
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