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[TowerTalk] Rohn joint bolts

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Subject: [TowerTalk] Rohn joint bolts
From: sawyers@inav.net (Steve Sawyers n0yvy)
Date: Sun, 11 May 1997 21:56:51 -0500
The bolts in a Rohn 25 tower are what keeps the towers sections from
jamming themselves together for a totally compressive load. So they are
pretty important from that stand point. 

If the tower has much bending stress on it (say the joint between the
top section and the one below it when you have a mast and a lot of wind
area above it) then the bolts are critical to keep the tension side of
the tower from pulling apart.

As far as why the shear strength is important - well - the tower base is
connected to the tower leg and the tower leg is connected to the tower
leg bolts and the lower leg bolts are connected to the tower leg
above.....

It is a chain of pieces that form the tower. The tower is only as strong
as it's weakest link. The bolt size and hole size and tower leg diameter
and tower leg wall thickness in a Rohn tower are very carefully designed
so that each link is of almost exactly the same strength as all the
other links. If you change the bolt material to stainless (normally I
love stainless with the appropriate application of ant-seize) you change
one link in the chain.

In your application, as you pointed out, the tower may not be loaded to
it's max so the stainless bolts MAY be acceptable. But to be sure, you
must know the maximum tower compressive and bending loads on your tower.

Hope this helps

de n0yvy steve

Roderick M. Fitz-Randolph wrote:
> 
> >Stainless steel bolts are not as strong as the SAE 5 grade bolts
> >that Rhon uses.
> >
> >SAE Grade 5 bolts are 120,000 psi UTS (ultimate tensile strength)
> >
> >A typical stainless steel bolt is rated at about 90,000 psi UTS
> _________________________________________________________________
> 
> I certainly can't argue with the figures above that were placed on
> the Reflector.... but somehow I just can't get it through my thick
> skull why that makes any difference.  The UTS of 90,000 psi is not
> being challenged on my tower, to my knowledge.  The guys, anchored
> by 4,000 lbs of concrete would probably IMO keep the tower upright
> with no bolts at all, after once installed.  Surely, 90,000 lb
> tensile strength bolts will keep the tower sections together.
> Admittedly, I am a retired electronic engineer and not a structural
> engineer but I do declare that I can't for the life of me see what
> difference in a tensile strength of 90,000 or 120,000 makes in that
> application.
> 
> This is not a flame: it is an earnest effort on my part to have
> someone who is knowledgeable explain to me (and perhaps there are
> others that would like an explanation) where the difference comes
> into play and how it affects the life/stength of the tower as it
> is installed (with the legs overlapping as they do on Rohn 25G).
> 
> Again, I believe that I'd a sight rather see a 90,000 psi tensile
> strength stainless bolt that is still 90,000 psi after 10-20 years
> than a 120,000 psi tensile strength steel bolt that is actually
> rusted through before 20 years (and don't tell me that doesn't
> happen in Florid!  I've taken down some old Rohn down there!!!)
> 
> If I need to be enlightened, please enlighten me!!
> 
> Rod, W5HVV
> 
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