[TowerTalk] Used Tower-Practical Strength Considerations

[K8RI on TT]

On 5/11/2011 9:05 PM, Wilson Lamb wrote:
> OK, so I dragged in 70' of used tower.  It was a "bargain".
> There's some light rust, but nothing structurally significant.
> Now to the questions.
What make, type, and size of tower?
"Light surface rust" with no pitting?
Can you check the inside of the legs to make sure there is no rust in there?
There are electronic "thickness gauges". I used to have to check 
pressure vessel wall thickness using one in a "tank farm". Very easy to 
use, probably expensive, but it'd be nice to find one to borrow.
The main thing that bothered me about taking down used towers was the 
though of rust inside the legs.
> Several sections have 1/4 and 5/16" holes.  I'm thinking someone may have
Most of the smaller towers use two sizes of bolts including the old 
American Steel...or what ever it was called.
There should be one 1/4" and one 5/16" at each leg junction.
> bored out some to get tighter fits on the bolts.  Also, some sections have
> ratty holes that look wallered out by being loose.  It's hard to visualizr
> this happening, except for a heavily loaded tower with a lot of load, may
> self supporting?
>    Is it common?

This is not uncommon when the bolts are not tightened/torqued down to 
the proper values.  Loose bolts causing elongated holes is probably more 
common than overloading.
> Have others seen it?
Many times.
> I have no good feel for how much of the bending load gets to the bolts,
The bolts receive shear force.
>   but
> it seems like the overlappiing joint would take most of it, especially at
> moderate load.  Does anyone have model results or experience with joint
> failures?

I've never seen one fail, but I've seen some in pretty bad shape. Most 
were caused by bolts that weren't tight enough and/or on joints that had 
been lubricated. Of course too tight a bolt will flatten the leg which 
is bad as it reduces the leg strength.

> I've seen dropped towers with serious bending nut never a joint failure.
>
> Has anyone bored the holes in 25G to 3/8"?
ROHN says not to bore holes even where the bolts are too tight to fit, 
but rather use a "taper pin punch" to enlarge the hole just enough for 
the bolt to fit. The were specific about not screwing the bolts in 
either.  The reason for that is to prevent removing the galvanizing, but 
that's not a problem where the holes are already enlarged due to movement.

Myself, I'd not use sections that had enlarged bolt holes that needed to 
go to the next size bolt.
HOWEVER there are engineers on here who know a lot more about that than 
I do.

> Could the loss of cross section be significant to joint strength?
Depends on the location of the brace.  It certainly reduces the strength 
of the tower to bending moment and if two of them are adjacent the 
reduction could be substantial.
> Has anyone seen bolts fail in shear?
Not in a 25G
> Should one use higher grade bolts, or is the leg material likely to tear
> before the bolts fail?
Even ungraded galvanized bolts are much stronger in shear than the thin 
tower legs.
> A couple of braces are detached at one end, although there is no visible
> bending of the legs.  I'm guessing this was caused by some sort of
> mishandling, but don't know.  I'm hoping that a nice MIG weld will be
> sufficient for reattachment??

Are you proficient at MIG welding?  It's probably not critical, but 
tower welding is not the place to learn. This is welding the rather 
thick brace to a very thin tube.

Clean the area well, and make sure none of the galvanizing gets into the 
weld. It'll pop and splatter all over the place as well as weakening the 
weld.  If MIG welding outdoors you need a very calm day or crank the 
cover gas flow WAYYYY up.

> I'm asking all this to see if there are problems that might bite me.

With towers there are always problems that can bite you, more so with 
used towers.
Rust and elongated bolt holes are not good signs that require added 
inspection and care. Never use over size guys on small towers. The added 
load on the bottom sections can be tremendous.
It's best to use original materials for fittings and bolts. Do not lube 
joints.

SS bolts require an anti seize compound on the threads which will change 
the required torque.  IMO it's best to stick with galvanized as strength 
should not be a problem.

I'm sure some of the more experienced will chime in here.

73 and good luck,

Roger (K8RI)
>   I
> don't plan to go very high, 60' max, but want to not worry about my friends
> or me climbing for installation and, hopefully in the distant future,
> maintenance.
>
> Thanks for any guidance or experience you can share.
>
> Wilson
> W4BOH
>
>
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