[TowerTalk] Tower buried section legs -- Buried in Concrete orBelow the Concrete?

[Jeff DePolo]

> You should know 
> that there aren't "hole in the base plate" pier pin bases made for 65 

All of the standard 65G guyed "catalog" packages come with a tapered base
section for use with a pier pin.  We have a 280 foot 65G at one of our FM
clients' sites with a tapered base on a pin, and that's over 20 years old,
so it's not something that was changed recently.

> and that Rohn supplies buried bases for 25, 45, 55, and 65 at 
> a fraction 
> of the cost of pier pin solutions.

We recently put up a 45GSR (solid-rod legs) on a pier pin.  It doesn't use a
traditional lattice tapered base section, but rather more of a tapered
"adapter" that goes on the bottom section.  The tapered section is only
about 2' tall (from memory).  The bottom has a rocker plate that is designed
to rest on a steel base plate that fits over, but is otherwise un-attached
to, the pier pin.  The tapered section is part number 45GSSRTBPP, email if
you'd like a copy of the print.  As I recall, it was appreciably less than
the cost of a lattice section.
 
>  From what I've seen and heard about tower failures, guyed 
> towers almost 
> always come down when a guy fails in some manner (turnbuckle, 
> insulator, 
> termination, guy corrosion, tree fall, vehicle contact, etc etc), not 
> sheared bolts or fatigue failures.

I'd hazard a guess that failures caused by rusted-out hollow legs occur more
often than naturally-occurring guy failures.  Of course, accidental (though
often preventable) guy failures caused by falling trees, vehicles,
vandalism, etc. probably happen the most often, but they shouldn't be lumped
in with failures that result from degradation over time as caused by natural
effects.

> I'm confident the PE did a great job for my buried base 65 
> tower, it is 
> a trivial structure compared to his day job.  

I have no reason to doubt that's the case, but unless there was a real good
reason to do it, I would almost always be biased towards a pier pin.  The
encased bottom section provides some resistance to axial rotation, and if
the tower is designed and guyed properly with respect to the design loads,
that should be a non-issue -- the guy wires (double-guyed if necessary)
and/or torque arms/triangles should be doing the work, not the base.  The
buried base could be argued to provide a bit more lightning protection, but
a properly-designed and properly-installed ground system makes that a moot
point as well.  If anything, the pier foundation would require less concrete
than a buried base as the only stress is compression with a pin.  

So, what's left that would make the buried base preferable over a pier pin?
Saving a few minutes of time when stacking the bottom couple of sections is
hardly a reason in my book :-)

73.

					--- Jeff WN3A


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