[TowerTalk] Parting comments

[Kurt Andress]

Pat Masterson wrote:

> You said:
> >Since, they spec deep footings, compared to the vertical footprint, one
> >might assume that they think that some kind of lateral loads are
> >present, and need to be counteracted by the obvious depth of their
> >footing.
>
> Most of us wouldnt assume this at all. People make deep tower footings to
> get below the frost line. Since Rohn has no idea where their towers will
> be built, they spec them for winter frosts. Where I live, if your
> concrete is less than 3' in the ground, it will start to "heave" in the
> spring and move around.
>  But, I do agree, that any  guyed tower will have some minimal lateral
> forces at the bottem when the wind blows.  -Pat

Hi Pat,Thanks for the comments. You are probably right. Here,we only freeze about
18" down. But my sandy soil looks to really like all of the lateral footing faces
if I don't use a pier pin base.
The lateral loads due to wind on the bottom span are pretty small. The additional
lateral forces caused by the base moment, caused by the tower leaning over, are
more. In some cases, pretty scary.
My whole point in bringing the subject up is that everyone ought to recognize that
wire rop, solid rod, and just about anything else elongates under load.
What made me first think about looking at it in the tower design was when the '92
Americas Cup designers were selecting rod rigging sizes purely on stretch, not
stress.
Rod that was strong enough to carry the loads was allowing the mast to bend too
much and driving the mast stresses up and reducing the buckling safety factors.
I fiddled around figuring how do the modeling and ended up finding the same thing.

I think this is why we need to be careful when looking the a Rohn recommended
design, for example, and saying to ourselves that they have a huge safety factor on
the guy loads.
The size may have been selected to reduce the deflections.

73, Kurt

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