At 14:21 2/22/01 -0500, Bill Coleman wrote:
[snip]
>For freestanding towers, while it is physically possible to have a base
>failure that results in a 100% fall, most failures like that are unlikely.
Why?
>Consider how trees fail. If you've ever seen downed trees after high
>winds or a tornado, trees rarely fail right at the ground. (Unless the
>soils gets so soft the roots just squish right through) Instead, trees
>tend to fail 10-25 feet up, with the mean being somewhere around 15 feet.
The structure of a tree is nothing like a hunk of steel. The upper parts
of the tree are much weaker than the old-growth wood nearer the base.
>That's the high stress point for a freestanding structure.
Not necessarily.
Think of the tower as a big lever with the fulcrum at the ground. Force is
applied to the lever with the object of turning the Earth. This requires a
lot of force and a heroic structure well-coupled to the ground. It is
unlikely that this can be achieved, so there is going to be a weak link
someplace. This can be part way along the lever, at the fulcrum or in the
coupling to the ground.
If the material strength is high enough and does not yield to failure then
the base will overturn (your tree root example). If the material strength,
someplace along the length of the lever, is insufficient for the load at
that point, then it fails there.
The force will always be highest at the fulcrum. That is why tubular masts
(and antenna elements) are tapered with larger (higher strength) materials
at the bottom. I suppose that ideally the mast should fail along its
entire length at once, with the result being a pile of metal confetti on
top of the concrete base. But than ain't gonna happen; there is always a
weakest point and there is no rule of thumb that can predict it.
This leads to the undeniable conclusion that you must consider the worst
case; the whole think falls over ("roots" and all) and the safety zone
better have a radius equal to the height of the tower plus the part
sticking in the ground.
IMHO
Wes Stewart N7WS
>
>Bill Coleman, AA4LR, PP-ASEL Mail: aa4lr@arrl.net
>Quote: "Not within a thousand years will man ever fly!"
> -- Wilbur Wright, 1901
>
>
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