N4KG response inserted below.
On Wed, 5 Dec 2001 "Joe Subich, K4IK" <w8ik@subich.com> writes:
SNIP
> > For (relatively) short guyed amateur towers under 100 feet the most
> > common failure mode is that of a broken guy support from excessive
> > horizontal forces. While the failure of a top guy of a multi-level
> > guyed tower might constrain the debris radius by allowing top
> sections to fold around lower guy supports, this would still not be the
> classic downward crumple observed in the TV tower.
Multiple guy points in each direction are the key to
allowing a tower to 'fold' in the event of a *single*
guy failure. N4KG
>
> This would only happen if all they guys on a single side failed at
> the same time. The *only* failure mode in which I could envision a
> multiple
> simultaneous guy failure is vandalism (the anchor rod is cut) or if
> the anchor block were to fail and the anchor pull out of the soil.
One scarry scenario would be if the anchor rod were
to RUST through, unseen by the owner. Fortunately,
this seems to be a rare occurance. This is a STRONG
argument for TWO anchors in each direction. N4KG
> If an amateur tower were, for example, have a leg rust out I would
> expect a crumple or crushing fall just like a commercial tower.
NOT exactly. Remember that for a guyed tower,
almost ALL of the forces at the base are vertical.
Fortunately and amazingly, even a single tower leg
seems sufficient to support a *short* tower but don't
bet your LIFE on it. N4KG
Case in point: I have two such towers, still standing.
One, a 55 ft TV tower with horizontal braces (NO
diagonal braces) and roughly 12 inch face width
where ONE leg rusted through at the ground where
I had left a string tied around the leg (who knows why).
I placed a plate under the rusted through leg, drove
3 ft pieces of angle iron half way into the ground
on the outside of each leg. Then I cross braced
the legs inside the tower using two pieces of 2X10
rotated to engage all the legs and nailed together.
Two, an 80 ft TV tower of identical construction
with TWO legs rusted through at the ground.
(Scared the H*** out of me when I discovered it)
Both towers were placed in the ground roughly
30 inches with NO concrete 20 years ago.
Again, I drove some heavy angle iron around
each leg, and crossed braced the bottom
opening using 2X8's in an H configuration
stacked on top of each other, up to the first
step, bracing the bottom of the tower in
all three axis'. Both towers are STILL STANDING
after 2 years, having survived some quite
high straight line wind events. N4KG
N4AR has a 200 ft Rohn 45 tower that SHEERED
the legs at the top of his concrete base when
a tornado twisted the tower with 4 stacked 4L20's.
It is still standing with an additional concrete
block poured around the bottom of the tower,
on top of the original concrete base. N4KG
> > I dare say if the TV tower had suffered the total loss of one of
> its upwind guy supports, it, too, would have fallen like a giant tree.
>
SNIP
>
> 73,
>
> ... Joe, K4IK
>
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