All this debate and conjecture about towers collapsing and the area covered
by the wreckage reminds me of a tower disaster that occurred in SE Saskatchewan
north of Carlyle about 15/20 years ago. This was a heavy duty TV transmitter
tower
of considerable height that acquired a tremendous load of clear ice during a
freezing
rain storm and eventually collapsed straight down. I saw the wreckage a few
days
later before any cleanup or salvage had been done and while I did not
measure the
diameter of the wreckage field, it was incredibly small. The whole tower was
piled
in a zigzag mess many feet deep and I don't recall any section longer than
about 40'
that was straight without being folded back on itself. The guy cables (big
stuff!) were
intact and lying mixed in with all the bent iron.
This same storm brought down several other towers across southern SK,
reportedly
with the same type of damage, but I didn't see any of them personally. I
believe the
Regina repeater group lost one of their towers in this storm also.
I also dimly remember the failure of an AM broadcast tower near Watrous SK some
years ago when a 50 year old heavy tower came down as a tornado passed nearby.
As I remember the newspaper article and pictures, one of the anchors failed
and the tower
was stretched out full length on the ground. The only way it could have been
laid
down this way must have been that there was only one anchor point per side,
which seems
odd because almost all towers have at least two anchoring points per side.
Perhaps it
was folded once - my memory of the picture would not be infallible.
Lastly, there was the vandalism of a TV transmitter tower near my home town
here at
Lloydminster (on the VE5/VE6 border) back in about 1978. On Hallowe'en evening,
some vandals cut the middle guys on a tower of about 400-500' height. It
folded exactly
in half and dropped straight away from the cut guys, so the top and the
bottom of the tower
were very close to each other and the middle was half the tower height away
from the base.
There have been several cases recently where surplus towers have been
deliberately
dropped under controlled circumstances. The ones I have seen involve cutting
the anchor
rods with an acetylene torch after taking the appropriate precautions and
the resultant
falls bear out my ideas.
I'd say that it would be very difficult to predict exactly what area that
the wreckage of a tower
would cover, because that would depend entirely upon what type of failure
had occurred. For
the furthest point of the wreckage to be the full tower height away from the
base would
necessitate total and instantaneous failure of all the anchors on one side
of the tower, otherwise
some folding will occur and the radius of the wreckage fall will be less
than the total tower
length. I would suggest that this would be the least likely failure
scenario. I would also suggest
that the wreckage radius of accidental falls will vary widely depending upon
what failed first and
subsequent structural member failures.
I have seen a few failures of self supporting towers of the type of the
Canadian Delhi self
supporters (very similar to the Rohn self-supporter) and never saw one where
any part of the tower
or antenna actually struck the ground. The tower would fold about 1/4 to 1/3
of the way down with
the upper portion dangling down alongside the remaining erect base portion.
Failure was always
due to unusual winds or lack of maintenance, such as loose bolts or rivets
from wind vibration of the
tower over many years. The occasional one was obviously overloaded, usually
with a sidearm
mounted antenna that would torque the tower and work the braces loose.
These towers are built of progressively heavier material from top to bottom,
so the bottom portion is
very sturdy. If installed with the approved concrete base, properly
maintained and not overloaded,
failure of the top part will only happen with very excessive and unusual
winds, and even then, the
whole tower will not collapse.
73
Bill - VE5FN
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