[TowerTalk] DAMAGED TOWER RESCUE

Eric Scace scace@uu.net
Wed, 27 Oct 1999 17:25:09 +0400


Hi Tom --

   I would consider this a very dangerous situation.  The internal condition
of the legs can not be ascertained.

   You referred to the "good leg" -- but, most likely, only the outside is
visibly 'good'.  There is a good probability that the interior has rusted,
but not yet rusted through.  That fact that you can "twist about the good
leg" suggests that the good leg has become detached from all other parts of
the structure below ground level -- and you do not know how far below ground
level this has occurred.

   The same risks exist on the second tower you described as having a
'single rusted through leg'.

   An engineer can not ascertain the safety of either of these structures --
there are too many unknowns.

   If it was me, I would not risk injury by climbing either tower in their
current condition -- even with bracing.  Tower and antennas and coax can be
replaced.  I can't.

   I'm not entirely clear about the wood bracing that you have applied but
it sounds like you've been mostly concerned about supporting the weight of
the tower.  However, on a guyed tower, the concrete base serves TWO
purposes:
   a) support, and distribute into the soil, the vertical loads from the
deadweight of the tower... plus the dynamic vertical component of loads on
each leg that are exerted by the guy wires when any horizontal force is put
on the tower (e.g., wind).
   b) to secure the base of the tower against horizontal movement.
Horizontal forces develop when a guy wire restrains the movement of the
tower.  Imagine a tower just resting on a smooth horizontal plate, and
guyed.  The guy level can be a pivot point, with the top of the tower
tipping one way and the bottom scooting out the other direction.

   From your description, it doesn't seem like horizontal movement is
sufficiently restrained.

   I think you have several safe options available:
a) cut the guys and drop the tower -- but you have already stated that can
not be done easily.
b) use a bucket truck to remove the antennas, and then remove the tower with
a crane.
c) build a form, above ground, around the tower's lower section and fill
with concrete.  A sufficiently large mass of concrete, distributed over a
large enough ground area, will be sufficient to support both vertical and
horizontal forces.  With proper steps during construction you can make sure
water drains out of the legs so that rust through does not happen again.
I'm not a structural engineer and can not give you a definitive answer for
the size and shape of the concrete mass.

-- Eric R3/K3NA

-----Original Message-----
From: owner-towertalk@contesting.com
[mailto:owner-towertalk@contesting.com]On Behalf Of n4kg@juno.com
Sent: 1999 October 27 Wednesday 10:03
To: TOWERTALK@contesting.com
Subject: [TowerTalk] DAMAGED TOWER RESCUE



Damaged Tower Rescue

I can hear it now -  K7LXC:

"I  TOLD  YOU  SO...Do as the manufacturer recommends !"

One of my in-the-ground towers has TWO rusted through legs
at ground level.  This tower is 30+ years old and has been in
place at this location for exactly 20 years.  It is a TV tower
with NO diagonal bracing, only horizontal steps 1-1/4 inch diameter
every 2 ft.  One set of steps is 4 inches in the ground, the next step
is 20 inches above ground.   There is NO lateral support from the
rusted through legs and the tower can be twisted about the good leg.
The strength of the remaining rusted stubs below the breaks is
questionable but obviously still minimally adequate since the tower
has not fallen or tilted.

To provide latteral support, I drove three 4 ft lengths of 1/4 by 2 inch
angle iron down alongside each leg to a depth of 28 inches.  Then
I inserted a 2 X 8 horizontally between the GOOD leg and the two
rusted legs, wedging in another piece of wood for a snug fit.  Finally
I inserted another 2 X 8 between the rusted legs and wedged in
additional boards for a snug fit.  The good leg is braced on 3 sides.
The rusted legs are braced on 4 sides.  After bracing, the tower is
rigid and cannot be twisted.

My concern is whether and how I should support the weight to
relieve the single good leg.  One idea is to simply stack 2 X 8
boards up to the first step (20 inches).  Another is to build a box
of 2 X 8's (or larger)and bolt the sides to each leg.

My goal is to brace the base well enough to make the tower safe
to climb and dismantle.  I considered just cutting the guys on one
side and letting it fall, but I would have to be very accurate in my
aim to get the tower to land between two guy anchors for two other
towers that are spaced about 20 to 30 ft.  And, I would prefer to
save the TH6, TA32, and TH3 mounted at 80, 60, and 40 ft.

FWIW, another similar tower (55 ft with 5L10's at 62 / 36 and 4L15
at 55 ft) with a single rusted through leg was similarly braced 18
months ago and is still standing.  I have climbed this tower with
no problem or uneasiness.  With TWO good legs it was of course
more rigid even before bracing.

Considered thoughts and recommendations are welcome.

de  Tom  N4KG
.......................

I added more bracing to the tower base tonight and it is quite rigid
latterally and rotationally.  I climbed it to the 10 ft level to remove
a TA33 and 155BA "temporarily stored" on the legs at 8 and 10 ft.
No problem.  My friends all tell me to get a bucket truck, crane,
or cut the guys.  There is only one direction I could let it fall and
then it would go across the upper two guys for a 120 ft Rohn 25
tower.  I suppose I could temporarily move the outer guys (90 ft
from tower) to the lower guy anchor (60 ft from tower) and let it
fall across the fence into my neighbors yard, assuming I get his OK.
His house is >150 ft away.

The bracing is as follows.  Legs are N, SE, SW.
A 4 ft 2X8 is laid flat between the N and SE/SW legs.
A short 2X10 is wedged in vertically against the N leg.
A short 2X8 is laid flat between the SE and SW legs, end against the
vertical wedge next to the N leg.  Additional boards are placed
vertically
against the SW and SE legs with another vertical board.  It looks like
an H from above.  Another vertical 2X10 is stacked against the N leg.
Another H is at right angles and above the first H, between the N and
SE / SW legs and flush with the bottom of the first step to provide
load support at each leg.
On the outside of each leg is a 4 ft piece of 1/4 inch thick 2X2
angle iron (painted steel) which is pounded in the ground 28 inches.

I believe the base is rigid and secure but my friends are all advising
caution and recommending against climbing.  But they are not
engineering types so are exercising caution in the face of unknown
territory.  My feeling is that it is safe to climb but I am open to
additional input.  I will call the bucket truck rental place to see if
they have something that will go to 80 ft, can supply an operator,
and whether the bucket can carry the load of assembled antennas.

Thanks for your considerations.

Tom  N4KG
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