The following is just my opinion based on years of experience and not
getting hurt.
> TowerTalkians,
>
>
>
> I need your expert advice. I climb for a friend who has a 100' Rohn 45
> tower. When the big Nor'easter tore through New England recently, a large
> tree fell on one of his top guys. Luckily, the tower did not collapse.
>
>
>
> Off the top of my head, I think the tree was at least 40 feet tall and
> the
> trunk was 8-12 inches in diameter. Maybe bigger. Way too heavy to move by
> hand. It was a deciduous hardwood, perhaps beech or birch, I can't
> remember.
> The main trunk didn't hit the guy wire, but at least one of the large
> upper
> branches did. When I got there, the branch in question (at least 5-6
> inches
> in diameter), was resting on the lowest part of the guy wire, just a few
> feet above the big grip connecting to the equalizer plate. Further up the
> guy, about 15 feet off the ground, there was a noticeable dip in the wire.
> Not exactly a kink, but a deformity that strongly suggests the tree
> initially hit the guy there and either bounced or rolled down to the
> bottom.
Probably skidded/slid
> Even so, the weight of the tree had caused the wire to sink at least 1/4"
> into the branch that was resting on the lower portion of the guy.
Which would accound the the cable being imbedded in the branch.
Taking into account the angle of the tree and how much mass extendid beyond
the impact point plus the speed imparted due to the wind, there is no way of
knowing just how much force was exerted on the system by that tree but as a
guess based on your description I'd say it was probably many tons.
<snip prodedure>>
>
> We noticed right away that the top guy was much looser than it should be.
> In
> fact, all nine guys have considerably more slack than they did when
> tension
> was last set with a Loos gauge last fall. My friend doesn't tension his
> guys
> particularly tight, probably somewhat less than the recommended 10%, but I
> climbed the tower last fall and felt the tension was at an acceptable
> level.
> The catenary was pretty shallow. After the tree hit, the catenary was much
> deeper and you could grasp one of the top guys and kind of whip it up and
> down. I'm not real experienced at this, but would guess anywhere from
> 6"-12"
> of slack will have to be taken out to retension the guys, with the top
> guys
> needing the most
When you say slack, are you referring to the catenary, or the cable is 6 to
12 inches longer than previous.
This could be caused by the anchors moving/tilting, or being pulled, or it
could be caused by stretch.
One other scary (but unlikely) possibility is belling of the tower where the
sections mate, or slite irregularities in the sections that would add up.
Even a bit of sinking of the base is possible. It would only take a couple
inches over all to make a big difference in guy tension and it would affect
all guys.
IF you are really talking stretch/length, then it's time to replace ALL the
guys and imspect ALL hardware, post haste. Actually ALL hardware should get
a careful inspection and the torque checked on all bolts. This is also a
very good reason for installing guys at a specific tension so you know
specifically how much change you are looking at. This is where "that looks
about right and it's straight" isn't good enough.
Any thing other than minor stretching means you no longer know the tensile
strength of those lines. Any piece that has been deformed should also be
replaced..
Also the reason for going to the specified tension will minimize tower
movement under these conditions. Guys that are not tight enough allow tower
movement and it's possible for resonances to develop that adds even more
pull on the guys as well as strain on the tower itself. I've seen relatively
small systems develop a resonance in high winds and literally fly apart
almost like an explosion. Higher mass systems can also do damage to the
structure that takes a careful examination to find.
Sight down the legs and make sure they are absolutely straight. Check tower
bolts to make sure they still fit tight. If you detect a leg, or legs that
are no longer straight, and I don't mean big bends, or bolts that fit loose
in the holes they may be the indication of serious weakening of the system.
Replace any turnbuckles that appear to be loose or worn and any other
hardware that appears to have changed.
When the tower is put back together, if it is found to be sound, make
certain that the guys are tensioned to specification. Don't accept, "It
looks good" for tension, or less than specified.
>
>
>
> We inspected the guys and anchors for damage. Aside from the mild dip in
> the
> top guy at the point of impact, there does not appear to be any damage to
> the wire, even where it cut into the branch. The anchors appear solid and
> there is no sign they have deformed or the anchor blocks have moved (this
> tower uses 3'x3'x1.5' concrete footings four feet down, per Rohn spec.)
> However, I was able to grasp an anchor rod and move it up and down a few
> inches. This was partly due to the guys being slack, but I think the soil
> is
> relatively loose near the surface, too. It's an area of forest that was
> cleared about 20 years ago and has that soft forest floor consistency.
> BTW,
> this is why the tree (and many others) fell -- the root balls disengaged
> from the loose, super-saturated soil and the heavy winds blew them over.
> Anyway, if I had to bet on it, I'd say the anchor blocks didn't move. My
> friend doesn't think so, either, and he's intimately familiar with them,
> having done the digging and pouring himself.
I'm using anchor blocks that weight over 17 to 20,000 pounds each 4 X 4 X 4,
5' deep and they are set in pretty solid soil, but I can gurantee they do
move ever so slightly with time, but it'll take a transit to see.
>
>
>
> The guys are broken up with insulators. I visually inspected the grips and
> could see the little stub of EHS sticking into the thimble area, as it
> should be. I don't think any of the grips slipped. I was not able to
> closely
> inspect the grips right at the guy brackets, but will do that through
> binoculars before climbing the tower. I think it's unlikely the grips
> slipped at the tower end.
>
>
>
> The tower appears to be perfectly plumb and there's no obvious damage to
> the
> legs or lattice. I plan to inspect the legs and welds through binoculars
> before climbing.
>
Very good idea.
Inspect as you go and "be careful". We almost lost one of the hams on
another reflector when the tower he ws taking down broke.
>
>
> It was a very windy day, and we were impressed with the fact that the
> tower
> wasn't swaying at all, despite the loose guys. It was rock solid (though
> I'm
> sure I would have felt some sway if I was riding the top.) This tower has
> a
> 40-2CD, 6m beam and TH-7 on a 12'+ mast and a TH-7 on a TIC ring at 60'.
> That's 25-30 sq feet of wind load. The 3/16" guys are at about
> 95'/60'/30'.
I avoid any even spacing for the guys going up the tower.
> Torque arms are used on the brackets. The tower sits on a pier pin. The
> tower and guys were installed about 20 years ago.
Using torque arms with the tower setting on a pin allowing it to pivot can
put tremendous pressure on the guys if the antennas start to whip back and
fourth and develop a resonant pattern.
>
>
>
> Questions: What caused the slack in the guys? Is it possible for EHS to
> stretch that much? If so, is it OK to simply retension the guys, or is
> there
An inch or two of length will make a very lage change in the catenary.
> a compelling reason to replace them? Is it likely that the impact point is
> weakened enough to require that section of the guy to be replaced? Should
> I
> look elsewhere for the source of the slack?
I'd replace that section regardless of the others.due to the change in
shape.
>
>
>
> Before I climb the tower, I want to be confident that it's in good order.
> Please, no speculation. This is a serious safety issue and I would
Some speculation is unavoidable as you are going up a partial unknown. Err
on the safe side.
One thing that is not speculation: If the catenary has changed then the
tension has changed and that means something caused it. Chreck the bolts at
the bottom of the tower. Take them out and check for a tight fit. Check or
twists in the tower. Even a fraction of an inch is an indication of undue
stress..Is the pier pin still the same distance above ground?
Were it me, I'd check every one on the way up, even with the added time and
effort. Something caused those guys to loosen. You will be dealing with a
numberof unknowns and not seeing the tower or general installation I have to
guess in a manner to err on the side of safety.
Remember a 100' 45G weighs almost half a ton (90# a section plus hardware).
With the guys properly tensioned it will put considerably moe load on the
base. A tower than moves will put added pressure on the base in a non
uniorm manner. For instance if the top guys are at a 45 degree angle to the
tower and tensioned to 600# then each guy will be 600Xcos(45) or 424# X 3 =
1273# added to the base just for the top set of guys. Then do the
calculations for the guys at 60 and 30 feet. The total for the tower plus
the top set of guys is over a ton at 2273#the other two levels will add at
least another 1000#.and we haven'd even counted antennas, rotator(s) and
coax. It depends on the soil conditions at the base of the pier, but I'm
surprised the base hasn't settled at least a little.
> appreciate answers from people with lots of experience, preferably with
> similar damage.
>
>
>
> Thanks & 73,
>
> Dick WC1M
Good Luck
Roger (K8RI)
>
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>
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