Fortunately, my tower is located such that tree limbs aren't likely to fall
on it. However, a friend of mine has a tower located on about 1/2 acre of
cleared land bordered by woods. A number of years ago, during a particularly
bad storm in which the ground got soaked and the winds were high brisk, a
birch tree pulled out by the roots and fell on one of the top tower guys. I
can't give you exact answers to some of your questions, but here goes:
What size/weight of limb hit the guy?
I don't know exactly, but I'd say the birch tree was about 30-40 feet high
and at least 8-10 inches in diameter. It struck the guy near the bottom of
the spray of branches at the top of the tree, probably around the 25-30 foot
level of the tree. As I recall, one of the larger branches of the tree hit
the guy, not the trunk of the tree. The branch was at least 5-6 inches in
diameter. I think the guy actually dug partway into the branch.
What was the tower configuration (height, type, and guy material)?
100' of Rohn 45. 3/16" EHS guys broken up into non-resonant sections with
insulators.
Which guy got struck?
One of the top guys. The tree landed on it about 2/3-3/4 of the way down the
guy.
How far did the limb fall?
See dimensions above. I'd say 30 feet at least, probably more. Much of the
weight of the tree was on the guy.
What was the result? Damage or no damage?
The guy did not break, nor did the tower buckle. The guy was pulled down
considerably from its previously near-straight run. We used a chainsaw to
carefully cut the branch and roll the tree off the guy. The guy sprung back
towards its original position, but was noticeably slack (i.e., the guy
stretched.) We tightened it as best we could. My vague recollection is that
we had to tighten the other guys a little, too. So they probably stretched a
little. Not long after, we replaced the slack guy with a new one. This
required installing a temporary guy while the damaged guy was removed. The
job was easier than I expected.
Hard to generalize an event like this. YMMV, as they say. You'll probably be
best off to take down as many trees near the tower as possible, or at least
remove the limbs most likely to come down on guys.
73, Dick WC1M
-----Original Message-----
From: John W [mailto:xnewyorka@hotmail.com]
Sent: Friday, December 02, 2011 7:29 AM
To: towertalk@contesting.com
Subject: [TowerTalk] Force required on guys to cause catastrophic failure?
Have your guys been struck by falling limbs?
This is a two-part question. There is a small survey at the end for those
who have had a guy struck by a falling limb or tree.
My main question is: Are there any engineers out there who can tell me how
to calculate the downward force on a guy that would cause a breakage or
catastrophic failure?
The reason I ask is that regardless of where I place my to-be-built tower, I
cannot avoid having guys passing through the woods, and I am in a high wind
area.
There are constantly limbs, and sometimes even trees of various sizes,
getting knocked down by the wind.
I realize that if a huge tree falls on a set of guys, it's probably all
over. That's a risk I'll have to take, and I plan to monitor tree health
closely.
What I am more concerned about is the large number of limbs, some of which
are fairly sizable, that fall when the wind blows.
I can measure the weight of a typical limb. They are mostly 2 lbs. to 5 lbs.
But let's say I want to plan for the occasional big one, say 75 lbs.
If it falls from a height of 80' before hitting a guy, I can calculate the
acceleration due to gravity and calculate the force it has using F=MA.
(I will have to break out the college physics book to do this, but I should
be able to do it!)
Now that I know the force, I can compare that figure against the force
needed to either a) snap the guy wire it hits, or b) put so much
instantaneous tension on the guy that it causes the tower to buckle.
(Sorry to cause the inevitable cringing here, but...) I assume the tower
would probably buckle at the point of attachment of the guy below the guy
that got struck.
It's also a reasonable assumption that it is most likely to be the top guy
that gets struck. Although it's also entirely possible that a tree off to
the side falls down, and the top of the tree strikes a lower guy in the
latter part of its trip to the ground.
In that case, the force on the guy would have both a downward and a lateral
component. It would also be a lot harder for me to remember how to calculate
the striking force in that case.
I assume that some of the other factors needed in order to make a correct
calculation would be the tower type (Rohn 25G, 45G, or 55G), the guy
material (which would be per factory spec), and the distance between the
struck guy and the one above it or below it.
I realize it's asking a lot for someone to provide the formula(s) needed to
make this calculation, but if there is anybody out there who would know, I
figured this would be the best place to find them! (Or is this something I
can ask the tower manufacturer?)
On a practical level, I'm sure there must be some of you who have had limbs
fall on guys. I'm interested to hear:
What size/weight of limb hit the guy?
What was the tower configuration (height, type, and guy material)?
Which guy got struck?
How far did the limb fall?
What was the result? Damage or no damage?
Thanks for all input,
John
W2ID
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