[Top] [All Lists]

[TowerTalk] Force required on guys to cause catastrophic failure? Have y

To: <>
Subject: [TowerTalk] Force required on guys to cause catastrophic failure? Have your guys been struck by falling limbs?
From: John W <>
Date: Fri, 2 Dec 2011 04:29:20 -0800
List-post: <">>
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 
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 
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,

TowerTalk mailing list

<Prev in Thread] Current Thread [Next in Thread>