[TowerTalk] Force required on guys to cause catastrophic failure? Have your guys been struck by falling limbs?

[John W]

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