Is there a guide specifying the conservative resistance of different trees to lateral load? I have seen a dam boom attached to 2 gigantic trees, one on each side of a river. My idea is that it would imply to have a biologist evaluate the type of tree, its health, root cover, etc.
I speak here more as a guy who grew up on a 60+ acre farm (more than half of which were covered by trees; not to mention the fact I have been cutting my parents firewood for 30+ years, my time working for a tree service, etc, etc) than a structural engineer. The bottom line:
trees are completely unpredictable. It could be rotted, infected with some sort of insect, disease, or what have you. Just about a year ago, a 40+ mph wind took down a (2 foot diameter) live pine at my parents place. It left me completely scratching my head......until I got into it and found the pine beetle had eaten it up inside and at the roots.
In the case you are talking about, if it is on the side of a river, if it is close to the edge, its root bulb may be cut off. (I.e. not a full radius.) To give an example about that: around here, everyone says "Wind never blows down poplars." And for the most part: they are right.....but guess where the only blow down poplars I have seen occurred? Right next to a creek/river.
Something else about your case, if you are using these trees as a "tail hold" for a crane....you may want to consider cutting the trees down and using high stumps for that. (With the proper notches cut in the stump for the cable.) Guys in the logging industry do that all the time to anchor the yarder. (But I don't know if that would fly with your situation.)
If you are interested in the percentages on this.....you may want to study the tree damage reports that follow hurricanes (many of which superimpose the % of damage on maps with peak wind gusts during the event). I have studied the ones for Hugo (in '89) quite a bit. There are a lot of variables.....but the bottom line is: nothing is guaranteed.
