The Physics of Rigging from Trees
If you have read much of anything that I have written over the last 30 years, you know that I do not like to say "don't do that," but rather, "if you do that, then this will be the result," or "you must contend with such and such." This same logic applies to rigging from trees, you can do it, but you must understand the forces and how to properly deal with them.
Before I get into this subject, let me say that I am not a fan of rigging from trees - in fact, I try to discourage folks from doing it. However, I know that no amount of warnings will dissuade a lot folks from doing this, so I think it is important that folks know some things that might make their rigs "safer" (note: there is no way to make it totally safe).
One more thing before I begin - the process of actually installing rigging can be very dangerous. NEVER RIG ALONE. Always have someone with you to assist, or call 911 if you fall. USE FALL PROTECTION WHEN YOU ARE RIGGING. Falling from the tree while working is a real danger. Think of climbing a tree in the same vein as climbing rock and "clip-off" to a sturdy anchor and/or use a belayer. Understood? OK, now to the physics stuff.
When we talk about forces, "tension" and "compression" are two terms that we hear about most often (especially tension). But rigging from trees often places a great deal of "torque" on the spot where the limb meets the truck of the tree. Torque is a turning or twisting motion that is often related to the use of a lever, which can be used to create tremendous amounts of force. In the case of rigging an aerial apparatus on the limb of the tree, the limb is the lever and the greater the distance that the apparatus hangs from the trunk, the greater the multiplying factor. Look at the diagram below.
This images shows a 100-pound weight hanging on a limb, 10 feet from the trunk. Torque is measured in foot-pounds, and is derived by multiplying the load/force (in pounds) by the distance (in feet) that the load is away for the pivot point - in this case, the trunk. So, in this example, the torque on the point where the limb attaches to trunk is 1,000 foot pounds (100 x 10). Now, if the load was an aerialist that was generating 700 pounds of shock load doing a drop, then the torque on the point where the limb attached to the trunk would be 7,000 pounds. See a problem? This is a LOT of force on that point.
Using a bungee pack is one way to reduce shock loads on your anchor point, and subsequently, the torque on the limb. You might seriously want to consider this if you are doing tricks that generate a great deal of shock load.
Now, the aerialist might be lucky and the tree limb could have enough "spring" in it to absorb a lot of the force and reduce the shock load. But, if the limb has too much "spring," it could bend and cause the aerialist to hit the ground, which would not be good. So, lets look at how we might rig the apparatus so that there is little torque on the joint, and the limb does not bend too much.
The image above shows a support line that has one end attached to the limb (just beyond the location of the apparatus) and the other end attached to the trunk. The angle of this line is important. Generally speaking, the closer to vertical, the less tress on it. But at the same time, the closer it is to horizontal, the stronger portion of the trunk it can be attached to. So, about a 45-60 degree angle is usually best. I would recommend round slings or wide straps be used to wrap around the trunk and the limb (in a "basket" fashion), so that the load is not on a narrow band around the tree. This will help protect the tree from a concentrated force on a portion of the tree. The line should be of a strong material with very little stretch and tensioned until it is snug (remember, this line's job is to support the load, not lift the limb). This is a good place to use a trucker's hitch, if you know how to tie one. The exact position of this line on the limb will help determine the percentage of the total load on the line, but it should support most of the load, with very little on the joint. Now the load is in "shear" on the limb, instead of there being a torsional force. This is much better.
Of course, the force on the line above is transferred to the truck of the tree. This then results in a horizontal force on the trunk. By using a second line, this horizontal force on the trunk can be transferred to the ground (or the trunk of a second tree) just as the vertical force on the limb was transferred to the trunk of the first tree. See image below.
Remember, both of the tension lines need to have very little stretch. A static climbing rope or a steel cable would work well for these lines.
The goal in all of this is to transfer the heavy load/force from a weakest member of the system (the limb) to a stronger member (the strong trunk of a tree and the ground. While this takes more materials and time to rig, it is much stronger and safer, and better for the tree. Of course, getting advice from an arborist is always a good idea. The arborist can advise you on the suitability of particular trees for your needs.
Remember, rigging from an engineered structure is almost always safer than rigging from a tree limb. Great care and planning must be taken when rigging any aerial apparatus, but even more so when rigging from a tree. If you have ANY doubts about the rigging or the ability of the tree to support the forces that you will generate, STOP and find another place to rig your apparatus. Aerial work is fun and beautiful, but neither are worth serious injury or death.