By Michael Tain
The goal of rigging in tree care operations may be any of a variety possibilities: to direct the fall of a complete tree being removed in a particular direction; to lower large pieces of wood or tops out of the canopy in a controlled manner; to slide line pieces of a tree being removed over a particular immovable obstacle; or to lift a limb or large piece of wood off a structure, just to mention a few. Obviously the multitude of rigging scenarios with attendant forces and factors involved are beyond the scope of this article, but an examination of some rigging forces and factors involved in many scenarios will help tree care professionals design and employ safer more efficient rigging systems.
* 2-to-1 force factor: The 2-to-1 force factor is one of the most basic forces present in rigging operations. A piece suspended below the rigging point will result in the rigging point experiencing at least twice the weight of the piece. The part of the rope leading to the ground, whether it be in a lowering device, the hands of a ground worker, or wrapped around the tree, must generate enough force to keep the piece suspended — thus experiencing approximately the weight of the piece itself. The other part of the rope, the one tied off to the piece itself, only experiences the weight of the piece. The only place in a static system that experiences a multiplication of the weight of the piece is the rigging point itself, whether it is an arborist block or simply a branch the rigging line runs over; and this multiplication is twice the weight of the piece. The addition of a dynamic factor to the system — a piece or branch dropping into the system — increases the forces at the rigging point dramatically, as gravity will multiply the velocity of the piece or branch prior to it being “caught” by the rigging system. This factor dictates that the rigging point, whether it be sling and block or branch, be the strongest point within the rigging system.
* 1-to-1 force factor (120 degrees): The 1-to-1 force factor gives riggers the opportunity to reduce the forces experienced by the rigging point simply by changing the angle at which the parts of rope exit the rigging point. In its simplest form, changing the angle between the two parts of the rigging line, or “spreading” them further apart, will reduce the forces experienced at the rigging point. This force factor will equalize at a 120-degree angle between the two parts of rope, at which moment both parts of the rigging line and the rigging point will experience only the mass of the piece being lowered. Although a useful factor to take advantage of, climbing arborists must remain aware of the possibility of creating a bending moment that may compromise the stability of the tree off of which they are rigging.
* Bending moment: A bending moment may be created when focusing all the forces involved in a rigging system at one point high in the tree, and creating horizontal or angled vectors that pull it in one direction or another. This bending moment, should it encounter defects or weaknesses in the trunk of the tree experiencing it, may have catastrophic consequences. Bending moments may be avoided in rigging systems through the use of redirects, which attempt to keep the force vectors traveling along the plane of the trunk.
* Bend radius: Bend radius refers to the optimal surface over which a rope should travel at a rigging point to lessen strength loss. Ropes work effectively under tension, thus, when they pass over something — whether it be a block, pulley or branch — some of their fibers are under compression rather than tension, and unable to help support the load. The smaller or tighter the bend radius, the more strength the rope loses, or the weaker it becomes. A bend radius of 8 to 1 is preferred for braided ropes, but a bend radius of 4 to 1 is widely accepted in the tree care industry. An example of a 4-to-1 bend radius would be the use of a block with a 2-inch diameter sheave or a 2-inch diameter branch, with 1/2-inch rigging line. Ropes of a twisted construction, such as three-strand, require a larger bend radius to retain strength — a ratio of 10 to 1.
* Elongation: Elongation is a factor often ignored or not taken into account when constructing rigging systems, yet it can be quite valuable if effectively employed. The ability of a rope to stretch or elongate under load can absorb force that would otherwise be experienced by the rigging point. Obviously, too much elongation will lead to the branch or piece possibly striking the obstacle the rigging system is meant to help it avoid, but, at the same time, too little elongation will result in the rigging point experiencing massive forces, sometimes with catastrophic results. The addition of more rope into a rigging system provides more fibers to absorb some of the forces generated, lessening forces at the rigging point. This can often be accomplished with a redirect at the base of the tree, to avoid creating a bending moment, and the rigging line run horizontally to another anchor point to a lowering device or “tree wraps” for control during descent.
The forces and factors described here are just a few of those present or possible in rigging systems, yet consideration and implementation of this knowledge will assist climbing arborists in safer and more efficient rigging operations. Quite often, the simplest rigging system is the safest and most efficient, yet, when presented with obstacles or site challenges that require more complex systems, tree care professionals must consider all the forces and factors these more complex systems will create and generate. To not do so may very well result in accidents and injuries.
Michael “House” Tain is a contract climber, splicer, educator and writer currently located in Lancaster, Ky. He can be reached via e-mail at firstname.lastname@example.org