As discussed in last month’s column on arborist blocks and pulleys, designing, developing and employing rigging systems is, by necessity, a part of most tree care professionals’ daily lives. Knowledge of what forces the various components of a rigging system will experience under load is not only important for the efficiency of the system, but also for the safety of the rigging system, the safety of the tree(s) involved as anchors in the system and the safety and well-being of the various members of the tree crew. An awareness of basic rigging forces, when they may or may not be present in a given situation, and various methods or techniques to lessen their severity will all assist tree care personnel in accomplishing jobs more safely and efficiently. These basic rigging forces will always be present to some degree, regardless of whether “natural crotch” or “false crotch” rigging techniques are being used, although increased friction, an inherent consequence of natural crotch rigging, may affect the level of forces present to some extent. Thus, whether dismantling trees “old-school style,” with a bull rope and wraps around the tree, or “progressively new school,” with all the latest blocks, pulleys and lowering devices, climbing arborists will be well served by knowing the forces they may be generating, and examining their simple or complex system for these forces’ possible effects. The alternative, not knowing basic rigging forces and not examining the system for them, can only lead to broken trees, broken gear and, sadly, broken lives.
2-to-1 (180 degrees)
The 2-to-1 force factor is one of the most basic forces present in rigging operations. As can be seen in the diagram, a piece suspended below the rigging point will always 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 the static system illustrated that experiences a multiplication of the weight of the piece is at the rigging point, whether it be an arborist block or simply a branch the rigging line runs over, and this multiplication is at least twice the weight of the piece. If the system illustrated becomes a dynamic system, one where the piece is dropped into the rigging, an exponential increase in forces will result. The effect of gravity on the falling piece will increase the forces experienced by both parts of the rigging line, and these increased forces will then be doubled at the rigging point. Measurements of force taken in real-world, worst-case scenarios have shown impacts at the rigging point of 10 times the weight of the piece. The presence and recognition of a 2-to-1 force factor in a rigging system can give climbing arborists much valuable information. Obviously, the choice of which branch or leader in the canopy to use as a rigging point should be affected by this information, given the doubled forces it will experience. In addition, typically, tree crews utilize their strongest sling and gear at the base of the tree to control the piece’s descent, yet the presence of a 2-to-1 force factor indicates that the base of the tree will only see half the force that the rigging point will, so the strongest sling should be aloft. Skillful and efficient management of the lowering line can lessen the forces experienced at the rigging point. Bringing the load to an abrupt and sudden stop will maximize the forces aloft, while gradual deceleration will minimize them. The use of a lowering device of some sort can often make this controlled deceleration easier. The choice of rigging line may also adversely affect the forces experienced by the rigging point. A rope’s elongation or elasticity assists in absorbing some of the forces generated by dynamic rigging situations; this energy absorption by the rope thereby lessens the forces multiplied by two at the rigging point. The use of a very strong rope with limited or no elongation will maximize the forces experienced at the rigging point, as the rope will absorb little or none of the energy generated by the falling piece being brought to a stop, maximizing the forces being multiplied by two.
Comparative component strengths
Although not directly related to rigging system forces, an important factor for climbing arborists to consider is which component, or link, in their rigging system do they wish to be the weakest. In most scenarios, the rope should be the weakest component. The failure of any component is less than desirable, but failure of the rope will be the least catastrophic. A component failure at the base of the tree will lead to an uncontrolled descent of the piece being rigged, and either gear or ground personnel moving rapidly upward into the tree. A component failure at the rigging point in the tree will also lead to an uncontrolled descent of the piece being rigged and the rapid descent of possibly both gear and climber. The failure of the rope will result in the uncontrolled descent of the piece being rigged, less than optimal, but still better than the other two options.
Rigging systems, whether they are simple or complex, have the potential to accomplish jobs quite quickly or slow them down immeasurably. They can make a hazardous situation much safer or make a relatively safe scenario dangerous, but if they are examined and evaluated through the lens of the forces involved, the strength of the various components and outcomes in the event of failure, rigging systems will, in the end, be of benefit and value to all who choose to employ them.
Michael (House) Tain is a contract climber, splicer, educator and writer currently located in Lancaster, Ky.