Rigging operations in the tree care industry can be incredibly complex operations involving multiple systems, truckloads of gear and numerous personnel, or they can be as simple as a climber, a branch manager, and a decent length of three-strand line. In both cases, and in the vast number of rigging scenarios that might fall somewhere between the two extremes, some knowledge of the forces present when rigging large woody debris is vital if breakage of trees, property or even personnel is to be avoided.
While a complete understanding of every force present in every rigging situation is beyond the scope or space of this column, and must be tempered with training and experience to be effective, a comprehension of some of the basic forces present will go a long way toward preventing breakage and accomplishing the task safely and efficiently.
Force versus strength
In technical terms, let alone the precise language of physics, these two terms mean very different things and are even measured in different units. However, for the purposes of this basic primer they will be used roughly interchangeably to give tree folk a basic idea of what forces can be expected and what strengths might be required to resist them.
How strong is it?
This value is often described as the tensile strength, breaking strength or minimum breaking strength (MBS), along with other descriptors. Simply put, this is a value determined by the particular manufacturer or fabricator for that individual item of gear, rope or equipment at which it will fail. The value may refer to a force at which the item will fail completely or distort to the point of being unsafe or unusable.
Regardless of which point the value refers to, the breaking strength is a one-time option. This means that the manufacturer has shown through testing that the item will survive to that amount of force one time brand new out of the box. This value should either be part of the packaging of the item or stamped or embossed somewhere on its surface.
Manufacturers test for tensile/breaking strength in different ways, so it behooves the tree worker who plans on flirting with the edge of the limits to understand how the gear was tested to arrive at those numbers. As an example, the breaking strength of some ropes do not reflect their strength straight out of the bag, rather they reflect their breaking strength after they have been stretched at a small percentage of maximum load to align all the fibers for maximum strength, thus the rope straight out of the bag may break at a lower load than stated.
Is safety really a factor?
Many manufacturers will use in their literature terms such as “safe working load” or “working load limit,” also known as SWL or WLL, when describing the use of gear, ropes or equipment. This is arrived at through the use of a safety factor and is a useful concept for tree folk trying to design and implement rigging systems.
The safety factor is simply a ratio that is used, along with the tensile/breaking strength, to determine what kind of loads can be handled consistently in safety. An example would be a safety factor of 10 to 1, which would mean the breaking strength would be divided by 10 to arrive at an SWL or WLL for a particular piece of gear. Imaginative tree riggers can quickly see how rigging line that’s good to 10,000 pounds one time quickly becomes less Herculean when a safety factor of 10 to 1 is applied.
Not only is the concept of safety factors important to arborists setting up rigging systems, it’s also important in the selection and/or purchase of gear, as an apparently gargantuan working load limit may be less appealing when the prospective purchaser realizes the manufacturer used a safety factor of 2 to 1.
Due to the concept of cycles to failure, discussed later in this column, a high safety factor will extend the life of a rope or component, while a low safety factor will decrease it. Both are viable options, but only if the tree crew understands the “trade-offs” they’re making with their safety factor.
Cycles lead to failure
The idea of cycles to failure as it applies to tree rigging is a fairly simple one to comprehend if one thinks of bending a coat hanger again and again at the same spot. As anyone who has ever attempted to fashion a poor man’s Slim Jim knows, enough bending at the same spot will break the coat hanger.
On a much larger scale, this is exactly what can happen to ropes and equipment through repeated excessive use. Every time a rigging line, block, pulley, sling, carabiner or whatnot experiences a load, it loses some part of its overall strength. The closer the load is to the component’s breaking/tensile strength, the more strength the component loses. As mentioned previously, that breaking strength is intended as a one-time option; a user should get it one time, but the second time is a definite roll of the dice, and each successive use at or near maximum load makes failure that much more likely. This is why higher safety factors result in greater longevity in rope and equipment, though at the expense of smaller loads of woody debris.
Bends are bad
The bend radius refers to the degree of bend put into a rigging rope when it passes over a branch or the sheave of a block. In short, ropes like to be very straight, with all the fibers in line with one another. Any bend begins to degrade their strength by preventing the fibers from working together and compressing some of them. A minimum bend radius of 4 to 1 is recommended for braided ropes, and an 8 to 1 bend radius is even more advantageous to retain maximum strength. Due to the different nature of their construction, twisted ropes, such as three-strand, should have a bend radius of 10 to 1. As an example, when using a .5-inch rigging line of braided construction the block should have at least a 2-inch sheave, so rigging designers can readily see how smaller and lighter is not always better when it comes to bend radius regardless of the strength of the small, lightweight block.
Two to one
Tree crews should always remember that the forces generated aloft at the rigging point, depending on the design of the rigging system, are almost always going to be magnified by at least a factor of two. A simple example would be a 100-pound piece being lowered to the ground. When the piece is hanging in the air it is putting a force of 100 pounds down on the rigging line, and thereby the rigging point. At the same time the struggling branch manager is holding the other end of the rigging line with, hopefully, a force of 100 pounds to keep the piece from falling. The rigging point is then being pulled from both sides by 100 pounds of force, resulting in roughly 200 pounds of force on the rigging point, or a magnification by a factor of two. This example also assumes a very controlled piece, without the usual factors of falls into the rigging line, dynamic loads, bending moments and the like.
Field testing has shown that loads brought to an abrupt halt in rigging systems can multiply the loads experienced by the rigging point by a factor of 10, leading to a 100-pound piece causing 1,000 pounds of force at the rigging point.
The weakest link
Modern tree industry ropes, gear and equipment have steadily improved in strength and durability through the use of new materials and manufacturing processes, an improvement tree crews everywhere are certainly pleased about. Yet with this increased strength comes new challenges in how tree crews should view and design their rigging systems. No arborist plans on a rigging system failing, but if it should fail, which component should be the first to go, or the weakest link?
In a basic rigging system that has a belay point at ground level, a rigging point aloft, and a rigging line in betwixt and between, the rope should probably be the weakest link. A failure at the belay point will lead to the piece falling and the belay device/system moving upward at a high rate of speed. A failure at the rigging point will lead to the piece falling and the rigging gear moving downward at a high rate of speed. A failure of the rigging line will only involve the piece falling. None are good options, but a failure of the rope is the “least bad” option.
Wood is wood is wood
This discussion of rigging forces has not delved into one of the most critical elements of rigging systems: the trees and their structures that will be experiencing all of these forces. This subject is one more readily explained in discussions of tree work risk/hazard assessment, but tree crews would be well-advised to not be so focused on the strength of the various components of their rigging systems that they ignore the large woody folk that are intended to support all this gear safely.
Rigging in tree care operations is a complex subject, a mixture of both science and art with 21st century materials and fibers in large living organisms that might be hundreds of years old, and as such it is a topic that requires continual study and training. The basic concepts discussed here provide an excellent starting point to understanding the forces involved when rigging, arborists and trees collide, and with training and experience will help tree crews rig more safely and efficiently.