Many a climber will spend extra time on the ground checking out their chosen tie-in point (TIP) from every angle, examining for strength, security and ease of use; considering the strength of the wood in the tree being climbed; examining the TIP for signs of decay or structural deformity; even preloading the TIP to evaluate its strength – and rightfully so since there is literally a life hanging in the balance.
However, when conducting this evaluation one factor is often ignored: the forces that will be exerted on the chosen TIP by the climber’s preferred climbing method. Not only may the chosen ascent method exert more or different forces on the TIP than it is ready to receive gracefully, but the change from an ascent system to a working or canopy movement system can change these forces radically.
Traditionally, tree folk have evaluated the strength of their TIPs based on their use and experience with dynamic climbing systems. In a dynamic climbing system the weight of the climber is roughly shared between the two parts of line, and both parts move as the climber ascends, descends or works throughout the canopy. While this knowledge of the strength required for a dynamic system is important and useful, tree crews need to understand that as the system changes so do the forces.
Many tree care pros are using different systems, with attendant differing forces, in the same TIPs. A basic understanding and introduction, or primer if you will, to the physics of climbing forces will help climbers not only better understand the impact of their systems on TIPs, but also help them make better choices about which TIP to use.
Species, species, species
While not truly part of the discussion of the physics of climbing forces, it is imperative that climbers understand the strengths, weaknesses and quirks of the different tree species they regularly climb. Suitable TIPs in an oak and a cottonwood are not going to be the same, and climbers who do not recognize this will soon be reminded of the error of their ways.
A dynamic climbing system, one in which both parts of the rope move and are attached to the climber, puts pretty easily understood forces on the TIP. The weight of the climber, along with his actions, such as bouncing, dropping or swinging, is split between the two pieces of line, approximately half in each part. This is, of course, affected by friction at the TIP, elongation in the line and other factors, but in general each part of the line is supporting half of the climber’s weight/force.
As mentioned, actions such as drops or bounces by the climber will affect this weight/force as gravity acts on their mass, increasing the force experienced by the TIP. This is just one good reason to keep slack out of the climbing system as much as possible, for the shorter the distance of the drop or bounce, the less force on the TIP.
The use of a friction-management system/device at the TIP will not change the forces endured by the TIP, though it will reduce the friction experienced by the line and the climber and may dissipate the forces involved somewhat better around the limb or the trunk. However, a climber who uses a separate line to lift a pulley into the canopy to provide a TIP has changed the forces felt by whatever branch the second line goes over as it returns to the ground to be secured. They have, in effect, taken the dynamic load split between two parts of rope and doubled the force that the TIP now bears.
Closely examining the system illustrates this: The pulley is experiencing the force of the climber’s body weight and movements within the system in a downward direction on the branch. The anchored part of the line must exert just as much force in a downward direction on the branch in order for the pulley to stay aloft, thus the branch sees twice the force of the climber’s body weight and movements within the system.
Static don’t mean stagnant
A static system in which both parts of the line are being climbed, such as footlocking, or even the use of a hybrid system with a floating anchor and a dynamic system on two parts of a static line will result in similar forces to a dynamic system on the TIP, roughly the climber’s body weight plus bounces and drops. The forces change though, much as they did in the dynamic discussion, when one part of the line is anchored and the other is climbed.
Once again, the forces are doubled, roughly at the TIP. As in the earlier dynamic climbing with a pulley on a separate line example, one part of the line is supporting the climber’s full weight and actions, so the other secured part will see roughly the same forces, resulting in twice the force at the TIP. Obviously, friction, rope elongation and other factors may lessen or increase this force, but, in general, a climber can guesstimate a doubled force in this system.
This in no way means single line use is unsafe or unwise. In fact, some of the most progressive and modern techniques that are both safe and efficient involve the use of a single part of the line. It simply means that climbers must evaluate their TIP with an eye toward what possible forces it might experience. In addition, many single-line systems, when properly set up, allow for the lowering of an incapacitated climber from the ground, a huge advantage over the traditional dynamic climbing systems.
Bending, vectors and whatnot
Though slightly beyond the scope of this basic introduction, tree crews would be well served to realize that TIPs that result in wide angles – meaning a force both horizontal and vertical in nature – may cause different stresses on the TIP. These types of forces, often referred to as vectors or bending moments, can stress the structural integrity of the tree, trunk or branch in ways it’s not prepared to deal with. If one thinks of pulling down in the same plane that the tree grows, and then pulling it perpendicular to that plane, one can realize the changes in forces that have been applied to the TIP.
New and improved
As always in the tree care industry, new information and research is becoming available and is in the process of being disseminated. As is often the case, this is typically driven by the development of new methods, tools or techniques from climbers within the industry. There has been some preliminary research done suggesting that perhaps a TIP in a static-type system with one part of the line being climbed does not see twice the load as previously thought. In addition, some research suggests that a dynamic system may experience more than simply the load of the climber.
While more information will certainly become available as the research progresses, currently a climber cannot go wrong by overestimating the amount of force the TIP will experience, whether it be in a static or dynamic system. After all, being “too safe” when a life hangs in the balance doesn’t seem to be that big of a problem.
This basic introduction to some of the forces in climbing systems should assist tree crews in better evaluating those TIPs and hopefully making better and safer TIP choices. The most important lesson is that spending a little more time looking for a good, strong, secure TIP for the particular climbing system/method being used may prevent a speedy, and painful, descent to the ground.