In this article, we’ll take some foundational rigging concepts and reintroduce them from a new perspective. Some of this information is well known and almost common knowledge, but some of it, I find, gets missed in early training or overlooked in the daily grind. Some of it could help us avoid the abuse we sometimes put ourselves and the trees through, and instead, allow us to enjoy the art of rigging before we get to that point in our career where we are too beat up to want to do the big removal.
We’ll address a few questions:
• What is rigging?
• What and where is force?
• What is mass, and how does it affect rigging?
• How can we control some of the variables in rigging?
• Why do we need to know math to be an arborist?
• How can we make this industry a little bit better and a little bit safer?
There are still some trends in rigging that are a little frightening, and there are still a lot of accidents and injuries occurring. That’s why it is so important for us, as professional arborists, to continue to strive to be better and to keep moving this industry forward. You have to be smart to be a professional arborist, because it requires an understanding of some advanced mathematic principles as well as a great degree of science. Who knows, you might just be the smartest person you know!
What is rigging?
To try and answer my first and what I thought was my most basic question, “What is rigging?” I looked up the definition. The only definition I could find was, “Lines and chains used aboard a ship, especially in working sails and supporting masts and spars.” There was another definition related to theater scenery. I tried many different searches, but none related to arboriculture came up. So it seems it is up to us to define arborist rigging.
What does rigging mean in our industry? Sometimes I think it’s just absolute destruction through calculated risk, but that goes against what I said earlier about arborists being the smartest people I know. Therefore, we need to break down the process and tools it incorporates to be able to define it.
Rigging is changing the orientation of something through the use of mechanical advantage. It is also, to borrow an idea from my friend, Tony Tresselt, creating “gravitational anarchy through the application of friction.”
We do this in many ways in our industry. There are many different tools used for rigging, from simple plastic wedges and all the beautiful and shiny mechanical-advantage systems such as pulleys, blocks, come-alongs and port-a-wraps, to the largest and most exciting pieces of equipment – cranes!
Rigging with mechanical advantage can include block-and-pulley rigging or aerial-friction tools to move an object from one position to another, usually lowering it down. Then there’s speed lining, which is still moving an object from one position to another, but with speed lining, there is usually an obstacle you are trying to work around between you and the truck or chipper.
There are many different variances to rigging, but to sum them all up, we are trying to move a mass in a controlled manner against the force of gravity while controlling the energy generated, and we need engineering to do that; for that we need math. We need to break down, based off of our own knowledge and experience, where our supports are. Will our supports hold? Are our angles correct? And are the ropes and blocks/tools strong enough to hold the load? Where is force and how can it be managed?
What and where is force?
Where is force? All around us, as Yoda explained. But when we are talking about arborist rigging, we have to look at where force comes from and how the forces are being applied in our rigging. One of the ways force can be explained is as a formula: Mass x Acceleration. Another way to look at it is as the energy of movement. What is the main factor that affects force in our typical rigging system? Gravity. Gravity is a big factor in everything arborists do. In tree work, we need to figure out how to manage gravitational acceleration, and therefore energy, through the tools and techniques we use. We have a lot of cool equipment to accomplish this.
What is mass and how does it affect rigging?
Acceleration is a key consideration in rigging. Mass, the amount of matter in a particle or object, has an effect on the ultimate force on our rigging system. We are removing very large pieces and small pieces and everything in between, so how do we manage both mass and acceleration?
I’m sure we all can recall a time we were rigging when our mass x acceleration equation didn’t add up and the energy created caused a near miss or an incident. I always look back to the start of my career when I naively told my boss I knew how to use a Hobbs lowering and lifting device and I ended up five feet off the ground, with melted skin from my hands to my armpit. This was my first lesson in mass x acceleration and in figuring out that they are two very tangible powers I have to respect.
Knowing how much something weighs is the first part in successful rigging and, thankfully, we have a green-log weight chart to help us with that. It’s not always 100% accurate. There are always variances due to moisture content, branch attachment and decay, but by using the green-log weight chart consistently, we can begin to get a very reliable weight estimation and eliminate the pure guesswork from this part of the rigging formula. If you don’t have the app or chart available, you should get one.
Understanding and knowing the weight of the pieces we are cutting is the first step toward success. This becomes a critical part of a rigging plan when using a crane of any type. Cranes are a rigging tool used frequently by arborists to safely remove trees. A crane is a lifting tool with known weight limitations. As long as we know the weight of a pick and we eliminate movement that increases force through kinetic energy, we can safely and efficiently take a tree apart.
Unfortunately, we are an industry with one of the highest crane-tipping/failure rates out there. We can do some awesome things to maneuver big tree parts with cranes, and when we know what we are doing, they make our jobs easier and safer. But we also need to understand the mathematical formulas that go along with rigging and crane use to avoid things going badly for the climber, the cutter, the operator and the grounds crew when we try to take too heavy a piece at too great an angle and the crane tips over.
Here’s the nifty thing; a crane can tell you exactly how much it can pick up, at the exact radius, with the specific length of boom in the air. There is a chart and a computer on the crane that does these calculations for you. When someone says the crane failed, you can say, “Cranes don’t fail unless the boom breaks due to an engineering malfunction. If a crane tips over, the failure was due to cutter and/or operator error.”
The only unknown variable in the equation is the weight of the log. The only thing we as arborists can do is educated SWAG (scientific wild-ass guess). But if we are using a crane or doing rope-and-tackle rigging using a scale of some sort for each cut, then discussing each cut as a crew and comparing weights of different picks will help us become more accurate in this critical part of the rigging process. Success starts with truly knowing mass, and I find that, on most crews, weight is rarely discussed until after something was too big!
Managing the mass
So how do we manage the mass? Cut smaller pieces! Seems simple and not nearly as much fun, so how else can we do this? We can do this by taking that mass and spreading it throughout the canopy by improving our rigging. So how do we improve our rigging? How can we cut down the amount of mass or the amount of load at a single point and control the acceleration? We add in multiple blocks or double-block rigging. As long as the angle of deflection is 90 degrees or less in the blocks, we can distribute the load more equally in the canopy.
This is also where we control acceleration. By eliminating friction from the canopy, we can more precisely control acceleration at the base of the tree with many of those shiny, cool tools – from the port-a-wrap to the GRCS (Good Rigging Control System) and all things in between. The larger the device barrel or bollard, the more friction; the more turns around said bollard, the more friction; and, finally, the bigger the diameter of rope, the more friction. Also, adding friction in the canopy through the use of rings or other tools is another excellent way to control friction, and therefore control acceleration. Combining aerial friction with blocks and basal friction adds the benefit of multiple rigging points and great acceleration control, and should be considered on all complex removals.
Dynamic movement and energy
We have discussed mass and acceleration. Now let us have a look at that final component, the force or energy generated from dynamic movement. How can we manage or dissipate this energy, and what are a few things that commonly seem to be overlooked?
We all know ropes are one of the key components in dispersing or assuming the energy in our rigging systems. Arborist rigging lines have certainly come a long way and now have greater strength, as well as the ability to elongate at certain loads to help with some of the dynamic energy that occurs in arborist rigging.
We tend to take for granted just how much our rigging lines do for us and do not replace or care for them as well as, say, our climbing lines. Keeping inspection records and in-service records for our rigging lines will help us track cycles to failure, so we know when to retire these critical pieces of equipment. There is a limit to the resiliency of these ropes, and once that is reached, they will fail! A wise person once told me I need to know the day before the day the rope is going to fail. That way I can retire it and save an insurance claim. Recording and tracking their use can get me closer to that date. Thankfully, rigging rope is not a finite resource.
The next seriously overlooked critical components are slings, especially those used for block pulleys and port-a-wraps. Slings are considered a rope and should be treated the same as any rope. And let’s face it, inspecting and replacing slings is a practice a lot of us should be doing more often.
There has been a lot of focus lately on the critical role slings play in the F = M x A equation (force = mass x acceleration). It seems that slings are loaded before the rope and, depending on the angles, see the peak force in the system. They are truly the front line of success in our rigging, and they are the most mistreated. I have seen slings girth-hitched on port-a-wraps for multiple years, where the sling literally had to be pried off with a tool. When was the last time you thought of the cycles-to-failure of your slings?
Another great variable we need to consider is the trees themselves. We know we have strength loss in ropes, and we know we have to consider our angles when it comes to our chokers and slings. We know we have to calculate wood mass and how we divide mass between rigging points. We have figured out friction and how to use our tools to help manage force and energy, but here is the problem with trees. We have really good ones and we have not so good ones, and when it comes down to the structural engineering of a tree, no two trees are ever the same.
Trees are living organisms, and the variables involved with them change with every tree, from visible and non-visible cavities to varying branch attachments, different moisture contents and many other factors. How much the tree can withstand is one of the most difficult questions an arborist faces, because there is no standard answer. The only thing we can do is to educate ourselves as much as possible, continue to increase our experience and use and understand the tools we have.
Trees have adapted to all sorts of environmental forces such as wind, rain, snow and ice loading. They are excellent at dispersing energy through movement and harmonics. They have been surviving under incredible force and load events for millions of years. When building rigging systems, we need to consider how placement of rigging points can incorporate more of the canopy and use the tree’s growth and the structure it has created to improve energy disbursement. Don’t build every rigging system or make every cut to fit your convenience; read the tree and its structural strength!
One of the coolest things I ever learned about rigging came from Rip Tompkins, a highly experienced climber and trainer in the industry. His theory was that by leaving one branch on the tree while taking the top down, that one branch will, through the damping effect, significantly reduce the amount of shaking the tree does when the piece comes over. It works! For better or worse, the tree, if engineered against its structure, will often fail before our tools.
The last thing we need to consider for energy or force dampening is the root system. There’s some very interesting research out there, where people have set up cameras to watch the ground when large logs are being dropped and rigged off of the tree and you can see the shock waves move through the soil. Roots have a dissipating and dampening effect to stabilize a tree during load events. Inspecting the root system before rigging is an important consideration we often forget about. Site factors such as roads and sidewalks that restrict root systems should be considered. The way the tree adapts to these restricted root zones affects the tree’s structure. Trees build reaction wood, and they move their center of mass around to accommodate their growth environment and support themselves. Building rigging that directs forces into the ground, into root systems, will help disperse energy and help prevent system failure.
What these variables add up to is tree inspection. Don’t skip this step. Completing a thorough tree inspection and understanding what we are looking at, and for, will give us the best chance at understanding all the variables that will affect our rigging when removing a tree.
We can use our knowledge and understanding to control as many variables in the equation as possible.
Concerning gear inspection, when in doubt, throw it out.
Last but not least, take smaller pieces. The number-one thing you can do to make yourself safer is to cut smaller pieces. Tree work is not a race; we are not competing against another crew to see who can do it faster. If you feel like you need to make more money doing it, raise your bid. Smaller pieces mean you’re not maxing out your equipment, you’re not wearing down your crew and you will all have a better chance of going home safe and sound at the end of the day.
Phillip Kelley is a safety team leader at Wright Tree Service and lead instructor with North American Training Solutions, a 10-year TCIA Associate Member company based in Douglas, Massachusetts.