The space was tight. The margin for error was even tighter. The crown of the once-illustrious beast that overhung the owners’ house – and the neighbors’, and the other people’s garage and the wires – is now on the ground. You’ve utilized all of your knowledge about rigging basics creatively to solve this puzzle. There may have been some near misses, maybe even a roof swept with brushy tips, but you and your team got it done. You may be out of the frying pan, but you’ve just jumped into the fire.
Now you’re staring down the barrel of the unforgiving trunk wood. It’s big. There are pockets of decay. The structures are still pretty close. The new pavement goes right up to the root flare. Now what?
Dealing with the wood of a spar is a whole different ballgame, especially when there are targets and structures right underneath. For starters, negative rigging involves the greatest force you can put on the remaining tree. It is also the most dynamic demand you can make on your rigging gear. Previous studies have shown peak forces reach upward of 10 times the weight of the actual piece. As always, you need to create a rigging system in which all of the components can withstand the maximum force you could possibly generate.
Take a moment to reread that previous sentence. It is critical. If you fail in this task, your rigging could fail. Rigging branches out of a lush or even a dead canopy is certainly high risk. Errors can lead to injury and property damage. It could be argued, however, that this pales in comparison to the devastation a much denser, heavier chunk of wood can cause as it basically bungee jumps into a rigging point toward fragile, or even robust, targets.
The weakest link
You are going to have to do some math here. This will require you to consult a green-log-weight chart so you can begin to estimate how much the wood weighs. These days, there are even phone apps that can provide this information and basically do the math for you, provided you pick the right species and dimensions. Be sure to take into consideration things like reaction wood, crotches/branch collars and water saturation, as these things can add greatly to the weight. You might be tempted to make calculations on sizes of timber based on space and spar or root conditions. While these are very important, your limiting factor (that you have hard data on to work with) is going to be the minimum breaking strength (MBS) and working-load limits (WLLs) of the gear available. It might be wise to start here and work backward. Keep in mind that your rigging system is only as strong as its weakest link.
Let’s say you have an arborist block that has an MBS of 50,000 pounds, with a sling and rope capacity of three-quarters-of-an-inch. The MBS of the sling and rope are both 20,000 pounds. At first glance, you might think you have the ability to deal with a maximum force of 50,000 pounds, but the MBS of the rope and sling are both 20,000 pounds. These are your weaker links and thus become your limiting factors.
Don’t forget that putting knots in ropes reduces the MBS, often quite considerably, depending on the knot and type of fibers you’re working with. For the ease of doing a rough calculation, let’s say the particular knots you’re using with the particular fiber types of your rope and sling reduce the MBS by 25%. So, 20,000 pounds minus 25% brings your MBS down to 15,000 pounds. As previously stated, the peak force of a piece of wood being negatively rigged can generate 10 times its weight at the rigging point. Let’s apply this to our example; 15,000 pounds divided by 10 gives you 1,500 pounds. Given the MBS of the hardware, you do not want to rig a piece any heavier than 1,500 pounds. Why? In a worst-case scenario, that 1,500 pounds of wood can generate 10 times its weight at the rigging point and produce a force of 15,000 pounds. That is exactly what we’ve calculated our MBS to be after the strength reduction caused by the knot used for attachment.
Suffice it to say, taking the time to consider the math involved can be critical.
Cycles to failure
Another factor of great importance is cycles to failure. The basic premise is that wear and tear ultimately will deteriorate your equipment. If you frequently approach the MBS of your equipment, you can expect its life span to be much shorter. This can be a terrifying prospect, as there is no definitive “life meter” indicating when a rope or sling is about to expire. It can happen suddenly and violently. It’s on you to inspect your gear often and become familiar with the signs of equipment fatigue. Retire your gear before it retires you!
The traditional way of bringing peak force down, so that we do not generate 10 times a piece’s weight every time we negative rig, is to “let it run.” Basically, decelerating a piece’s fall through a well-timed belay on a lowering device can have a drastic effect. Unfortunately, in a tight-quarters environment, there is not much room to sufficiently decelerate. The shorter the spar gets or the closer you get to the targets, the less time and room you have to work with.
Even though we have found ourselves in a “no-run” or “snub-off” situation, we still have options.
We can build our rigging system with more robust components, add more components/rope into the system, take smaller pieces, fortify the targets or employ a combination of all these things.
Getting hold of bigger ropes, bigger slings and heftier hardware is self-explanatory.
Adding more components leads us into looking at some different techniques. The first time I ever saw anyone set up two independent negative-rigging systems intended to be used in conjunction to handle one piece of wood, it was Reg Coates (an experienced climber and rigger now living and working in British Columbia, Canada). He effectively spread the peak forces over two sets of gear. It’s an ingenious solution, but it requires quite a budget. A slightly more economic option came to me via the April 2015 issue of TCI Magazine in an article called “DBR: Double-Block Rigging,” written by Gareth Tudor-Jones.
Essentially, by running the rope through the first sling, then through a second sling on the piece to be rigged, and terminating the end of the line above the first sling but below the notch, you create a reverse mechanical-advantage system. This system incorporates more force-absorbing rope and slings into the system. The nature of the setup also makes things easier on your equipment. While the mechanics of letting the load run change drastically, it can be perfect for a “no-run” scenario. Various combinations and variations of these techniques are possible.
The sky’s the limit, or maybe I should say the ground’s the limit. No rope-based technique can help you when you reach a height that is too close to the ground to negative rig. Now we’re going to have to get creative.
When it comes to saving concrete, brick or even wood, a well-crafted “crash pad” can work wonders. This often consists of, but is not limited to, tires, pallets, plywood and brush arranged or layered in a way to absorb impact. You also will want to cut or slab the wood into smaller bits. In some cases, you may even have to halve or quarter those bits. The most fragile scenarios, like decks built around trunks, will require more mincing.
If you’re at that special height where you’re not high enough to rig without a piece hitting the fragile ground, but you’re too high to drop even small cut chunks, you may have to get ladders and sturdy shoulders involved.
Kudos to the crews that take on the tightest of situations. Your wits must be as sharp as your saws. You must be as creative as you are tough. But remember, the mantra of “when in doubt, rig it out” changes to “when in doubt, mince it out,” because we cannot calculate the MBS of the spar or the condition of the urban-disrupted root plate. Be conservative with your calculation, but even more so with your safety.
Lawrence Schultz is an ISA Certified Arborist and an ISA Certified Municipal Specialist working as a contract climber in the San Francisco Bay area of California. This article was based on his presentation on the same subject during the TCI Virtual Summit 2021 in January. To listen to an audio recording of that presentation, click on the play button below.