Tree people know all about rigging.
We dismantle trees with rigging systems of lesser or greater complexity, depending upon the situation. Sometimes, when the situation allows for it, we will rig speed lines, zip lines, Tyroleans, Flying Foxes… a range of names basically describing the same thing: a tensioned line between two anchors allowing to move a mass from the higher to the lower anchor point avoiding an obstacle below the structure being dismantled. The image below from a report commissioned by the UK HSE/ Forestry Commission, Evaluation of current rigging and dismantling practices used in arboriculture, illustrates the elements which might be employed in such a set-up.
Rigging a speed line can be fairly complex, involving floating anchor points, putting anchor points in compression by rigging beyond the high anchor or combining lowering systems with the tensioned line. The forces that a tensioned line can generate onto the anchors can be considerable, therefore speed lines are never used in a dynamic fashion, i.e. limbs or stems are never snatched directly onto them.
So far, so good.
I saw some imagery the other day of a Tyrolean at a tree care event as part of the family fun part that kids and spectators could zip down on. I was concerned by the fact that this was being run on a single line, on a single sheave pulley. Why is this an issue you ask? I think the following video of a rescue exercise in Peru that went seriously wrong is quite self explanatory…
There have been a number of very serious incidents, one of them resulting in a fatality, with Flying Foxes in the past years rigged by Scouts or similar youth groups here in Switzerland that led to insurances clamping down hard on the practice and refusing to ensure your event if you were planning such an installation as part of an activity. Only recently have they been allowed again, albeit under the condition that you fulfill certain, quite stringent, criteria and conform to best practice guidelines (see image below), such as doubling up the lines you are running the trolley on.
The incident in the video above could have been prevented if a backup had been used… as it was, it was the brake line that saved the two climbers – but only just. Further an appropriate pulley shall be used that can handle the sorts of speeds that are to be expected when running the speed line.
I was discussing something similar with a friend the other day who is rigging an installation for a big art show in town next week. He has planned the whole thing around 6 to 8mm Dyneema hollow braid lines that have a very high breaking strain – on paper. The installation will be anchored onto massive concrete pillars and sturdy steel collars with eye bolts welded onto them. Yet the span they are rigging across is very wide, about 45m per side, and the rigging is quite complex involving bridles and floating anchors, so whilst the weight of the strips of fabric suspended off the lines may be negligible, the over-all mass, combined with the high tension required to keep the whole thing at the height it needs to be at makes me wonder where the weak points in this system are and whether the specced materials are up to it. In the end we decided that the best thing to do would be to to integrate a load cell into the assembly to be able to keep track of the loads and to prevent overload and/ or failure.
The point I am trying to get to here is that I find it hard to understand how we can be so diligent and thorough in our professional life when it comes to rigging lumps of wood out of a tree on a speed line, to then in our spare time rig up something that is obviously inappropriate and potentially not up to the task – and then use it to transport people?
In both cases I believe we should attempt to always integrate backups where and when possible, to strive for the highest degree of control possible at all times and be very wary on the overall load on the system due to high loads on anchor points.