How much do you weigh?
How much do you weigh? As an aerialist you could weigh around 1000 lb!
This was a statement offered by Brett Copes in a workshop I attended held at the Circus Warehouse in New York last month (OK, I may be paraphrasing a little!). Brett specialises in stunt and performance rigging. He works with dozens of aerialists and aerial shows a year and has been rigging for Cirque du Soleil since 2009 with “O” and IRIS.
Brett is an engaging and entertaining speaker who provides a very practical stance on advising aerialists about rigging safety. One of the aims of his lecture was to provide some simple guidance to make performers who rig for themselves safer. To do this part of his workshop covered some figures that can be used to calculate how much force your equipment and rigging points would need to handle to safely support you.
If you are a silks performer and have to rig for yourself how do you know that what you are hanging from can support you and your performance? How do you calculate what load your rigging (anchor) point should be able to support? Your own body weight plus a little bit? Double your weight? Does that give you an adequate factor of safety?
Leaving aside differences between US measurements and the metric system. I’ll do some creative rounding for the sake of this article.
If you weigh 45 kg or 100 lb you may need a rigging point that supports something in the region of 3000–5000 lb or 1200 kg to 2200 kg. Yes, that is 2.2 tonnes!
How do we get from the 100 lb or 45 kg that you would see on your bathroom scales to 1000 lb to be used as a figure for your weight? And how do you then get to a figure of over 2 tonnes for your rigging point?
The answer to the first question is of course shock loading. When you fall and that fall is arrested suddenly the load, i.e. you, has greater mass than your static weight. Try jumping up and down on your bathroom scales if that’s not clear (please don’t try billing me for a new set!). You could then try falling on your bathroom scales from 6 metres to see what forces are applied in your act (DO NOT TRY THIS AT HOME - obviously!).
For a simple calculation I’m going to use the following formula, there are other ways but this is simple enough:
W x (1+ DFalling/DStopping)
Figures for this formula are in feet and pounds. The force applied to your equipment (and your body) can be calculated by a combination of your weight, your falling distance and your stopping distance. If you don’t want to work this out for yourself go and check out D2 RigCalc or get their app for your iPhone or Android.
OK, let me throw in a couple of examples: A 100 lb silks performer, dropping 5 ft in a silks drop with the stopping distance over half a foot due to the stretch in the silks. The shock loading for this calculates to being 1100 lb. That is around half a (metric) tonne.
A 100 lb trapeze performer dropping 2 ft with a stopping distance of 0.2 ft calculates out to the same 1100 lb. Or, if you are me at 72 kg or 160 lb, put in your own figures!
So from these examples it looks like it is possible to generate forces of over half a tonne, so your rigging point and equipment is recommended to support a figure greater than this so you have a margin of safety. So how much should that be?
Brett had a great analogy here related to a cliff edge- if you are supported by something that would break with your biggest drop, say you are applying your 1100 lbs shock load and your equipment can support only 1100 lbs, you are right on the edge of the cliff. The idea is to move you back as far from the cliff edge as possible so you have a reasonable factor of safety.
There are different institutions that give different guidelines, Cirque de Soleil work on a guideline of 10:1, ANSI the American National Standards Institute give a figure of 7:1. The usual recommendation for professional artists is a safety factor of 3:1 to 5:1.
So the in short you weigh 1000 lb and to build in a safety factor you will need a rigging point that can support 3000 lb to 5000 lb or you weigh 45 kg and your rigging point is usually recommended to support a load of 1200 kg to 2200 kg.
Don’t forget it’s not just the point you are rigging from that we need to consider but all of your equipment to which that force is applied.
For a circus school like Aerial Edge our over engineering factor is greater but the forces are also much less for the average student. For crazier students I have to get my calculator out.
Thanks to Brett for a great lecture, any errors or hyperbole are my own!