How much do you weigh?
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?
To be able to work out how to safely rig equipment for circus, one of the elements we need to understand is what sort of forces are being applied to our equipment during performance so we can be sure the equipment is strong enough to take it.
If you are a silks performer and have to rig for yourself, how do you know whether what you are hanging from can support you and your performance? How do you calculate the 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 margin of safety?
If you weigh 50kg then depending on what you are doing and what safety guidelines you use you may need a rigging point that supports over 2500kg, yes that’s 2.5 tonnes!
WHAT? WHY? How do we get from the 50kg that you would see on your bathroom scales to 2.5 tonnes?
The first part of the answer is shock loading! When you fall and that fall is arrested suddenly the load, i.e. you, has greater ‘weight’ than your static weight. Try jumping up and down on your bathroom scales if that’s not clear (please don’t try sending me the bill 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!).
At a very high level of acrobatic skill in certain disciplines you may be generating forces greater than ten times your bodyweight. That means now you weigh at least 250kg. Some professional performance companies have found equipment rated at ten times the performer's bodyweight failing due to the shock loading generated by the performer. However it must be said this is an extreme case.
If you are held up by something that would only just support your biggest drop, say you are applying a shock load with a peak force of 250kg and your equipment can support only 250kg, you are right on the edge of the cliff. One extra eclair or one additional kg of force and you are plunging straight over the edge and making a messy landing.
The best idea would be to move you back as far from the cliff edge as possible so you have a reasonable margin of safety. This is the second part of our equation. The simplest way is to use multiples of the load, we can therefore call it a factor of safety. A commonly used factor of safety in performance is 10:1.
If your peak dynamic load is 250kg and you add a factor of safety of 10:1 on top of this it means that your rigging point must be capable of supporting loads up to 2.5 tonnes. If we use this method we should ensure all of the equipment will also support the same forces as well as the rigging point itself.
Before you start to get too self-conscious and start thinking about a diet, let’s look in a little more detail on the subject of load, weight and force in the next article.
I've added a document in our resources section which covers some standards around factors of safety in performer flying