Prolyte Campus blog: Unbalanced, Asymmetric, Uneven Loading?
Many audio techs and electricians will think: Ah, Okay, I know about that. But in rigging and trussing the word does not seem to be as familiar as it should.
Some people refer to unbalanced loading (of chain blocks or electric chain hoists) when these do not (all) lift an identical load. That however should be called uneven or asymmetric loading, as a balance -by definition- involves a single pivot point.
Lifting identical loads in the entertainment industry is exceptional anyway, even on a ‘uniformly distributed’ loaded truss, supported by two points. Electric cable weights will build up the amount of load towards the direction of the dimmers and/or power source positions, thus the support closest to that will be taking more load.
A real 50-50% distribution only happens in a projection screen frame, a backdrop or a (closed) front curtain, when these are on two evenly spaced support points.
A uniformly distributed load on a truss (or a fly bar for that matter) supported in three positions, will result in the one in the centre taking much more load than the two outer ones.
A further explanation of statically determinate and indeterminate is beyond the scope of this column, but the centre support will take half (or more), even when the load is uniformly distributed. This is clearly another example of uneven (asymmetric) loading, although some people will still speak of unbalanced loading.
When should we speak of unbalanced loading?
Considering the single pivot point, unbalanced loading in trusses happens when one side of the cross section of a truss is to absorb more load than the other side. This is something quite commonly done.
However, this is a situation where the loading tables of the manufacturer are not valid anymore. So please take this into account on your next gig!
For further reading: Prolyte Group Black Book 2015