Prolyte Campus blog: Yes we can (do better slinging)!

For the fans, one more blog on (scary) slinging found on the World Wide Web (read here the first blog on slinging and here the second blog). Although at first impression one might say the guys rigging that sound system took extra care in covering the rear of the cabinets with plastic, thus reducing electrical risk. Question could be: why leave the chain motor un-covered? And where are the horizontal restraints for the line array system to prevent it from swaying in bad weather.

The issue: The real concern however is the basket method of slinging the chain motor point the truss. This one looks like nobody did notice the deformation in the lower chords. Being too focussed on the sound system perhaps? A more detailed look at this deformation shows a standard basket method, however applied to a truss type that cannot deal with it. Trying to establish the sling route in the truss module gives the situation shown in figure below.

The answer: Application of this B024 basket hitch to this in this position is totally wrong. The internal cross strongly increases the force that compresses the lower chords inwards. No horizontal bracing is present at that point so the chords deform drastically.

The verdict: At this position the truss module bracing structure is accepting a type B011 basket with ease. The sling could be much shorter as well. For stability reasons one (B013) or each (B012) of the chords could get an additional full wrap. This increases friction thus reduces risk of movement of the sling and keeps the motor centred, and the truss load balanced. Even the B001 method is to be preferred here over the internal cross-basket (B024). Each type of truss requires a different method of slinging, also depending on the position in the span.

 

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