Prolyte Campus blog: Yes we can (do better slinging)!
Recently I got some pictures of bad deformation of a master grid truss at the slinging position. A number of 50 cm size trusses were used as lifting beam for serious loads from sound and screen points. It was not a Prolyte truss but looked very similar.
The trusses were used as master trusses (individual lifting beams), rather than as one master grid. This was done to avoid making bridles to the lateral relatively weak structural beams of the venue.
The issue: The trusses were suspended from 2 tonne motors, and subsequently 1 and 2 tonne motors were to be rigged from the master trusses. Slinging of the truss was done in very straightforward baskets. The basket methods were the variations B 001 and B 011 from the basket slinging classification *. The trusses were sling-supported close to the end of the modules, and from the span inverted down to the chain motors hanging of them.
The answer: Know what you are doing. Manufacturers (like Prolyte) always state that the support should be at the ends of a span and close to or in a node point. Thus being next to the end bracing that has vertical and horizontal braces in each side, plus the internal diagonal or stabilizing corner plates in that area. Slinging next to this is never a real problem. On the body of the truss the word is different. Slinging must be at the node points, with braces present in each direction of applied force. In a basket that will always be two directions in a 90 degree - or less - angle to each other.
The verdict: Each type of truss in cross section or in bracing pattern has its own number of preferred and discouraged or even not allowed methods. Too often people think this is the best method and we should apply that to all truss types.
For example if method B024 had been applied the risk would have been considerably higher for a complete failure of the chord and subsequently the whole truss. Similar accidents have already been recorded, where mistakes in slinging were (partly) to blame for a collapse.
* Over 200 basket variations have been described with a single sling on a square truss.
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