Prolyte Campus blog: Rigging LED screens
Rigging and such, a series of blogs by Rinus “Rhino” Bakker
Rigging LED screens
LED screens are heavy, not as heavy as in their early days, but the current tendency to use bigger systems isn’t helpful to bring the total weight down. It’s not to be expected to see a real decrease in the LED-screen weights that have to be rigged. So, how to rig them properly?
For a start, the use of (smaller sized) triangular truss is out of the question. In general, triangular truss uses 25% less material than same sized square trusses, but also have an approximate 50% lesser loading capacity. Triangular truss is normally used for smaller loads, like textiles, and has a more restricted application range than square truss.
Square or Rectangle: How to balance your LED screen load?
So, square truss is the advice, but how?
For lighting technicians, it is relatively easy to distribute the fixtures over the two sides in the cross section of the truss. However, a LED screen load is a very narrow and long load, by definition a line-load. And a straight line is impossible to distribute among two bottom chords.
When only one chord is used the worst possible unbalanced load is the result (see fig 1). Manufacturers will immediately say that their load data are not valid anymore is such a situation. Bending and shear forces are taken into account, but a torsion force across the main axis of a truss is not foreseen in the standard loading tables.
As an alternative the slinging suspension could be positioned over the Centre of Gravity, but again this will lead to a 50% reduction of the truss capacity, as one side of the truss is only acting as a stabilizer structure for the loaded ladder side of the truss.
(This ‘single sided support’ is not possible when the truss is assembled between two sleeve blocks of a ground support).
Diamond orientation use of square truss?
Even though this practice is observed on a regular basis when LED-screens are rigged, the answer can only be: ‘No’. (Fig. 2).
A multitude of parameters are at play here, leading to this negative advice.
The most important one is the fact that lots of (mainly plated, but definitely other types as well) trusses do not have diagonals in all sides and thus do not allow for such orientation.
The second is that lots of trusses are missing internal diagonals, resulting in relatively easy deformation of the square cross section into a rhombus shape. Specially the bi-conical connection trusses made with narrow tolerances are susceptible for this type of deformation. Responsible brands, like Prolyte, will tell the user that using truss in a diamond orientation does reduce the loading capacity dramatically and will cause a real hazard.
Square with a simple basket set-up
There is no rocket-science needed for a simple solution (fig. 3). The drawing shows a square cross and a B55 (or B56 or B57) basket variation across the top chords. Those baskets obviously must be at the nodes or at least at the ¼<>¾ (“under” quarter or “over” third quarter) positions, and definitely not end up right in the middle of the chords in between the nodes. Simple but effective.
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