|24 12 2017
LED-screen data in the Risk Assessment
When rigging of LED screens is planned, it is mandatory to do a Risk Assessment, see also a previous blog on this topic. In order to reduce any possible or foreseeable risks, we need all available data of the equipment involved. Obviously when talking rigging for LED-screens the optics and electronics are of less importance, but the strength and stability - as well as the applied design factor are. This could be a problem, not much data on the structural design of the screens is available.
Manuals often fail to give such information, although weight, size and placement of top-brackets can be found. To get more data, I performed some tests on a top bracket, equipped with one big eye bolt and two smaller ones plus an empty module frame. The bracket would be on top of a 12 LED-modules high row. The bracket and frame held out well, yielding only at “WLL x 2” a force equivalent of 24 modules.
However, it’s hard to find out whether all manufacturers design their brackets as lifting equipment or not. The most outrageous combinations of using many or just very few eye-bolts can be found when doing a quick look in Google image. So be sure and check before use.
The only thing consistent is the fact that the LED-screens are very rigid within the plane of the structure. The type of load that is transferred to the truss is completely different from sets of drapes, lighting fixtures, etc. at identical spacing. A truss span loaded with a LED-screen will give puzzling read outs in load cells, much different from what is to be expected in a statical indeterminate distribution.
Riggers only have the truss data
Looking at allowable truss
loading and the resulting deflection, in combination with a LED screen that is more rigid than the truss from which it is supported, we have an issue. The result will be that the centre of the truss span is closer to the LED screen top side than outer ends of the truss span.
As a result of that, the slings in centre will have more slack than on the outer ends. This is completely opposite to what we know! The outside lifting points will get more load than the centre lifting point. By how much? That’s difficult to tell, when truss type, load type, position and sling length variations all can play their own role.
To compensate for deflection, each lifting point could be equipped with a turnbuckle, but how can we check if the tension is identical in all of them? Tightening one means loosening the neighbouring two…
- Use a clearly bigger truss that has lesser deflection. In this case the best assumption is to verify the truss for a TPL load and check if those eye-bolts on the screen would still be loaded safe.
- Set up a system of LED-Screen brackets that have a static determinate system, which will result in a true TPL.
- Make sure the slinging is positioned in the node points of the truss. Read more about this in a previous blog about how to rig a LED screen.