ProlyteCampus blog: Rotating a truss from Square to Diamond?
What is the effect of using a square truss in a diamond cross section shape? A type of use often seen in relating to hanging (LED) screens, as a solution to create a centralized hanging point. (see fig.1).
Before you start
Please realize that for square truss, used as a diamond shape, the loading capacity of that truss is reduced significantly, almost to the point of that of a flat truss.
If no intenal braces are in place, the truss may start to deform under loading. (see fig.2).
The truss position in a truss pod
Any proper design of a square or rectangular truss type will have a system of internal diagonals that prevents the cross section of the truss to deform under conditions of for example: - unbalanced loading; - sloppy slinging, - rotated orientation. That last thing will happen in a truss pod that is raked: two sides will be in the raked and two sides in a rotated position. (see fig.3). Support reaction forces in raked trusses were discussed in the previous Prolyte Campus blog.
In such a raked set-up the loading tables are no longer valid. But the manufacturer should not immediately ‘punish’ the user for doing a truss pod design like that. In Europe a manufacturer is expected to keep in mind that ‘abnormal’ use can happen and that this shall not lead to immediate failure.
Internal diagonals help the user in the return on investment
Why is that, one might ask?
Well, let’s have a closer look at a square truss used as a diamond shape cross section. The load – like lighting fixtures or LED screens – will be applied to the lowest chord, and slinging at the support points in most case will pass around the two side chords, or –one should not hope- the top chord only. The effect of this is a vertical pull and a horizontal compression, forcing the shape to become a rhomboid. (see fig.2). Internal diagonals can absorb those forces, but as these are not as abundant as the regular braces, their capacity to prevent such deformation is not unlimited. Even a brand like Prolyte, that manufactures trusses equipped with internal diagonals, will tell the customers that diamond shape use of square truss shall reduce the load to about 66% of the loading tables.
And what about those brands that don’t have any internal triangulation? Where will they limit the capacity of the diamond shape use? From lots of user experience and feedback it turns out those brands often are causing more problems in assembly. Not too hard to imagine, because when the conical connectors, bolt holes or fork ends are machined with high tolerance accuracies, even the slightest deformation, can render a module difficult to assemble. Trying to get it mounted, using hammers, ratchet straps and donkey kicking will start to degrade all the truss units it is assembled to as well. The example of such extreme deformation is shown in fig. 4.
So please keep this is mind, next time you use a square truss section as a diamond shape. Although diamonds may be a girls’ best friend, that doesn’t necessarily account for riggers.
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