|24 01 2018
In practice, truss loading is something more than what loading tables might say and most of the times not a simple question like summing up the UDL and PL on a truss span, as the below drawing might make perfectly clear.
What do we see in the drawing?
The horizontal truss is assembled from several modules (1) into a span (2) between the supports points (2A). Supports can be a variety of things: a truss tower, a sleeve-block, a hoist, etc.
This span is loaded (almost ideally) in UDL (3) by a number of identical fixtures¹.
However that UDL load is combined with two PL (point loads) at more or less random positions. These positions can be chosen by means of inserting a 3-way-T corner (4) inside the truss, or using truss clamps attached to the lower chords (6). Obviously the ‘T’ shall be designed and manufactured according to CWA 15902-2, specifying that corner modules inside a span shall have the same load capacity properties as the straight modules. The trusses (5) hanging down vertically can also be used to mount fixtures to. This vertical load of these fixtures shall be equal on each side of that truss, as shown on the left vertical truss and not as in the one on the right where the 4 to 1 factor will cause additional bending stresses in the T-corner. A similar unequal effect can come from asymmetry in boom arms or outriggers as in (8)
As far as loading of those vertical trusses themselves is concerned: that can be many thousands of kilograms. Always much much more than any horizontal truss of the same type is able to hold safely. But as a totem or tower it still is considerable, but limited earlier because stability and buckling effects.
Although a reliable truss manufacturer will give you all the loading data, and the good ones even the technical specifications, there’s still a lot of work, and calculating to be done before you can rig your show in a safe way. Never underestimate the way that loads can easily accumulate and cause un-safe situations. When in doubt, never hesitate to ask for help from a structural engineer. That’s what they are there for anyway.
A final question
As a rigger I shall not be interfering with workmanship in disciplines like sound, light or screens and visuals, so I leave the question to you: Why I did mark two different colours for the fixture hooks/clamps/brackets…….. Think about it!
¹ To be 100% ideal – thus meeting the manufacturer loading table UDL data - the fixtures shall be evenly spaced and placed on both lower chords.