>>Any discussion of structures failing is hardly relevant, since in a CNC machine we are aiming to obtain a certain level of stiffness, not just aiming to ensure the material does not fail.

Used an example of failure as an extreme example of how lighting trusses are only stiff in certain directions and almost without strength in others. It is made in a certain way to exploit weight savings for very specific reasons.

It may not be obvious that lighting truss has standard outside diameter chords , similar to aluminium scaffold pole, but the wall thickness is much thinner, the strength being compensated for by the solid rod welded in a triangular form between chords.

Not expecting catastrophic failure of a moving gantry on a router to be considered a high risk.

As said it may not be immediately apparent that trusses secret is generally thinner that would think walls on main tubes, like a staging system also linked to.

But then have already said all this.........