Apex DN: one bottom
chord in tension, two top chords in compression.
Apex UP: two bottom chords in tension, one top
chord in compression.
Tension is weaker than compression (although with
truss tube by a very small margin), thus it's better to help it out by doubling the number of chords in tension. (Except for very short runs, or not-heavy loads.)
Does that make sense?
On
This Old House, when a plumber must drill a hole or notch a joist, Tommy says to make it in the upper third of the joist. Notching in the lower third effectively turns the 2x10 into a 2x6 or whatever.
... The company I work for always rigs apex down, as it's much easier to hang movers from one pipe than two. ...
Point DN also gives one a
flat area to
lay cabling, but
Point UP using cable hangers solves that.
http://www.citytheatrical.com/Products/cable-hooks# One commercial product, but not difficult to fabricate oneself. I'd at least add a
safety cable to the above; when I built some years ago I drilled holes to accept a
scaffold pin to capture the top
chord.
Most movers' clamps can be positioned to use two chords of 12"
truss. OR, stagger the fixtures, half on each
chord.
I can't tell if you're using the term "tri
truss" as a brand name or generic type. Read what the manufacturers' of triangular
truss websites say, and follow their instructions. Some state that their
truss is equally strong and can be used either way.