I'm imagining those steel buildings, where all you see is steel beams and puffed-out plastic wrapped fiberglass insulation. If that's what you have, then there is NOTHING between that fiberglass and the sky but that single layer of sheet metal.
Naturally, you do not want to punch any holes in that metal. I've hung lights by attaching beam clamps to the lighter steel framing pieces. This is also a good place to use SO cord for the whips; MC seems to impart a twist to the fixtures, leaving you with a pretty sloppy look.
I was really impressed with the T-5's I installed. In a bay measuring about 50x80', with the lights mounted 22 ft. up, a dozen 5-lamp fixtures lit the place brilliantly.
In another place, I found the same fixture to be more than a match for the 400w Metal Halide lights that had been there.
IF you are lucky enough to have two layers of sheet metal - that is, steel-insulation-steel - for the roof, then you can mount directly to the inside layer. Nothing will really hold besides toggle bolts.
