Best answer may be: Check mfg specs or Apply common sense. 8' fluor channel fixtures are surface mounted at 3 points; suspended at 2 points (each fixture) It's a 'no definite' answer as the layout is vague by your description above.
With hanging 8' strip lights I usually support the first one at both ends and the far end of each succeeding one.
Here is a job I finished last year, almost 600 fixtures.
That said I recently hung some 8' custom linear lights in offices and that jobs foreman had me do the same as I described above. IMO the fixtures being cast where to week for that and should have had three supports a piece for each fixture as they had been designed.
I guess what I am saying is follow Johns advice.....apply common sense.
Bob Badger Construction & Maintenance Electrician Massachusetts
Bob: NICE.... BTW, quite a while back...like you said, 2 on the first; chase nip/ln thru end ko's, one on each additional. I didn't want to write a novel on the many ways.... Now the 'old' 8' INdustrial type with the full reflector shade...nice old heavy mag ballast, and a housing made from real live steel, not recycled Pepsi cans (man's fixture), that was a definite 'two chainer'.
Back to my serious mode....any shot to get good old common sense into the NEC??
Thanks for the input everyone. I agree on the 2 on first fixture then one a foot or so from the end of all the rest; however, bar joists don't fit that senario. I guess my actual question was can you miss a hanger on a fixture or must you have a support on every fixture? Checked manufactures specs, told me nothing. The store is a Wal-Mart Superstore.....many many fixtures.
Bar joist locations not 'matching' a place for a support are solved by bridging b'twn bar joists with strut, or another suitable alternate. What about the plans/specs? Most, if not all will provide a detail on the REQUIRED method of installation by the Architect/Engineer of record.
Your 'client' is not a small operation, so you should have details available, or submit an RFI.