Looking for some suggestions on supporting some heavy light fixtures(300lbs). The fixtures will mount flush with a grid ceiling. There is about a 4 foot gap betwen the grid and the roof line. The roof is framed with wooden trusses.

I would start by lag-bolting some unitstrut across the trusses to carry off several. Then threaded rod down to another strut at your desired elevation. Then figure a fastener that will meet up with the mounting bracket supplied by the fixture.
http://www.unistrut.com/

Not often you'll see a heavy fixture in a grid cieling.

Anyway... Nice big heavy fixtures need something subtantial. Try spanning to structural framing with DEEEP strut, with horizontal anchoring via L brackets, and hanging the fixture via 1/2" rod. Good idea to add a safety cable too.

Good idea to get a good cut sheet of the fixture first.

You might want to talk to the structural inspector before you start hanging something heavy from a truss, particularly the bottom chord of a truss. He may want engineering.
YMMV on that but they get "funny" here about engineered assemblies. Maybe it is because the truss companies give inspectors free CEUs.
You will distribute the weight better if you span several with a 2x4 and "all thread" down from that.

Thank you for the replies guys. The fixtures arrived yesterday and I had a chance to look at them today. Much closer to 120-150lbs. Makes my life alot easier.

I am going to use 4 1/4" superscrews with 3/8" rod attached to each. With 5 feet of strut bolted to the rod. Anyone had any problems with the superscrews? The Dottie catalog says they are rated for 1900lbs. of pull strength. Anyone have the pull strength rating for a 1/4" lag bolt into wood? I guess with wood it would have to be a mean average.

when I have to hang something heavy I try to spread the load over and much area a I can. Lag bolt can take a big vertical load but can cause sag in a single truss. Spread the load over several trusses and less sag. I also try to extend to a load bearing wall when I can. (Over kill is good) Solid strut is stiffer/better for heavy loads. Rod from shaky so cal.

I'm always leary of supporting heavy objects with fastener vertically. Although the pulling tension rating of the fastener is 1900 LBS, whats the rating of the material, whats the rating of the material it is in. (I guess you had the same question. For wood, it depends on the wood... And , depends on conditions too, what if there is a flood, and it gets soggy, not that it would, but it could.) When you have the fastener horizontal to the load, you're then dealing with the shear strength of the fastener, and material, which most often will be much higher.


Just for refferance fixture supports need to be independant of the box for it.

I often say when explaining the extra work involved for hanging chandelliers, "You have to build it as if you could have sex hanging from it. Not that you would, or suggest that you do"

[This message has been edited by e57 (edited 07-01-2005).]