I have an attic framed with W style trusses. I would like to use the center section for storage (spread load) and/or some machinery installation (200-400 lbs point load). The load would not exceed 20 lbs/sq ft, and could be kept to 10 if needed. Is that a sane idea?
A key part of the question is ‘when do you call in a P.E.’. I’m not sure where the line between ‘standard practice, use the table’ and ‘not normal, call a structural engineer’ sits with trusses. Even if I do call a P.E., I’d like to get a feel for what people do in the field. It will help me figure out what questions to ask.
Details of truss. Roughly 24′ span, 2×4 material throughout, 24″ spacing. I’ve not been able to find any manufacturer or grade stamps on the truss, but I may not be looking in the right place; suggestions are welcome. The house was built in 1997, if that helps.
One of my concerns is that trusses are ‘value’ engineered to use the minimal material. If putting weight on the interior span isn’t part of the design, then there may not be *any* capacity for loads there. I took a structures course many moons ago in engineering school, but that’s only made me aware of how many things I don’t know about wood trusses. Simple beams; no problem, just do the math. Trusses make me nervous.
One of my assumptions about the point load from the machinery is that I can build a platform out of 2×6 or 2×8 crosswise to spread the load onto multiple trusses, and treat that as a distributed load. Is that true?
If the truss can’t handle the machinery load, I guess I need to frame some sort of support for it. There is a hallway running perpendicular to the trusses down the centerline. It doesn’t seem to be framed as a load bearing wall, but could be beefed up to act as one, and carry the load straight to the foundation. Given the loads we’re talking about, I don’t think I need to worry about reinforcing the concrete slab (on grade) that the hallway framing sits on.
Thanks for any suggestions!
Replies
can you give us a drawing? I wouldn't add any weight to the trusses whatsoever, other than maybe sheetrock. I had the same issue in my garage - I put in a platform directly under the trusses, and stacked stuff on top of the platform, between the trusses. That way no weight bears on the trusses. Kinda hard to see in this pic, but the bottom of the trusses is at a little over 8', and the top of my platform is about 7'6"
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Trusses are generally designed solely to support the weight of the ceiling on the nottom chords and nothing else. So I'd all but guarantee you that they weren't designed for any added loads.
But the only way to tell is to check out the trusses and see.
If it was built that long ago, the truss plant probaby won't be able to find any records.
If you want to take some pics of the joins and grade stamps, I'll tell you what I can. But no guarantees.
BossHog, thanks for your kind offer, but don't bother. I've decided that I'm not going to use the trusses at all. My original gut-check had me concerned about the idea; and you all pretty much confirmed it as a bad plan.So I'm going to play it safe. The air handlers will get their own separately supported platform floating free of the trusses. A nuisance to frame, but it will make it easier for me to isolate the vibrations from the rest of the house, so I'll come out ahead in the end.I'm going to install a short catwalk to allow access to the equipment; I assume it's safe for the truss to support the short term load of a person walking around on them.