Wood Truss Bearing on CMU
Wood Truss Bearing on CMU
(OP)
I have a project involving a simple wood truss bearing on cmu. We are expanding the building and want the new truss profile to match the existing. The truss mfr is telling me the heel of the new trusses needs to be higher to meet current design loads. The contractor wants to bear the truss directly on the cmu (no wood sill plate) in order to match the existing profile. Is this an option? Would a bearing plate with an uplift clip suffice?






RE: Wood Truss Bearing on CMU
RE: Wood Truss Bearing on CMU
Mike McCann
MMC Engineering
RE: Wood Truss Bearing on CMU
RE: Wood Truss Bearing on CMU
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Consider separation or treat wood in contact with masonry.
RE: Wood Truss Bearing on CMU
boo1 made a good suggestion, make sure that the truss is not directly on the masonry unless it is made of treated wood (highly unlikely unless it was specified). You can best break this with a treated sill, or as his attached sketch shows with a moisture barrier such as a steel sill.
With the higher heel height on the new truss will you now need to have the last course of masonry be cut to match the top chord elevation of the existing structure? If that is accurate, then perhaps you could cut the last course by an additional 1.5" to allow for a continuous sill. Of course, if the contractor is still opposed to this detail you still have the steel barrier option (and I imagine others as well).
There are several good manufactures of truss clips available, but I happen to have a Simpson Strong-Tie catalog right here. The 2006 version I have shows many good applications for roof trusses bearing on masonry walls.
You can download their current literature here...
http://www
Pages 139-147 address attachment to masonry.
If that link fails, simply go to their homepage at www.strongtie.com.
I agree with several contributors that truss blocking (picture frame blocking or solid wood depending on the heel depth) would be required to transfer lateral loads into the masonry wall. Don't forget to specify the nailing into the blocking and follow your shear path all the way through.
RE: Wood Truss Bearing on CMU
I hear the concern about bearing of untreated wood on masonry. I think this comes from foundation designs where there's a chance for water to wick up from the foundation below and sit between the sill plate and the top of foundation wall. I don't think this will happen at the top of a masonry wall. It couldn't hurt to spec. the trusses as PT, but probably overkill. Another solution, you could wrap the end of the truss with a small piece of W.R. Grace Ice & Watershield. I/M/O - it's not necessary.
RE: Wood Truss Bearing on CMU
RE: Wood Truss Bearing on CMU
RE: Wood Truss Bearing on CMU
Never, but never question engineer's judgement
RE: Wood Truss Bearing on CMU
You can isolate untreated wood from masonry lots of ways for applications that are mostly dry already. Ice & Water Shield is one way, or a metal bearing plate, or even plastic sill seal foam.
RE: Wood Truss Bearing on CMU
RE: Wood Truss Bearing on CMU
Mike McCann
MMC Engineering