wood truss bearing on cmu

huangyhg

wood truss bearing on cmu
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?
you should be able to bear directly on the cmu if you provide anchorage for uplift.  you just have to check bearing stresses in the cmu. plain or reinforced cmu? asd or lrfd? mortar type?  all makes a difference.  check asce 5-08 buidling code requirements and specifictaion for masonry structures.
the only peoblem i see here is how to transfer any lateral shear forces to the cmu wall with no plate.  is this wall a shear wall too?
mike mccann
mmc engineering
i don't see a problem bearing directly on the cmu.  simpson makes clips for this application.  you will need to provide a way for the shear force in the diaphragm to be transferred to the cmu wall (assuming you are using it as a shear wall).  most of the time the clips do not transfer this lateral force, so blocking between the trusses should do the trick.
straps are shown in fig 9.
you can check the truss shop drawings for anticipated uplift on the truss ends, or simply develop your own calculation.  commercially available straps may be satisfactory depending on the loads.  multi-ply girder trusses or long span trusses usually seem to have the largest uplifts.  
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...
this is totally acceptable. like others have said, you'll need a special connector to handle the uplift. bearing won't be a problem on the masonry. as far as shear transfer goes, you should be blocking between the truss ends to transfer the lateral force from the sheathing into the masonry. the blocking will need its own connectors from the wood to the masonry. you should never count on the truss ends to transfer lateral shear forces, even if there was a plate.
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.
we always spec, and typically see from all truss suppliers, a metal plate shop applied on the heel of the truss to provide a moisture barrier between the concrete/cmu wall and the wood truss.  simpson also makes some plates that can be applied in the field prior to erecting the truss.  this is standard here in florida.

regarding matching the existing profile versus meeting design loads, ask him what has changed between then and now.  roof gravity loads have not changed in a load time, so i have to wonder why they can't match the existing.  if bearing or shear (up or down) is the problem, he could double up the ends of the truss or use a higher grade wood.  it sounds to me like they want to use the lowest grade possible, rather than provide what you need.
how do you deal with non treated wood to masonry connection?
never, but never question engineer's judgement
i sometimes see the need for deeper heels due to increased insulation requirements. don't forget that the ceiling heights will not match between the new adn the existing if you lower the bearing. perhaps sprayed in foam insulation could be used at the eaves rather than increasing the heel.
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.