I have a roof condition where we have to truss, can’t stick it, and a one-big-truss situation isn’t possible because of transport limits.
So, we’ll have to piece the trusses. See the attached sketch.
My question is, with only 3-1/2″ of bearing width at the interior bearing locations (“IB” on the pic), what will be do at that location to give the wing trusses “A” bearing?
Should we simply offset them, and deal with the shift on centering with scabs or something, re the sheathing? Or is there some clever way to keep trusses in line?
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This may be naive, but the truss designer knows you are doing what you plan, right? Seems like offsetting them will maybe screw up the engineering of the truss as a unit and make it weak. I may be totally wrong. I'm sure others will know more. I can't remember what we called them (girders? something like that), but they were trusses that ran perpendicular to the others and the others were placed against it and nailed. They were usually pretty substantial--sometimes doubled and had wide top and bottom chords. Anyway, maybe you could place a parallel top & bottom chord truss (web truss?) on the walls and run the other trusses into them at right angles. Would be similar to the idea of a piggyback truss, but horizontal instead of vertical. Hope this makes sense to you.
Hi Bob,
Just read your question. I am a Structural Engineer and Building Inspector for the City of Fairbanks. In my oppinion, off-setting the trusses would be the best way to go. There are other ways (joist hangers, shear straps, etc.) that could work here but I personally always prefer to get direct bearing whenever possible. The problems with off-setting the trusses will only become evident in the sheathing right at the off-set. You may need to add some blocks to the side of the trusses at the plywood end joints right at the off-set but once you get past it, you'll be on lay-out again and off and running.
I'm curious - How long is the whole truss assembly?
Some plants won't build anything over a certain span. Like the plant I work at now only builds up to 60'. But our main plant is set up to build up to 80' trusses in one piece. So it could be that a different truss company could build the whole thing as a unit.
If you HAVE to do it in pieces - I'd definitely lap the trusses. The plywood offset problem isn't all that big of a deal.
Lapping the trusses also allows you a bit more "slop" if everything doesn't line up perfectly. Truss "A" or "B" can be shifted a hair to make things work out better.
If you're absolutely determined to make them line up, I can sketch out a detail that would work. But I wouldn't really recommend it. It would add more pieces to the truss, and therefore add more cost. I just don't think it's worth it.
A fundamentalist is an idealist with whom you disagree.
Here it is. I don't think it needs to be pieced, after talking to the truss plant.
Definately lap, a lot less work and the truss manuf. should actually design all components pretty well independant of each other for loading.
Example: The center section may require 2 x 6 top and bottom members with webs as required to be structurally sound without the small trusses at either end. And the small trusses at each end may only require 2 x 4 top and bottom members with webs as required to structurally sound as if they stood alone.