Truss Bearing On Dbl Top Pl

[dhoward26]

I have always specified the attached detail when I have an architect want trusses to span more than about 35ft and ground snow loads are greater than or equal to 25psf. I haven't ever been questioned on it until recently when a contractor was all up in arms (not surprising). When calculating the point load from a truss on the double top plate, the double top plate fails in bending when the truss bears greater than 3" from the adjacent stud as shown on the detail. Are you guys providing this detail?

The argument I would make against it is that the dbl top plate would have to bend the plywood as well in order to fail. What are your thoughts?

 

[AELLC]

I think your bending analysis is conservative compared to mine. Are you using the bending stress "flatwise" increase?

When truss spans are too long, I specify a triple top plate of DF-L#2, where as we normally use (2)-2x Hem-Fir#2.
If that doesn't work (or the studs get overstressed) , then it is studs at 12" oc.

 

[AELLC]

BTW, there is somewhere in the IRC that allows double top plate and studs at 16" oc without calculations, unless truss span > 40'

I sometimes have to resort to that when designing tract homes.

 

[CANPRO]

As AELLC noted, depending on where you are, you should be able to justify this with a code reference. With that said, we have a note on our drawings that requires the studs to be aligned with the truss.

 

[dhoward26]

Okay. I was using flat wise bending with a simple span in the past. I just tried doing a multiple span with (4) spans and it works with lighter snow loads.

Thanks for the input

 

[BAretired]

Usually trusses are spaced at 24" o/c so in order to coordinate the studs and trusses, the studs would have to be spaced at 24" centers also (or 12" but that would mean every second stud is loafing). Could space studs at 16" centers and add a stud at alternate trusses.

Top plates in bending are not a good idea. They aren't nailed sufficiently to be considered laminated and each plate has to be spliced somewhere, weakening the member for bending. Reliance on nailing plywood, OSB or drywall to support trusses is not a sound practice either.

I prefer to have a stud directly under each truss.

BA

 

[dhengr]

Dhoward26:
As usual, BA gives good advice. But, we have also been doing this for years and years, and it has worked within reasonable limits. I haven’t run this top plate bending check, to my satisfaction, in years; and with the changes in allowable lumber stresses, the truss span length won’t have increased. I would guess you should be checking this situation at 30-35' truss span. The top plate is usually 2-2x’s, on the flat, and continuous, no special nailing unless you spec. it, and one of them may spliced over an adjacent stud. In my neck of the woods a 2x6 stud wall at 24" o/c makes a pretty good wall, and then you can line up the studs and the trusses, and this is good. You can get buy with a single top plate then, but you need steel splice plates to provide some tensile continuity at the butt spices in the top plate, over a stud.