Diaphragm Chord in Deep Bottom Bearing Wood Trusses 1

[sundale]

Does anyone have any good ideas about providing a wood diaphragm chord in 3'-8" deep bottom bearing parallel chord wood trusses on a stud bearing wall? I normally design the stud wall top plate for this force, but am not happy with OSB diaphragm tension and compression forces going through the OSB wall sheathing to get into the top plate that is > 3'-8" away. Detailing a double top plate up near the diaphragm boundary seems better, but this does not look easy to build.

Also, does the permanent end bracing for such a system want to be wood or strap x-bracing? I know this is the truss manufacturer's deal, but would appreciate any comments here. Thanks in advance. Eric

 

[DaveAtkins]

I would detail a mechanism to transfer the chord force down to the top plate. Personally, I think the OSB could do this (remember, the force in the OSB wall sheathing above the top plate will not be the entire chord force, but only the shear in the diaphragm (greatest at each end of the chord, and zero at the middle of the chord). If you are still uncomfortable, use a diagonal strut in each truss space (but I think that would be overkill).

DaveAtkins

 

[sundale]

Thanks Dave. I think that that's a good idea and one can view the 3'-8" portion of the wall above the top plate but below the roof diaphragm as a vertical sub-diaphragm. I'll block and nail this wall sheathing sufficiently to drag in the chord forces.

 

[bjb]

I share your concern. Could you also require a 2x band at the top of the truss similar to a parallel chord wood floor truss? That could be deisnged as the chord, and it also could help with provising some stability.

Regarding permanent end bracing, that is usually the responsibility of the building designer, not the truss designer, according to the "Commentary for Permanent Bracing of Metal Plate Connected Wood Trusses", by the WTCA. The truss designer has to indicate where individual truss members need to be braced.

 

[RARWOOD]

Sundale

Check past postings on the wood section of this forum, I think your question has came up several times before.