Interior wood shear wall sheathing

[jeffhed]

I have been looking at our details lately for interior wood shear walls that line up under a roof truss. For a shear wall that is between roof trusses, the wall is framed up to the bottom of the roof sheathing and the entire height is sheathed. For a shear wall directly under a roof truss our detail shows sheathing on the wall and on the side of the truss all the way up to the bottom of the roof sheathing. It seems to me that if the truss has been designed for the appropriate drag force and edge nailing has been installed through the roof sheathing into the truss, that the sheathing only needs to extend to the top of the shear wall (truss bearing level). I am curious as how many sheath to the underside of diaphragm or just to the top of the shear wall when the shear wall is directly below a truss?

 

[msquared48]

If you sheath the truss too, you are creating a hard point in the roof sgtructure and you WILL read it outside. NOT a good ides.

The best way is to frame and sheath to the underside of the trusses using Simpson DTC clips to allow the roof trusses to deflect, but still transfer the shear force.

If the wall does not line up under a truss, intermediate framing transverse to the wall will be needed between the truss bottom chords.

If the wall is transverse to the trusses then small pony shear walls above the shear wall top plate need to be installed between the trusses to the roof diaphragm. Blocking needs to be installed at the roof diapragm and the same DTC clips used at that point to attach the infill shearwall top plates to the blocking.



Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[jeffhed]

Thanks msquared48. I didn't think that sheathing the truss was necessary, it didn't really make sense to me. We specify the DTC clips for non bearing shear walls.

 

[JoshPlumSE]

If the force from the roof diaphragm has to make its way into the shear wall then there has to be a clear load path. We can all certainly agree on that.

I'd say sheathing the clearest, easiest load path would be to provide sheathing for the truss as well. Could this be overkill, yes it could.

In lieu of the sheathing, the truss must be absolutely design to transfer it's lateral forces into the top plate of the shear wall.... Most trusses are not designed this way.

 

[JoshPlumSE]

Msquared -

When you say "read it outside" you're saying that the deflection of truss will be substantially different than the other trusses in the roof (and be noticeable to the visible eye) because it is so much stiffer. Is that correct?

This seems like an un-related concept. The truss (even without sheathing) lines up with the interior shear wall and would be continously supported by it. Right?

The sheathing certainly stiffens it up some. But, I would think the continuous support of the wall (whether or not the sheathing is there) is probably a greater factor in creating this "hard point". Therefore, the hard point gets created no matter what you do.... if the truss and wall line up like this.

 

[msquared48]

Josh:

Sorry for the belatged reply.

Yes, the difference in the deflections of the trusses, sheathed and unsheathed, will be read on the roof, particularly with tile or metal roofs. I can even read it on comp.

The use of the DTC clips will allow the truss to deflect vertically as it wants to, same as the other trusses, but still transfer the shear force. The top of the shear wall below should be 1/4" or so below the truss to allow this to happen. Normally, residential trusses can serve as drag struts to the shear wall with no modifications needed.

Unless the shear wall is also a bearing wall, it should not be allowed to function as both.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[woodman88]

Another way to transfer the shear is to nail a 2x12 onto the top of the wall and than to the side of the shear truss. I have also seen details that nail plywood to the top of the wall with a 2x4 nailed 3/8" or 1/2" high on the truss to nail the plywood to.

Garth Dreger PE
AZ Phoenix area