roof diaphragm and cathedral ceilings

[bjb]

I have a wood framed building with cathedral ceilings. The rafters are 2x10', with an LVL ridge beam. What I'm looking for are comments on my design approach for transfering lateral loads from the exterior walls into the sloped roof diaphragm. The slope is 8:12. I calculate that the resulting in-plane shear load in the roof diaphragm is the horizontal load divided by the cosine of the roof angle. I check the shear capacity by only using the actual length of the diaphragm from the eave to the ridge. I then transfer the load in the diaphragm into shear walls, accounting for angle of the roof. All comments pro and con are greatly appreciated.

 

[DaveAtkins]

I typically use Simpson hurricane ties to transfer lateral force into the rafters, which then transfer the force into the roof sheathing. Check with Simpson to be sure their hurricane ties are acceptable for this steep slope. DaveAtkins

 

[Trussdoc]

I think you may be adding extra steps for yourself. The shear load enters as a horizontal force at the low ends of the rafters, and is resisted as a horizontal force by the shearwall below the gable or drag truss. If the horizontal force is increased to account for the pitch of the roof, there is also actually more material there (in the roof diaphragm) to resist this force.

The Simpson clips have capacities from 120 pounds on up, depending on the type of wood used. I would think that you need to be concerned only with the horizontal component of the shear force, as the vertical is taken by the uplift resistance of the clip, or bythe framing below. I might be a good idea, however, to check with Simpson or USP as to the effect of pitch is on their product.