Roof Diaphragm

[strctma]

If you have a gable style roof and you have the wind acting perp to the ridge, how does the diaphragm work? you need a compression and a tension chord however at the ridge these chords are not continuous. also if the gable end walls have sloped double plates how does this get tied to the walls that have level double plates. i haven't been able to find good details.

 

[DaveAtkins]

I don't worry about discontinuity at a roof ridge. I did a structural analysis once on a roof at a ridge vent, because the gap was something like 4". I found that all the truss top chords in weak axis bending could handle the horizontal shear in the diaphragm.
The sloped top plate at each gable end does not need to tie to the level top plates at the truss bearing walls. Is that what you were wondering about in your second question?

DaveAtkins

 

[strctma]

Thanks for yourprevious post. Do the sloped top plates
tie together at the ridge? I think they would need to. If they didn't how would I have a continuous compression or tension chord? When you ran your analysis did both halves of the roof act as cantilevered diaphragms?

 

[DaveAtkins]

Apparently you are thinking about wind load PARALLEL to the ridge. Do you have trusses at 2'-0" oc? If so, use the top chord of the truss closest to each end wall as your diaphragm chords. Truss top chords are spliced together with gang nail plates.

DaveAtkins

 

[tricalcim]

So, the top chord would be designed for the tension due to bending? I would think you'd be tying the Bottom chord to the end-wall top plate? Is this typical design for gabled diaphrams?

 

[DaveAtkins]

Generally, the top chord of a truss is adequate to support the gravity loads on the truss AND resist diaphragm chord force, because the load duration factor is 1.6 for wind load, but just 1.15 for dead plus snow load, leaving some reserve capacity for the chord force.

DaveAtkins