Sloped roof acting as a diaphram 1

[rittz]

How does a sloped,sheeted roof (roof trusses 4:12 +/-) relate to the classic diaphragm modle which is presented as horizontal beam with roof edges acting as chords and the sheeting acting as the web. The picture I see is a flanged beam with a large buckle in the middle and I have difficulty expecting that this condition can replace a flat horizontal diaphragm provided by sheeting the underside of the trusses.

 

[hokie66]

a2mfk,
There are a variety of solutions, but yes, the gypsum board is recognized as one method. Sometimes diagonal bracing is used, occasionally plywood or fibre cement board as a diaphragm. But the ceilings go in after everything is framed, so the diaphragms only span between bracing walls. The interior partitions are used as bracing walls as required. Typically the bracing walls have plywood or fibre cement bracing panels, with gypsum board overlayed. We typically only have ribbed steel roofing or concrete/clay tiled roofs, with sarking under the roofing. No continuous sheathing.

 

[rittz]

My thought is that if there is ANY logical method of analysis at all, msquared48 has stated it in his last post. But I like a sheeted ceiling best anyway.

 

[BAretired]

With a diaphragm of equal size to either side of the ridge, the shear forces in the diaphragm should be zero at the ridge anyway.

I don't think that is correct. When wind pressure is applied normal to the ridge line, the diaphragm shear along the ridge varies linearly from a maximum at the gable walls to zero at midspan. In the absence of blocking, the shear must be resisted by the top chords of the trusses because there is nothing else available. The plywood panels are not connected to each other.

BA

 

[msquared48]

BA:

Speaking of lateral EQ and Wind loads only...

For shears parallel to the ridge line, the statement is true, assuming equal diaphragms, and equal loads. Just look at the shear diagram - the ridge is at the center of the diagram where the shear is zero. The endwalls should have a double top plate to function as the chord member to make the diaphragm work.

However, for the transverse lateral shear direction, what you say is true. But with the rafter/trusses, coupled with a continuing of the diaphragm plywood to the ridge, with blocking and holes for the venting, it will function to carry shear.

Speaking of the vertical load / deep beam issue only...

I still feel that for this load case, and there are others, it is not necessary to have the diaphrams linked at the ridge if each diaphragm is partially functioning as a deep beam. A portion of the load will flow through the rafters to each diaphragm and to the endwalls.

Mike McCann
MMC Engineering

 

[hokie66]

Diaphragms...I call it "trickle theory".

 

[msquared48]

Oooooo... Slick topic Hokie.

Now we'll discuss how to fix the holes in a diaphragm.

Mike McCann
MMC Engineering