Sloped Roof Diaphragm with Ridge Vent 2

[jamie2000]

I am analyzing a large 3.5 to 12 roof as a plywood diaphragm. I realized that there will be a ridge vent down the center of the diaphragm that will essentially cut the roof in half (a few inches of the plywood will be removed to provide the ventilation at the ridge vent). Does this cause a major discontinuity of the diaphragm? Or is this not a concern since the stress is assumed zero at the middle (deep beam concept)? The trusses will still be intact, so will they provide the continuity?

Thanks.

 

[JAE]

Deep beam concept: the max. shear is at the mid-height of the shape (the ridge of your roof) so the horizontal shear will be lost at this point and the diaphragm will act as two separate diaphragms of smaller "depth". Chord forces would develop along each edge of the ridge vent opening.

 

[boo1]

umm. Then the shear diaphram is unblocked.

 

[jamie2000]

Is the solution to design two "sub-diaphragms" and put blocking along each edge of the ridge vent? The blocking would also be diaphragm chords and will be designed for tension.

 

[JAE]

That sounds appropriate to me....remember that blocking is discontinuous so some means of transferring tension would be required.

 

[jamie2000]

The other half of the problem is what to for the other direction. The diaphragm that resists the shear from the gable end walls is also in two pieces. The problem at the ridge is that I don't have a shear wall running down the middle of the building to resist the drag forces transmitted from the diaphragm edges. My proposed solution to this problem is to attach another lower diaphragm to the bottom chords of the trusses to resist the shear from the gable ends. My theory is that this also provides top support for the end walls, where as without the lower diaphragm, I would need to cross brace between the trusses to support the top of the end walls. Does this sound reasonable?

Thanks.

 

[boo1]

In 20 years of construction I have never see blocking at the ridge vent.

 

[jamie2000]

BOO1,

I haven't seen it (or specified) it either. That's why I asked the question - I was wondering if omitting the plywood at the ridge vent was actually a problem. Can the truss transfer the forces across the opening? I sent an email to APA asking this question. I will let you know if they reply with any useful information

 

[JAE]

boo1 - you are probably quite correct that most wood construction doesn't use blocking at a ridge vent. What do you think - is most of the construction you are referring to basic housing where there are sheetrock ceilings and lots of interior walls that make a ridge-vent issue moot?

For a larger, more commercial type structure where you might have a large, uninterupted space below, I would guess that if there was a ceiling (gypsum board) attached to the bottom chords, you would again be able to use the ceiling as the diaphragm (if it worked) and ignore the discontinuous roof sheathing.

With no ceiling, though, I'd think you'd have to deal with the blocking and load path. Your thoughts?

 

[haynewp]

In the case where wind is blowing on the gable end, instead of putting in a new subdiaphragm, is it possible to design the roof as 2 cantilevers? Consider the endwalls braced by the roof? I haven't dealt with this situation before, I am just wondering why not.

 

[jamie2000]

I contacted APA. They emailed to me a document called "Introduction to Lateral Design" which discusses how to deal with ridge vents in roof diaphragms. For unblocked diaphragms there is no issue because there is no requirement for connection to be made between unsupported edges of adjacent panels.

For blocked diaphragms you do need make connections for the unsupported edges. APA recommends "partial ridge venting" which involves placing blocking at alternating truss spaces. To carry the shear load, the nailing is doubled at the blocked spaces.

 

[boo1]

Truss ridge joints, are not blocked either.

 

[ahmedsonyz5]

What about "Reducing Shear Stresses in Reinforced Concrete Shear Wall with Carbon Fiber plastic" any papers or research?