Wood Gable Roof Diaphragm & Connections to Shear Wall (Revisit)

[hdn32]

Dear fellow Engineers,

I am new with wood design in general.
While trying to put together details for roof diaphragm to shear wall connections, I have some questions about the topic.
Thru my research in this forum, I found these two terrific threads (thread507-388238 and thread507-285903). Unfortunately, it appears that there is no clear consensus on the topics.

In threads thread507-388238, KootK's concern about the shear at the ridge (disconnected roof sheathing) at sections closer to gable end walls. IMHO, this should not be a concern (with proper tension tie between rafters at ridge) if we look at the rafter(s) as drag strut(s) for wind perpendicular to ridge load case. For wind parallel to ridge case, the shear load would be zero at section (thru/ along ridge)

In thread507-285903, Bigmig brought up concern of possible lack connection capacity for roof diaphragm to shear wall connection due to Architect's ventilation. It is still my question/concern now although I am aware DaveAtkins had posted that "Omitting diaphragm edge nailing is not an issue if you use the diaphragm capacities for unblocked sheathing" (second to last post on the thread).

Unless I am missing something, even with unblocked diaphragm we still have to transfer shear load at diaphragm boundary to shear wall. The load path would be: Maximum shear (from diaphragm boundary/ roof sheathing) to blocking/rafter and to top plate and to wall sheathing.

If blocking is omitted , how can we transfer maximum shear from roof sheathing to shear wall sheathing? Can anyone please provide additional advice/reference/detail on this connection (beside KootK's detail in thread507-388238)?

Thank you,

hdn32

 

[hdn32]

Koots: You are correct. I did not think about that. Have you tried to compare the unit shear = [chord force (T = C = M/W) / building length] with the shear flow using the beam equation abobe?

 

[hdn32]

jayrod: yes

 

[KootK]

unit shear = [chord force (T = C = M/W) / building length]

.

It's important to recognize that that formula is not the unit shear. It's the total shear divided by the half span which is different. At least that's the case in the elastic domain which is where diaphragm design usually resides.

I haven't compared my shear flow method to anything else because, frankly, I don't think that there is anything else. It's just:

- Unit shear parallel to load = shear / building width.
- At any location, shear perp to load = shear parallel to load.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jdgengineer]

How much ventilation do you need at the ridge? If rafters are spaced 24" oc, we typically put in 12-16" long blocks and cluster the diaphragm nailing to the block at the ridge. This allows some space for ventilation and to allow for diaphragm shear transfer.

To be honest, almost everything we work on now is spray foam so it isn't really much of a concern these days (for us at least).

 

[hdn32]

This is great

The second point on my original post concerning of the lack of blocking between rafters for transferring shear load from diaphragm to shear wall when wind is parallel to ridge. For wind perpendicular to ridge, I do not concern about the connection between diaphragm and shear wall (perpendicular to wind).

Kootk: Do you imply that for wind perpendicular to roof, i should be concern about the diaphragm to shear wall (perpendicular to wind) connection in addition to diaphragm to shear wall (parallel to wind) connection?

 

[KootK]

Do you imply that for wind perpendicular to roof, i should be concern about the diaphragm to shear wall (perpendicular to wind) connection in addition to diaphragm to shear wall (parallel to wind) connection?

Nope. What you need at the perp wall is something...anything that can soak up the horizontal shears coming out of the diaphragm. The vast majority of the time, that something is the diaphragm chord and the internal resolution that jayrod mentioned. Once in a while you do use walls in place of chords for this purpose however. This recent thread of XR's is an interesting example where that concept was explored: Link. Scroll down to around where the sketch below resides.

As a matter of pedantic interest, I've always wondered about the case where your perp line has a chord and shear walls connected to the chord. Where does the load go when it has that choice? Something stiffness based, no doubt, but, really, who the heck knows. And it seems to not cause problems which is the standard that underpins most of what we do whether we choose to admit that or not..

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[hdn32]

KootK: Thank you

 

[KootK]

You're most welcome hdn32.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.