Diagonal sawn lumber as roof diaphragm 2

[EngDM]

Does anyone have a resource of a design example for a roof with diagonal decking installed at a 45°? CSA O86 doesn't provide much of anything aside from some commentary on it, and the only diagram they give is for a shearwall.

O86 instructs me to design the boundary members (chords) for the out of plane component, but since the decking members don't have shear transfer between adjacent members I can't quite figure out my load path to determine the loads on these members. O86 gives an equation, but it appears that it is assuming that the chords are one continuous member from one shear collector to the next, whereas I have edge purlins that start and stop. That is to say, the bending moment diagram of the decking would be based on the overall length between collectors, but each individual edge purlin is 1/4 of that length since I have 4 bays.

Similar to the above, I can't see how without shear transfer between the planks, that load in the N-S direction actually makes it out to the shearwalls on the left and right for the option 1 framing. If I draw the UDL and try to follow the load path, it almost seems that the left wall never has any load dragged into it. Is the assumption that the load goes axially into the plank, then at purlin intermediate supports it jumps over and makes it way out?

My shearwall orientation is 2 endwalls and a central wall, so a large H shape essentially, but for wind in the direction parallel to the single central shearwall the load path doesn't appear to actually make it to the center unless I change the decking orientation at some point to start collecting it into the middle.

I've attached a crude sketch of the framing, intended shearwalls are currently shown red. The north and south walls are full of openings and can't be relied on as shearwalls (I haven't drawn the openings). Planks are drawn for orientation only, not drawn to scale.

 

[EngDM]

With my limited understanding I think in the 45 degree layup it acts more truss like while straight layup does exhibit the small moment connections you noted before. I think you have the Building Structural Design Handbook while not an extensive coverage it's more than I have seen anywhere else starts on page 966.

View attachment 7539

Interesting. CSA O86 doesn't permit horizontal plank diaphragms, only at 45°. But in the SDPWS 2021 they do have tables for it.

 

[KootK]

What I find particularly interesting is that the diagonal sketch indicates curvature not of the chords, but of the struts. And that's somewhat intuitive if we are doing the weird truss thing. So, presumably, the struts need to be designed for out of plane load as well as the chords. And with much greater spans.

 

[EngDM]

What I find particularly interesting is that the diagonal sketch indicates curvature not of the chords, but of the struts. And that's somewhat intuitive if we are doing the weird truss thing. So, presumably, the struts need to be designed for out of plane load as well as the chords. And with much greater spans.

View attachment 7540

Yea, in the code excerpts I posted above it says exactly that. I think it is because the planks on those elements don't actually continue down to the bottom chord of the deep beam, so their axial force translates into bending of the boundary elements. I presume the same thing happens to the long side when looking at wind in the other direction, but the bending is restrained by the joists/purlins such that any bending is insignificant.

 

[KootK]

Yea, in the code excerpts I posted above it says exactly that.

The O86 seems to say that. The SDPWS2021 seem to say something else: that it's the chords that need attention over the cord spanning over the members distributing load to the diaphragm. So is it both?? O86 also seems to not be explicit in defining the load that the struts ought to be designed for. So one may need to truly understand this setup in order to design it properly.

If the whole diagonal thing is down to just using the boundary members in weak axis bending, the whole thing doesn't really feel that diaphragmy.

 

[KootK]

And fine, design the boundary struts for some weak axis. That's no so hard. But you'll also need lateral connections that go someplace at the ends of those struts. That could result in some objectionable hardware in your average deck.

 

[EngDM]

And fine, design the boundary struts for some weak axis. That's no so hard. But you'll also need lateral connections that go someplace at the ends of those struts. That could result in some objectionable hardware in your average deck.

They specify that you need strapping or something at the corners, else that resists the deck boards pulling away from eachother probably to help resolve your concern. You still have to strap or fasten the chords together if they aren't a continuous spliced member.

 

[ChorasDen]

I have the same gripe, so I bought a soft copy. Copyright prevents me from sharing it, unfortunately. Looks like Scribd has it as the third hit on Google; I don’t know how reliable that website is. What a weird time in history to be an SE.

The entire PDF is loaded into an i-frame viewer. What does that mean for you? That means that there is a zoom button on the bottom of the viewer window that allows you to zoom into the PDF directly, this prevents blurriness. I assume you were trying to ctrl+mouse wheel to zoom in, which is a browser based zoom, and does not actually expand the contents of the i-frame. There are also other creative ways to get a better, more zoomed in view of the code from the AWC website if needed, but I'll leave that to the more adventurous among us...

 

[EngDM]

The entire PDF is loaded into an i-frame viewer. What does that mean for you? That means that there is a zoom button on the bottom of the viewer window that allows you to zoom into the PDF directly, this prevents blurriness. I assume you were trying to ctrl+mouse wheel to zoom in, which is a browser based zoom, and does not actually expand the contents of the i-frame. There are also other creative ways to get a better, more zoomed in view of the code from the AWC website if needed, but I'll leave that to the more adventurous among us...

No zoom button on my chrome, just fullscreen and presenter mode. No spyglass or anything.

 

[ChorasDen]

No zoom button on my chrome, just fullscreen and presenter mode. No spyglass or anything.

Ah, how interesting. There used to be one, but I do not see it anymore either. I'll have to explore. @phamENG originally pointed the zoom feature out on a previous post, wonder if he has an option.

 

[phamENG]

@phamENG originally pointed the zoom feature out on a previous post

And I started to post it here, too, but when I went to grab some screenshots to show how it's done, the zoom buttons were gone. They're probably seeing decreased revenue from people being able to view it online whenever they want to. Having a PDF is the only way to be able to zoom with good fidelity, or have a big monitor. When I go to full screen or slideshow mode, I can read it pretty well. But I have PDFs of all of them, so I don't worry too much about it.