Cross-grain bending with bounded diaphragm

[masterdesign]

We have a project with overhangs that are desired to be very thin. It is a mixed wood/steel structure. To accommodate this, I am thinking of using an HSS steel frame with 2x4 infill. Each interior face of the HSS frame will have a ledger for boundary nailing. However, the steel frame itself will have some rigidity which may preclude diaphragm action. Neglecting the steel frame rigidity and assuming the wood framing does in fact see diaphragm action, would this configuration impart cross-grain bending to the ledgers that should be considered? These are just overhangs outside the primary structure and they will only see very minimal lateral load - wind on the fascia which is 6" max depth. Overhang width is 3'-0" (short dimension in sketch).

 

[driftLimiter]

Assuming that your 'chords' are positively anchored to the structure, i think the answer is No.

Diaphragm behavior would require that shear force is resisted by the fasteners. If you are relying on the ledger itself to hold this thing to the building then the answer is yes thats classic cross grain bending of the ledger.

But I'm not really following your line of reasoning as to whether this is a wood diaphragm or not, and I'm not seeing how that is going to effect the force in the nails.

Can you describe the loading direction and support conditions of this thing so I can better see what you are thinking regarding cross-grain?
Generally I would use the ledger to transfer shear forces back to the building, and chords to transfer the tension/compression from the racking of it.

 

[masterdesign]

Thanks for the quick response.

The HSS8x4 is part of the primary structure. The HSS3 1/2x3 1/2 which are perp to it are cantilevered off the face of that. So that is how it is attached to the primary.

Definitely not using the ledger to hold this to the primary.

My comments on the steel frame is that, laterally, the frame will be rigid and resist racking. In this case, the wood roof sheathing does not see any lateral load (no diaphragm action). My question is assuming we ignore the rigidity of the steel frame (laterally) and therefore assume the wood roof sheathing does see lateral load.

 

[driftLimiter]

So if it acts like a diaphragm I would consider a 3 sided diaphragm where shear is transferred by nails on 3 sides. I'm still not visualizing the cross-grain concern.

 

[masterdesign]

Well the confusion here is likely that I am unclear on when cross-grain bending occurs. I know that if this whole overhang wanted to pull away from the structure and the ledger is the only point it is attached, that is classic cross-grain bending. I guess my question is: if this overhang wanted to rack laterally due to wind, engaging the wood diaphragm, would the nailing into the ledgers (chords) along the short sides induce cross-grain bending. Thank you for helping me to figure out my real question!

 

[driftLimiter]

I'd say the answer is squarely no. The chords are assumed to be stiff compared to the flexibility of the diaphragm.

Some inspiration : https://www.structuremag.org/article/tackling-the-3-sided-box-diaphragm-one-engineers-perspective/

 

[JoshPlumSE]

Technically, I think you do see some cross grain bending. I just don't think this is the type of configuration the code is concerned about.

a) The nailer on the diaphragm side of the HSS would experience cross grain bending as the diaphragm moves away from the HSS.

b) The nailer on the far side of the HSS from the diaphragm would not.... The beam is constrained from failure by the presence of the HSS. It would be like the diaphragm being connected to the OUTSIDE face of a tilt up or masonry wall.

There is no concern that cross grain bending would detach this nailer from the wall can cause the roof to "pancake" the roof. Isn't that what we're really preventing against with this provision?

That being said, I think you'd still need to check the through bolt anchor for the diaphragm anchorage forces.