Cross-grain bending when using rafter/joist hangers

[PVPE24]

Hey everyone,

Had a quick theoretical question about rafter/joist hangers inducing cross-grain bending if anyone has some insight. I've been working through some calculations for the SE exam and noticed an inconsistency in the analysis of hangers connecting to ledgers.

When analyzing the withdrawal capacity of the connection, you assume an eccentricity caused by the location of the application of the load. In doing so, wouldn't this introduce cross-grain bending in the ledger? I'm wondering if this is common place to ignore this effect in industry, or if I'm viewing the analysis incorrectly in terms of the effects on the ledger. Attached is a diagram showing my analysis and the cross-grain bending. I have been told to assume the d value is about 5/6 of the actual depth from the top nail to the bottom of the ledger, meaning there is a small compression block near the bottom with this assumption. Thanks for any guidance!

 

[Aesur]

Typically every ledger has an H clip at the top attaching to each stud and is therefore in torsion between the attachments. The H clip serves two purposes, direct transfer back to the stud for what you show, which is rarely considered from what I have seen, and for out of plane attachment to provide a load path from the stud to ledger to sheathing without eccentricity therefore not creating cross grain bending.

 

[BR0]

It seems like you don't have your boundary conditions drawn in for your ledger. You would have cross grain bending like you drew if the ledger was supported at the bottom only. A lot of ledgers to studs would have several connectors along its height that would resolve the tension force and reduce any cross grain bending to near 0.

 

[PVPE24]

Thanks both of you, very helpful context. When drawing/thinking about the ledger connection to the studs, that is going to transfer the tension in the ledger so cross-grain bending goes away. It still leaves the question if the joist connection to the ledger was in between the connection of the ledger to the studs, but that seems easy enough to design around by aligning the joist and stud spacing or providing blocking. New detail is attached that shows how I'm visualizing the connection tension transfer through to the studs with a simple dowel type connection (though to your point Aesur, could have used some hardware).

 

[BR0]

I think between the studs I would consider it torsion and not cross-grained bending. However wood doesn't handle torsion very well so it might be considered equally bad.

 

[driftLimiter]

Typically every ledger has an H clip at the top attaching to each stud and is therefore in torsion between the attachments.

Are you saying an H clip on the top edge of the ledger back to the stud? I've not seen that detail but it is intriguing.

I really don't feel like for typical ledgers this cross-grain concern is reasonable. The intent of the calculation 5/6th of the depth is to properly check the anchors. I've seen the problem come up several times in study and not once was there a check for the ledger to resist this moment or mention thereof. That doesn't mean it is not resisting the bending, I think it just implies that cross-grain failure hasn't posed a problem in this context.

It can't be worse than hanging framing from the side of a large girder for example. There would be way more torsion there.

 

[PVPE24]

Yeah totally agree. I like the idea that its not a failure mechanism that has posed a problem and is therefore not something that needs to be considered in this connection. Thanks again everyone!