Help me to understand Cross-Grain Bending in wood members

[jvvse]

Would anyone like to help educate me on the concept of cross-grain bending? I've known, and avoided, only 2 cases where this might occur:

1) A wood diaphragm is nailed to a wood ledger, which is bolted to a concrete/CMU wall. If the wall tries to pull away from the diaphragm, the anchor bolts will produce cross-grain bending in the ledger.

2) A wood framed shear wall has no hold downs and so uses the sill bolts for uplift resistance. As the wall and sill plate try to lift, the bolts holding them down will produce cross-grain bending in the sill plate.

I have searched the web, searched this site, and talked to other engineers, but haven't found anything other than these 2 cases. I feel like if I ever have tension on a bolt through the weak axis of a wood member, I'm going to have cross grain bending. Yet somehow wood members are OK being used in weak axis bending. Confusing!

I appreciate any help.

 

[msquared48]

Never been there BA, and my morning coffee made me finally realize that fact. Duh...

You know, this discussion brings up the long standing and unrecognized for a long time problem of CGB in the sill plate of shear walls, and I had forgotten about that - one of the problems highlighted at Northridge. If my memory serves me correctly, and that is problematical, I admit, this quake had primarily vertical motion that would have loaded the anchor bolts in tension as well as any shear wall holddowns. It was that force that made the anchor bolts act more in tension than shear, creating a cross-grain bending situation. If the anchor bolts only see shear, there is no cross grain bending, but who can predict what a seismic event will do?

The same force is present when the wall of a building wants to pull away from the roof diaphragm. No difference in the force, only the respective member and connection sizes.

Mike McCann
MMC Engineering

 

[jvvse]

I don't recall many studies about the effects of Northridge on wood framing, other than the tuck-under collapses. Regarding the vertical motion, I talked to a member of the SEAOSC Seismology Committee about it and I think their stance was that the vertical motion really wasn't significant. The recorded motions are high but the post-earthquake studies didn't really show much damage from vertical motion.

Back to OP (me!), I know how to avoid CGB in the cases I mentioned, I'm just trying to understand the concept. So far everyone has done a great job and I feel like I understand it a lot more. I guess my example with the plate would help reduce cross-grain bending as it is what NDS SDPWS recommends for shear wall sill bolts, but of course we do not rely on it for any strength. This became a great thread guys, I appreciate it!

 

[msquared48]

jvvse:

Not to be contrary here, but I have my doubts about that.

This was a blind thrust fault, not a known slip fault, with far more vertical acceleration than a slip fault. That, coupled with the local soil anomalies that amplified the vertical and horizontal responses, generated the loads that caused the problems seen, in my opinion.

Mike McCann
MMC Engineering

 

[jvvse]

The recorded vertical motions were indeed high (around 1g), but much of the damage was determined to be caused by horizontal motions and not vertical. For such high motions I would expect most wood-framed buildings to receive a lot of damage, but it just wasn't the case. Certainly it's something that needs to be studied more, but I think it will take another major earthquake to get money rolling for these studies.

 

[TDIengineer]

From what I saw the major culprit of damage was two things...

3 story apartment buildings with parking below.. which meant discontinuous shear walls and soft story.

The other was inadequate diaphragm thickness for a topping slab over pre-cast beams in a parking structure.

I don't think cross grain bending came into play in either of those.