Help me to understand Cross-Grain Bending in wood members
Help me to understand Cross-Grain Bending in wood members
(OP)
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.
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.






RE: Help me to understand Cross-Grain Bending in wood members
Consider a 2x10, the grain of the wood, the cell structure of the wood, runs the length of the member. This cell structure is very strong along its length, parallel to its orientation, thus bending normal stress, tension, in this orientation is pretty strong. This is the orientation that we normally think of as this member being strong, either as a spanning joist or a plank, but that’s another variation also, a function of grading, etc., plank vs. jst. This same grain or cell structure is pretty weak across its (perpendicular to its) orientation. Thus any loading which causes tension across the grain is a very weak loading orientation. Due to this tension, the member will just split along the grain, rip the cells apart along their boundaries. This latter situation is why wood splits easily along the grain, with a splitting wedge. Your two examples are good examples, and any others... you should know them when you see them, so you stay out of trouble.
RE: Help me to understand Cross-Grain Bending in wood members
That's the concept I'm having trouble understanding. The name as well, cross-grain bending. From the two examples I gave, the only thing I understand is not to split a wood member in half. Yet if I use a large bearing plate on the end of the bolt, will that prevent cross-grain bending?
Quick sketch I did of the load which causes cross-grain bending, from what I understand.
Link
RE: Help me to understand Cross-Grain Bending in wood members
Bending about the strong or weak axis induced stresses that are parallel to the grain of the wood.
The minor axis bending your refer to in your opening post induces stresses that are 90 degrees from the stresses induced by cross grain bending.
RE: Help me to understand Cross-Grain Bending in wood members
RE: Help me to understand Cross-Grain Bending in wood members
RE: Help me to understand Cross-Grain Bending in wood members
It is the force of the wall wanting to pull away from the diaphragm that is creating a force out of line with the ledger anchor bolts, creating an eccentric moment on the ledger that is inducing the cross-grain bending phenomenon that encourages longitudinal splitting of the ledger due to rotation, not shear.
Pant, pant, pant...
Mike McCann
MMC Engineering
RE: Help me to understand Cross-Grain Bending in wood members
So the cross-grain tension is more about the internal forces being in tension, rather than say a force perpendicular to the grain. From what you are saying, would I consider any kind of torsion to be prone to cross-grain bending?
Also I modified my original sketch with a plate. Say a bearing plate the full width of the member. Would that still generate cross-grain bending?
Link to sketch
Link
RE: Help me to understand Cross-Grain Bending in wood members
RE: Help me to understand Cross-Grain Bending in wood members
In that case, the wood member would still feel cross-grain bending but it would be reduced because the steel plate is carrying some of the applied load and spreading it over the width of member. If the plate is sufficiently strong to carry the load by itself and sufficiently rigid so that deflection is negligible, then cross-grain bending of the wood member could be virtually eliminated.
BA
RE: Help me to understand Cross-Grain Bending in wood members
Link
RE: Help me to understand Cross-Grain Bending in wood members
If the bolt force is P and the member width is L, the plate must carry a moment of PL/4. Then, you may be justified in ignoring cross-grain bending.
BA
RE: Help me to understand Cross-Grain Bending in wood members
The UK used to state categorically that no CGB was allowed. I will have to check the IBC.
Mike McCann
MMC Engineering
RE: Help me to understand Cross-Grain Bending in wood members
RE: Help me to understand Cross-Grain Bending in wood members
BA
RE: Help me to understand Cross-Grain Bending in wood members
RE: Help me to understand Cross-Grain Bending in wood members
RE: Help me to understand Cross-Grain Bending in wood members
That was my point with my post. We shouldn't be designing for CGB. We have no allowable values to design to.
In the OP's case, any number of tension ties, holddowns, etc could be used at the sheathing to remove the eccentricity in the connection.
RE: Help me to understand Cross-Grain Bending in wood members
Mike McCann
MMC Engineering
RE: Help me to understand Cross-Grain Bending in wood members
BA
RE: Help me to understand Cross-Grain Bending in wood members
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
RE: Help me to understand Cross-Grain Bending in wood members
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!
RE: Help me to understand Cross-Grain Bending in wood members
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
RE: Help me to understand Cross-Grain Bending in wood members
RE: Help me to understand Cross-Grain Bending in wood members
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.