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.

 

[dhengr]

Jvvse:
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.

 

[jvvse]

"any loading which causes tension across the grain"

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

 

[JayZ12]

In your sketch the stresses induced by bending are normal to the grain of the wood. There is virtually no strength in that direction.

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.

 

[Splitrings]

There are three axes of your wood member; the strong axis, the weak axis and the third axis which NDS refers to as the z axis(looking at the end grain). Your connection is causing moment around this z axis (looking at the end grain of the ledger). Weak axis bending causes tension and compression parallel to the grain. Bending about the weak axis causes tension and compression perpendicular to the grain. A picture would be worth a thousand words here.

 

[Splitrings]

This is the best image I can come up with quickly. In this image the weak axis moment is My and the strong axis moment is Mz. This leaves the third axis which is the x axis. Moment about this axis in your case causes tension perpendicular to the grain. On the face against the concrete wall, above the anchor.

 

[msquared48]

To avoid cross grain bending at a ledger situation, in addition to the anchor bolts through the ledger beam, metal straps are added that are nailed off to the wood diaphragm and anchored to the wall, concrete or otherwise.

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

 

[jvvse]

Thanks a lot guys. This helps a lot.

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

 

[JayZ12]

Yes, your detail still induces cross-grain bending.

 

[BAretired]

The member in the detail is not in equilibrium under the forces shown. If an equal force is applied at the bottom to the left, the member would be in equilibrium provided that the sum of horizontal forces equals zero.

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

 

[jvvse]

The plate I added was just a bearing plate. Maybe I oversimplified it. Adding the bolt back in, but it has a bearing plate the width of the member, would there still be cross-grain bending?


Link

 

[BAretired]

Any amount of bending in the plate permits an equal amount of bending in the member, so the member will still feel cross-grain bending. If the bearing plate is designed to resist the bolt force by itself over a span of the member width, there would still be cross-grain bending but you would not be relying on it for strength.

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

 

[msquared48]

BA:

The UK used to state categorically that no CGB was allowed. I will have to check the IBC.

Mike McCann
MMC Engineering

 

[wannabeSE]

The 2008 Special Design Provisions for Wind and Seismic (SDPWS) requires plate washers on shear wall bottom plates to alleviate cross grain bending. See commentary section C4.3.6.4.3. A read only pdf of the SDPWS can be downloaded for free at

http://www.awc.org/pdf/2008WindSeismic.pdf.

But, I agree with BAretired that it depends on the load. In shear walls, holddowns are typically designed to handle the uplift.

 

[BAretired]

I would certainly discourage anyone from using CGB in design. There are better ways to resolve the problem. But for existing situations, the article cited by wannabeSE may be worth considering.

BA

 

[Splitrings]

How can you design for CGB when no allowable stress values are listed in the code? NDS doesn't list values.

 

[Jerehmy]

You design to avoid it. The wood handbook has average ultimate values, and they're not very high.

 

[Splitrings]

@Jerehmy
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.

 

[msquared48]

My I phone is always thinking that it is smarter than me. I hate that and am considering firing it. I meant UBC, not UK.

Mike McCann
MMC Engineering

 

[BAretired]

I was wondering when you traveled to the UK, Mike.

BA