Hi There,

Got a couple of question about green wood behavior. Thought this might start an interesting thread.

Questions on checking, and bowing. Although checking is natural, and I have no problem with it, it seems that if the checking is predictable it might help me understand wood further.



These questions are based on all things being equal, such as the fortune of having the heart project all the way through the timber in the same point. I understand this could be a scenario that is not always possible.

1. Rectangular timber: Is the checking in this timber going to check on the two faces in the x-axis? If this is right, is the checking going to always start first at the face(s) closest to the heart? Could the checking also happen on the faces furthest from the heart (y-axis)?

2. Rectangular timber: The heart is closer to one face on the y-axis. Could this keep the face furthest from the heart from checking? Any possible bowing from this?

3. Offset heart: Face very close to heart. Which way is this baby going to bow? Like a "C" or "D"

Has anyone heard of relieving grooves to negate checking?

Alot of questions.

I think you need to assume also that this timber is free floating in space, with no direct sunlight, and no heat reflecting surface from below or the sides. Sun exposure, air movement, and if it is stacked tight to other timbers (or a wall, ground, etc.) will affect the wet and dry sides...

I'm waiting for Don Ps. Insight here... as he'll throw down a good deal of knowledge if this interests him.

timbers usually check on the side the heart is closest to. Usually... A lot of mortices on another face could throw it that way. Or if the face closest to the heart is knotty, the path of least resistance may be towards another face.

And yes, I've heard of running a kerf down a non show face to throw the checks that way. Same principal as a control joint in concrete (it's gonna crack, so let's make it crack where we want it to.) Haven't done it, though.

As for bowing, I've not been able to see a correlation between heart location in the timber and direction of bow -- I just looked over our cull piles (plenty of bowed timbers) and noticed that they are going both toward and away from the heart. And I noticed that some of the timbers where the heart was only an inch or so from a face were dead straight.

Thanks for the insight Gabel.

I think I'll try a dado for 12/2, and I'll report later.

Hi mo. Good topic I like wood.

1. Most likely the timber will check on the x-axis, closest to the heart. Unlikely it will also check on the y after that, as the side checks will be pushing the wood together towards those faces (does that make sense? I'm kinda bad at descriptive writing...)

2. First part: yup, but never guaranteed. Second part: depends on way too many factors - drying conditions, species, how it was milled, how it was stacked...as Gabel mentioned, "I've not been able to see a correlation between heart location in the timber and direction of bow"

3. See 2, second part.

The Japanese will kerf the tops of their timbers and then wedge them open to keep the visible faces from checking. It doesn't need to be too deep - an inch in a 10x is plenty. I have done this before - unfortunately I haven't seen the finished product, now about 3 years old. It would be cool to know whether the kerf has made a significant difference.

We have sent some cedar and larch frames to Hawaii, and they would be checked when they left our shop, but after a few weeks on the Island, the checks would close up because of the moisture there. That's pretty cool...

Keep in mind that I've worked mostly with DF,larch and cedar, not hard wood...

Peace
*K

I agree with everyone. All things being equal (uhhh, right) the check will take the shortest path from heart to face. If you remember the concentric rings of cells standing around the campfire analogy. As the cells lose moisture and pull together tighter, shoulder to shoulder, but are restricted to being able to move in only half as much as they shrink in shoulder to shoulder width this creates stress in the shoulder to shoulder connection. The weakest point in the circles is where we have cut more of the concentric rings away and that stress concentrates along the rays. When tension perp becomes greater than the strength of the wood a check forms.

A check is sort of like a tear in a piece of cloth, the cloth absorbs and distributes your efforts until you get a tear started and then all your effort gets concentrated to that one area and it becomes quite easy to rip. Same with wood, it is worth taking pains to try to prevent checking whenever possible, much easier said than done.

In piece 3 above the stick will likely bow like this ")" right off the mill due to growth stress. But assuming it doesn't, the larger proportion of juvenile wood on the right hand face will begin to shrink lengthwise as it dries, due to the off axis microfibril angle. As the right side shrinks it will try to pull the timber into this shape "(". There's several forces at work, that's why stating anything definitively is hard to do. Most forms of warp in straight grained wood after milling would be due to reaction wood either juvenile, compression wood or tension wood. Some exceptions that come to mind are cupping and slope of grain.

That is all fine in a perfect world. There are branches, zones of reaction wood, slope of grain, spiral and any number of other variables in there. Everyone crowns horizontal wood up. We also like to have any knots near an edge up on the compression edge rather than down on the tension edge. Ever notice that you almost never can do that? The slope of grain around the knot pulls length out of that side as it dries, makes it shorter and that face becomes concave, the bottom.

TG brings up a good point. I stickered a load of white oak boards onto the trailer right off the saw and drove out to deliver on a bright low humidity day. It was only a few miles but by the time I got there I had checked the mess out of the wood. By the time we had talked a few minutes and then unloaded, the checks were gone. I had dried the surface cells very rapidly and they shrank over the still swollen core until the shrinkage force exceeded the wood strength, and it checked. When I stopped, the high moisture of the wood's core re-equalized. Those surface cells swelled up, closing the checks. Looked ok at that moment, but I had torn the fabric. I had a kiln operator show me the same thing, wood with dirty streaks in it. In their yard as the checks were open the dust filtered in, when the checks closed the dirt was trapped.


This is a section I cut off a 6x6 post and noticed a few hours later, after it had dried. Since the length was not restrained the shrinkage was able to go into the third dimension rather than forming a check. Also notice the convex faces, the tangential grain shrinks more than the radial grain. Imagine what happens if I push down on the "tent", a check would have to form for it to lay flat, as it would in a timber of any length. Hopefully just showing another view of the stresses.

Very well put Don. Thank you for that fine illustration of what' s going on with the stress in wood as it dries.

This is a very cool explanation! Visual aides and metaphors really help the spacial thinkers...

Thanks Don

Great thread guys...and gal. I'm soakin it up!

Thane

Hey Don, Very interesting stuff, thanks for taking the time to do that!

"In piece 3 above the stick will likely bow like this ")" right off the mill due to growth stress. But assuming it doesn't, the larger proportion of juvenile wood on the right hand face will begin to shrink lengthwise as it dries, due to the off axis microfibril angle. As the right side shrinks it will try to pull the timber into this shape "(""

I understand that this could probably be a question that leads to a complicated answer. But here goes.

Could you explain the growth stress factor? Does juvenile wood shrink longitudinally at a greater rate than the "mature" wood? And at what point does one submit to the other?

Although this theoretical it would be great if we could use this knowledge in a practical example. But like you said, there are a lot of other variables that are at work.

For instance say that "3" needs to be a post, with an interior brace. Could you rightly take what we have so far an orient it correctly (leading to a whole nother' discussion on "bowing: out or in"?

I don't know the strength of these forces in relation to joined material, but I wonder if we could realize that the post is going to bow in than the brace is in compression, but if it wanted to bow out, than this might not be as good, since we would be relying on a peg for tension. Is this last paragraph frivolous? I guess this last scenario depends on how long the timber takes to go from mill to bent....?

Sorry for all those questions ladies and gentlemen...