Wood Movement and internal forces

[Dave MacArthur]

The topic comes up fairly often, thought I'd write a blurb up with a pic so I can point to it in future when folks ask about wood movement.

I've been meaning to write this anyways so this may be a useful time.
Wood Movement

1. Boards are usually cut to rough length first, reducing all the bow/curve a long board might have that will cause machining/safety issues as well as wood loss when trying to straighten. Length meaning distance along the grain. Wood doesn't move almost at all along it's grain length. Trees don't get taller and shorter with the weather/moisture.

2. rip boards to rough width next, width being distance across the grain, usually on a flat-cut board. This width can change with moisture, so it's best to oversize. Boards can also curve after cut, so that you have to joint the edges (one concave and the opposite edge convex) straight, thus losing width.

3. re-saw boards to rough thickness. Thickness is usually talking about "quarter-sawn" or radial grain direction, that is from the center of the tree towards the bark; but this is most common only because most wood is "flat sawn" or "plain sawn", making it's thickness radial. However, in a quarter-sawn board the grain directions of width and thickness are reversed.



Who cares what grain direction thickness and width are? Well, YOU do, because this is what causes boards to move. First there can be internal stresses in the wood. However, the center of a board can also have different moisture content than the outside. Consider a slice across the trunk of a tree that produces a board. Most hardwood trees we use for boards now are not that big in diameter, and therefore the width of our boards is not that big. Consider a 24" diameter trunk, a decent walnut. Now imagine a 12" slice, see it going across the circular grain lines... can you imagine that one side of the board has the growth rings going quarter-sawn (outer edge of board), and the other side of board where it passes above the center/pith will have flat-sawn tangential ring grain? Because of this, the two sides of the board will move differently as the board dries.

As wood loses moisture, it must shrink both radially (center to bark) and tangentially (around the circumference of tree and any growth ring). The wood shrinks much less radially than tangentially. I had some rough math-based calculations here, but as some disagreed with the exact numbers, I'll reduce it to something not arguable that still explains the point: wood shrinks much less in a radial-grain direction (quarter sawn) than tangentially around the growth rings (flat sawn). While this applies of course to the change across the whole trunk, any "slice" of that trunk must and will display the exact same differential changes. This is the basis for 'quarter sawn stability'--the "width" of a quarter-sawn board is all radial grain, while the "thickness" is tangential grain, so the QS board will change width only much less than a flat-sawn similar board. It will change thickness much more than a flat sawn board, but most boards are only 3/4" thick in use or less, so it's not as apparent or usually critical to our joints.

Now, when you slice across a trunk to make a board, you can see that one side of the board will be more "quarter sawn" than the other, this results in different movement rates. In a THICK board, the different movement rates due to radial/tangential grain differences can be significant just across the thickness of the board. Draw a picture of the end of a tree with rings on it and start slicing out thick boards on paper and look at the ring/grain orientations on different edges/faces of the board, it becomes obvious.

So, you have a board. It dries. It moves, and soon all it's forces shrinking one part, expanding another, twisting here and there due to different shrink-rates, all balance out and it stops moving. But does it stop because it's dry and won't move again? NO. It stops because a layer of wood on each side of the board is exerting it's own force, in fact every small discrete chunk of wood in the board is exerting it's own force vector, and they all add up and move until ... balance. Someone squares that board up roughly and sells it to you.

Now, you take a saw and slice off half of the total balancing forces, and maybe at the same time even expose some wood of different moisture content that begins drying and exerting new forces! So, board twists and warps until all the discrete forces are equalized again. Small milling from all sides of a board will tend to result in small removal of forces that are close to equally balanced, resulting in little or no board movement. Slicing a thick board in half can not result in differential changes, as the thickness of these boards is a significant distance in comparison to the critical grain-line changes.

[Dave Gaul]

Wow! Great write up and explanation! Thank you Dave!

[Cody Colston]

As a tree absorbs and loses moisture through it's seasonal cycles, it must shrink and expand both radially (center to bark) and tangentially (around the circumference of tree and any growth ring) in equilibrium to keep from ripping itself apart and creating fissures.

Dave, A good write-up but I disagree with the above statement. Wood shrinkage doesn't occur until the MC is below the fiber saturation point of the cells, which is around 28% MC. A live tree will never go below FSP thus it will never change dimension due to moisture content.

Here's an excerpt from Gene Wengert's book, "Drying Oak Lumber" May 31, 2001

The living tree uses water as the medium to convey food between the roots and the leaves. The living tree, therefore, contains large amounts of water. For every 1 pound of wet, living wood, about 55 to 60 percent is dry wood (that is, cells) and 40 to 45 percent is water. Some of this water is contained in the cell lumen and some is contained within the cell wall itself. Water in the lumen is called free water while water in the walls is called bound water. These terms arise because the free water is not held chemically within the cell, while the bound water is held by hydrogen bonding. Free water evaporates as easily as water from a tea kettle; however, bound water requires a little extra energy for evaporation. Further, and most importantly, when the bound water leaves the cell walls, the cells shrink. When all the free water and all the bound water has been removed from a cell (at temperatures no greater than 217o F), the cell is at 0% MC; this is also called bone-dry or oven-dry.

[Jeff Duncan]

Hmmm, that's an interesting explanation....not sure how much of it is totally accurate though? There's some good stuff out there on wood movement....Hoadley's Understanding Wood Movement is considered by many a must have. I highly recommend it to anyone who wants to understand in depth wood movement.

In it's simplest explanation wood moves due to moisture moving in and out. Movement from the changes in relative humididty will never stop, unless you place your finished piece in a climate controlled room that never changes relative humidity. This is why you want your wood to acclimate to an environment as close as possible to where it will be placed, while also accounting for movement from seasonal changes.

Internal stresses in wood can be minimized by buying good quality lumber that has been properly dried, (that part is very important), from a reputable source. Reaction wood, (wood which grew in a way to cause stress, think of a tree growing with a significant lean to it, or wood which was dried improperly), should be avoided if possible IMHO as it causes a lot of headaches.

Again this is a very simplified explanation, but it gives the basis for working with a natural material. Anyone working with wood should have a basic knowledge of it in order to avoid a lot of headaches form improper construction techniques.

good luck,
JeffD

[Andrew Pitonyak]

For every 1 pound of wet, living wood, about 55 to 60 percent is dry wood (that is, cells) and 40 to 45 percent is water.
And this is why one guy I know who runs a mill tries to harvest wood before the sap starts flowing in the spring.

[Dave MacArthur]

Some folks pointed out my 1 page explanation wasn't totally correct, and indeed they're right, so I did some editing--my thanks, I do want the thread to be useful and at least not misleading.

I removed some numbers /calculations to avoid saying something in question, so the main thrust of the post doesn't get lost. In any 1-page synopsis of an idea that (literally) is often covered in hundred-page books, you must simplify; and when presenting a complex idea to someone who hasn't read the hundreds of pages, easily-understood concepts that approximate the truth are first used. Of course, this means that anyone can then say, "This is totally a simplification of chapter 49!"... which is true. I'm not intending to replicate Hoadley's work (a good book for sure!), but rather to have a simple thread to link to in response to the literally hundreds of posts a year we see where someone says, "What? Wood shrinks and twists, I have to account for what?". I'm thinking they'd read a short post, while they might not read "Understanding Wood".

Cody's point on live-wood water content and FSP was interesting, and not something I knew--thanks! And I guess that only the movement of "dead wood" way below living water content is really of interest to us, so his point is doubly valid.

I'd love for some other folks interested in the subject to chime in with some posts to help flesh out the thread a bit. Some topics that others with more experience might be able to cover are below (and I'll try to search for relevant similar posts and link to them also):

1. comments on milling a log, why trunks are sliced like they are, why quarter sawn is less common.
2. why wood shrinks less radially than tangentially, alternating layers of cells found radially vs. around a growth ring and their differing properties.
3. Why boards cup opposite the "smile" of their grown rings
4. "reaction wood", tree limbs, why there are stresses in the wood.
5. Differences in wood movement between different species, some thoughts on variable-season growth rings vs. constant season growth and the result on movement.

Just some ideas, love to year your thoughts and to maybe make this a more-useful thread to point folks to. Thanks again for the constructive feedback!

[Dave MacArthur]

Here's a decent thread that just popped again today on handling cupped boards:

Lumber Cupping (http://www.sawmillcreek.org/showthread.php?153826-Lumber-Cupping)

[Cody Colston]

Dave,

I've been doing a lot of studying on wood movement, specifically as it relates to drying. Hoadley's book is almost a must-have for anyone who works with wood and it will dispel a lot of misinformation that gets repeated on ww forums. Another good source of info is WOODWEB. Dr. Eugene Wengert is the resident expert there, offering his vast knowledge on the subject. He will even answer e-mails! the WOODWEB knowledge base is a very good source for answers by Dr. Wengert on wood properties.

[Cody Colston]

And this is why one guy I know who runs a mill tries to harvest wood before the sap starts flowing in the spring.

Many species actually have a higher MC in the winter than they do in the spring and summer. There is no all-around best time to harvest wood specifically for a lower moisture content. It varies with species. Cutting timber in winter when the temperature is cool/cold does help retard fungal staining.

[Jeff Duncan]

Well I'll take a stab at a couple...
Quarter sawn is less common and more costly because it's harder and more time consuming to cut it that way. If you think about how a log is sliced the easiest way is flat sawn, just keep running it through the saw without turning. With quarter sawn you must keep rotating the log as you cut.

Stresses in wood are sometimes caused by the way a tree grows. If you think about the old pictures of redwoods, large diameter trunks that grow straight up, that's a happy tree for lumber. Now think about some of the trees you might see in your neighborhood, maybe split near the base into 2 or more trunks that lean out a bit more diagonal than straight up. There's stresses on the wood in that tree. The bottom of the trunk is being compressed by the trees own weight, whereas the top of the trunk is being stretched out. These stresses incurred in the tress life are still in the wood when it is later milled into lumber.

Again as you mentioned yourself these are overly simplified answers to what's a complicated subject. I think the idea of having a simplified overview is a good one. But there are a lot of aspects that will be tough to break down into a paragraph or two.

good luck,
JeffD

[Chris Fournier]

"Jeff Duncan

Well I'll take a stab at a couple...
Quarter sawn is less common and more costly because it's harder and more time consuming to cut it that way. If you think about how a log is sliced the easiest way is flat sawn, just keep running it through the saw without turning. With quarter sawn you must keep rotating the log as you cut."


Truth is that most any commercial mill tries to maximize the amount of highest grade lumber that they can get out of any given log and around here they call that "sawing for face". As a log passes through the saw new wood and new defects as well as new dimensions are exposed creating a pretty complicated puzzle that reveals itself at a jogging pace through the saw blade. A good sawer, now usually aided with a laser and a computer will flip a log many times as the computer tells the sawer what cuts are available as the sawing operation goes on. All of this activity for flatsawn lumber with a smattering of quartered material, but at the highest grade that the log would yield thus the greatest profit. The grade of lumber is so important that after the lumber comes off the saw there is a fellow waiting to trim the length of boards around defects. A shorter defect free board is most often more valuable than a long defective board. Defect being relative to the grading system.

Guys with manual portable mills don't really love all of this activity and often given the nature and intent of furniture makers they can "slab saw" or "livesaw" the customers log and literally just start at the top and work down. This leaves you with wide boards and all the defects including the heart.

In southern Ontario the lumber mills beef up their log inventories during the winter for a few reasons. One its the best time to work in the bush. Two there are spring road restrictions when it comes to trucks and heavy loads. Three, there is indeed less staining and degrade. A lumber mill owner friend of mine has also told me that the logs contain less moisture when cut in the winter. This makes sense to me as the flow of sap moving from the leaves to the roots is finished.

[Dave MacArthur]

If anyone looks to purchase Hoadley's "Understanding Wood", it's important to note that there is a 1980 version and a 2000 version, and you almost can't tell the difference. Sometimes the 2000 edition is called "2nd ed", sometimes it's called "Rev E". It was around $23 a year ago when I bought it from Amazon, but they tricked me with a bogus picture of 2nd ed then shipped the original 1980 version. Had to return for the newer one, which has a section on plywood and engineered wood beams in it.

You can get used copies also for like $8, a very worthwhile purchase.

[Cody Colston]

3. Why boards cup opposite the "smile" of their grown rings

Short answer: Because the wood nearest the bark shrinks more. That's also why it's almost impossible to dry "wafers" without a crack developing.

A lumber mill owner friend of mine has also told me that the logs contain less moisture when cut in the winter. This makes sense to me as the flow of sap moving from the leaves to the roots is finished.

According to Dr. Wengert, most species will have 1% - 2% higher MC in winter than they do in summer. Also, moisture (sap) is transported from the roots to the leaves during the growing season. It doesn't go anywhere when the tree is dormant but remains inside the wood cells.

[Larry Edgerton]

Stresses in wood are sometimes caused by the way a tree grows. If you think about the old pictures of redwoods, large diameter trunks that grow straight up, that's a happy tree for lumber. Now think about some of the trees you might see in your neighborhood, maybe split near the base into 2 or more trunks that lean out a bit more diagonal than straight up. There's stresses on the wood in that tree. The bottom of the trunk is being compressed by the trees own weight, whereas the top of the trunk is being stretched out. These stresses incurred in the tress life are still in the wood when it is later milled into lumber.

JeffD

I just sawed up the Ash on my property, and this was obvious in one particularly curved tree. It was about a 24" tree that had grown curved out away from the 48" trunk behind it. We laid it down and cut out the curved section for firewood as it is dangerous on the mill, and as fast as we cut it it started splitting across the grain, and shifting back to straight. In ten minutes the inside of the curve had shifted a quarter inch in on both ends of the ten foot curved log as it tried to straighten itself out, the outside of the curve being under extreme compression, the inside under tension, while holding up the tree, and that load now being released. It was interesting to watch it try to correct itself in just a few minutes.

Larry

Ash is one of the species that is recommended that it be cut in late winter. I'm not sure on these trees if this is an indicator because even though they were leafed out last year, they were infected with EAB, but the boards which we sawed up the next day were an average of 22.5% M/C.

[David Nelson1]

Very interesting thread guys and thanks fpr taking the time to write and respond. This place is awesome!

[Chris Fournier]

Wood cups opposite the smile because of the fact that the relatively flatter grain is on bottom of the smiley face. Flatter grain, more shrinkage. More shrinkage vis a vis the opposite face, cupping. If you cut a quarter sawn piece from the heart to the bark, the wood near the bark would not shrink appreciably more than the wood near the heart.

The only reason that we get so much quirky movement or "wood shrinking more" as Cody put it (crown, cup, twist and combos there of) in lumber is the infinite variations of grain orientation that we saw out of our logs. If wood shrank as much radially as it did tangentially we would not see this kind of movement, just a reduction in dimension from wet to dry.