Shrinkage...

[mrcaptainbob]

Sure, a board shrinks. But I just heard about some guy milling half way down a willow log and stopped there to point out the 1/16 or so difference in length. That much, that soon??!! Okay, so, it was willow. But still. I would guess then that differing brands of wood would suffer different shrinkage? A fresh cut 20' willow would be as much as an 1/8" shorter within minutes of bei9ng cut. Hard to believe.

[Tom]

Somebody is BS'ing somebody.  When you rip a log, the boards will all be the same length.  The shrinkage won't take place radially, until water leaves the cells of the wood and never much difference longitudinally, even after drying.  There may be some apparent difference in size because of the wood bending due to internal stresses, but  it will still be the same length if you take into consideration the bend.

Yep, hard to believe.  Keep on thinking that and read the wood handbook. 

[Mooseherder]

How cold was it? ;D

[Brucer]

So here's something to cogitate on.

When a tree is growing, the trunk is carrying the weight of the branches and leaves. This compresses the trunk.

Each year, a fresh layer of wood grows on the tree -- this isn't carrying any of the weight that accumulated in previous years. However, all the additional weight due to mew growth is supported by the entire trunk. As a result the centre of the trunk is carrying a lot more of the weight of the tree than the outer surface.

Then you cut the tree down. No more gravity compressing the trunk.

With no gravity to compress the trunk, the trunk grows longer (ever so slightly). This puts the outer surface into tension while the centre is still under compression (but not as much). Basically the tensile forces near the outside just balance the compressive forces near the centre.

Now you saw off one face of the tree. That removes a bunch of tension on the cut face, but not on the opposite side of the log. This tends to make the centre of the log arch up toward the cut side. If you keep sawing away on the same face, there's less and less tension on that side and the wood moves even more. The effect is most pronounced on Free-Of-Heart timbers.

Most of us have seen this phenomenon, especially with the weaker woods like WRC or Birch. Even strong wood like Douglas-Fir moves a little if you don't rotate it as you saw.

So what happens to those boards you sawed off the outside of a log? They were in tension before you cut them, but once they were cut free from the log they were able to relax -- and shrink along the length. The shrinkage is caused by releasing tension, not by the wood drying.

Is it enough to be visible? I haven't a clue -- never thought to look for it. But I will this summer.

[Ron Wenrich]

But, doesn't the tree also grow wood on the outside to help support the new growth?  They add new support to the structure every year. 

Shrinkage in length is very minimal.  Lumber shrinks in width and thickness as it dries, but not in length.  If that was the case, you wouldn't be able to find two 2x4s that are the same length.  They are double end trimmed before they are kiln dried. 

Beech will shrink more in volume than willow.  Just because its soft doesn't mean it shrinks more.

[SwampDonkey]

Woolly surface on sawed lumber is a sign of radial tension in hardwood and removes the set of a blade rapidly. [Source: "The Structure and Properties of Tension Wood", Holzforchung, 9(4):97-103 (1955)] Also, identify by use of safranin and green stain on thin slices of wood. Normal wood is red, tension wood bright green. Chlor-zinc-iodide can be brushed on a larger sample, tension wood turns blue-gray or bluish violet, normal wood yellow brown. Radial compression wood in softwood is indicated by darker than normal late wood in the region of fastest tree ring growth, seen in x-section (end grain).

With severe longitudinal compression you end up with "brittle heart" and the loss of strength in the lumber produced from it. The breaks in the cell walls can't even be seen with a normal microscope. No mention of any appreciable longitudinal shrinkage due to this defect from the "Textbook of Wood Technology", 4th ed (1980).

Maybe there is more current literature on "brittle heart" on the internet.

[Hilltop366]

Ok Mooseherder were thinking of a Sienfeld episode too when you read the title.

[mrcaptainbob]

Well, the story goes that the sawyer stopped at midpoint of the saw to show the guy the difference. No chance for drying here. However long it took him to cut maybe eight or so feet and stopped the saw. The difference of the board end and the log was almost a 1/16". This was willow, of that makes a diff.

[Brucer]

But, doesn't the tree also grow wood on the outside to help support the new growth?  They add new support to the structure every year. 

Yep, but the new growth in the branches is supported by the entire trunk, not just the new wood growing on the outside of the trunk. The earliest growth in the trunk carries a disproportionate share of the weight of the tree.

[backwoods sawyer]

I did just finish cleaning a bucket of clams but I smell something fishy.

[Tom]

We have a lot of heart rot in the water oaks in the swamp and the wildlife uses them for a home.  It makes the trees so happy that they wave their limbs, twigs and leaves.  It's not so bad in the early morning, when everybody is asleep, but, in the afternoon, the breeze from all that waving can be annoying.  It cools things some, but isn't always a pleasant thing, because, in the late summer and fall, there is so much exuberance that they cause winds and storms.  I hate it when they all get together and cause a hurricane.   I'm convinced that it is the wildlife playing inside of the hollow trees that tickles them into this mode, because the summer and late fall is when you see the birds and squirrels playing so much.

So, when you see the trees waving their branches, don't think that they are just having a good time, all of the time.  I think it's a conspiracy and they are planning a take-over of the world.  You'll notice that it all begins with the waving limbs.  Eventually, they can run everybody inside of the house. 

I'm enjoying a very quiet time right now.  I can hear the thunder of a storm on the horizon, but the trees are being very still.  I'm hoping that they don't beckon that storm over here.  If I'm nice, maybe they will behave.  :P

[Texas Ranger]

 ;D Nothing like common sense to get to the heart of the matter, er, since they are hollow, hollow heart? 8)

[Stan snider]

Welll. . . . . . Brucer said it was something to cogitate on and I believe it. A log from a leaning tree will exhibit strange behavior on a mill and the processes would be the same, only amplified. The next chance I get to cut a strong leaner I will fully analyze this. In the meantime it will be a source for some deep rumination. ??? Stan

[red oaks lumber]

so i don't need my board strecher anymore? good i'll put that next to my brass magnet.

[Ron Wenrich]

Leaners have tension wood on one side, and compression wood on the other.  I think that's a bit different than what Brucer is talking about.  

I would think that the tree produces as much wood as is needed to support the new growth, or maybe more.  Growth is a product of crown expansion.  If the crown doesn't expand, then there is little formation of new branches or leaves, and the need for new growth really isn't called for and doesn't harm the support.  

As Tom points out, I've seen some hollow trees that stay supported.  Is the support on the outside of the column or the inside?  And if the heart is in so much compression, wouldn't the logs actually get longer when you took off the weight?  

[Stan snider]

Ron I may be all wet,(considering we have had a couple of inches of rain then 6or8 inches of snow then light rain now, that is likely),but the same forces that keep a tree from blowin over in a breeze would be acting on the leaner just arranged differently in the trunk. If this theory was any good at all I'm thinking that would be where it is displayed. This would seem to be what makes a plane sawed board cut from just under the bark turn into a ski. As the wood matures maybe it assumes more pressure supporting ability. I too will testify that hollow trees don't all fall over. Hollow trees grow really slow usually so old wood can be close to the cambium. More rumination needed. >:(  Better weather also.  I need skis to go feed those hungry cows or pontoons to keep from going through the slush and mud. Maybe one of those fishing tubes would be good. Not fun.   Stan

[SwampDonkey]

Compression wood is in softwood and tension wood is in hardwood in the radial direction. The other type of compression is from gravity in both types of species causing extremely microscopic slippage in the cell walls. This causes brittle heart in extreme cases, but as far as I can tell from the literature there is no appreciable shrinkage longitudinally. As Ron points how, how would it shrink when being sawed when it was under compression to begin with? The other longitudinal shrinkage from drying is so insignificant that it's not even considered.

Now don't Google "brittle heart" or you will might find some bleeding heart lyrics, try adding "wood" in the search string. :D

If you look at the cross section of some trees you will find that the compression or tension wood won't show up until later from phototropism and from gravity forces on a tree leaning out over a hillside or from a  tree that got creamed by another falling on it and remained alive. I have seen a spruce or a fir get creamed and laying flat on the ground and as the tree grew beyond that point the tip growth keeps growing towards vertical. Also a tree that does not go elliptical in cross section does not mean it didn't grow tension or compression wood. In that year of growth there is only going to be a certain amount of wood put on and if it is thin in the ring on one side it will have the balance put on the other. As Ron said is a function of crown expansion.

[Brucer]

Leaners have tension wood on one side, and compression wood on the other.  I think that's a bit different than what Brucer is talking about.

It is, but Stan's right, the effect would still be the same, regardless of how the stresses got there.

I would think that the tree produces as much wood as is needed to support the new growth, or maybe more.  Growth is a product of crown expansion.  If the crown doesn't expand, then there is little formation of new branches or leaves, and the need for new growth really isn't called for and doesn't harm the support.

Does a tree grow to support an expanding crown, or does the crown expand to support more growth? I think it's a part of a feedback system where each one feeds the other. Douglas-Fir and Larch don't tolerate shade well, so they are always poking above the tops of the other trees in the forest. As the forest grows taller, the Fir and Larch have to grow higher in order to survive. All part of a system.

As Tom points out, I've seen some hollow trees that stay supported.  Is the support on the outside of the column or the inside?

The support comes from whatever wood is there. When the heart rots out (as it does on WRC), the outer wood picks up the load. According to my hypothesis (that's this month's big word), the outer portion of the tree would normally be carrying less of the load, so it can easily adjust to picking up the slack if the center rots.

And if the heart is in so much compression, wouldn't the logs actually get longer when you took off the weight? 

Yep, they do. Even if the compressive stress is uniform across the trunk, the tree will still get longer when it's cut down. The question is, how much longer does it get? Is it enough to measure with a tape?

This is true of any material. As you apply a stress to it, the dimension changes in proportion to the stress. For materials like steel and concrete the change is so small you need special instruments to measure it (strain gauges, for example). Wood is 15 - 30 times more elastic than steel, so it might actually be possible to measure the change with a tape.

As Ron points how, how would it shrink when being sawed when it was under compression to begin with?

It's only under compression when it's standing, due to weight of the tree above it. When you cut it down, there's no weight pushing down on the trunk, so the stresses redistribute themselves.

The other longitudinal shrinkage from drying is so insignificant that it's not even considered.

Y'know, that's an interesting point. The literature suggests that a piece of wood going from 30% MC to oven try shrinks less than 0.1% along it's length. Doing some math ...

  8' long stud, green = 96" long.
  Dry it to 10% MC and it will shrink 0.067%, which is 0.064".
  In other words, 1/16"  ??? .

Now don't get me wrong, I don't for a second believe that a piece of wood is going to go from 30% MC to 10% MC while you stand and watch.

I don't even know if releasing the stresses by sawing will cause a board to shrink noticeably. But I'm pretty sure the stresses vary across a log -- compression in the centre, tension on the outside. It's the only explanation I've come up with for the way a softwood timber moves as I saw it.

EDIT: I fixed a math error regarding the stiffness of wood relative to steel.

[SwampDonkey]

Still when you compress something it's not going to shrink when it's relaxed. If it's under tension, then yes. Tension growth is only in hardwoods. And the test I explained in an earlier post is proof of tension wood.

[Tom]

I suggest that you do a test to justify your hypothesis, Brucer.    I think I have a way you might be able to measure the tree without spending a lot of money on expensive equipment.

Drive a nail at the base of the tree. Climb the tree to a still-stable place in the crown and drive another nail.  Tie a piece of non-stretch string (dacron maybe) to the top nail. Climb down and tie the other end to the nail at the bottom.  Cut the tree down and see if the string breaks.   Nah, I'm kidding.  Measure the string  and then cut the tree down.   Measure the distance between the two nails.  If it is greater then the tree expanded.  I don't know what the knowledge will prove since compression isn't usually released over a long period of time.   I would think that the important measurement would be derived from the shrinkage of drying.

[Brucer]

Measure the string  and then cut the tree down.   Measure the distance between the two nails.  If it is greater then the tree expanded.  I don't know what the knowledge will prove since compression isn't usually released over a long period of time.   I would think that the important measurement would be derived from the shrinkage of drying.

I thought of that. Just one little old problem. It's not that I'm afraid of heights ... I'm afraid of falling.

But first ... I dug up some information on stress and strain in wood. I'm going to do the math and see if there is any possibility at all that would will shrink enough to measure with a tape.

If it's under tension, then yes. Tension growth is only in hardwoods. And the test I explained in an earlier post is proof of tension wood.

"Tension wood" is a type of wood growth. What I'm talking about is altogether different -- its tension in the wood. My roof trusses are made of softwood, but the bottom chord is under tension due to the loading on the roof. The top chord is under compression, and it would be even if it was made from hardwood.

[SwampDonkey]

The sole evidence of those stresses in a living tree or fresh cut is going to be "brittle heart" or "shake" not longitudinal shrinkage. Those are from fibre separation.