That is interesting, I have usually attributed those kinds of checking differences to different species rather than grain orientation within a species. For instance red pine, our cousin of scots pine, tends to check with many smaller checks where eastern white pine tends to relieve its drying stresses in one or two major checks. I'll start paying more attention to grain orientation and checking within a species. I don't think mild spiral is a huge deal as long as the grain within a piece doesn't become too short and weaken it. In the round the grain never "runs out" and is fairly balanced. As we saw or hew a timber we cut across the grain and also create places for the stress to concentrate as it dries. Thanks Ken, more to think about.

My experience is with log more than heavy timber, a lefty in a wall of righties is trouble, its unwrapping in a different direction from the rest of the wall and hard to keep put.

Mo,
The switch from juvenile to mature wood is gradual and in pines is normally considered to happen around the 20 year old period. Nature varies alot as does the age of a part of the tree. To drop in one more level of magnification, individual cell walls are made up of several layers of microfibrils, bundles of filaments. The S2 or middle and thickest layer of the cell wall is the one of most interest usually. In juvenile cells the microfibrils align at an angle to the upright axis of the tree, they are slanted at an angle of up to around 30-40 degrees. As the tree matures the MFA comes more in line with the axis of the tree. This is below "grain" level but I think it does have an effect on grain, or the orientation of cell to cell up the tree. I suspect this is due to the proximity of the apical meristem and its auxins. As the tip moves farther away from a section it "matures" and produces straighter alignment, further away yet and the spiral reverses. (That's all pure speculation on my part)

The MFA does affect shrinkage and direction. Shrinkage occurs as the water bound between the microfibrils leaves the cell walls. This is the "bound water" as opposed to the "free water" that is within the cell lumen and doesn't affect shrinkage as it leaves. As the water leaves the microfibrils can move closer together, this is the level where shrinkage occurs. If the fibrils are oriented vertical the wood shrinks in width and thickness...normally. If the wood has a large proportion of juvenile or reaction cells with a steep fibril angle the wood shrinks lengthwise as it dries. If a board has juvenile wood along one edge and normal wood along the other, the board bows as it dries. If the grain spirals the board twists.

Wealth of knowledge, much thanks. What an extensive subject!

Don, do you know of a good book to read regarding what you are talking about. Something that would not require a chemistry or biology degree, but at the same time provide the information. I have learned a little about material science (trees, metals, etc.) so I would not be completely in the dark with terms.

Thanks, mo

An index to the Wood Handbook is here;
http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/fplgtr113.htm

The search function on the FPL site links to thousands of research documents.

"Forest Products and Wood Science", Haygreen and Bowyer is a good one. There is a course description and syllabus with good slides here;

http://legacy.ncsu.edu/WPS202/syllabus.html

I was lucky enough to work around students from this curriculum interning in the wood industry for some years, they thought we were teaching them, they were giving us a good basic wood tech education.

Hoadley's "Understanding Wood" is another good book.

I was bumping around in the computer looking for a labelled cutaway pic of a cell, no luck yet, but I found a short piece I wrote several years ago for another forum that skates on the edges of some of what we've been talking about, off topic but hopefully it'll be neat to you too. Wood's cool, and plumbers don't get to burn their mistakes. Feel free to bring us back to the topic if anyone wants to

Reaction Wood

Reaction wood is formed when, for some reason, the main stem of the tree is not vertical. It is the wood formed by the tree in trying to right itself. In softwoods compression wood is formed on the underside of the leaning stem. In hardwoods tension wood is formed on the upper side. If the center of the stem is considered the point of rotation, compression wood is pushing up on the stem or tension wood is pulling the stem upright. Imagine pulling on a stem, the bottom, compression, side would become shorter as the upper, tension, side would lengthen. I find this analogy helps me understand other characteristics of this wood.

Compression Wood

The underside of most branchwood is compression wood, holding up the branch. Weeping or drooping branches would be an exception. Compression wood contains more lignin and less cellulose than normal mature wood. Lignin is what makes cells stiff, or strong as in a strong column. Individual compression wood cells, or tracheids, are typically shorter by almost a third, blunt or even folded on the ends, and rounder in section. It is normally about equal in strength to normal wood, although denser. Compression wood shrinks about ten times more lengthwise than normal wood, 1-2% vs 0.1-0.2%, this is its major drawback. Compression wood tracheid walls typically contain a primary later of somewhat random microfibrils followed by the fairly horizontal S-1 layer, this is relatively normal. The major ply of the cell wall, the S-2 layer, that controls most properties such as shrinkage, lays at a much flatter angle than normal. As bound water leaves the spaces between the microfibrils and they draw closer to one another this causes more lengthwise shrinkage than in cells having a more vertical microfiber angle.

Tension Wood

Tension wood is trying to right the stem from the upper side of the pith, by pulling on, or restraining, it. Tension wood is low in lignin, high in cellulose. Cellulose is the long, straight chained, glucose polymer in wood. This makes a cell that is strong but supple. This analogy can only be taken so far as in reality tension wood is weaker than normal wood, so should not be used in critical structural applications. In cross section tension wood cells walls are thicker than normal, often with the inner secondary cell walls detached from the primary layer. In the secondary layers is an abnormal, thick, gelatinous layer. The fibers in this layer are arranged nearly vertically. Again the analogy of the cable being pulled taught with fibers straightened out or pushed to a flatter angle as in compression wood. Often this gelatinous, or G, layer us pulled loose out of the "shell" of the primary cell wall and dangles there as a tough fibrous "fuzz" that heats saw blades and makes a flat smooth finish very hard to attain. Shrinkage is high in tension wood but for a different reason than in compression wood. Since the inner layers of the cell wall detach so frequently the orientation of the primary layers' microfibrils determines the direction shrinkage will take. This is typically a more horizontal angle than that in the secondary lamellae of normal wood, so the longitudinal shrinkage in tension wood is greater than in normal, although normally about half that of compression wood.

For the topic we started on... Chapter 4 of the wood handbook starting at 4-28 is a good read, really the whole section on Natural Characteristics Affecting Mechanical Properties is worth reading. Notice fig 4-4.

The slides on the NCSU site covering softwood rays and resin canals are good too.

While I was looking at those slides another couple of wood tech things came to mind. First as you look at rays and tracheids you are looking at the plumbing of a tree, tracheids, vertical softwood cells, take care of up and down movement. Rays take care of the heart to bark transport among other duties.

Second, you probably know that wood shrinks about half as much radially as it does tangentially. While looking at the rays, notice the grain direction they have within the wood. They don't want to shrink lengthwise and so reduce shrinkage in the radial direction. The microfibrils winding around the pit pairs on the radial faces of the cell wall are another reason.

Hi Don,

Great replies. Clearly you have been burning the mid night oil on your researches.

A thought for consideration.

With a spiral whole timber if this comes under significant tension then the tendancy would most likely be for the checks to open up as the timber starts to straighten itself.

Would the opposite be true i.e. if a timber is under compression e.g. a post would this tend to close up its checks and hold itself together as loads increase ?

Regards

Ken Hume

Thanks for the kind words Ken,

One other neat factoid I came across while reading some time ago. Archers seek tension wood, that rubbery G layer makes a more elastic bow. They've been seeking out tension wood for alot longer than we've had microscopes. I'd appreciate it if you kind of kept an eye out for where the oldtimers used spiral grain, were they "placing" it?

Thinking about a spiral grain compression member, I think I'd rather have spiral in compression than tension or bending. I think we're talking about loads that would be way above "allowable". Would it tend to buckle sooner in a taller more slender column than straight grain? Kinda like trying to push a slinky down, it wouldn't take as much for it to squirt out the side?

I guess with a short enough column maybe checks might move a little, but then would you crush whatever is delivering that load before the checks think about closing?

The Japanese have been using relief cuts for centuries and maybe millenium. On an overhead beam, the cut would be on the top, on a post it would be against the wall.

Does that make a well built house a boat?

The closest I dare hope to get to either one is a Grumman canoe and an Airstream trailer.

Motorhomes float not and it takes a biiig tractor to fish em out.

The relief cut works if its put in early, before the check finds another place to relieve the drying stress. I built a log home that had been kiln dried, then their planing equipment also relief cut it... the check had already happened.

I was reading last night, not sure if or how it applies. A concrete column can be reinforced either by standing vertical rebar and placing individual loops of rebar around it or by spiraling a continuous rebar up the vertical rods in the column. The spiral column is slightly stronger.

TOIVO...............THE REASON YOU REJECT LEFT HAND SPIRAL LOGS IS THAT THEY WILL TWIST LIKE A BARBER POLE.IN A CONVENTIONAL LOG WALL THIS CAN POP THE LOG RIGHT OUT OF THE WALL.IN A TIMBER FRAME APPLICATION,ONCE MILLED THE LOG COULD TWIST AND CAUSE STRUCTURAL PROBLEMS.
[1]IN A MORTIS TENON APPLICATION IT COULD BRAKE THE JOINT AND PIN.
[2]IF YOU PLACED INTERIOR AND EXTERIOR CLADDING ON THEN IT TWISTED YOU WOULD GET AN UNSIGHTLY BUMP IN THE WALL.
[3]IF IT WAS USED AS A SILL OR TOP PLATE IT COULD HEAVE THE WALL OR RAFTERS
[4]IF USED AROUND WINDOW AND DOOR ROUGH OPENINGS FOR FRAMING.IT COULD JAM OR BREAK THE WINDOWS OR DOORS.
[5]ETC.

CORDWOODGUY