Predicting cant movement: UPDATE 5/1/2007

[beenthere]

The wood won't shrink any until it goes below the 30% mc, so expect some shrinkage yet to come.

[TexasTimbers]

Well it is below 30% by four points. And on the end by a bunch more than that. So what do you think - is it a pretty good indication that no movement can be expected?
I think I have noticed that most wood moves right smart quick and not many suprises down the road. But I have little experience and defer to yours. Just tell me what your crystal ball says. ???

[beenthere]

Not a good indication it will not warp......it won't warp until the wood gets below 30% when shrinkage takes place....however wood does not shrink along the grain (practically speaking), but compression wood will.  So if there are areas one side of the beam with compression wood, some shrinkage and resulting warping could yet occur as it dries.  Note the crooked boards that twist AFTER they come from the lumber yard where they were tied up in a wet bundle. When they dry, they warp.

The quick movement you refer to is the relief of stresses in the log when sawing away one area and leaving an imbalance.....called stress relief. 

[TexasTimbers]

.....it won't warp until the wood gets below 30% . . . .

I'm not too bright remember so bear with me. You keep saying it will not shrink until it gets below 30%. It is below 30%. Do you mean movement will not take place until it gets alot below 30% ? Because 26% is quite a bit below 30%. If no detectable movement has occured maybe it won't start moving, if it is going to, until it gets below say 20%?
I would think by the time it hits 20% - end of summer I am guessing - what movement will have taken place to a noticeable degree will have done so or telegraphed the severity to come at least.
Not trying to make this difficult just trying to understand why you keep saying nothing will happen until it goes below 30 and it is well below that and the string says there has been 1/8" deflection maybe.

[Engineer]

AHA!  Which direction is that 1/8" deflection?   ;D

[beenthere]

kevjay
The surface may be less than 30%, but just below that surface, likely the wood is well above the 30%.  Surface drying usually causes just surface checking.
Also, if there are areas of compression wood, which can shrink longitudinally more than normal wood in pine, that is likely what will cause warping. Giving it some time, will tell.
Predicting what lies below the surface.?.....can't do. 

[TexasTimbers]

bt, that made sense. Don't know why I didn't remember surface MC is way behind interior MC. I have like a 24" section I whacked off. What can I do with that to help us decide what the internal MC is? Whack off a cookie and rip it in half ???

Engineer, really I am not sure it moved at all. My marks were washed off by rain so I am guessing the width of the string at most, or inability to place the string where it was. The beam is inside now and any movement from here on out will be catalogued precisely. I knocked it down to a 9 7/8" x " 10 7/8"  and ran string along the top bottom and sides and it is straight as an arrow starting out. From now on any movement will show and I did away with using marks.

[Engineer]

I think the best way to check the internal MC is to slice the beam in half lengthwise, check it with your meter and report back to us. 

Then get yerself a whole bunch 'a glue.......  smiley_goofy_face

[Don P]

Above about 30% the cavity within the cell, the lumen, is full of free water. That's the first moisture to leave. What you have left after that free moisture has left, is a cell wall made of saturated fibers. At that instant, that cell is empty and the cell wall is holding all the water it can, its at the fiber saturation point. From then on, as the moisture bound between the fibrils of the cell wall leaves, those fibrils move closer together. That's why shrinkage doesn't begin till a cell gets below about 30%.

The top left shot shows a shell below fiber saturation point, the core is above FSP, the intersection of the 2 layers is technically at fiber saturation point, about 28% in cypress I think. The core is showing its free water in the cell lumens so I know its above FSP. Those pins are 3" and only read at the tips, so I can read the gradient as I go in.



The wood will stop losing moisture when it reaches equilibrium with the surrounding relative humidity, that's going to be around 12% give or take about 4 points. So the entire range of shrinkage happens between 30% and 12% not between 100% and 12%. Until the piece of wood reaches equilibrium all the way through, shrinkage is happening. But a dry shell is pretty strong in restraining movement, but the juvenile wood around the heart shrinks lengthwise and dries last, kinda like post tensioning a slab, pulling the cable tight from end to end within the timber. If the cable is lower than center it bows up, if the cable is high it bows down. ...in theory, trees are slow readers  ;D