The check follows the grain, spiral grain causes spiral checks, straight grain checks straight. The "cleavage plane" if you want to call it that, is along the rays or resin ducts in the wood. Those are the cross grain cells that transport from bark to heart and are a natural weak spot. you can follow grain direction in softwood by watching the direction of those ducts or in hardwood by following the "chicken scratches' of the rays.
Just thought, lumber graders have a small pocket tool, a pick, that follows the grain and shows spiral. Should we try to source them?
There used to be a shear value assumption in the NDS for a checked timber and increases for unchecked material. That was changed a couple of ewditions ago, well they had also blown the math.
Never seen a timber check through although I've seen lumber do it. A timber typically checks to the heart.
