Non cracked timber (Kirf cut)
Timber will check because of drying ratio is different from across the growth of rings (radial) or along the growth of ring (tangential). If timber contains the heart of tree, it call boxed hear, if not it call Free Of Heart Center (FOHC). Depends on this grain ratio but boxed heat timber will check guaranteed and we try to use FOHC as much as we can.
Because of the budget is limited, we often use air dry timber, moisture content below 19%. These are mostly locate the interior BC, many of trees killed by fire, dead stand for while so it is dry. But these logs are not big, depends on log source, we have to use boxed heart timber.
Boxed heart timber, usually will check toward nearest face of the timber from the center. 99% this will be side of the beam. When timber check, it will check along the wood grain. All the tree has twist, it is just matter of how much. Timber grade system allow more than what we expect even in structural grade, so we have our own grade to apply for the frame. However, when timber check, check follow the grain, timber will twist as much as grain is twisted. This is the reason why you see some of the beam twisted.
To prevent this, we apply kirf cut. This is the cut from the face of timber being covered, it need to cut all the way to heart of the timber. More the timber is green, all the check will focus on this one cut and will not show any other check on side. But air dried timber, often check is already proceeded, so some of the check can not be eliminated but it will not twist.
Here is one picture that shows our inventory. This timber had been dried for 6 month now, starting moisture was 20% now it is 13%. Dimension has been reduced but you can see how much this cut has been opened up. If we plane to square and use this timber as structural member now, it will be stable.
Timber will check and that is in engineering consideration already so this kirf cut should not sacrifice the strength of the beam. Actually, it will increase the strength as the beam because the side check will be in nutral axis which is the strength of the beam what relying on.