While that is a significant size check, it doesn't seem all that unusual from what I've seen from green oak. As someone else said, the thicker it is, the larger the checks are going to be. It also depends on how it was cut from the tree - the ring pattern in that post. Steve Chappell's book shows how checks can occur based on how it was cut.
It's hard to know if you have a problem with that floor joist mortise without knowing the shape of that mortise, it's dimensions and where the check is relative to the bearing surface. Is the bearing surface sufficient and will it split further? is the question. That gap you show sure is ugly unfortunately. That looks like a full 1/4 inch. I'm wondering how it could be that much if the joint was properly fit to start with and the peg holes were draw bored. Were they? If that were the case and the joint were tight fit to start with, I wouldn't think it would open up THAT much.
So what will prevent things like this? I am now working with some white oak that is seasoned 5 or 6 years. It is stable and not bad to work with a sharp chisel. Maybe oak should be cut oversize and allowed to season and hence stabilize, then remilled to size? I'm also thinking that there must be techniques to minimize checking and movement in oak. Anyone have any knowledge here?
Recently I took a timberframing class where we had red oak stock for the braces. This stock was 3 inches thick if I remember correctly and was fairly dry and stable. We ended up being short of material but they had an oak log and a band saw mill on site and cut come stock from the log. As we began to cut the newly cut stock, it was noticeably more wet/green than the other stock we worked with. The pieces were cut for tenons and a curve cut on the stock for looks. The following day as we began the raising, you could see that the newly cut stock was starting to move - twist a little bit and small checks were starting from the wet wood drying so quickly over two days. The question I asked myself was...what had been done differently with the first stock we started with so that is was very square and stable? I concluded that there must have been some technique to drying that stock, or it had cured and was then cut to dimension. Any insights from anyone with timber knowlege of this type is appreciated.
