... in the formulas where it says 4D for edge distance
First of all, when you say 4D for edge distance are you talking about the peg placement in a tenon or on a post, in the mortise?
This is very important. The peg distances have to be right for both halves of the joint. If you put the peg hole too far into the post it is closer to the end of the tenon. If you put it too far from the end of the tenon then it's too close to the side of the post....
Deciding where to put the peg is something that has to meet code if your frames are going to be reviewed by an engineer or let the engineer decide where they should go.
Sometimes, I've seen at lease one frame this way, the posts were actually made bigger in width and depth to allow the proper peg placement both in side distance and tenon end distance.
Everything has to be taken into consideration in order for it to be strong and safe.
Normally, we were told, at a week long engineering course, that tenon thickness is one quarter the beam width. So an 8x8 has a 2" thick tenon. A 6x6 has a 1 1/2" thick tenon. And then the pegs are usually 1/2 the tenon thickness. So that means a peg going through a 2" thick tenon is a 1" diameter peg. And a peg going through a 1 1/2" thick tenon is 3/4" in diameter. But each joint has to be reviewed and it has to be determined if these "rules of thumb" are ok.
Now that we have somewhat of a standard rules of peg sizing we can then look at the distances needed to make sure the placement is correct. I think that the distance from the end of the tenon to the peg hole, to prevent relish blow out, is very important, as this blow out is "with the grain" and it isn't as strong as the side distance of the peg hole as it is placed on/in the mortise of the post. For the peg hole to fail in the post the peg hole failure is cross grain not with the grain.
To me, common sense would say that the with the grain failure would happen before the cross grain failure. But without searching for the peg report results, I don't have any solid facts to back up that assumption.
When the engineer made that speech at the conference and explained his methods of placement of the pegs using the diameter of the steel bolt and not the diameter of the peg, seemed to make very much sense to me.
I'm not an engineer, nor do a play one... anywhere....
I'm just interested in timber frame engineering.
