Kerfing

We currently kerf the face of boxed heart material that is covered by a exterior wall or floor to control the check. The kerf is 1/2" deep located in the center of the timber. We discount the width of the mortise from the width of the timber for all our calculations. Do I need to futher reduce the size of the timber for the kerf cut?

hiya freestate,

wish there were more of those.

I am no engineer, but if you take away the surface area in the level plane (im assuming load capacity), I think you have it right.

Tell me, has the "relieving groove" been working?

Later,

mo

Yes an interesting question for the fact if you force the check will it continue along the kerf into the pith. But if you let it naturally occur it is spread out, as in, not in a continues crack in the same plane. We are told the natural checks are not a serious defect, but could a forced check be? In a post I would suspect not, but a load carrying beam maybe.

On the other hand a check has no material removed from the section, it is just rived. Where a mortice or peg hole has removed wood from the section, this needs to be accounted for. Is the 1/8"x1/2" enough to be calculated and inserted into the equation? Maybe.

Tim

Correct me if I'm wrong.
If the kerf is in the top side of a beam then it is in the compression zone and a 1/8x1/2" kerf in there would have little or no impact on the strength of a beam.

I'll stick my neck out and say that you could cut much deeper into the top surface, all the way to the center, without affecting the strength of a beam. I would rather focus the checking on the top surface than have a big check in the sides or the bottom.
Perhaps this will get the attention of an engineering type?

You would think that if a check runs vertically through the center of an 8x8 beam then you would have two 4x8 beams with the same strength as an 8x8.
I may also sticking my neck out.

The top surface has much to do with the structure of a beam. If you have tusk tenon mortices coming into both faces and a kerf running down the top center you have destroyed the I-beam effect. Width of the top cord does make a difference. In effect the mortices cut most or all the way through the web in each half, the structure is nearly gone. I wouldn't kerf a beam as I have described, for that matter I don't kerf timber at all.

How is that for laying it on the chopping block. I hope the axe is sharp.

This reminds me of the time my wife and daughter had had enough of the rooster, the boys were selected to take the head off via the block and axe. When I came home that evening I had to finish the old bird off, Lincoln was his name, had flinched, resulting in in his comb being the only thing removed, he was very bloody and still strutting the yard.

Tim

You never have the same strength when piecing 2 beams together. See Ben B's recent article in the TF journal on keyed beams.

No engineer here so this is just IMO

I don't recall that the TF standards under development mention this but did remember it in the recently published ICC log standard. So here's their poop

302.2.4.3 Kerfing:
where kerfing is provided in logs used in walls the depth of the kerf shall be no deeper than H/2. The sum of the depths of the kerf and cope shall not axceed H/2. Where beams are kerfed, the net section shall be used to determine the section capacity.

I think the last sentence says what you need. I don't think 1/2" is going to accomplish much in a rectangular timber. In log work 1/4-1/3 depth is typical. You are encouraging a check, a failure, and directing it in the case of a beam to a better, safer,face. No need to be shy about it.

But in who is to say what the better, safer face is? Do we say the face which is not seen is the best location, out of sight, out of mind? For that is what is happening when we are trying to hide the check. Why not kerf down the apex of the timber, it would have to be deeper?

This bring up spiral grain or what appears to be spiral grain. I often see timber checks running at an angle to the face, these do not continue around the timber they are sectional like a dotted angular line running down the timber. I then think about splitting firewood or peg stock. It splits easily if the grain is clean, but a twisted wood does not, it is a struggle. With this concept is a straight grain with checks weaker than a gnarly one? If it runs out it should be but if it doesn't? Also is there a difference between checks, grain and spiral are they related?

Tim

The tendency in a rectangular timber would be to check on the wide face lowering its shear capacity, although this is seldom pushing the limits. The "safer face" in my mind is along the vertical axis of the beam. A built up vertical girder is fine, but you wouldn't under normal conditions stack a bunch of boards and expect to make a stout beam.

I thought about spiral grain but... it is restricted by grade and shouldn't matter to the kerf IF it is within its limits.

Checks follow the grain, spiral or straight. If you see checks it is easy to follow the slope of grain. I imagine with your dotted line of checks you are seeing checks forming around the weakness at rays. I suspect shear is better in interlocked grain but bending is lower...Again all IMO.

Ben B and Firetower have a presenation up at their site.
http://www.ftet.biz/index.php?action=resources.vwp2

Here's another link to the white paper for the engineering geeks:
http://www.ftet.biz/index.php?action=resources.wp01

Vertical check, Mike. Two 4x8s side-by-side: Technically, I think that is as strong as an 8x8, unless they twist and buckle sideways from being too thin. For example, I'd say 8 1x8's side-by-side are not as strong as an 8x8, unless they are attached to each other to keep them from buckling sideways. CB.