Where to best use knottier logs in post and beam

[Coastallogger]

I saw something recently about using perceived flaws in wood to be a strength like using knotty wood in applications where splitting is an issue.

I am currently sawing out a king post and beam barn (to be joined with structural screws) out of white spruce and it got me to thinking: Where should I use my knottier wood?

For rafters and purlins, the answer is clear to me to use the clearest wood.

But for posts and collar ties, it got me wondering: does knottyness make any difference at all in the strength of a post? Could it actually be a huge benefit for the strength of a collar tie if a screw bit into a dense knot and act as an anchor? Do knots affect the tensile strength limits of the collar tie? What about the king post?

Any thoughts?

[Den-Den]

I believe knots always detract from strength and that it is a judgement call to decide if a particular piece is suitable for its intended use.  With that said, small knots that are not in the edge of a joist are only a small concern for me (if that small of a concern is a problem, I should have used larger joists).  Small knots in a post are not much of a worry either.

My process is to imagine cutting out the knot and then decide if that would be a structural issue.  If not use the piece as is.  If cutting out the knot would be a structural issue, use it for something else even if that is firewood.

[Don P]

When we were building log homes, that was before timberlocks and such, we were using 3/8" lag bolts to pull the logs down. If I had one that was being ornery I would run a lag down right beside a knot in the log below. The denser interlocking grain gave better withdrawal, or pull down, strength. I also broke lots of lags  . For tensile and bending knots are not good. In compression they will help buckling to occur so they are not improving strength in any of those but the "best" place for the knottier pieces is in a column, in compression.

I'd have to look but I think grading rules do look at a hole the same as a knot, or close to it. But! do not line up knots in mortises thinking that "hiding" the knot by removing it is a good thing. The grain is already swirling around a knot. You want good long continuous straps of fiber in the timber. A knot is already playing heck with that, don't make it worse by locating a notch in that zone, you'll just be cutting out more straps.

[Don P]

More thoughts, a "short column" fails by crushing where a tall slender column fails by buckling. The short column line is 1:12. So for instance an 8"x8" post 8' tall is probably not going to fail by buckling, it will fail by crushing. In that instance geometry is really more important than grade... as long as you don't get too carried away with obscene knots.

Then, on the use of 3/8 lags back in the day. If you look in the Wood Handbook in the connections chapter, up to a 3/8" lag can be run into lower density woods like EWP or WRC without a pilot hole. Larger diameters require a pilot. We were drilling a pre-bore through the upper log with a counterbore to allow for the lag head and washer as well as anticipated shrinkage, adding a pilot would have added yet another step so most log home companies that connected that way went with 3/8" fasteners. I think they had a steel tensile strength of about 45,000 psi. We tried to build "dollar bill" tight and broke hundreds of lags. In good wood they had more grab than tensile strength. When the Timberlocks, etc came on the scene they needed no pilot, no prebore, they were slick shanked and no counterbore, the head sank itself. The big bonus was the steel was 190,000 psi tensile. I very rarely break a screw anymore. The smaller diameter does not have nearly the withdrawal/pull down strength though. A big block Chevy with bicycle tires. I still carried a few thousand lags with me for that withdrawal/pull down strength for ornery logs.

Here nor there, she's cleaning house and I'm trying to look productive... alas, busted