Help estimating log wall settling

[Edvantage]

I just ordered windows and am designing the spiral staircase for my log home. the general consensus seems to be 3/4" settling per foot of log wall height. I cut logs in March of 2019 and peeled them. Walls where stacked December-March of 2020. The long grooves where overscribed 1/4". In August and September  the roof went on. Since May It's settled 1". Logs are red pine and average 18"  butts are up to 24" smallest tip is 14". Wall height is 102". If an 8' wall might settle 6" and if these logs have been down going on 2 years and already moved and inch how much more might they shrink? How much should I allow over a 4' tall window? I'm thinking of leaving 2 1/2" over windows and 4 1/2" over doors. The spiral stairs is wrapping around my middle column it may have to get adjusted as the house settles. planning to make the first tread 3 1/2" high and put a temporary step there. Does this sound reasonable? 

 

[Don P]

I don't think you'll be too far off, the stairs are always the big bugaboo.
Settlement is a function of shrinkage, plus slumping and compaction in coped log construction.
I made a shrinkage calc from the equations in log standard here;
Log Wall Shrinkage Calculator (timbertoolbox.com)
I never did a slumping or compaction calc but can scan those pages if you want to go there. It is a function of contact area, cope size, kerfing or not and whether the kerf is on top or in the cope, etc. Prescriptive is 3/16" per foot of wall + shrinkage.

I'm not sure the result of those machinations is much better than rules of thumb. A number of us wrote in when they were writing that section but they had their own music in their heads.

[barbender]

All I will say is, great looking build👍👍

[Edvantage]

I did cut kerf's. I was surprised the roof weight did not give any measurable compaction. We have not had any significant snow to add any weight. Very mild winter. Especially compared to last winter when snow was a constant battle.

[Don P]

Compaction is really a function of bearing area and load vs compressive strength. If you divide it out the roof itself doesn't deliver too many psi's. Add a significant snow load and it may crush to a wider bearing area and lower some. As the wood goes from green to dry the compressive strength goes up considerably... fun stuff to try to estimate.

If the kerf is on the underside with the cope it is better protected from water but causes the cope to spread and increases slumping. The standard recognizes this. If the kerf is on the upper side it tends to balance the cope spread and can eliminate the cope spread. More fun stuff to try to estimate.

[Edvantage]

I cut my the kerf on top. Never heard of kerfing the cope. 

[Edvantage]

First window installed. I'm going to cut cookies out of window cutout logs. If I measure the diameters and mark the measurement points and dehydrate them in oven this might give me an idea how much log shrinkage I might get. Any thoughts?

 

[Don P]

Yup, I've done that. Well we used a microwave in the rv. What I didn't have that would help, use a set of good scales if you have them, weigh and measure along the way and get numbers for around 7% if possible. That won't take compaction or possible cope spread/slumping into account but will give you some idea on shrinkage numbers. I'd keep it a few inches thick to avoid shrinkage going into the third plane and skewing the shrinkage numbers higher, if it's too thin it will pop out to the side in the middle and shrink more.

A big part of it is where the check forms in the wall and in the sample. If you got good control of the check direction then it works, if the check is in a random place then it might not be as good a predictor. My argument when they were writing those tables and equations is that they are based on the wood handbook shrinkage tables which were small thin samples. A log is a good bit different, it is more "locked" by the competing radial and tangential shrinkage and then depending on where a check opens shrinkage has certainly occurred but overall dimension may not have changed. It is a pretty complex set of possible variables to try to get a hard number out of.

The best answer I've heard to the question of "when will it be done settling" was "when the ridge hits the ground"  :D

[firefighter ontheside]

My home was built with green Red pine and Scandinavian scribed with Swedish coped corners.  I was told to expect 5/8" per foot of shrinkage.  I'd say that was about right.  I started with 10' tall walls and they have come down about 6".  Id say most of the shrinkage happened in the first 5 years.  My logs look just like yours.  I'd say you are right on the money for your space above doors and windows.  Just make sure that you account for the space that will be taken up by whatever you use as insulation as it compresses.  I used a squishy styrofoam that the log builder supplied.  It was able to compress down to about one inch.



 

[Edvantage]

I was surprised how tight the edges of my long groove had compressed and the amt of contact on the edges. Vertical saw cuts are from roughing out for 2x4 spline. I ran a smaller slot behind the buck and ran 5/8 threaded rod up thru the header log and anchored in the window sill log. Will run rods thru to basement on both sides of doors. Not sure if thats neccessary or not.

[Don P]

I can't see much compression coming from that kind of work, nor slumping. Man, look at those rings, beautiful timber and workmanship. Technically the rods are needed for a continuous load path. Alien spaceships need to be able to beam up the entire structure in one piece.

[firefighter ontheside]

Threaded rod is a good idea to keep the joints tight as the logs dry.  My house has 3/4 threaded rod about every 10 feet in the exterior walls.  In the basement the rod ends have big springs that kept tension as the logs shrunk.  I had to tighten every so often and cut off extra rod.

[Edvantage]

Thanks for the input guys. I need the input as I have nobody looking over my shoulder keeping me on track. 

[Stephen1]

I would say your shrinkage is right on track, allow a little extra never hurts. 
I followed B. Allen Mackie on my build,  Scandinavi  full scribe, I never bolted anything together.
I always smile when I had the Building inspectors and engineers asking about the bottom row of logs being bolted to the foundation, yes Sir. there is the threaded rod and bolts.  Nobody ever asked if they went all the way to the roof.  If I have a hurricane here,  the roof is going to leave me chilly in bed in the loft :D 

[Edvantage]

 I'm probably overkill with the rods.  Any 2x4 stick framed house would be flattened long before a house like this will start to move. My teenage boys have been my crew building this. We all laugh when we pickup a 2x4 feels like a toothpick. As far as aliens beaming the house up - well you never know. The way 2020 went who knows what's next!

[firefighter ontheside]

My house survived a straight line wind event at 90mph.  No damage.  Had some water coming thru where the gable end wall sits on top of the log wall.  Pressure from the wind forced water thru and it also came thru the key hole in my door, but no structural damage.  

[Edvantage]

That's impressive. Had similar winds come close last summer. Flattened the forest 20 mile long narrow swath. Made me think more about rebar.

[Edvantage]

Per request heres an update: I have not been on ff for a while mostly because I have moved to my log home and do not have internet access. Just measured my settling space over my windows. Started at 2 1/2" now about 2". Thats  four foot tall windows.  All of my logs have checked in the kerf on top of the log. Except my top log did not get kerfed. It checked on the bottom and slumped. Have been heating with woodstove 2 winters. Temp in loft runs 80 degrees. Since october my overall wall has moved 1/4". Total settling since 2020 about 3". I've have a couple of showers that don't have much settling space left. Making me a little nervous.