foundation question

[comanche]

"Allowable side grain compression is safely about 350 psi so that looks ok at this point" - what's that??


I have talked to a few cabin people around here. Many of the cabins are built just on cement blocks and are not sinking. One friend told me his cabin has less play on his pier foundation than his house does with his concrete foundation. I am on a tight budget and I see two options for myself- maybe you could think of another
1. 4 sided logs & 8' ceilings
2. Add 12 x 12 concrete blocks in between each pier for a total of 14 piers and 14 blocks on the perimeter.

[Don_P]

"Allowable side grain compression is safely about 350 psi so that looks ok at this point" - what's that??

The crush calculations are checking the compressive strength of the side grain of the girders sitting on the post tops... you don't want the posts crushing into the beams.

The beams can take a side load of about 350 pounds per square inch before the wood begins to crush, so I was checking to see that the post wouldn't crush into the surface of the beam, it won't.

Normally a concrete footing is poured beneath a pier or foundation wall to spread out the load over a larger ground area... snowshoes for posts, to keep them from sinking deeper into the ground under load. I don't know your ground bearing capacity, you estimated it at 3000 psf. Tallying up the loads you should design for I was coming up a good bit higher than that. All I can tell you is if it were mine I'd either have an engineer check it or add more posts, on footings. If you aren't crushing into the beam up top, the post would not crush into a pair of treated 6'x6's under the bottom of the post, and they would spread out the load as well. You can also wait and see and jack it up if it sinks and deal with it then.

Do brace the posts diagonally in both directions, big X's with 5/8" galvanized bolts are typical on beach piling houses. A heavily loaded tall post is much more likely to try to tip.

You can do 3 sided logs as well.

I've done several flat on flat log homes. One trick, the sides of the logs shrink more heighthwise than the center, opening gaps on the inside and outside edges. If you take a power plane or log wizard and remove a channel of wood from the center bottom of each log as you stack them it'll make them bear on the outer lips. Helps keep the water out.

[rwanders]

Don P, I've seen several builders, when using flat on flat logs, lay sill-sealer (fiberglass about 1/2" thick) between each course-----does that seem worthwhile?  Like your thought about the relief channel too. 

Also, do you favor spikes or drilling and driving hardwood dowels through the logs? I have done both and have heard pros & cons both ways.

[comanche]

Thanks for all the great advise- I'm heading up to Talkeetna for my spring break- no internet, no phone, just some ice fishing gear and some books. Have a good week.

[comanche]

One last thing I forgot to mention, that's probably pretty important. I'm building with beatle kill which is going to be considerably lighter than most other wood. Hopefully doing this 3 sided with more posts will prevent too much shrinking. Also looking forward to hearing your opinion on spikes or lags- I was looking at the Oly's Lag screws and am leaning that way.

[MountainDon]

or these

http://www.mcfeelys.com/search.aspx?q=headlok&src=M8GLSCW&s_kwcid=TC|4266|headlok%20screws||S|p|4712748385&gclid=CMnc686moaYCFQTNKgodDCfOog

[Don_P]

I'm a fan of 3/8" lags. I prebore a 1/2" dia hole through the upper log with a 1-1/4" counterbore about an inch deep to recess the washer and head. The lag drops thru that log and is drawn tight to the log below. When the log shrinks it can slide down. I've done them with foam sill seal and backer rod in between but after we started doing the relief channel we used the expandable foam tape, like emseal on the outer edge of the channel. I've used olys and they are better than spikes but less pull than real lags. They do have incredible tensile strength for their size but the smaller screw strips out in low density wood much easier than a full sized lag so they just don't generate as much pull as a larger fastener. When we've used them I have lags handy if there is something that needs more pull down. The oly log hogs are somewhere in between an oly and a lag. The highest density wood in the log below is right beside a knot, don't go in the knot but right beside it  . The fastenalls are basically the same thing as olys, they are all high tensile lo-root screws. When we set a 2 sided log on the wall we went by centerline of each log and then looked at the outside. If any ledges were visible on the log below we would scribe them with a pencil, remove the new log and drawknife those back under the new row. You want to make slight drip edges not water catching ledges. It still needs caulking between the logs. And of course the longer you can let the logs dry after milling the less movement there will be in the finished wall.

If you can build a scrap board or plywood dip tank using screws and construction adhesive and dip the logs fresh off the mill in a borate solution it will help with bugs and decay fungi. I use Solubor, an ag soil boron ammendment that is chemically identical to TimBor the listed chemical for treating wood. It has low mammalian toxicity but inhibits wood consuming insects ability to digest what they eat and it halts rot fungus. Set up on a plastic sheet to catch any drips.

As long as you are kicking options around, I've also done several that were 3 sided log siding 3-5" thick. Basically a heavy slab with 1-1/2-2" flat top and bottom edges that allowed us to spike or oly log to log and to a conventional 2x6 stud wall. Good insulation and easy to repair if a log ever goes funky. We've done log interior, panelling and drywall, opens up more possibilities inside.

Edit;
found this pic I had drawn on the computer.
First drawing is a 2 sided log just after sawing. Middle is the log after it has dried... tangential grain shrinks about twice as much as radial grain, so on a cut like this the sides shrink more than the section through the middle. A check usually opens on one of the cut faces, this becomes the bottom face so it drains rather than catching any leaks. Third pic shows the relief cut.