Hi all!
Plans have changed a little, and we're now planning on building our small vertical log cabin on piers. I'd like to try protecting the piers from frost with rigid insulation and building right up near grade, on a shallow pad of crushed rock. I'd rather not pour the 18 piers, due to the cost and setup time. It would be a whole lot easier just to set some precast blocks down and level with shims. Do I need to strap the 8x10 sills down, though? Earthquakes aren't common here in the UP, and the cabin is going to be tucked away in the woods. I would rather secure the place down, but all the precast blocks are made to connect to a 4'' post, not an 8'' beam. I thought of installing little 4x4 posts on each of the piers and simply chainsawing them off at the level height for the sill beams. Wouldn't those little posts be kinda precarious, though? I can just see all the piers tipping over in the same direction and setting the cabin on the ground..
Thanks once again,
Andrew
How about drilling a hole with a rotohammer and inserting a rebar to hold it all together . Add diagonal braces from the posts to the beams both directions -- it should work fine.
Check out Pa Kettle's foundation.
The logs could make it a bit extra heavy - I don't know anything about your soil and piers - just that heavy stuff sinks in wet soil. Possibly you would want more piers than a normal house of the same size is all I'm saying.
Just chiming in here with some thoughts based on my limited experience:
For your piers, why don't you get a 18" diameter sonotube and cut it into 18" sections and pour four or six of your own piers. That would require about 4 bags of concrete per pier. Then use CB66 post bases set into the piers.
For the beams, you could switch from the 8x10 beams to 6x12 beams and then set the beams either directly into the post base pocket or if you had to adjust for elevation, use 6x6 pt posts.
I would think that a wider pier would bear the weight better.
my $.02...
I don't think 6 piers would cut it... The cabin is 32'x16' w/ an 8' deck. Like Glenn said, the sills are going to be under a lot of stress, especially due to the vertical log construction. I'd rather not try to muscle enormous beams around with no mechanical aid, either.. If I go on 8' centers, I'll need 18 piers.
Glenn, are you talking about concrete block with a wooden post on it, and drilling rebar down through the two? I'd have to borrow the drill, but it could be an option..
I really like the idea of wood posts that can just be sawed off at the right height. I've used the sonotubes, and I can never seem to get them on the same plane, no matter how much I tinker with them.. Not to mention they're way too expensive. :o
I also thought about skipping concrete altogether and just using hefty cedar bolts or chopped up telephone poles. Only problem is they wouldn't last as long..
EDIT
How about this... A 2'x2'x6'' deep gravel pad, topped with a couple inches of XPS. A 16''x16''x4'' footing of stacked pavers on the foam, and an 8''x8'' hollow core block to top it off (hole facing skyward). Fill the block with redi-mix and stick a chunk of rebarin the center, with a few inches protruding. An 8'' round post with a hole in the bottom goes over the rebar and sits on a shingle. Posts get trimmed, sills get spiked, and we can add diagonal bracing later. Sound like it would work?
I was just talking about that method to keep things together - yes drilling a hole in the block. This is not a best way to do it-- this is a way to get by. You may want to consider your expected future for the building and how long you want it to last as to how much time to spend improving the foundation -- longer time - more work. My dads house was put on wet soil 70 or so years ago -- it has been that many years of jacking and re-leveling up to a foot of settlement or so over the years.
The gravel trenches should be drained to daylight somehow so water does not build up and freeze and tear things apart - about 10% random movement every time it freezes. I see you were talking holes only -- if filled with water and froze they will move. Sufficient drainage would be a key consideration to keep from heaving.
Foam under that much weight would likely simply be smashed.
Don't they pour frost-protected slabs directly on foam, including the thickened load-bearing edges?
The gravel holes being only 6''-8'' deep and near a ravine I doubt would fill with water, but it still might be a cause for concern.. It's only a cabin that we're trying to build for cheap cheap cheap, but I certainly can see what you're saying. We're hoping to build the shell for under 5k, but would rather not have the whole thing turn into a giant indoor matchbox car derby ramp, either.. ;)
The difference is the amount of area you place the weight on. The piers are more of a point load where the slabs spread it out over a large area. Calculating the total floor loads, dead loads, live loads and dividing it all out could tell you what load was on each pier, but I didn't go that far. Just guessing off the top of my head.
As Glenn mentions you should total up your dead and live loads for the roof and floor (I assume one level) and then divide by the number of spaces between the piers (there will be end piers that each carry 1/2 the load of the interior piers. This should be the load carried by each set of piers on each side so 1/2 to each. Check my math here. You want the tributary load getting down to each pier that is carrying weight from both sides. Now divide this downward design load by the soil bearing capacity or upward resistance force of the earth.
Most building codes limit this to 2500 lbs/SF unless soil tests can prove you have better soils. The best soils may be 4 times this and the worst 1/2 or 1/4.
For example, if you have 6000 lbs as the design load on each pier then you would need 346 square inches of footiing area or a bearing pad just over 18.5" each side if square. If you are making this out of concrete (unreinforced) it should be 9" or more deep. This should be in well drained soil below frost line.
As shown in this article http://www.countryplans.com/foundation/ you can also build this footing with crushed rock.
PS - You never put insulation below a footing. Footings must rest on undisturbed soil. You may want to insulate above the footing but only if you were needing to put it above frost line and you know the soil is well drained. Some members have done this successfully. See: http://countryplans.com/nicolaisen.html
It actually seems to me that the soil is more stable near the surface in this clay than when you dig down. If you punch a 4' hole in the ground anywhere on the property, you get about 6'' of water after a few hours, no matter what time of the year. We had to replace two crushed plastic septic tanks within a couple months apart, subject to ground water. We finally installed a concrete tank w/ drain tile around the perimeter. I guess we have a pretty treacherous soil, and it just seems like staying on the surface and protecting from the frost is the cheapest, easiest way to go. If I put 2'' of foam around the surface of each pier, approx. what radius do I need? Perhaps building an insulated skirt around the place would help the ground underneath stay warm.
Thanks for the help on the load calculations. I'll do some math and try to figure out the weight, but it might be tough to guess what those log walls would come out to..