foundation question

[comanche]

I have been enjoying the forum for last few days and really appreciate the information I learned just reading off of other people's posts. I haven't been able to find an answer applying to my situation yet and was hopeful someone could help. First, the background:

My girlfriend and I are building a 2 sided log cabin- I have a Logosol M7. It will be 32 x 24 with an 8 foot deck off the long side. I put the in treated timber pilings last fall and will begin building as soon as some of the snow melts (Homer, AK - might be awhile). I want as much of the materials to come from the land and since I have the mill I don't see any reason I couldn't do this (besides plywood flooring).

My question is what is the best way to do the floor joists/beams/etc? The pilings are arranged 5 x 4 with another row coming out front for the porch. Would you recommend running beams all the way out and connecting to the porch. Thanks.

-Brandon

[rwanders]

Have you considered making your porch 10' wide?-----you would be amazed how that extra 2' changes the feel----especially on a long porch. Are you going to cover it?  I built a 10x24 double decked porch on my cabin at Willow----wouldn't have one uncovered in Alaska----too many days when rain or snow renders them useless. Picture of cabin can be found in Project section ---search for "1 1/2 story cabin nearing completion in Alaska"  posted in 2009.  Wide eaves also are great for keeping snow from piling up against the cabin walls----mine extended 4' on a 12/12 pitch roof.

[comanche]

I will draw the extra 2 feet into my plans tonight- thanks. I am planning on leaving the long porch uncovered because it is on the south side of the house, but the porch will wrap around to the east and will be covered on that side. I am planning on using some wide eves as well. Thanks for the advise. How high is is from the floor to your ceiling joists? I am thinking 9 ft. for mine.

[Don_P]

Although it's tempting to run the joists from under the house out under the porch it is hard to make that joint watertight. If you are insulating the floor then it stands a real good chance of having water get back into that hidden cavity and causing rot. I also prefer to drop the porch floor a couple of inches below the main floor to keep leaves, blowing snow and water out a little better.

[MountainDon]

A big second on that. No matter what steps one would take to try and prevent water from getting under the house itself, remember "water always wins".

[comanche]

Ok, seperate the porch. Check.

How about the flooring for just the cabin? I've been scowering the web and books trying to find something that would fit. I was thinking about running 4 x 8 beams around the perimeter and down the middle then running 2 x 6's across. How much of a gap between the joists? Thanks!

[Squirl]

The joists would be severely undersized for that kind of span.  The framing book listed on the homepage has great charts on joist sizing and spacing. There is also a span calculator on the homepage.  Beam sizing depends on the distance between the posts.  More space requires bigger beams.

[MountainDon]

.... a 2 sided log cabin-

Does that mean a cabin constructed with log walls, sawn flat on two sides and left round on the other? That had me puzzled at first, but that is what I'm thinking. Right? Wrong?

If log walls are meant it is my belief that a foundation that is more solid is required. Something along the lines of the method used by John (redoverfarm).

from this...



to this...



Sorry for the drift I've caused if I've misinterpreted what is being built

[rwanders]

I will draw the extra 2 feet into my plans tonight- thanks. I am planning on leaving the long porch uncovered because it is on the south side of the house, but the porch will wrap around to the east and will be covered on that side. I am planning on using some wide eves as well. Thanks for the advise. How high is is from the floor to your ceiling joists? I am thinking 9 ft. for mine.

My ceiling is a cathedral though I do have a 10x24 loft at the rear of the cabin with a 4' wide catwalk down the center to the upper porch. My walls are 2x6 and 12' tall. The roof is a 12/12 pitch so the peak of the ceiling is 24' off the floor in the 14x24 open area--it is 15' above the loft area since there is a 3' knee wall in that area (regular 8' ceiling height under the loft area).  All that height makes for a pretty impressive space but, it also presents some construction challenges. The peak of the roof is almost 29' off ground level. Framing, sheathing and installing a metal roof on a 45 degree slope can get pretty exciting unless you have no fear of mean ole Mr Gravity.

Are you planning a one level cabin?  In that case, 9' ceilings will definitely make your cabin feel more spacious----it may be just an illusion but it works and the extra cost is minimal.  The planning stage is a lot of fun----I stayed up late many nights drawing scale drawings of numerous designs before I firmed up my plans.  One item---I found that pocket doors are real interior space savers for a small cabin and they are inexpensive and the kits are easy to install.  As noted by others above, be careful about beam and joist sizing----too small can lead to serious problems.

[comanche]

Mountaindon: I am milling two sides and stacking the logs on top of eachother. My plan is to stick with the timber piles for foundation. This is a ways off the road. Don't think I'll be getting a cement mixer and that many bags back there. I havent heard or read until now that it wouldn't be able to sustain the weight of the logs. I will have to increase the size of beams and joists I guess.

Rwanders: thanks for the advise. I'm doing a 1/2 loft. Right now thinking 9 ft ceiling and a 45 degree angle roof on that. I'm trying to keep everything as basic as possible. But it seems like every night I find something new to add or some modification I find from people like you. I like the idea of a knee wall. Maybe I'll pencil that in one of these days. Thanks!

[MountainDon]

I understand about the mixer trucks not having access; same boat here.

As far as the weight of the structure I'm a little out of my element when it comes to logs, but I do believe the weight will be more than the typical stick built I'm used to. I can do math though.   What kind of trees? What is the planned cross section or dimension across the sawn flats? What is the size of the timbers used for pilings? Any idea of the bearing capability of the soil?

What's the snow load; I know it could be a lot.   

So, if I understand correctly you are planning on three rows of piers, three girders/beams on top of them. I'd run the girders down the long dimension and place joists over them with overlaps over the center beam. There is a handy joist calculator on the AWC website. It is for graded store bought and kiln dried lumber. For an example, DougFir #2,  2x8  lumber is rated for floor joists to a 12 ft 9 inch span;  2x10 of the same can span slightly 15 feet 7 inches. That is with them spaced at 16".

It'll also do ceiling joists and rafters.  Speaking of rafters what have you envisioned for the roof?

[Arky217]

My girlfriend and I are building a 2 sided log cabin- I have a Logosol M7.

Comanche,
     I also have a Logosol M7; I used it to cut all the lumber for my house.
You may be interested in looking at a post I made on the Forestry Forum site.
It has a number of tips about using the M7 and about chainsaw milling in general.
Here is the link http://www.forestryforum.com/board/index.php/topic,39153.0.html

[comanche]

Arky - Thanks! Great post. I'll need to get my area as well organized as yours. That looked great.

MountainDon: I will be using Spruce and each log will be about 8 -10 inches from flat side to flat side and about 12 inches going the opposidte direction. The pilings are pretty sturdy - most around 10 inches in diameter. They will be arranged in 4 rows with 5 piers in each row- each 8 ft apart from the center of the pier. For the house alone there are 20 piers (plus another 5 for the porch, but I'm not concerning myself much with that these days). That website will definitely help. From what I am hearing/seeing I probably want to span less than more with my joists.

For the ceiling I am doing gables with 2 purlins and a ridge pole. Something like this:

http://www.alaskaantlerworks.com/images/HPIM3256.JPG

What an awesome community there is here. I really appreciate all the advice.

[MountainDon]

If the first beam on the outside walls are continuous or if more than one log is used, they join over a pier then the walls will be like one beam on top of another, and good. I'm not sure how the floor gets fitted in there. Like I said, no log experience, other than Lincoln logs many many years ago. 

[rwanders]

Are you aware  of the differences between a ridge board and a ridge beam?-----I suspect you will need to plan on a ridge beam which will require proper support at the gable ends and possibly somewhere in the middle-----it will allow you to have a clear span in your open area and also in the loft. Several members are competent to provide technically correct information for your planning but will need to know some level of your design desires to point you to the spec information. If you have access to a friendly structural (civil) engineer or to a very experienced builder, I would advise you to talk with them. Foundations and roof framing are very critical design elements that you really need to nail down (no pun intended). Small errors in wall or interior framing can often be tolerated---errors at the foundation and roof framing can really cause you unending problems and even lead to catastrophic failures if basic structural elements are incorrectly done.

It's not rocket science but it is a science and "common sense" may lead us down the wrong path. It took me a while (and talking with an engineer and a very good builder) before I understood that my design desires simply mandated a ridge beam instead of a simple ridge board.

[comanche]

Yep, there will be a ridge pole (entire log- not milled) spanning the length of the cabin (excited for the challenge of getting that up 20+ ft in the air) plus 2 purlins that will run midway up and support the rafters. I do have access to a skilled carpernter- will be renting him out by the day here and there to help with the tough issues.

[MountainDon]

Okay. Four rows of five piers. So one row under each long wall and two rows under the inner area.

Spruce is about 35 lbs/cu ft. Less if well dried. I'll use 35 lbs/cu ft.

A 32 ft long wall x 9 ft tall and if one foot thick would equal: 32 x 9 = 288 x 35 = 10080 lbs. Resting on 5 piers = 2000 lbs per pier. That is just the one long wall load, not including any roof material or snow loads, or any interior floor and contents loads. That also does not make any allowance for door or window cutouts, but my conservative nature dictates to leave that as a fudge factor.

A 10 inch diameter pier has a cross section area of 78 sq inches = 0.5416 sq ft.  2000 lbs on 78 sq inches = 3692 lbs / sq ft
A 12 inch diameter pier has a cross section area of 113 sq inches = 0.7847 sq ft.  2000 lbs on 113 sq inches = 2548 lbs / sq ft


If the logs averaged 10 inches in width the wall weight would be 8400 lbs or 1690 lbs per pier.
Load on 10 inch pier = 3120 lbs / sq ft
Load on 12 inch pier = 2153 lbs / sq ft

If the piers are smaller in diameter the load goes up.


Are there concrete pads poured under those piers or are they bearing directly on earth? If bearing directly on earth those figures will be the loads per sq ft of earth contact. If on hefty reinforced poured concrete then the weight per sq ft of footer will be less (assuming the footing is larger than the pier cross section.

So the question is what is the bearing capacity of the local soil and what will the total weight of the cabin plus contents and potential snow load be?

[comanche]

Snow load down here can be alot- the last few winters have been fairly mild. When I checked the averages it was around 57 inches per year. I will have a metal roof which will help keep that to a minimum. The soil is gravel w/ clay which has a bearing capacity of around 3000 psf. I guess it looks like I am nearing the capacity, huh? By the way, thanks for doing all the math for me, MountainDon. One thing that might be on my side is that I will have 8 windows (10 if/when I put on the dormers) and they are all pretty large besides the one in the kitchen. I am thinking to help with the weight I could run big 10 x 6 beams down the rows of 5 piers. and then connect beams with 8 ft. floor joists. (??) Then put a lot of cross bracing on?

[MountainDon]

I'm not an engineer. I don't see how loads from the outer walls and piers can be transfered to the inner rows of piers. I don't think floor joists can do that. But like I said; no engineering degree.

By all means the piers should be braced along both the length and width axis'.

Once you know the window sizes you can cut out some of their weight; but double pane glass adds up too. Then look at the end wall loads, roof and snow.

[rwanders]

Don is right about transferring outer wall weight to inner piers and beams----doesn't work that way. You may be able to build your outer foundation to handle the weight as long as your footers there are big enough to spread out the psf load you will have.  If concrete is out, you may take a look at all wood foundations----uses soil-contact treated lumber and they actually work well in colder climates. You really need to consult an engineer familiar with the problems of log construction---I have owned two log homes----the problems are many but are solvable.

[MountainDon]

Info I found on Homer, Alaska indicates 45 lb/sq ft snow load

http://books.google.com/books?id=8hck2Po2RS0C&pg=PA27&lpg=PA27&dq=snow+load+homer+alaska&source=bl&ots=DcSLBS7jqN&sig=sR1JcFZAi5f4yWGR4eWkDaeFVUw&hl=en&ei=9AJ3TePyJY-ssAOV-IDIBA&sa=X&oi=book_result&ct=result&resnum=5&ved=0CDIQ6AEwBA#v=onepage&q&f=false

[MountainDon]

Twp early morning thoughts.

1.  I never ran crush calculations of the timbers where they lay across the pier tops. Top of my head says they're okay, but.... (no time to run numbers this AM)

2.  There may be a way to spread the sidewall loads to the inner piers. If large continuous length girders were laid across the width, in five rows, and then the long outer walls built on that, the load would be shared by the inner rows of piers.

Once again, not an engineer, just a thought.

[comanche]

MountainDon,
That's exactly what was going through my mind through the restless night! Running beams around the perimeter, then laying beams on top of those and running them across so there are 3 beams going across the inside. Just thinking about it tells me that would hold more weight. I need to find an engineer and run this by him- it'd be nice to see this work and not have to rip out and start a whole new foundation.

What do you mean by crush calculations?

[Don_P]

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.

I'm not an engineer either and I do think one might be a good idea. I do enjoy a good problem though.

If you ran beams perpendicular to the ridge across under the floor system, then put the beams parallel to the ridge on the perimeter on those it will help but I'm not sure at what point they would "hook up" and take a significant portion of the load, nor what the floor flatness would look like when they did. In other words these would need to be big honkin beams with very little deflection to do what you are proposing. We can try to explore that a little later.

Lets see if there is enough footprint in the ground to carry what you propose first.

Since there are gables I'm going to stick to 1' thick walls at 9' tall and figure its going to average out right now. 32x24 is 112 ft of wall x 9' tall=1008 cu ft x 35pcf=35,280 lbs of log walls

Lets figure the roof load; Assuming 2' overhangs on the 32x24 footprint it makes 36x28'=1008 square feet. Multiply by snow+dead load-60 pounds per square foot. 1008x60=60,480 lbs on the roof.  

The floor should be figured at 40psf LL and 10psf DL so 50 psf. 24'x32'x40psf=38,400 lbs

5' of the porch, half, is on the 20 piers we have. quick and dirty 50psf floor+60 psf roof-110 psf x 5 x 32=17,600 lbs

Add the loads=151,760 lbs total design load.

Divide by 20 piers=7588 lbs/pier

Even if we can effectively distribute the loads uniformly over the existing piers it looks like it stands a good chance of sinking. You can go real world and reduce things like the floor LL but it's probably still overloaded for the ground footprint IMO.

Doing the crushing math 7588 lbs/78 square inches of post top=97psi. Allowable side grain compression is safely about 350 psi so that looks ok at this point.

[MountainDon]

Thanks Don, that's a lot of weight.