Foundation Plans for a 16x24 Cabin, ponderings...

Started by Strix, October 02, 2016, 04:26:23 PM

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Strix

Hello all,

Long-time lurker here, who is finally starting to plan his first build. So far, I've intended to build a very similar foundation to this:



What I am wondering though, since this image suggests piers, is that if I wanted to maintain the same support points both on the perimeter and the center two piers, How would I build a full foundation?
I am dealing with a code frost depth of 60" in northern Minnesota, so I have to go down a ways. I am NOT dealing with code however, in the area I will be building. State-wide soil survey data suggests surrounding areas are clayey-loam so I have assumed a 1500lb/sqft soil bearing capacity. The water table is a good 10 feet below the build site.

My goal in asking is that many users on here seem to try the pier foundation method, and there are ample warnings of "doing it right the first time" with regards to piers being less stable than full foundations. I want to do it correctly so nothing moves.

What I've imagined this entails is building a perimeter footer of concrete down below the frost line which is 16" wide and traces out the 16x24 and cross-members. Then, attached to this are 8"x8"x16" cement masonry units, hollow, which I build up to grade. The footer is crudely represented here in red overtop the original pier image...



Understanding that this is probably the most backward way to get support to that center floor area, and that I could do with just a perimeter, is this the "right" way to do a full foundation that is sturdier than piers? I understand with the concrete continuous footer as is, I would have ~144sqft to spread out the weight of the cabin onto. About 550#/sqft for the cabin if soil capacity is correctly 1500#/sq. I do not plan to use the foundation as a basement due to flooding concerns: I'll backfill around the perimeter with gravel and the inside will be filled back to grade minus enough to leave crawl space. With soil inside and out of the cement walls, do I need to fill the CMU's with rebar and cement or can I attach them with mortar and have a strong enough structure?

Any pointers or pictures? Going to ask lots of questions as I soldier forward before doing anything dumb or costly here... I want my final cabin to stay standing!

Best regards and thanks in advance!

-S




NathanS

#1
Yeah, definitely want a continuous perimeter foundation. On the interior, there is nothing wrong with piers to support loads that can't span the 16'.

The diagram you have doesn't really make sense to me once you convert it to a perimeter foundation, though. You would want a built up beam to run parallel to the 24' wall, and want your joists running the 16' length.

That beam doesn't necessarily need to be midspan, either. If you have a second floor that would need support from posts, you might want those posts offset from center to not interrupt the space downstairs. For example, in my house, the posts that support the second floor are located at the corners of the stairwell (this is just one example of why John Raabe's plans are a great starting point for your home or cabin).

Sounds like you are planning a crawlspace. I would set the beam below the joists, and run the joists straight across -resting on top of the beam- the full 16' feet.


Strix

Good to know. Continuous Perimeter is much more stable to frost and earth movement compared to piers as I know. Running the cost on it all doesn't come out too much ahead of piers either, which is encouraging.

Is what you're saying that as the image shows, the beams span the short length and what I should do is instead only have built beams span the 24' length and joist between?
I want to avoid spanning the full 16' as a continuous piece of lumber so I can use 2x8 for the joists instead of 2x10 or 2x12... I can still do this if I rest the joist on the beam and not at the same level as the beam, I think.

Any reason why joists shouldn't run the short direction? I see this a lot on here and can't figure out why other than convention...

I am okay with the posts in the middle thirds where the piers are in those images mainly because I want to run a post to the ridge beam of the roof. I understand this will reduce the already manageable wall thrust from my planned 12/12 sloped roof.

Back to the foundation though... I will definitely run around the outside 16x24. Is it okay to leave the hollow-core CMU's unfilled and still have enough structural strength to hold up the building? Or do I really want to fill those cores? Do you suppose frost can get a hold of the sides of the walls and still heave the structure? I have 0 intuition on what would happen, other than that I know there will be some deep frost most years.

ChugiakTinkerer

Congrats on taking that first step, and welcome to the forum!

NathanS has described exactly what  I was thinking about the perimeter foundation and having the support beam running east-west rather than north-south.  Putting your joists on top of the beam means you don't have to mess with joist hanger brackets, although you will want to tie the joists to the beam and sill plate.  For your interior support beam and piers, you can also break it up into two separate beams if that helps for wall placement.  That's only useful for when the walls are load bearing, but if you're looking at a loft or 2nd floor then having bearing walls makes things a lot easier.  You'll want to bring your CMU wall about 18" above grade to keep water and pests out of the wood framing.
My cabin build thread: Alaskan remote 16x28 1.5 story

ChugiakTinkerer

For standard floor loads you can run a 2x6 on 16" center with a span well over 8', almost up to 10' for most #2 graded lumber.  A continuous 16' 2x6 joist supported by a 6" beam in the middle and resting on 6" sill at each end only has an unsupported span of 7' 3".  No problem with a 2x6, and would be much stouter with a 2x8.  Without that support beam in the middle then you would be looking at a 15' span and that would take some very tall joists.

By the way, the International Residential Code is available online at http://codes.iccsafe.org/app/book/toc/2015/I-Codes/2015%20IRC%20HTML/index.html

Joist span tables are in Chapter 5, but the beam spans tables are covered in Ch 6 under Girder and Header Spans.

[EDIT] Corrected the span to above to 15' not 15"  d*
My cabin build thread: Alaskan remote 16x28 1.5 story


Bob S.

If I had to go down 60" I would go another 2' and pour a 4" slab and have a full basement. If you do not want the stairs just go with a bulkhead, Cheap space. Bob 

NathanS

Quote from: Strix on October 02, 2016, 06:43:26 PM
Good to know. Continuous Perimeter is much more stable to frost and earth movement compared to piers as I know. Running the cost on it all doesn't come out too much ahead of piers either, which is encouraging.

Is what you're saying that as the image shows, the beams span the short length and what I should do is instead only have built beams span the 24' length and joist between?
I want to avoid spanning the full 16' as a continuous piece of lumber so I can use 2x8 for the joists instead of 2x10 or 2x12... I can still do this if I rest the joist on the beam and not at the same level as the beam, I think.

Any reason why joists shouldn't run the short direction? I see this a lot on here and can't figure out why other than convention...

I am okay with the posts in the middle thirds where the piers are in those images mainly because I want to run a post to the ridge beam of the roof. I understand this will reduce the already manageable wall thrust from my planned 12/12 sloped roof.

Back to the foundation though... I will definitely run around the outside 16x24. Is it okay to leave the hollow-core CMU's unfilled and still have enough structural strength to hold up the building? Or do I really want to fill those cores? Do you suppose frost can get a hold of the sides of the walls and still heave the structure? I have 0 intuition on what would happen, other than that I know there will be some deep frost most years.


A lot going on here. Chugiak offers some good insights already.

The reason to run a built up beam the 24' length is because it is much less work to do it that way. You just have to dig/pour a couple piers and then nail together your beam. Just for this example, if you went with 2x6 joists ( actual height 5.5") you could set the top of the beam down 5.5" and simply run a 16' 2x6 the length of building (the bottom of the 2x6 would rest on the beam where they cross). This is faster than running joists the other way, because you will have to do piers for two separate beams and then you are also going to have more than one piece of lumber for your joist span. It's not a huge deal, but it is definitely faster and easier to do it the way I am saying.

As for leaving the CMUs hollow, this will vary somewhat with local conditions. Hollow CMUs have the compressive strength to easily hold virtually any 2 story house. The reason you would fill the CMUs is if you have rebar reinforcement in them. The purpose of the rebar is for shear (horizontal) strength. In a basement, you might have soil piled against the wall 5 or 6 feet higher than the floor - that weight is horizontal pressure that usually requires rebar, which requires filling those cores with rebar. Rebar also gives a building a better chance of surviving an earthquake.

If you are going to do a ridge beam, you'd likely need a post or two to support it along that 24' span. In that case, you'd want some piers centered under your foundation.

It may sound like this is a lot of stuff to think of, but think of the process as something iterative. Roughly design your foundation, first floor, second floor, roof. And see how one impacts the other. Go through this process dozens of times and eventually you will have a cohesive plan. Posting it here you can get some useful input, but keep in mind we are all amateurs as well. It is very wise to keep you building as simple as possible, and stay within the prescriptive code even if you don't have a code officer stopping by every once in awhile.

ChugiakTinkerer

Quote from: NathanS on October 02, 2016, 08:58:57 PM
... but keep in mind we are all amateurs as well.

Guilty as charged!  :)

If you have the property already, it might be worth your while to do a little light excavating to see what the soil conditions actually are.  I've had to relocate my cabin site because my original location had way more clay than I had assumed.  Discovered it when digging the outhouse pit.  The top foot or so was a nice mix of gravel and clayey sand, but underneath was clay down to at least four feet.  In checking out the new site I used an 8" earth auger, a mini excavator would have been much nicer!

I think the point I was trying to make is that a perimeter footing below the frost line should be able to hold the foundation wall in place, but that may require the rebar and concrete fill in the CMUs.  You might at this stage consider having the concrete and earth work hired out.  Talking to local builders can help you decide if that is work you want to do yourself.  But it would also expose you to what the builders are doing in the area and may help you zero in on a foundation design.

My cabin build thread: Alaskan remote 16x28 1.5 story

Strix

Quote from: ChugiakTinkerer on October 03, 2016, 12:06:28 PM

If you have the property already, it might be worth your while to do a little light excavating to see what the soil conditions actually are. 

I do have the property already.
I'm actually planning to do this soon before the ground freezes... There are two sites I've got in mind but one of them is in close proximity to the property electric and well water... Would make a lot of the DIY work simpler if I can access those! No big deal either way though. I'm anticipating some clay composition in either spot due to the local soil surveys of surrounding areas. Would be good to know for sure!


Strix

Did some excavating on one proposed site, which is where electric and water are already established on the property. Findings were not surprising but also not great.

Location is about 7 feet in elevation above a nearby creek, and about 5 feet down, there is sandy clay and water. It hasn't rained much in the area; fairly certain the water table is just that high at the moment. (would be higher in summer...

This poses a question: is it worth the headache of trying to get a basement in the ground if I'll be fighting water most of the year at this site? Would it make more sense to just do a perimeter foundation as discussed previously? Besides the more remote site I've spotted, where I would need to get separate electric installed, there is no higher ground here. The proximity to the creek really makes finding drier space difficult.

Has anyone on here built up the soil/gravel around their foundation so that they didn't build below the frost line, but instead raised the grade up higher? Seems like a pain in the ass...


NathanS

Without knowing the specifics of your site, a water table at 5 feet in October would scare me off of a basement. Water barriers can be very expensive, and if you don't have the grade for a daylight foundation drain you could have a sump pump running non-stop.. which could require a back up automatic generator etc.

Try researching frost protected shallow foundations. The engineering booklet has provisions for unheated structures. Not in the code book but easy enough to understand.

I have flashbacks to my own build thinking about high water tables. I could not keep up with bailing water and setting my foundation forms. Only thing I threw the towel in on so far. (I contracted out all excavation though, and have no regrets)

If you had a better building site I guess you could always dig yourself a new well in an afternoon.  ;D If you already have an electric meter you could probably trench the electric over to the new site yourself too? You'd have to add up all the costs and pros and cons I guess.

One corner of my footing was almost even with grade when I started. Then dirt was bermed up against it during backfill. If you have an on site source you would basically be paying an excavator for a day of labor.

ChugiakTinkerer

I explored the frost protected shallow foundation as well and what I did find suggested they wouldn't work in my application.  It looks to be a nice method for building slab on grade and requiring minimal earth disturbance.  The limitations I found were that it isn't for places with permafrost, annual mean temperature below 32 F, or for crawlspace foundations.  If none of those apply then you should look into it.  Here's one reference, I didn't look for more recent literature: https://www.huduser.gov/publications/pdf/fpsfguide.pdf

The house where I grew up in California had a poorly built addition that was a slab on grade.  I think they just walled in a concrete patio and made it a huge rec room.  Despite my dad's best efforts to manage surface runoff away from the house, every heavy rain we would get water seeping up and puddling on the floor.  I spent a lot of winter nights as a teen mopping up water with a towel and wringing it into a bucket.  I'm sure this built a lot of character in me, but to this day I wouldn't touch any kind of slab as close to a water table as you're describing.
My cabin build thread: Alaskan remote 16x28 1.5 story

Strix

Good to know, good to know...

I could forego the basement then and still be kosher at this site. I definitely don't need the room as it won't be a primary residence, and there will be less excavation to do if I don't need the interior of the foundation wall for space...

The other site I've found is ~20 feet up from the creek and the water table "should" be much much lower if basic hydaulic conductivity is the only constant considered between sites. I, of course, would need to dig to prove this. This site is ~1000 feet straight-line path from the electric meter on the property; it would make as much sense to put in a second pole on site as it would to trench to it. (Sadly)
I'm thinking maybe this is the better spot based on what I'm reading. I can simply dig a well and have electric put in anytime during the construction. (and haul water/use a generator if I need it before either of the two are done)

My property is in Minnesota, latitude of 46 deg... It gets quite cold. Neighbors have basements right next to this site too... Lots to think about here!

Only downside to this site is there's a restriction on my proximity to the creek which requires a building permit. No code requirements though... That may, be a pain in the ass too. Haven't considered it enough yet to know how much of a pain it will be.

low277

Hey Strix!  Another Minnesotan!   What part of the state are you in?   You are ahead of me.  I might be doing some foundation work in 2017.

I will be watching as I have lots of questions about foundations myself.
NW Minnesota


Strix

Low,

I'm in Pine Co, on the southern end of Nemadji. And as you may have read, the water table is right beneath my feet where I propose to work.

I've decided to demo the house (collapsed, broken, built forever ago) on the wet lower area I've mentioned and to wrassle with the water table around the area and build up the grade around the foundation/basement.
Soil is marginally well drained actually, but the damn water table is so high, it's impressive. Our well only goes down 10 feet and I cannot run it dry...
The main reason I even want/need a house here at all is because I'm developing timber stands on the property and it absolutely sucks to not have anywhere to stay except a tent. Call me picky, but I don't like tents much.

Will be experimenting with some trenching to control water and also get some sweet needed fill to build up around the foundation. Also, so the trees don't have wet feet all spring and rot... All the needed work piles up and really makes you realize just how little one person can get done on their own!

ChugiakTinkerer

Quote from: low277 on October 27, 2016, 08:52:35 AM
Hey Strix!  Another Minnesotan!   What part of the state are you in?   You are ahead of me.  I might be doing some foundation work in 2017.

I will be watching as I have lots of questions about foundations myself.

Welcome to the forum!
My cabin build thread: Alaskan remote 16x28 1.5 story