Foundation Question

Started by retiredmarine, August 13, 2018, 12:04:15 PM

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retiredmarine

building a 20'X40' small house, one story wood structure. I've been convinced to use a footing and stem wall vice piers.  My plan is to do a formless pour down to 24", frost level, then come up to my house height with block.  I'm pouring because it's just quicker than building forms and laying more block.  Questions come in rebar placement - the footing will be 24" deep and 12" wide for 8" block.  Thoughts?  Thanks Chris

ChugiakTinkerer

It all depends on what your local code is.  It's likely a version of the International Residential Code.

The IRC requires footings extend at least 12" below the frost line.  If your frost depth is 24" then you want the bottom of your footing trench at least 36" down.

The width of the footing depends on the compressive strength of your soil.  If your soil has some sand or gravel to it you can probably get by with a 12" wide footing, but if in clay or loamy soil you want 16".

Rebar may be required in the footing and in the concrete block wall.  The vertical rebar will need to be placed in the footing before the pour.

Forms are very useful for putting the rebar precisely where it needs to be, and also as a reference for ensuring a level pour.  Without forms you'll want to find a way to ensure your footing is level across the entire perimeter.
My cabin build thread: Alaskan remote 16x28 1.5 story


Don_P

No, a footing simply needs to be below frost depth. If the maximum frost depth is 24" that is your minimum footing depth. A footing is never shallower than 12" though, I think that is where you mixed those two together.

Rebar in the bottom third of footing depth, reinforcing steel at least 3" from soil always.

For grade pins I just drive rebar into the trench bottom every 4-6' and shoot the tops of those pins tapping them down to the top of footing level, I can then run a straitedge across the concrete from pin to pin.

retiredmarine

Don,

Are you worried about the grading pins introducing water into the concrete?  That's what stopped me from just driving the vertical block supports into the ground before the pour.

Chris

Dave Sparks

If the rebar rusts before its designed life, the result can be not acceptable.  Rebar simply can't come in contact with the soil and must be contained in the cement.
The pins won't be a problem as they are mostly not in contact with the bulk of the rebar.
"we go where the power lines don't"


ChugiakTinkerer

Quote from: Don_P on August 13, 2018, 08:48:25 PM
No, a footing simply needs to be below frost depth. If the maximum frost depth is 24" that is your minimum footing depth. A footing is never shallower than 12" though, I think that is where you mixed those two together.

Rebar in the bottom third of footing depth, reinforcing steel at least 3" from soil always.

For grade pins I just drive rebar into the trench bottom every 4-6' and shoot the tops of those pins tapping them down to the top of footing level, I can then run a straitedge across the concrete from pin to pin.

I should have said I was working from memory.   d*

The darn ICCC web site is locking down free access to the IRC again.  Not cool.  So I couldn't get online and confirm my recollection.  ???
My cabin build thread: Alaskan remote 16x28 1.5 story

MountainDon

Quote from: ChugiakTinkerer on August 14, 2018, 12:01:02 PM
I should have said I was working from memory.   d*

The darn ICCC web site is locking down free access to the IRC again.  Not cool.  So I couldn't get online and confirm my recollection.  ???

What url are you using?   I have access to2015 through this link...   (pacific northwest national lab)
https://basc.pnnl.gov/resources/2015-irc-international-residential-code-one-and-two-family-dwellings

I only looked at chapter 3 & 4 and they opened, let me scroll down the page.
Just because something has been done and has not failed, doesn't mean it is good design.

MountainDon

Just checked and found that from this page, clicking on the state on the map,
took me to where I could access the IRC pages that were applicable.   Maybe they have reworked their site and your older link is now invalid for free access?


Just because something has been done and has not failed, doesn't mean it is good design.

Don_P

Quote from: retiredmarine on August 14, 2018, 05:28:14 AM
Don,

Are you worried about the grading pins introducing water into the concrete?  That's what stopped me from just driving the vertical block supports into the ground before the pour.

Chris

Your reinforcing steel needs to have 3" or more of concrete cover to keep oxygen out of the rust equation, the concrete is always "wet". A grade pin is not reinforcing anything... that said rust is no different than ice, it can spall concrete. I haven't seen it cause a problem but that's anecdotal. I have seen wooden grade pins in footings create a pathway for termites into the inside of block and then up into the house. So rebar became the grade pin of choice, for a typical 8" thick footing I usually use 12" pole barn nails if the soil is firm. If this is of concern all the pin top is giving you is a reference for the top of the concrete. That can be the top edges of the form, grade pins or any other way you can get a quick reference as you pour.


ChugiakTinkerer

Quote from: MountainDon on August 14, 2018, 01:47:19 PM
Just checked and found that from this page, clicking on the state on the map,
took me to where I could access the IRC pages that were applicable.   Maybe they have reworked their site and your older link is now invalid for free access?

I was going through the main public page that you linked here.  I use the I-Codes link at the bottom to see the full list of current codes.

On Monday I was getting a NO Access message, no matter which way I tried.  It's working now, thanks for confirming that.
My cabin build thread: Alaskan remote 16x28 1.5 story

JRR

Are you saying you plan on the footing being 24" thick?   I would think anything over 8" thick would be "much"!  My municipality just requires the footing width to extend 8" (min) on each side of footing wall ...or, 24" minimum; as the footing wall width is 8" minimum.   I don't think my local code yet even mentions re-bar in footings, but I like a lot of steel.  I think galvanized is worth the extra cost.  I like the re-bar to be wire-tied very rigidly and supported off earth with pavers or stones.

I like to use sacrificial forms.   Strips of corrugated metal roofing about 6" wide (tall).   Wired to re-bar pins, with top edge at desired elevation.  Extra work, and my design may not be Kosher  ... but so far no one has complained, and sure makes pour day a bit more easy.


retiredmarine

Quote from: JRR on August 15, 2018, 11:29:03 AM
Are you saying you plan on the footing being 24" thick?   I would think anything over 8" thick would be "much"!  My municipality just requires the footing width to extend 8" (min) on each side of footing wall ...or, 24" minimum; as the footing wall width is 8" minimum.   I don't think my local code yet even mentions re-bar in footings, but I like a lot of steel.  I think galvanized is worth the extra cost.  I like the re-bar to be wire-tied very rigidly and supported off earth with pavers or stones.

I like to use sacrificial forms.   Strips of corrugated metal roofing about 6" wide (tall).   Wired to re-bar pins, with top edge at desired elevation.  Extra work, and my design may not be Kosher  ... but so far no one has complained, and sure makes pour day a bit more easy.

JRR,  Yes Instead of laying 5 rows of block and dealing with forms I was planning on going formless and pouring the first 24" to grade. Then going up the remaining 24" above grade with block.

JRR

OK, I can see that, especially on a near-level plot.  Probably cost effective.

Don_P

I've done that on a couple of jobs where we were doing rubblestone walls. top level wasn't even critical just pour to a few inches below grade and start up with the rock. Definitely a no brainer there.

I use the junky boards from the sawmill for one shot form lumber, it's just sort of taking a detour on the way to the bonfire pile  :D


retiredmarine

If the ICF's weren't so expensive I'd excavate 16" wide and use them to form the whole 24", letting the trench walls support the first 24".  But with their cost, added rebar I think the block would be cheaper.


retiredmarine

Just got a quote for the foundation.



The quote included:

Excavation to 24"
Back fill
16" footer
Poured concrete wall to 24" above final grade
The center wall poured to a heights to match the beams
2 steel frame, steel access doors
2 metal vents
sill bolts on 6' centers
Beam pockets for center beam and the short 17' opening[/li][/list]

The area is Southern Missouri and the cost is $7020.00

Thoughts?


Don_P

Time or money, it certainly isn't unreasonable.
You shouldn't need a midwall unless I'm missing something, that can be spot piers supporting a girder inside the perimeter foundation. Unless there is something else going on needing it.

Anchor bolts every 6' AND within 1' of sill board ends, plot your plates and add those board end bolts as well.

CabinNick

Your quote sounds reasonable.  I am getting ready to contract a similar foundation - slightly smaller perimeter by a handful of feet and wall will be 8" shorter than yours.  I had one quote for $10,000 and another for $18,000 for excavation, foundation and backfill.  My build site is remote, is pushing the limit of distance and road access for cement trucks and contractors are hard to come by in my area. 

NathanS

Sounds like a very good price to me. If you rented the forms and did the work yourself you might not come out ahead much, especially after paying an excavator to come out twice.

Consider putting foamboard on the inside of the stem wall and heavy duty poly over the ground - sealed to the stem walls - and no venting. That is the currently recommended best building practice. You then heat and cool (condition) the crawl space with the rest of the house. I don't know all the details for a southern build - I am guessing termites are always a consideration.

It's so much cheaper to build in the south, less insulation, less footing depth, no snow load... better winters  ;D


Good luck, looking forward to seeing the progress.

retiredmarine



retiredmarine

One more.  If I leave the midwall I'm thinking I'll just make sure the girder (3 - 2X12's) are PT so I can sit them on the midwall.  But if I did that the girder would not be attached to the midwall just sitting on it and in the girder pockets in the walls.  thoughts?

Don_P

yes, it would be attached to the top of the wall... but if you have a wall why are you putting a girder on it  ???

retiredmarine

 d*  Dear Lord!!!  Originally there were piers there so I was always going to put a girder there but when the concrete guy said he could pour a mid wall almost as cheap with the excavation and all.  But your right no need for that girder now.   d*

Don_P

As the fellow said when the Lull rolled over
"You'll have some of that on these big jobs"
Our motto in times of trial ever since   ;D

I would punch one or two access holes in that wall so there will be short sections of raised girder over those voids with joists hangered in on each side.

If at all possible I like to put a thin ~2" thick, bull floated, rat slab in the crawlspace. There is no load to speak of, it makes for a drier and easier to work on space.

retiredmarine