Block basement construction details?

Started by jbos333, March 21, 2013, 06:56:10 AM

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jbos333

Hi everyone,

The time has come for me to submit plans for a building permit. Per the town code enforcement officer, I do not need "stamped" plans for my build as it is under 1500 s.f. and it is conventional construction. So, I am going to draw my own plans....I already have a good idea of room layout, etc.

What I need help with right off the bat is foundation details. Not so much the 3 walls that will be fully/partially backfilled but the "walkout" wall foundation or "frost wall". Is the frost wall just poured in a width equivalent to the footers on the other 3 walls.....in my case I believe it would be 18" wide x 28' long x 48" high (the foundation dimensions are 28 x 40 with one 28' wall being the walkout).

If someone familiar with this could chime in and/or suggest a book I could buy, please let me know.

Thanks!

Redoverfarm

Your wall demensions will be 1/2-2/3 of the footing demensions.  I normally use a 18" footing and 8" wall .  The frost wall as you refer can be a 1/2 wall or full height wall with the rough door demensions blocked out(if pouring) or blocks laid to that doorway opening and then headed off with a lintel above the doorway opening.

http://www.google.com/search?q=lintel&hl=en&qscrl=1&rlz=1T4TSNA_enUS401US438&tbm=isch&tbo=u&source=univ&sa=X&ei=DgFLUbP0Dtam4AOh9oDQDg&sqi=2&ved=0CD4QsAQ&biw=1093&bih=434


jbos333

Thanks for the response, Redoverfarm.

Yes I was thinking of at least partially framing the "walkout" instead of a full block wall. My confusion is with the footer on the walkout side- is that done just the same as the other walls i.e. 18" footer 48" deep below finished grade, then block wall constructed to whatever height desired above grade?

I did get a couple quotes on poured walls/footers, and it almost seems like they were going to take an 18" wide excavator bucket, dig down 48" below finished grade then fill that entire void with concrete and rebar for the walkout side. Does that make any sense?


Redoverfarm

Yes the footing is the same as the other 3 walls.  You are distributing the same amount of weight from the house down to the footings on that wall.  Not really sure of your geographical location but 48" is extreme for this area.  Maybe the frost depth is greater where you are.  Normally the footings are poured and then a second pour for the walls.  Rebar reenforcement for both.  Poured walls are a little more watertight for a lack of another definition in comparison to block walls.  Also they are less prone to settling cracks that you often see in blocks

Squirl

48" down makes sense.

Filling it to the top with concrete and rebar is just a labor saving method.  Rather than pour a footing, and use masonry or forms to pour a wall, they just pour up to the top. 

So 1.5 x 4 x 28 / 27 = 6.22 yards of concrete.  Around me concrete costs around $100 a yard delivered.  About $650 in material for one wall, and it is done with very little labor.

  The rebar should be minimal and just along the bottom and a few pieces horizontally.  With an 18" thick wall vertical rebar is definitely not required by code.  If you have a 6" wall with even backfill rebar is not required by code.

Footing sizes are determined by ACI 318 or TABLE R403.1 of the IRC:
http://publicecodes.cyberregs.com/icod/irc/2009/icod_irc_2009_4_sec003.htm

With block just used for the foundation and light frame construction there is no necessity to go wider than table R403.1, but many masonry guides like to suggest it.

I did a little guide to reading the tables.

http://countryplans.com/smf/index.php?topic=12374.0


jbos333



Thanks for the info. The tables etc. will help for sure. I'm not 100% sure what the unbalanced fill will be but it looks like 8" block will work ok. I guess I will start drawing it up using that info. I will go over the drawing with the Amish guy who will be pouring the footers and laying the block. If he has any suggestions or improvements I will add them and then go over it with the code enforcement guy.

Should I bother with pilasters on the 3 backfilled walls? Or is it unnecessary? I'm trying to save money by going with CMU vs. poured walls (it will be almost 1/2 the cost) but I don't want it to fall down 10 yrs from now either.

Redoverfarm

#6
Just make sure that you have a well installed perimeter drain below the top of footing.  Don't skimp in this respect as it can be a major cause for failure.  They make some pretty good products (http://www.superseal.ca/dimpledmembrane.html) now that will shed the water away from the wall and down to the drain.

Don_P

I've been trying to model a local hill and used this as an exercise in putting a foundation in it, not perfect yet but it shows the idea.


This is how I've done walkouts and garages over a frost depth footing using a frost wall under the opening and steps as you go around and pick up more cover. I like to have return walls beside the opening as drawn to help butress the sidewalls.


SouthernTier

Jboss333:  Glad you are getting going (semi-) neighbor.  Will have to stop over and check out your progress someday if you don't mind.

Meanwhile, I am still in the design phase myself.  My last post on my design was on this exact topic:

http://countryplans.com/smf/index.php?topic=12801.msg168325#msg168325

Didn't get any comments back, so I guess nobody saw any major flaws in it.  My design looks similar to Don_P's study.

My concern in that post quoted above is also the unbalanced fill part.  Might have to get an engineer to sign off on that, not sure.

Also, if your are going prescriptive to avoid getting an engineer to stamp it, then the max step in the footer is 2', which can pose a challenge, so that's something to consider for the footer in the walk out portion.

My rear crawl space would require a >2' footer step, which is why I am considering doing that part of the foundation as a PWF.


jbos333

SouthernTier,

Thanks for the link to the other thread- I must've missed that before. I wish I had/ could use design software like that! The funny part is, I have a college degree in CAD- but I've been "out of the loop" for so long it's all new to me now! So I'm using the ol' drafting board and/or graph paper.

As usual, I'm running out of time to get my drawings done, permit approved, excavator booked, Amish mason booked, etc. Spring is already here, sure doesn't look like it though.....

I hope I will have made some progress this spring so you can stop by and check it out.