Concrete post and beam - footing/post all in one pour?

Started by SardonicSmile, August 31, 2009, 01:07:13 PM

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SardonicSmile

So, my question is.. wouldnt it be easier to make my posts the same size as my footers and go straight up? Usually the footer is poured, and then the (smaller) posts are poured on top and tied together with rebar. It would be easier because I can just use one large tube and put it straight down into the footer holes.

I know it will take more concrete, but It just seems like a good idea. Can this be done?

Ernest T. Bass

Seems to me the only disadvantage would be cost... Depending on the size footer you need and your frost depth, you might need three times as much concrete. If you need a dozen piers or more, that adds up really fast.

You might also have a little less uplift resistance depending on a bunch of factors, but I'm not sure if that's really worth mentioning.. Just a thought.

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Redoverfarm

SardonicSmile The main purpose of a footing is to spread the weight load out from the pier.  Without a larger footing that weight would not be distributed and may cause a failure in the pier settling.  Most footings are at least 1/3 or 1/2 larger than the piers. Think of the terminology as your foot and leg.  If your leg didn't have a larger platform (foot) to support it chances are that you would sink in the ground more often. ;D

Squirl

I can't think of any reason other than cost.  I am not an engineer though.  I guess it depends on how deep you are.  If you are only going 1 ft. the extra concrete maybe small in the large scheme of things.  Usually you are looking at triple the concrete.  You may need more rebar to, I am not an expert.  Say you have 1ft to pour above the footer and you have an 8" post and a 16" footer.  The post would normally take .34 (pi*r^2*height, 3.14*(4/12)^2*1) cu/ft concrete.  If you made it 16" it would take 1.4 cubic ft. (pi*r^2*height, 3.14*(4/12)^2*1) or 4 times the concrete.   I programmed it into a spreadsheet so I could play with it. Now if you do this for 4 ft posts for 12 posts this can add up. At $3.45 per 80/lb bag of premix it would cost $96.29 for 8" posts and $385 for 16".  Still small in comparison to the total cost of the house, but adds up quickly.  If you have bad soil and have to go with 24" footings and 12" posts you are looking at $200 vs. $800.

poppy

Sardonic  w* to the forum.

When you ask questions like this we need to know the frost depth and soil type.

But to try to answer the question anyway; It's pretty easy to do it in one pour by either using something like "Big Foot" on the bottom of a Sonotube; or just waddling out the bottom of the hole if the soil is stable enough.

If the hole is pretty deep I would use re-bar in any case.


Don_P

This is a javascript pier volume calc I made awhile back. I took to writing in javascript rather than spreadsheets because it was easier to share formulas online with them;
http://windyhilllogworks.com/Calcs/piercalc.htm
(that one is pretty simple inside if you want to look at the guts Squirl. When its open right click and hit "view source", course if you know how to program these infernal machines you'll probably be ROFLYAO  :D)

The footing doesn't have to be wider than the column. Don't forget the soil on the footing has weight. As long as the footprint is large enough to bear the load in the soil type it can be full size all the way up. Most folks don't realize, by the codebook, in 3500 psi soil you can support a 2 story house on an 8" wide footing, in 3,00psi soil you can support a single story on an 8" wide footing, in other words just pouring an 8" wall right on the dirt.

JRR

Don_P is right about the capabilities of firm soil.  Here's another link that supports the thinking:
http://www.concretenetwork.com/concrete/footing_fundamentals/footing_dimensions.htm
.
Note, that soil capacities are usually expressed in "psf" instead of "psi" ... it gets confusing!

kyounge1956

Quote from: poppy on August 31, 2009, 06:34:55 PM
Sardonic  w* to the forum.

When you ask questions like this we need to know the frost depth and soil type.

But to try to answer the question anyway; It's pretty easy to do it in one pour by either using something like "Big Foot" on the bottom of a Sonotube; or just waddling out the bottom of the hole if the soil is stable enough.

If the hole is pretty deep I would use re-bar in any case.

I kept having a mental picture of a smaller sonotube standing up in a wider box or simply in a larger diameter sonotube, then both filled with concrete at the same pour, but Poppy's idea is simpler & I think probably better (no layer of sonotube between the wide and narrow parts of the pour as you would have had if you had followed my mental meanderings).

Don_P

QuoteNote, that soil capacities are usually expressed in "psf" instead of "psi" ... it gets confusing!
Thanks for the correction JRR, I had soil as hard as... concrete d*


Beavers

Sardonic,  w*

Just a couple of non-expert thoughts from me.  ;D

I think the reason most of us choose the Post and Beam foundation is because of cost and ease of building.  I'm building a 12x16 house to live in while I work on my larger house, and I used treated 4x6's for the piers.  If you go that route it is cheaper than most other foundations.   But, if you are going the concrete pier route, you might want to run the numbers on using a full concrete block crawlspace.  

I started the foundation for my larger house this spring using concrete piers, and by the time I add in all the costs of the things like... bracing the piers with angle iron, the pier to beam connectors (that aren't cheap if you get the good ones), building a plumbing enclosure to keep the pipes from freezing, insulation of the floor (I want to use the sprayed in foam, got a bid of over $2000), skirting, and the cost of the beam to sit on top of the piers, the concrete block crawlspace is at least even, if not ahead of the post and pier foundation in terms of cost.  

I'm not saying don't use the pier and beam, just that you might want to price out the other options first.  Of course there are a lot of variables that could make one or the other foundation type a no brainer.

Ernest T. Bass

As far as getting a pier w/ footing in one pour, I just wanted to mention a system that worked real well for us. We propped up the beams (with pier brackets already attached) over the holes on blocks of wood and got them nice and square/level, and then hung the sonotubes and rebar with wire from them. We simply measured from the bottom of the beam to the bottom of each hole and subtracted about 6''-8'' for the sonotube lengths. As long as your concrete isn't too soupy, you can fill the tube up without losing it all through the bottom, once the footer is filled up.

Hope I explained that well... Brain's in a fog right now.

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MountainDon

Quote from: Ernest T. Bass on August 31, 2009, 11:24:39 PM
As long as your concrete isn't too soupy, you can fill the tube up without losing it all through the bottom, once the footer is filled up.


Your concrete should never be soupy. Soupy means too much water, too much water = poor strength.
Just because something has been done and has not failed, doesn't mean it is good design.

Squirl

Also, I don't know if  you have to deal with code inpectors.  My area requires inspection of the footing before the post.  I'm not saying it can't be done without it just check with the inspector first.