Concrete post footing and general post guidelines.

newbie_07May 12, 2008

I thought it would be wise to post my plans for my deck posts to get any suggestions. Please let me known if anything I'm doing is overkill or insufficient. I live in Central Texas.

I plan to dig a 2' 2" down. The 2" is for gravel. The 2' is for concrete.

I wanted to use the forms from Quikrete (http://www.quikrete.com/ProductLines/QuikTubeBuildingForm.asp) but the VA code doc says my footings should be 18" in diameter but the Quikrete forms come in 12" diameter at most. What should the diameter of my footings be?

I plan to let the forms come out above the ground 2".

Quikrete recommends that I use

QUIKRETE 5000 Concrete Mix (http://www.quikrete.com/ProductLines/Quikrete5000ConcreteMix.asp). How many bags will I need per footing?

Once the concrete cures I plan to drill holes for wedge anchors. I'll use 1/2" x 3" wedge anchors. Then I'll attach Simpson ABA44 post bases (http://www.strongtie.com/products/connectors/AB-ABA-ABE-ABU.asp).

I'll use pressure treated 4x4s for my posts. They will be about 22" high. I read somewhere that this may be short enough to cause the posts to crack... I don't know that I have a choice.

I'll attach the posts to the beams using Simpson BSC2-2/4Z (

http://www.strongtie.com/products/connectors/BC.asp)

Again, any comments welcome.

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newbie_07

I found http://www.quikrete.com/PDFs/SPEC_DATA-QUIKRETE5000.pdf which says that 1 80 lb bag of concrete yields 0.6 cubic feet of concrete. If I used holes that were 1 ft accross and 2 ft deep that would leave me needing about 2.6 80 lb bags per hole.

That can't be right.

    Bookmark   May 12, 2008 at 11:31PM
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aidan_m

42 80 lb bags make one cubic yard, or 27 cubic feet. That is 0.64 per bag. Concrete weighs 125 lb per cubic ft. You already know how to calculate volume. I think your numbers are right.

When making concrete footings, the concrete needs to bond with the earth. The best way to do this is have the wet concrete fill in the entire hole and then vibrate it. If you use sonotubes, lift them up so the bottom is only a few inches from the top of the hole. Dig the hole so it is widest at the bottom. I know central TX has hard limestone bedrock. You can stop digging when you get to bedrock. That's deep enough for a deck foundation. It ain't goin nowhere. No need to use gravel in the bottom of holes.

Set J bolts in the wet concrete if you are confident enough to get them in straight lines. The wedge anchors need really hard concrete to work. You can use epoxy and threaded rod in green concrete, but not wedge anchors. The simpson ABU44 are great for the post bases. I always use them. Don't use simpson joist hangers in your project, just bear all members on top of one another. It is much stronger.

    Bookmark   May 13, 2008 at 12:40AM
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john_hyatt

Hilti makes a two part silca/cement to set threaded rod Man is that stuff strong,fast set up too. J.

    Bookmark   May 13, 2008 at 8:01AM
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riles_j

I just got done doing a whole lot of piers like you are describing, I think I had about 25. Mine were 18" diameter and 2 foot deep as well. I'm no expert, but here are few things I came across:

- I'm not sure how many piers you have but with 25 I knew I wasn't going to be hand mixing. I also had a long haul since I couldn't let the concrete truck on my driveway. In the end I ended up getting a pump truck to pump the concrete to my piers. That cost me some extra $$$ but worth every penny in my situation. If you only have a few piers then you can hand mix, but that is a pain. You might want to look into a concrete mixer.

- I bought 18" Sonotube and cut it into 6" slices with the thought being that I sink 4 inches into the hole and let 2 inches stick up. Trouble is when I augered my holes the tubes didn't fit in nice and tight and I had to drill 2x2 on each side to hold it in place. This worked OK, but in hindsight I would have just run the tube all the way to the bottom of the hole to make it easier to level and you use less concrete too.

- I used a couple of the Quikrete tubes and they are not nearly has heavy duty as the Sonotube. Mine saw a good rain before I could get concrete poured and the Sonotube held up great. The Quikrete ones did OK, but they are a bit flimsier.

- I sunk J-Bolts in the wet concrete and that worked out great for me. Then I came back with Post bases.

- I used 6x6 posts. The bases are more expensive, but that did allow me a little play in my construction in case I didn't get all the piers and J-Bolts in a perfect line. They are a little trickier to cut though. In the end I was glad I used the 6x6.

riles

    Bookmark   May 14, 2008 at 11:07PM
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aidan_m

Riles,
I commend your knowledge gained from this single experience. If I need enough concrete for a truck, I always get a pumper. You did a good job on your footings. The 2x2 is absolutely necessary to hold the forms in place.

Your idea about running the sonotubes to the bottom of the hole may seem logical, but it's not good. The concrete needs to bond with the earth. I see lots of DIY set the sonotubes all the way down the hole. This just makes a heavy anchor that will want to continue to sink into the earth. The only thing that will save this footing is if there's stable bedrock underneath. Lift the sonotube up 12" from the bottom of the hole so the concrete can completely fill the sides of the hole and not just rest on the bottom. It bonds to the ground and can withstand a side loading and uplift, as well as compression.

    Bookmark   May 15, 2008 at 12:50AM
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newbie_07

riles_j,

your posts were only 4 inches below ground? that doesn't seem very deep at all (unless your posts were less than 1 ft above ground).

The VA doc says go down 2 ft and I emailed quicrete and they said my depth should be 1/2 my above ground length (for me, that would be about 1 ft).

Anyone have any suggestions about depth?

    Bookmark   May 19, 2008 at 3:40PM
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riles_j

newbie,

Sorry for the confusion, but my post were 2' deep and they stuck up out the ground 4-inches.

Depth of footers is dictated by frost depth in your area not by the above ground length. The frost depth in my area is 18-inches. Quikrete should probably stick to concrete and stay away from engineering advice. Your local codes should dictate the depth of your footers.

riles

    Bookmark   May 20, 2008 at 10:02AM
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dooer

12" diameter footings should be fine, unless your local code says otherwise.

Rather then try to anchor a bolt in the concrete, after the fact, why not use a column base like Simp. CBSQ. Set some string lines and place them during the pour. If done right, this will be stronger then the other methods. Just don't let your concrete set up before you place the brackets.

Mark

    Bookmark   May 20, 2008 at 2:38PM
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slippery_biscuit

Just a note to agree with dooer on the use of Simpson post ties. String a line and set the support into the concrete. Post in place with a couple of lag bolts (or thru-bolt).

Not sure how you folks are using the j-bolts for post setting. Attaching L-brkts, maybe? Or a U-shaped brkt. The Simpson will do that with one less step.

    Bookmark   May 23, 2008 at 11:28PM
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newbie_07

I'm a bit confused about using concrete forms.

Seems that folks do not put the tube all the way to the bottom of the hole. Instead, they only use the form on the top 6-12" inches of the hole so that the concrete below that bonds with the earth.

However, the Quikrete directions (http://www.quikrete.com/PDFs/Projects/QuiktubePillarFoundations.pdf) and the Black and Decker deck book bot indicate that the form should be used for the full 2 ft of the hole.

Which is correct?

Thanks.

    Bookmark   May 24, 2008 at 3:33PM
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aidan_m

"Quikrete should probably stick to concrete and stay away from engineering advice."

Said Riles.

I work with soil and structural engineers everyday. Keep the sonotube up 12" from the bottom of the hole and you'll get a good bond. You can use 8" sonotubes to save concrete. Put 2 pieces of 1/2" rebar in center each pier. Keep the rebar 2" away from the sides of the sonotube.

An 18" footing means the bottom has to be 18" wide. It should be shaped like a bell, or an upside-down mushroom, so the soil around the concrete is preventing lateral loading and uplift.

    Bookmark   May 24, 2008 at 8:57PM
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diynj

I would have guessed that the form should extend below the frost line at a minimum.

My thought being that if the footing were to have an irregular shape it would provide a better hold for frost heaving.

    Bookmark   May 27, 2008 at 8:16PM
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