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davidshammer
11-12-2007, 09:29 PM
I have been installing decks for years in Atlanta, and another contractor and I were discussing footings for a deck. I said to him that I install re-bar in mine and he looked at me like I was crazy!

Does anyone else use re-bar? Have I been doing over kill all these years?

smtitmas
11-12-2007, 09:44 PM
the short answers are yes and no

S. Donato
11-12-2007, 09:58 PM
i use it either that or i make a smaller grid work out of concrete mesh (the wide chicken wire one) and stick that in the tube.

is there a better way??? what is overkill???

Jordan
11-12-2007, 10:20 PM
Footings less than 3:1 (width to thickness) don't require rebar for structural capacity under "normal" conditions. What the rebar will do is help keep the footing in one piece if things don't go just right (too much water, weak spot in the soil, un-level base, too fast a cure). In larger footings, it matters because the footing has bending forces in it; in a small footing, there's really no bending, just a shear cone and direct bearing.

There's nothing wrong with leaving out the rebar in a small, simple spread footing, but the footing is less likely to fail if something goes wrong with the pour.

The same is true for strip footings, by the way. A 1' thick plain concrete footing will fail if you get a 2-3 foot soft spot; put a couple #5s or three #4s in it and it will probably span a 6-8 foot soft spot. How confident are you in the soil on your site?

davenorthup
11-12-2007, 11:48 PM
All mine get 4 sticks of 1/2" bent at the bottom for the base. I usually use 10" sonotubes with base. We go down 5 feet minimum for frost and have seismic and (usually) poor soil issues to deal with.

Lavrans
11-12-2007, 11:54 PM
Yep, I stick rebar in my footings too. Even if they don't need it, there's always extra chunks of steel lying around and I'm too lazy to sell them...

David Meiland
11-13-2007, 12:29 AM
I put about 15 plain concrete piers under my house. They're 24" dia. x 12" thick and my engineer gave the same explanation as Jordan. There just isn't that much weight on them...

The place I put a lot of rebar is in slabs.

BeachBoy
11-13-2007, 04:55 AM
Think of it this way: What you don't want is for the footing to fail by cracking in a circle right around the post. The weight of the structure would then push the post and the 'concrete plug' down through the rest of the footing resulting in uneven settling. The rebar makes sure that can't happen.

davidshammer
11-13-2007, 07:04 AM
Hey gang, thanks again. This has reinforced my decision to continue to use re-bar in my 1' footing. I install a seat with four horizontal,two up and two at the base with a "seat" on the floor and four verticals tied together.

"normal" footing depths are 24" unless we don't hit stable soil, then we go deeper. In Atlanta, it's normally 15" and you hit hard, compact clay.

Tom Anderson
11-13-2007, 10:08 AM
Think of it this way: What you don't want is for the footing to fail by cracking in a circle right around the post. The weight of the structure would then push the post and the 'concrete plug' down through the rest of the footing resulting in uneven settling. The rebar makes sure that can't happen.

Bingo! That's why I do it, anyway.

George Roberts
11-13-2007, 07:28 PM
And here I am thinking that the concrete is stronger than the soil it is resting on ...

The soil is going to fail not the concrete.

smtitmas
11-13-2007, 09:02 PM
Think of it this way: What you don't want is for the footing to fail by cracking in a circle right around the post. The weight of the structure would then push the post and the 'concrete plug' down through the rest of the footing resulting in uneven settling. The rebar makes sure that can't happen.

i agree about using rebar in footings but are you saying that a 10" diameter x 36" deep footing will crap out a plug of concrete from the weight of the deck? that would never happen. perhaps a footing would separate from the slab and settle below grade but that's not what the OP was asking about. he wanted to know about deck footings.

clydewater
11-14-2007, 07:29 AM
Just out of curiosity, what size sonostube piers does most everyone use for decks?

Do you pour a separate footing under the sonotube or just use a 10, 12, 14" etc. sonotube?

How does the soil type impact your decison on the piers?

I am asking since I see around here in NJ, there are many decks being built with sonotube piers only (i.e. no separate footing of a pier if large larger diameter) that are marginal at best in supporting the design loads on our soil types.
Thanks
Dick
PS Yes I use rebar too since that was the original question

davenorthup
11-14-2007, 11:35 AM
Just out of curiosity, what size sonostube piers does most everyone use for decks?

Do you pour a separate footing under the sonotube or just use a 10, 12, 14" etc. sonotube?



We generally use 10-12" and add a base similar to Bigfoots bases - all in one pour.

Timbersmith
11-14-2007, 02:56 PM
Every single deck we've done requires steel reinf. in the footing. Probably because of our snow load.

George Roberts
11-14-2007, 03:56 PM
clydewater ---

You need to do engineering to determine the size of the footings. 2000#/sqft is the allowed load on soil around here - about 40sqft of deck per sqft of soil bearing.

Jordan
11-14-2007, 04:26 PM
Think of it this way: What you don't want is for the footing to fail by cracking in a circle right around the post. The weight of the structure would then push the post and the 'concrete plug' down through the rest of the footing resulting in uneven settling. The rebar makes sure that can't happen.

Sorry, thanks for playing. Rebar placed in your footing generally does not increase the punching shear capacity of the footing. In fact, that's the reason I mentioned 3:1 limits in my post. The shear cone is nominally 45 degress, though some will go as wide as 60 degrees. I usually presume you have a bearing element about the thickness of your footing (6x6 is a little small for a, 8" thk x 2' footing, but it's close), so you have nearly a direct-bearing connection to the soil. Technically, the punching shear area is determined by taking the outline of your post and offsetting it by 1/2 the footing depth. Punching shear will almost never control your footings, bearing capacity (for small footings) or bending capacity (for large ones, where you need steel) will.

worthy
11-14-2007, 04:44 PM
For decks on new homes, I've been using 10" sonotubes, no footing, no rebar, usually in clay, at least 4' below grade. I've seen some I did 20 years ago--the p.t. is in bad shape, the piers seem solid. But I guess now I'll be upgrading!

clydewater
11-15-2007, 08:47 AM
Hi,
Thanks for the comments on how you do piers.
So here is an example based on some of the info that has been submitted.
10" Sono tube = approx .545 SQ FT of area
@ 2000 PSF = 1091 bearing capacity
If as above 40 Sq Ft of Deck w/ 40# LL + 10# DL = 50# TL = 2000lbs to be carried by 10" Sono Tube, if there is no footing, such as a big foot, then the pier is half the area required to carry the design loads.
Of course if your bearing capacity is higher, then this is not an issue.
I created a quick spreadsheet I will post if anyone wants it to do a quick sizing of round piers.
Thanks for your info.
Dick Owen

Sweep8
11-23-2007, 11:07 PM
Do most builders just guess at this?

Lavrans
11-24-2007, 01:18 AM
Do most builders just guess at this?

Yes.........

David Meiland
11-24-2007, 08:29 AM
I wouldn't call it guessing, but in this jurisdiction you are allowed to assume 1500PSF min. bearing for any soil and required to assume certain loads for a deck, therefore you can size piers yourself if you want to.

Jordan, with a 6x6 post and an 8" thick x 24" dia. pier, what would the required load be to punch out a cone thru the footing? I've certainly never seen that, and would imagine that a simple crack across the footing would be a helluva lot more likely.

Allan Edwards
11-24-2007, 09:04 AM
I do piers on all of my homes, they are of course engineered and based on geotech soils report. Obviously this would be overkill and not required on most decks. The piers have bell shaped feet (bellbottoms) that spread out to 36”-48” and bear on soil that has been determined (by soils testing) to carry the anticipated load.

My point is that a pier or sonotube footing is only as good as the soil it bears on. Which may be OK in most cases, but I would not assume anything. Another thing I’ve learned over the years is that you need a large circumference area of the pier to get any benefit in terms of bearing strength from skin friction of a round pier or sonotube footing. We do 12” piers and the engineers tell us the skin friction is negligible, that you need 48” pier to get any benefit. Again, I’m talking houses and not decks, but perhaps something to think about.

As to rebar, my understanding that in general rebar in a beam simply holds the concrete together when it does crack.

Phil H
11-25-2007, 12:31 PM
David,
I returned to school for engineering and recently studied reinforced concrete design, so take my statements with caution (no practical experience). The equations for punching shear are difficult to write in this forumn because of the sub-scripts and greek letters. A quick search using Google and "ACI shear 11-35" yields some good explanations and a preview of the ACI 318-05 code http://books.google.com/books?id=oa8XnnvX6BgC&pg=PA182&lpg=PA182&dq=aci+shear+%2211+34%22&source=web&ots=321NsNAYjT&sig=MmTHe40khRBdXUFxhF03y7zPS-Y

Using ACI equation 11-35 and 2500 psi concrete, I calculated 57,600lbs before applying a strength reduction factor (phi) of 0.75 for shear. With the stength reduction factor, it would be 43,200 lbs. Thus, it is extremely unlikely for your post to punch through the footing. It would fail in bending or one-way shear (break a side off). Here are the values I used for punching shear strength.

d - effective depth - since there is no reinforcing and the footing is cast against soil, the code requires reducing the thickness by 2 "[ACI 22.4.8]. d = 8" - 2" = 6" (ironically, this is the minimum thickness for an unreinforced footing per ACI 22.7.4)

bo - critical perimeter - the edges of the post are offset d/2 to find the perimeter of the section that is assumed to fail in punching. Using a = 6" (the sides of the post) bo = 4 ( a + d/2 + d/2) = 4 (6 + 6/2 + 6/2) = 48"

fc' - 28 day compression strength of concrete. I used 2500 psi concrete for the calculation. You may be using higher strength concrete.

Vc = 4 x sqrt(2500) x 48 x 6 = 57,600lbs

David Meiland
11-25-2007, 02:29 PM
Thank you Phil. It sounds like I would need to almost my entire house on a single steel 6 x 6 column before we would see punch-thru. I figure it weighs about 25 tons.

S.Joisey
11-25-2007, 06:14 PM
Hi,

I created a quick spreadsheet I will post if anyone wants it to do a quick sizing of round piers.
Thanks for your info.
Dick Owen

I'd be interested in your spreadsheet, Dick,

Scott

nonameisgood
11-26-2007, 09:44 PM
Pier design doesn't rely on the same bearing values as footings, so the APPROXIMATE value of 2000 psf probably wouldn't apply.

Reasons for rebar in piers, particularly in hurricane or seismic regions is uplift and horizontal shear. Rebar helps keep piles from breaking and allowing the deck to lift or shift sideways.

The biggest problem with putting steel in concrete is that it is generally rusting of the steel that makes the concrete fail with time. Only put rebar in the footing or pier if you are going to consolidate the concrete well, and if you provide adequate concrete cover around all sides of the steel, including the bottom (3 inches clear cover when buried.)