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Old 07-16-2013, 07:24 AM   #1
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Carport post footer


Hello all,
I am replacing three carport posts that previously had no footing beneath them. I am using 4x4s and the posts are about 7 feet apart. I have removed conrete from around the posts and am digginging new footers. The footers will be 24" in depth (I am in Northern Virginia). I'm not sure the load of the carport, but I have attached a picture of the carport.

Two questions:

Is an 8" sonotube form wide enough, or should I be using a 10" or 12"?
And should there be a bell shape at the bottom or is the straight tube enough?

Sorry for the beginner questions, but I've never dug a footer before.

Thanks alot!
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Old 07-16-2013, 07:51 AM   #2
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I'm not a concrete guy, but 8" is to small in my opinion---10 to 12 will give you a more solid footing---and belling the bottom of the hole will add even more to the strength---lets see what someone more experienced has to say----Mike----

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Old 07-16-2013, 11:24 AM   #3
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Thanks for the input Mike. I'm curious to heare what others have to say.
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Old 07-16-2013, 12:48 PM   #4
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TBick... From a NON-ENG GC in Colorada... Just my best personal opinion...

Technically, it's a soils engineering question... but assuming you are not on any excessive sandy (old river bead)or any strange expanding soils (bentonite).

1) From visual look, it appears you have a relatively nominal dead load and I assume not much if any live load (does it snow in WV).

2) I think you are structurally fine on a 8" sono.... however with 4x4's, I like a 10" as easier to land on, and I just think they look proportionally better/appropriate.... and cost is virtually nominal... plus you get some more footing.

3)24" depth...I assume is below any frost/freezing depth for your area (has to do with freezing upheavel). Assuming you are below frost depth, anything you can bell-out, is just additional footing. (I wouldn't bother going with any "big foot" form..... considering my above assumptions.)

4) I like to hang 3 vertical rebar in any sono pier for xtra streangth (shear)

Don't know your post base, but gently round/slope the top for water runoff.

Good going.... I personnally hate digging post holes.... I often dig a little hole the night before and watersoak the hole for easier digging.

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Old 07-16-2013, 01:30 PM   #5
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MTN Remodel - Thanks for the excellent advice. 10" tubes it is. We do get a bit of snow in Northern VA, but not too much. And thanks for the tip on the rebar.

Regarding the hole digging, I got a cheap electric demo jackhammer off Amazon that works quite well for digging holes. It has definitely saved me some time.
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Old 07-16-2013, 01:48 PM   #6
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tbick, I think you'd be OK if you just followed the Ffx Co deck footing guidelines:

http://www.fairfaxcounty.gov/dpwes/p...ks/details.pdf

How do you plan to support the carport roof while you're digging the footing holes? And how do you plan to bust up the concrete?
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Old 07-16-2013, 02:37 PM   #7
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Thanks for the reply huesmann. The guidelines you reference you are useful. They do recommend a rather substantial footing and pier, with the footing a minimum of 17" round and a 12" round pier (This is just for the smallest beam span and joint span, which I would exceed). Do you think that is overkill?

I have supported the carport with 4 4x4s (not pictured). I have removed the concrete around the posts in order to dig holes for the footing. Once I install the footing I will backfill around the footing and intall the post. I will then hire someone to come in and remove the rest of the concrete, so I can lay a new patio.
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Old 07-16-2013, 02:51 PM   #8
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I am a geotechnical engineer. The depth needs to be greater than the frost depth and depth enough to not be disturbed by erosion. As long as the soil is compacted, and not too wet, it should have a nominal capacity of at least 1500 pounds per square foot. A 10 inch diameter hole will almost give you this capacity. For uplift loads I would not case the hole. The rough surface between the ground and concrete provides more uplift capacity than a smooth surface of a form. If the 4x4's are going to be embeded below ground into the concrete I would drive some 20 penny nails into all four sides of the 4x4's to help the wood bond to the concrete.
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Old 07-16-2013, 03:17 PM   #9
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Hi SCengineer. By "uplift loads" do you mean attached to the top of the footing via a post base? That is what I'll be doing. So then you're saying that I should not use a form, rather just fill a 10" round hole with concrete?
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Old 07-16-2013, 03:44 PM   #10
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Uplift loads would be wind or some thing trying to pull the foundation out of the ground. With this type of structure these are higher than the downward loads. I would pour the concrete against the soil. The only time forms need to be used is when the soil will not stand up by itself. If you need to you can form the above grade portion.
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Old 07-16-2013, 04:23 PM   #11
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Quote:
Originally Posted by tbick View Post
Hi SCengineer. By "uplift loads" do you mean attached to the top of the footing via a post base? That is what I'll be doing. So then you're saying that I should not use a form, rather just fill a 10" round hole with concrete?
And a little dig-out (bell-out) will significantly help with any wind uplift.

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