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pier and footing question

 
pollinator
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I wish I was building a timber frame but I definitely don't have it in me. Sorry to ask this question here but it seems the most appropriate place. I need to pour four piers for my 8-ft by 20-ft shipping container tiny house. I'm in possession of 12-in diameter sonotube. Do I need to put it on a footer? Frost level is 18 in and I'm going 3 ft deep. If I need a footer, Is there any way to create that without needing two separate concrete pours? I don't have the equipment or the back to makes the concrete myself so I have to order it from 30 minutes away. I would use Redi Base or Bigfoot footers but everybody seems to be out.
 
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A couple of questions;
- how are the holes being created?
- why the sonotube?

chat about sono tubes

The use of reinforcement in the concrete is encouraged, and the Utube vision above has another talking about reinforcement, called reo, following.
It is confusing , so watch carefully.
Essentially reo needs cross bars in the concrete to prevent the steel bars pulling out.
The reo serves two functions, gives strength to the pier, and provides a place to attach the container to as well.

Dont feel bad about mixing concrete by hand.
I stopped that years ago, premix is often better value and fast. A 30min delivery is very good.
Think about pouring any slabs you may need at the same time, since there is usually a minimum amount of premix you need to order to get a good price.
 
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Hi,  I made my box footings with steel in both directions. There was wood 1x1 over top of the form with wire over them to suspend the rebar. The tubes were placed in the middle of the form on top of the rebar with rebar vertical in the tubes.  I had to wheelbarrow the concrete and shovel it into the tubes.
 
John C Daley
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If the pier goes into the ground, the foundation will resist any high winds trying to blow the container away.
It can be a problem sometimes.
 
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Do you NEED footers on piers? Maybe.

There are "bell" augers that will dig a wider footer like a big foot without digging the whole hole bigger.  

You can do a tapered hole by hand fairly easily when starting with a twelve inch hole.  A tile spade or post hole digger can enlarge the bottom of the hole plenty to help with uplift.  I think it is a better option than bigfoot footers, as all the dirt stays native packed and you only need sonotube for above ground and maybe 6-12 inches into the ground.
 
denise ra
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I am digging the holes with a sharp shooter and post hole digger. I definitely have high winds. I need the sonotube because the container is not going to rest on the ground but 6 inches or so above the ground. I like the idea of digging out the hole more at the bottom. Since I only need four holes I can do it all by hand. I've got two whole dug already the soil is loam and undisturbed other than the holes. I do plan on using rebar. I guess the rebar form won't be able to be bigger than the 12-inch hole I'm digging. I am attaching a metal plate that has hooks off the bottom into the pier with the rebar. Then the container will be welded to the metal plate. See the photo.
Screenshot_20210502-153016.png
[Thumbnail for Screenshot_20210502-153016.png]
 
denise ra
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Does anyone know of a rule of thumb for how deep and wide the footer should be. I am assuming the footer will be entirely below the frost line which at a safe bet is 2 ft. Really it's 18 in.
 
denise ra
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If I use my sharp shooter to dig the footer, how do I keep the sonotube from sliding down into that space?
 
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You could screw the sonotube from inside onto a few 1x2s that extend across the hole and bear on solid ground. Maybe a grid of four 2' or 3' long pieces that will hold the sonotube firmly. You can adjust the assembly precisely before concrete arrives, just be sure to anchor it so the jostling of pouring doesn't shift it. It is easy to get something misaligned during the work of a truck pour and not notice until too late to fix it.
 
denise ra
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Here's a better picture of the plates that Will be attached to the rebar in the sonotube.
Screenshot_20210430-153825-2.png
[Thumbnail for Screenshot_20210430-153825-2.png]
 
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Will the container be unusually heavy or just a tiny home?  The standard cabin I built (14' x 34' with Wisconsin snow load) used ten 6" sonotube piers.  According to the architect I didn't need a footer.  Based on the much larger footprint of a 12" circle I'm guessing you don't need a footer.  But it officially depends on the load bearing capacity of your soil.

I did put a simple footing under my piers by getting a landscape block that was an octagon shape and dropping it into the hole and putting the sono tube on top of it.

 
denise ra
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It's only a tiny house so not particularly heavy. I started digging out the lower foot of one of the piers today and it was pretty easy to get about 4 in of soil out of the bottom one foot. That's four inches along the outer diameter of where the pier will be. I might go look at tools tomorrow and see if there's something I can use to get a little more and then I'll call it good. Unless a rule of thumb shows up that tells me different.
 
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