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roundwood and timber framing

pier and footing question

denise ra

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posted 5 years ago

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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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John C Daley

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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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Arthur Angaran

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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.

R Scott

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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.

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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

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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.

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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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Glenn Herbert

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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

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Mike Haasl

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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.

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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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Jan White

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Hey Denise, what did you end up doing here? Was your intention just to hold up the container or to hold it down? I see on your other thread you've got the piers in now. I meant to respond to this topic earlier, but things were crazy here. If you're trying to hold the container down in a tornado, you'll need big footings, preferably pretty deep, and rebar through everything. Hopefully that's not what you were aiming for, cause it sounds like it's a bit late :(

denise ra

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Footings 3-ft deep And one foot across with rebar and a weld plate. It won't go anywhere in a smaller tornado which is What we have had here in the past.

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Jan White

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denise ra wrote:Footings 3-ft deep And one foot across with rebar and a weld plate. It won't go anywhere in a smaller tornado which is What we have had here in the past.

It's a 12" pier all the way down though, right? It doesn't get bigger at the bottom, does it? That sounded skimpy to me since we'd talked a bit about wind when building our place. We've got a 4' overhang on our roof and while we're nowhere near tornado country we can get a bit windy. I asked my husband who's a structural engineer. Just off the top of his head he said 3' footing, 12" thick. He said the skin friction on a straight pier isn't much. You can potentially lever out a 3' with your body weight. No idea what your soil's like though.

denise ra

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12 inches by 36" with as much of a bell-shaped footer as I can dig out of my soil which is undisturbed. I will have a shed roof with minimal to no overhangs to avoid the lift. Truthfully, I have no idea if that's adequate or not.

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John C Daley

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A better method maybe to use the tube for only the last 6 inches of the hole and the extension above.
That way you fill the hole completely, increasing its resistance to uplift

Jan White

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Well, it sounds like it's all done anyway, so no use worrying about it now. Hopefully my husband is just being overly cautious. He also said if it was him he'd make the piers six feet deep. I know you've got a berm (berms?) in your design, so maybe that will help with uplift as well.

John C Daley

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I am not sure how you could dig piers 6 ft deep by hand.
Is it possible?

denise ra

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As someone who just dug four 3-ft piers, I can tell you that you will not dig 6 ft piers by hand unless you're planning on filling in the dirt around them later.

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Mike Haasl

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I'm not sure if this counts, but I dug an 8" diameter, 23' deep hole a few weeks ago by hand.

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denise ra

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Mike Haasl, how?

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John C Daley

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Hand auger, long pipe and a gantry?

Mike Haasl

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John got close!

Seymour AUA2 post hole auger with extension pipes and couplings to increase the depth in 5' increments. No gantry or gin pole, just lifted the whole works out of the hole. After 15' it helps to have a second person to "catch" the basket end as you slide your hands up the pipe while the assembly tips over and you try to get to the fulcrum before the handles slam into the ground. The second person shakes the dirt out of the basket and guides it back into the hole as you spring the assembly back up to vertical. It's a very good workout.

A gantry or gin pole with a pulley system would've been nice but we got the hole augered in under an hour so it was done before we'd've figured out how to make a gin pole...

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Jan White

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If doing it by hand, you would have to dig a wider hole to get to six feet. My husband says undisturbed soil versus soil removed and repacked around the pier doesn't really make any difference. If you're worried about soil settling later on, you'd just take more time to tamp really well as you filled around the pier.

We dug our piers by hand. We've got bedrock close to the surface, so only two of them got to the frost depth of 30". We went a full three feet or a bit more just cause. In our sandy, rocky soil a two footish wide hole was fairly easy to dig to that depth. We could have gone a bit deeper just with a shovel.

denise ra

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Mike Haasl - wow!! What was that deep hole for?

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John C Daley

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Denise, I have a technique for insulating the outside of containers.
Materials;
angle brackets
4 inch C or Z section steel lightweight sections length of container [ rails]
- you will need 3 or 4 to suit the container height
90mm thick polystyrene insulation often used for packing, cold rooms, but can be purchased.
- Rockwool can be used
Metal sheeting cut to length to suit the height of the container [ coloured if you like]
- flashing to suit for weather proofing
Reflective rolls
Pockets full of self drilling Hex Head metal screws
gap sealant

Process
Fasten angles if needed to wall in a horizontal line to hold the C section.
- space to suit the insulation size
Attach the C section to the brackets. [ laying flat with its legs facing up]
- If Z section is used the brackets are not needed to fasten the Z section, but they help in holding it straight whilst you fasten them to the walls.

Install insulation properly, no gaps etc if possible, using the rails as support, and glueing to wall.
Seal any gaps you can
Install reflective roll
Attach the metal sheeting to the rails, and finally the flashing.

I believe its the best and easiest way to insulate.
You finish with a watertight and airtight, very effective panel system coloured the way you want!

Mike Haasl

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denise ra wrote:Mike Haasl - wow!! What was that deep hole for?

We were checking to see if there was water down at a depth we could retrieve with a pitcher pump. It's easier to auger a hole than to pound a sand point so that's what we did. When it's sandy that is...

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I agree. Here's the link: https://woodheat.net