Camper to Container Home

This is the post I am the most intimidated to write because it has to do with all the details my hubby and the other brains behind this operation have been thinking about, while I have just been excited to see the overall progress. 😃

Today I'm going to share with you the specifications of the concrete supports on which we placed the containers and the engineering reasoning behind it all.

We wanted to put a "basement" container in the rear stack for a storm shelter, so we had to pour columns to support the front two containers and bring them up level with the two in the rear. (Hindsight, it would have been cheaper and easier to use another container instead of the concrete columns.) So we poured 6 foot high columns for the front stack and dug down and poured footers in the back that are even with ground for the rear stack.

The amount of weight soil can support depends on the type of soil. Clay can support about 2,500 pounds per square foot, sandy loam about 3,500 psf. Our soil is close to sandy loam, we did not get it tested, if you aren't planning much margin of safety, you should probably get your soil tested. The containers are about 10,000 lbs each and we stacked 3 on the back. So we dug two concrete footers for the back stack 2 ft wide by 9 ft long. That gave us 18 sq ft and multiply that times the amount per square foot the soil would support gave us:

3,500 (Sandy loam support amount)
X 18 square feet (size of footer)
X 2 (number of footers)
= 126,000 total weight the footers and soil will support

Since we have 3 containers on the back, at 10k lbs each, (30,000 pounds) after you add contents and framing of a few walls and doors, plus snow on a winter day, we figure there is still quite a bit of margin safety here.

Another factor is the stress of frost, freeze and thaw, on the structure of the concrete. In Oklahoma the frost line is 18 inches. So the footers needed to be deeper than that.

We also wanted to hit rock bottom if we could, and (as I said in my last post), we hit solid rock at between 36- 42 in.

Another factor to consider is the tensile strength of the footer. (The stress of twisting and pulling.) Rebar is the solution to that problem. The footers that were level with the ground would not have to withstand the force of wind, etc so the tensile strength did not have to be as high. The amour of rebar needed to be about 5% the total weight of the concrete.

The columns would receive more stress from wind, etc so we increased the amount of rebar to 15-20%. They spaced it about every 12 inches vertically and horizontally and needed to be about 3-4 inches from the edge of the concrete.

The mistake and what we learned:

As I mentioned in a previous post, on the day we poured the footers for the first time, we didn't have the forms set correctly. When they started pouring the concrete for the first column footer (front right), and had filled the hole and were about halfway up from the level of the ground, the form bowed out about 6-8 inches. They stopped. They knew the other form was not secured correctly either, so instead of filling that form, they just filled it to the level of the ground and then filled the holes for the footers for the rear stack.

At the time my husband didn't know anyone with a lot of concrete experience, so there was a lot of discussion about what to do next.

We learned to drill holes for additional rebar to be put in and set with anchoring epoxy. Then formed around the "mushroom" as my husband called it, added more rebar, and poured the column.

So we have one column that is about the right size for the containers and one column that could probably support a bridge for a 4 lane road!! 😂

Here is John and my 2 year old to show how massive that pillar is!

We decided to frame the outside, spray foam insulation and side our house with metal siding. So, we built a stem wall of cinder blocks to support the frame and give the storm shelter more structural soundness. We live at the end of Tornado Alley, so we wanted to have a place to go. Also just north of where we live I saw a hotel building knocked flat by straight line winds!

The footers for the cinder block wall could have just been 12 inches wide, but the bucket on the digger our friend owned was 18 inches wide. So once again we are overkill on our safety margin! These footers aren't supporting the weight of the house at all, only the siding and framing.

The cinder blocks are filled with concrete and rebar was placed in every other block for additional tensile strength.

The next challenge was to anchor the containers to the concrete. We decided to use a 1/2 inch thick, 16 inch square steel plate under each container corner. We drilled 4 holes in each plate 12 inches apart and bolted the plates to the concrete with 4 wedge anchors. The wedge anchors have to be at least 10 inches apart. Then we welded 16, 3 inch angles that were 3/8 of an inch thick to the container corners.