Two questions come up more than any others when people ask me about containers. The first is about insulation, and the second is about foundations, which I'm going to talk about today.
When it comes to foundations, it's important to first understand the nature of containers and how they work structurally. A container can span it's entire length and more. If you could securely hold one side of the container in place, it could hang the other 40 feet with no problems. Someone I know even did a project once where they welded two containers together front to front, and made a container bridge that spanned 80 feet. That seems a pretty amazing feat, until you look at and understand the structure of containers. After that, it seems a simple thing.
A container, is in basic form, a giant box truss. Think covered bridge. The small square top rail, and small beam along the bottom of the container act as a top and bottom cord on a truss, while the steel siding acts as a webbing. When you take a high cube container, you then have a 9.5 foot tall truss, and it's no surprise it can span such a distance.
So why is that important? Well, in it's unaltered form, a container can span it's full 40 feet, sitting on just it's four corners. Think of containers stacked in shipping yards. They are fully loaded with cargo, but when stacked, all of the weight is sitting on the four corners, and the container spans the entire 40 feet. The normal load of a house or occupied building is going to be significantly less than that of a fully loaded cargo container. So by theory, you only need to support the 4 corners of the container. You can even come in a few feet and allow the end ends to hang off as much as you like, creating a cantilever.
The problem comes, and things get complicated, when you start to cut openings into the containers. I know of very few container projects that don't cut out as much siding as possible. Cutting out the siding in a container for a door or window, is no different than cutting out the crisscrossing webbing in a truss. Something that is incredible strong suddenly becomes very weak. There are a few ways to fix the problem. The first is to create a series of rigid reinforcement (such as the beams and posts we put around windows and doors). For windows especially, this works great, and adds a lot of strength. The real problem for foundations is when you cut out doors or giant openings. Even if you add strong posts and beams, that only helps with the upper portion of the container. If the opening extends to the floor (which is the difference between doors and windows) then you only have a 6 inch beam running along the bottom. This may be sufficient for a small door opening, but not for a major 20 foot or 30 foot opening. When you have a larger opening, there are two main ways to reinforce. The first is to add more steel to the bottom beam, such as a steel channel. The second is to add more footings across the span. I will typically at a new footing every 7 feet on those spans, sometimes on top of adding the steel c channel.
So in theory, you only need to support containers at the four corners, but in reality that is rarely the case. The important thing to remember, is that whenever you take away structure (or webbing), you have to compensate for it. You either need to add structural reinforcement, or additional foundations. Also keep in mind, that if you just add additional foundations, then you can expect alot of flexing and bending in your container when you pick it up and move it.