The Optimum Container Foundations
If you have looked at multiple container builds you will quickly recognize the breadth of foundations available for a container. The optimal foundation should always be designed and stamped by a professional engineer. Such a design should consider soil capacities and weather conditions in the area. This article will describe how containers interact with foundations and general designs.
Containers are the opposite of stick frame construction. In a wooden house the load is transferred down walls to a bottom plate with a relatively large surface area. Load bearing walls are spread throughout the house. Metal construction is much different. The containers have extreme point loads on the corners as the main frame does not naturally rest on grade. In my personal house, roughly 400,000 pounds of weight rests on 1.35 SQFT of surface area. Point loads are best kept on deep pier foundations that meet strong soil conditions.
If your project is in an area with a rock foundation, such as limestone, the considerations are much smaller. It is a stable foundation at a shallow depth. Drill a pier into the rock and set the home on top. Unfortunately, a majority of homes are built on expansive clay, especially in Houston.
There is extensive research on these soil conditions, but the important consideration is the assumptions made when the research is finished. Houston has sunk multiple feet in the last few decades. The City naturally moves, but man-made influences such as pumping underground water out for human use has shifted the landscape. Weather and seasonal effects also move soil. Clay absorbs water and then dries out. These shifts pressure shallow foundations and cause concrete, which is weak in tension, to crack.
It is interesting to travel much further north and experience the same soil conditions. Northern climates require slightly deeper slabs due to frost depth requirements. The possibility of ice expansion forces foundations deeper. There are noticeably more older structures further north than in the south. Engineers are still certifying slab on grad foundations in Texas knowing that they will fail. It’s a race to the bottom in search of a better price.
Large Area Foundations
Containers should never utilize a slab foundation. The key consideration being a point load. Slabs cannot handle the extreme stress. Some engineers will require a footer or multiple grade beams on a container due to this reality. It is a viable workaround, but the idea falls the same. The engineer is simply trying to increase the surface area of the foundation to accommodate the load.
A better solution for this idea is a block and base foundation. Level out the ground below the structure, compact the soil, and place concrete blocks at intervals. Shim the blocks to level the container when setting the structure. The unfortunate reality is that consumers hate block and base. It is written off as cheap, but it is a far superior foundation. With minimal leveling repairs every decade the foundation is good indefinitely.
The best container foundation is and always will be deep piers. The concept is straightforward: deep piers reach down to a depth where soil conditions are more stable. The stability yields a more consistent and higher load capacity depending on the size of pier and depth. The extreme point load of the container will be accounted for. The best part of this foundation is that soil shifts substantially less at deeper depths. No longer is weather, frost, or seasonal shifting part of the equation. The foundation will perform consistently for decades.
For larger structures, helical piers should be considered. I personally utilized them for my four-story home. They are the gold standard of foundations. The pier involves a metal shaft with helixes that are drilled into the ground. The torque required to drill them can be mathematically equated into the allowable load. Thus, installers can verify the strength of the foundation during installation.
The only downside to helical foundations is the expense of installation. They are usually double the cost of concrete. While they do handle a higher load, it often isn’t required for residential structures which are not comparatively heavy enough to need it. I’ve tried to work with installers to utilize more piers with cheaper grade steel at a shallower depth, but I’ve been unable to find a company to do so. Eventually the technology will become more ubiquitous and we will see a shift to it. The ICC only recently added helical foundations to their documents in the 2000’s. The foundation was first utilized in the mid 1800’s, but it’s taken a while to catch on.
Whatever foundation you ultimate decide on, be sure to have a structural engineer certify it. Our engineering firm is standing by to assist.