Seeing several houses made from shipping containers gave me a good indication that shipping containers are strong enough to support loads that a designed home will put on them. But, I really didn’t know just how strong they are.
Shipping containers are required to withstand maritime loads equivalent to hurricane and an earthquake forces at the same time. So, before being altered with openings for new doors and windows, the shipping containers are much stronger than they need to be for home design.
In order to better understand how and why shipping containers are very structurally sound for home design, let’s look at the main structural components and know about the rules that are followed while manufacturing and using shipping containers.
Shipping Container structural “Kit-of-Parts”
The floor of a shipping container is designed to support up to 65,000 pounds. That is the equivalent weight of over 6 school buses! The floor is constructed of 1-1/8″ marine grade plywood over bent sheet metal floor joists that are spaced around 12″ on center.
These floor joists are spanning between bottom rails. The joist connect to the back side of the bottom rails.
The bottom rails run the length-wise direction of the shipping container. These rails are channels and made from bent metal. The dimension of the bottom rails are approximately 7″ high and has a 2″ wide top flange. The siding and corner posts are welded to them.
The corner posts of a shipping container is designed to support 165,000 pounds of gravity loading when 8 more shipping containers are stacked on top of it.
At all eight corners of the shipping container are corner castings. The corner castings are cubes with 6 sides like a standard gaming dice. They all have 3 sides that are welded into the other parts of the container. The other 3 sides have holes in them.
When containers are connected to each other, twist locks are inserted into one of the three holes of the corner castings. Another container is placed on top of the container with the twist lock, the twist lock sticks out partially into that top container and a lever twists its lock in the top container’s corner castings.
Twist locks are amazing. They are only about 12 pounds in weight. But, they can connect two containers while supporting up to around 100,000 pounds of load in certain directions. They are really strong relative to their own size and weight.
Here is a quick YouTube video that shows how a twist lock works: https://youtu.be/Sz8smq6ddok
The top rails run parallel to the bottom rails and along the length-wise direction. They are 60 mm x 60 mm x 3 mm (approximately 2-3/8” x 2-3/8” x 1/8” thick). The siding and corner posts are welded to them.
The siding is usually painted 1.6 mm to 2 mm thick (minimum 1/16″ thick) corrugated Cor-Ten high strength steel (Fy = 50 ksi). Cor-Ten is a product that is designed to rust only to a point that the rust actually protects the steel from losing hits structural integrity.
Cor-Ten has been used as an exterior finished material in buildings too. An example of a building that used Cor-Ten is Knights of Columbus Tower designed by Roche & Dinkeloo, in New Haven, Connecticut.
Corrugation helps strengthen the steel siding. Each “bump up” of the corrugation is called a “flute.” Each flute is about 1-3/8″ deep. They repeat every 11″.
The siding provides lateral load support and helps to hold up the corner columns from bending side to side. When removing the siding for door and window openings, additional support, such as lintels and beams, will need to be designed.
The doors are considered to be at the “rear” end of the container.
The door header of the rear end is at the same height as the top rail of the long sides. The roof is welded to it.
The door jamb acts as a corner post on either side of the rear end. It also supports the hinges of the door.
The door sill is flush to the finished floor of the container. It is designed to support the weight of fork lifts carrying large loads into the container.
At the front end, or “nose”, of the container there is a nose header, column, and sill.
The nose header is welded to the roof, column, and siding. It is at the same height as the top rails.
The nose column is probably one of the weakest structural elements of the shipping container. If relies heavily upon its connection to the siding to get its strength. If the siding is removed from this column, then additional reinforcing will be needed in order to support this column.
It is important to note that the top of the nose sill is higher than the finished floor of the container. So, if containers are joined nose to nose, the nose sill could be a tripping hazard. Some designers intentionally joint containers rear to rear (where the doors are) in order to avoid this problem.
The roof of a standard dry shipping container is not designed to support very much point load. Therefore, many designers will add another layer of support, like roof joists, to support the weight of equipment and roofing material added on top of the containers.
The roof joists will usually span between the top rails and bear upon the top rails and/or additional structural members like beams and columns.
High Manufacturing Standards and Testing Procedures Contribute to High Structural Integrity
To maintain high structural integrity, shipping containers need to be manufactured under standards set up by the “Convention for Safe Containers” often referred to as “CSC.”
When looking at various containers to use for your home, you might also see that a shipping container is referred to an ISO container. This is because shipping containers are also tested to meet stringent code 1496 of the International Organization for Standardization (ISO).
Due to the ISO strict guidelines, the containers are tested just after being manufactured, right before use, and then again during its use.
Following that, they are also inspected after its use too.
If there are any structural defects, perhaps the container was damaged when in use, then they are either repaired or discarded.
Are Discarded Shipping Containers Good for Home Design?
Many of the discarded shipping container are good building blocks for a shipping container home.
The older the container, the better chances of it having more dents and scratches. The dents and scratches often do not diminish the structural integrity of the shipping container. These are usually less expensive than newer and less worn shipping containers.
Also for some designers, the dents and scratches are not a problem because they will be covered and concealed with an additional exterior and interior finished layer.
If the designer doesn’t want dented containers for the home they are designing, then the containers can be purchased with little or no previous use for a premium cost.
DIY Home Builders Should Include a Professional to Help Design Their Home
Every project is different. Every project has its own parameters to consider. Weather, budget, construction time line, soil, labor capabilities, construction material availability, zoning laws, and building codes are some of the many things that will affect the structural design of a shipping container home.
It is important to know the basics of the shipping container structural components. And, it is also important to include a structural engineer to assist in the design of any shipping container home.
Written by Larry Lane, A.I.A.
