What Affects the Structural Integrity of a Shipping Container?
Purchasing new containers or used containers is a great idea for so many reasons, whether you use them for mobile offices and classrooms, or as storage in your backyard. Steel shipping containers are very durable and are designed to last for years, decades even, if they’re cared for properly.
Many people choose to make modifications to their shipping containers. However, they may not realize that those modifications can compromise the structural integrity of the container, resulting in long-term repercussions. In order to safely store items and valuables inside the container, the structural integrity must remain intact. It also needs to stay sound enough to handle safe alterations you may make in the future.
Annual routine maintenance, particularly to the roof and doors, will help to extend the life of your container. But there are many things you have to take into consideration that can negatively impact the structural integrity of the shipping container.
Where a Shipping Container Gets Its Strength
Before you cut, stack, or set a container, it helps to know which parts actually hold it up.
- Corner posts and corner castings: Each container has four vertical corner posts and eight corner castings, one at every top and bottom corner. The castings are where the container is lifted, stacked, and locked down. The posts carry the weight of everything above them.
- Top and bottom side rails: These long steel rails run the length of the container. The bottom rails carry the floor and the cargo.
- Cross members: Steel beams under the floor that support the wood flooring and tie the bottom rails together.
- Corrugated walls and roof: The ribbed steel panels aren’t just a skin. The walls work as shear panels that keep the box square and help hold the roof up.
Containers are built from weathering steel (often called Corten) and tested to the ISO 1496 standard. Loaded containers are designed to be stacked up to nine high on a ship. That strength only holds as long as the load path through those parts stays intact.
What Affects Structural Integrity? 5 Factors to Consider
There are five main factors that affect a shipping container’s structural integrity. They are:
1. Improper Stacking of Containers
Whether you’re stacking containers for storage purposes or you’re looking to build multiple container complexes, stacking is a common practice. When done right, it isn’t harmful to the containers. But when the containers are incorrectly piled on top of one another, they can result in structural damage as well as a dangerous situation for people around them. As such, you should always stack units at the corners (not diagonally), tying them with twist locks to ensure a multi-level shipping container building that is structurally sound.
A few more rules keep a stack safe:
- Line up casting to casting. The top container’s bottom corner castings should sit directly on the lower container’s top corner castings. That keeps the weight on the corner posts, not the roof.
- Start with a level base. A stack on uneven ground puts uneven loads on the posts of the bottom container.
- Put the strongest container on the bottom. A container with bent corner posts, heavy rust, or large openings cut into it shouldn’t carry another container on top.
- Bring in an engineer for occupied buildings. If people will work or gather in a stacked structure, the design needs to be engineered.
2. Too Many Modifications
Alterations are a common undertaking among shipping container owners. Perhaps you would like to transform the unit into a garage space or workshop, or maybe you want to add security upgrades to your mobile office so it’s safer at night when you’re not there. Whatever the case, changes to the original container are sometimes necessary and should be allowed to happen. However, you have to follow certain guidelines and precautions to ensure it remains structurally sound even after the modifications.
For instance, you can’t cut out too large a space to accommodate more doors or cut too close to the corners, as this will lead to an uneven distribution of weight and a precarious situation. Keep in mind that every shipping container is lifted, stacked, and secured by its corner castings, located at each of its eight corners. Let’s say you cut out a corner casting to make a desired modification design change. If you did this without consulting with a structural engineer, you could be sacrificing the container’s structural integrity.
Drastic cuts to a wall, for example, are another way to compromise structure. If you were to cut out an entire wall, this would cause the roof to sag, while the floor would become almost spring-like. The walls carry a lot of weight in addition to the corner castings, and they are necessary to maintain the container’s shape.
To keep a modified container sound:
- Frame every opening. Windows, roll-up doors, and man doors should be framed with steel to replace the strength of the wall that was removed.
- Leave the frame alone. The corner posts, corner castings, and top and bottom rails should not be notched or cut. The International Building Code’s simplified design method for single-unit containers (Section 3115) prohibits it outright.
- Plan large openings with an engineer. Joining two containers side by side or removing most of a wall requires added steel beams and posts to carry the load.
- Check local rules first. Requirements vary by location, so confirm with your local building department before you start.
Interior changes like partitions, shelving, insulation, and lighting generally don’t affect the structure, as long as they’re not welded to or cut into the frame.
3. Frequent Sea Travel
The deterioration process is hastened with frequent travel and exposure to the ocean air and water. All that salt can cause corrosion, and objects in the water can result in dents. If buying a used container that has been in the ocean, make sure you fully inspect the unit, checking for signs of rust, wear and tear, and other damage. Keep in mind, if you buy a used container, this means it likely traveled at sea many times. You may want to take a look at “one-trip” containers, which have only traveled once across the sea.
A used container that has been exposed to the ocean is more affordable and may be perfectly suited to your storage needs. But if you need a structurally sound container to turn into a classy mobile office, art space, or yoga studio, you’ll want to go with one-trip or new containers, especially when people’s safety is at stake.
Salt exposure doesn’t end when the container leaves the ship. A container placed on a coastal site, near the water in places like Wilmington, Charleston, or Savannah, faces the same salt air every day. Keep the paint touched up and check for rust more often if your container sits near the coast.
4. Poor Site Preparation and Uneven Ground
Where you set your container matters as much as how you use it. A container sitting on soft or uneven ground can settle at one corner. That twists the frame out of square, a problem known as racking. The first sign is usually doors that are hard to open or close or won’t seal.
Ground contact also traps moisture against the bottom rails and cross members, which speeds up rust on the parts you can’t easily see.
To prevent both problems, make sure the ground is as firm, solid, and level as possible before delivery. Placing 4″x4″ boards, railroad ties, or solid concrete blocks under the container keeps it from sinking and lets air circulate underneath.
5. Roof Damage and Overloading
A container roof is built to shed water and to keep cargo dry. It’s not built to be a deck. Storing heavy materials on top, or walking on it often, can dent the roof panels.
Dents are more than a cosmetic issue up top. A dented roof holds standing water, dirt, and debris, and those low spots are where rust eats through first. The same goes for the floor. Heavy items concentrated in one small area can damage the flooring and cross members underneath, so spread heavy loads out. See our guide on keeping your shipping container roof in tip-top shape for maintenance steps.
How to Check a Container’s Structural Integrity
Whether you’re buying a used container or checking one you already own, walk through these points:
- Overall shape: Look down the length of each side. The container should be square, not leaning or twisted.
- Doors and seals: Open and close both doors. They should swing without forcing, and the gaskets should seal. Stiff doors can point to a racked frame.
- Locking bars: The vertical bars on the doors help keep the box square. Several bent bars is a warning sign.
- Corner posts and castings: Check for bends, cracks, or heavy rust. These carry the most load.
- Floor and cross members: A floor that’s heaved, soft, or uneven can mean damaged or missing cross members underneath.
- Roof: From inside, look for daylight, dents, and patches. From outside, check for standing water.
- Rust: Light surface rust is normal on weathering steel. Flaking, bubbling under the paint, or holes are not.
- Data plate: Most containers carry a CSC Safety Approval plate near the doors. It lists the manufacture date, maximum gross weight, and allowable stacking weight. An expired plate doesn’t matter for ground-level storage, but it matters if the container will ship cargo again.
For ongoing care, see Your Guide to Shipping Container Maintenance.
Match the Container’s Condition to the Job
Not every project needs a perfect box. Our appearance condition grades make it easier to match a container to how you’ll use it:
- New/One Trip: The right choice for offices, classrooms, stacked units, and heavy modifications.
- A Grade: A good option for modifications, especially if you plan to paint the unit.
- B and C Grade: Good value for general storage. C Grade is our most popular grade. It’s not very pretty, but it’s still a structurally sound unit.
- D Grade: Rust is becoming structural. It’s still wind and water tight and works for locking up 4-wheelers or lawnmowers, but it isn’t a candidate for cutting or stacking.
Not sure which way to go? Read when to buy a used shipping container instead of a new one, or check our container specifications for weights and dimensions.
Frequently Asked Questions
Are the sides of a shipping container structural?
Yes. The corrugated walls help keep the container square and support the roof. You can cut openings in them, but large openings need steel framing, and removing most of a wall requires added support.
Can you stack used shipping containers?
Yes, as long as the containers are in good condition and stacked corner casting to corner casting on level ground. A container with damaged corner posts, structural rust, or large cut openings shouldn’t be used as a base.
Do I need an engineer to modify a shipping container?
For small changes like a vent or a framed window, often not. For removing large sections of wall, joining containers, cutting near the corners, or building anything people will occupy, yes. Check with your local building department as well.
How long will a shipping container stay structurally sound?
With regular maintenance, a steel container can last for decades. Keeping it off the ground, keeping the roof clear, and treating rust early do the most to extend its life.
Contact Transocean Equipment Management
Whether you’re on the hunt for new containers or used ones, we can hook you up. Please contact us at (910) 483-7828 for a free quote. See our service area to find out where we deliver.
