Container glossary
What is a corner casting?
A corner casting is the hollow block of cast steel at each of a container’s eight corners, with openings through which it is lifted, locked and lashed. The whole weight of a container and its cargo passes through these fittings, and through nothing else.
How eight fittings carry the whole load
A container is a frame with skin on it. Four corner posts join the top castings to the bottom ones; the corrugated walls and roof stiffen the frame but bear almost none of the load. When boxes are stacked, the weight of everything above runs down the posts, casting to casting, to the deck. When a crane lifts, it holds the four top castings and the cargo hangs from the frame.
Each casting has openings on its outer faces: one facing up or down, which takes a twist-lock or a spreader, and two on the sides for lashing gear and hooks. Their shape is set by the standard ISO 1161 and their positions by ISO 668, identical on every container of a given length. That is the entire reason a box from any maker fits any ship, wagon or chassis.
What the corners mean when a container sits on the ground
For anyone placing a container on land, the castings are the foundation plan. The bottom four project slightly below the floor frame, and the box is meant to rest on them: four level supports under the corners are structurally enough, which is why piers are the usual choice over a slab. The foundation guide gives the options and the levelling tolerance.
They are also an inspection point. When buying a used box, look at each casting and the welds around it for cracks and for signs of repair. Damage there is structural, unlike surface rust on a wall panel — the buying checklist treats it as a reason to walk away.
Supporting or stacking a container anywhere but its corners
The mistake is treating the container as a solid block that can rest on anything. Set an upper box offset on a lower one, or hang it across the middle of two others, and its weight lands on a roof or a top rail that was never meant to take it. The steel deforms; in a stack it can fail. Corner on corner is the only arrangement the structure was designed for, and it is why a 40 ft may stand on two twenties but not on one.
On the ground the same error is quieter. Supports that leave one corner high or low twist the frame, and the first symptom is doors that bind or will not latch. Owners blame the doors and pay for a repair, when the cure is a shim under a casting.
Frequently asked questions
How many corner castings does a container have?
Eight — four at the top and four at the bottom, one at every corner. This is the same for a 20 ft, a 40 ft and every special type, including flat racks and tanks.
What are corner castings made of?
Cast steel, made as separate fittings to the standard ISO 1161 and welded to the corner posts and rails. They are far heavier in section than the wall panels because they carry the load of the whole stack.
Can a container be lifted without using its corner castings?
Only where it was built for it — for example the fork pockets found in the base of many 20 ft containers. Otherwise the castings are the designed lifting points, and lifting by slings around the body or forks under the floor risks damaging the box.