Shipping container structure

Containers are designed to carry cargo. They are designed to be lifted vertically from above by cranes and transferred from ship to shore and between other forms of transport such as trains and lorries. Therefore, they need to have strength in the top corners where they are engaged by the twist locks of a container lifting crane. The corner castings in the top corners of the container are therefore points of strength. 

The strength is transferred down through the corner posts to the corner castings at the bottom and then through the floor. The floor is constructed of steel cross members approximately 6 inches (150 mm) deep, which are approximately 20 inches (508 mm) apart (laterally across the container) and give transverse strength and support the floor. The cross members are welded at each end to longitudinal beams which run the length of the container between the corner castings. This all means that the strength of a container lies within the corner posts and the floor. Structural alterations or damage to these components will weaken the container.