Shipping 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 twistlocks 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 (150mm) deep, which are approximately 20 inches (508mm) apart 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 shipping container lies within the corner posts and the floor. Structural alterations or damage to these components will weaken the container.
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The sides of a shipping container are in nearly all cases, corrugated. The depth of the corrugation is usually 1inch (25mm), which means that 2inches (50mm) is lost from the external width dimension. The back (blank end) of a shipping container is also corrugated and the doors are around 2inches (50mm) thick meaning that approximately 3 inches (75mm) is lost from the length.
The main reduction from external to internal dimensions is with height. The floor of a standard shipping container has an underside clearance of approximately 6 inches (150mm) and the floor has a thickness of 27mm (1.1 inches). As the roof is corrugated another 1 inch (25mm) is lost resulting in an internal dimension of around 8 inches less than external – 7ft 10 inches (2.39m), though this can vary slightly either way.
Through the door height is further reduced because of the steel top rail above the door which is a part of the structural integrity of the container which is 4 inches (100mm) thus reducing the entrance height to 7ft 6ins (2.28m), though this can vary slightly either way.