Do you want to know how stacking shipping containers on ships or land is done? Do you need to know what shipping container schematics and door markings are used in the shipping container stackability process? Do you require information on what shipping container stacking regulations apply?
Recent developments have caused increased interest in the topic of shipping containers and their stacking. During the COVID-19 pandemic and lockdowns, there was a lot of news about shipping containers being banked up in ports across southern California due to congestion at the Port of Los Angeles.
Super cargo ships, such as the Evergreen Ever Ace with a capacity of over 24,000 twenty-foot shipping containers, are now deployed on a regular basis. These growing extremes in the ocean freight industry are causing ever-bigger challenges for the stacking of shipping containers on land and container-stacking on ships.
This comprehensive overview on stacking shipping containers covers:
- How shipping containers are made stacking
- Container stacking on ships
- Stacking shipping containers on land
- Shipping container stacking regulations
Stacking shipping containers on ships or land including regulations
How shipping containers are made for stacking
The key structural feature that allows shipping containers to stack is that the external dimensions are standardized worldwide. Making all containers a uniform size is the major reason why stacking is practical and efficient. The next major structural feature are the load-bearing vertical corner post elements. They are designed to hold loads in excess of 60 tons on each corner, or a total weight of 240 tons for each. The sides, roof and bottom of a shipping container are not load-bearing in any way. They have other features, such as air vents, to provide ample airflow within the container to prevent mold and sweating.
Shipping container roof and walls
The container roof needs to be durable as it is the part most exposed to the harsh elements of the seas. It comprises an inflexible grooved exterior made of weathering steel sheets that are welded to the container structure for greater sturdiness. The covering is susceptible to denting and rust blistering, which can result from a crate falling when being stacked or knocked when being transported.
The lateral faces are like the top cover. They are made from welded grooved weathering steel sheets and designed to resist impacts when stacked or moved. The walls are surrounded by thick steel beams welded together as a box frame to ensure adequate support. Despite being strong, the walls are prone to rusting and dents.
Shipping container doors and floor
All regular shipping containers have identical doors made from high-strength low-alloy weathering steel. These doors can open outwards to 270 degrees, making it easy to load and unload cargo. All doors have rubber gaskets to keep them watertight and airtight. This also prevents rodents and other pests from getting into the container. The performance of the doors is governed primarily by its lock rods.
A shipping container can weigh between 5,000 and 10,000 lbs and hold cargo five or six times that. The floor is structured to hold heavy payloads and be safely lifted by crane. Comprising several layers of steel and marine-grade plywood, it is strong and watertight. Sub-floor slots allow insertion of forklift tines for easy container transportation. There is risk of sagging and cracking if inappropriate materials are used to construct the floors.
Shipping container and door components
Shipping container stacking twist locks
All containers have eight solid metal casting corner fittings with standardized hole sizes. Twist locks, also known as stacking pins or cones, are inserted to stop the container from slipping out of position. No maintenance is required and they are not affected by long-term exposure. The corner fittings can be used for lashing with ropes or lifting by crane.
How high can shipping containers be stacked?
When containerized shipping started in 1956, long before twist-locks and the quality of steel now available, shipping containers could only stack boxes five or six high. Nowadays they can regularly go to eight but can stack as high as ten or twelve!
Shipping container schematic diagram showing components
Container stacking on ships
There are considerable differences in the considerations and techniques used when stacking shipping containers on ships compared to on land within a container park at a port.
Cargo ships need special engineering
As cargo ships have increased in size and hold more containers, their deck openings have also become larger. The hull girder must be designed so the hull structure does not fail under load from the torsion and bending caused by wave action. The upper extremities of both sides of a container ship are strengthened by elevated girder webbing sections. This creates a torsion box that extends along the ship from stem to stern.
Two types of on-board stowage
The two types of storage on a container ship are lengthwise and horizontal. Stowage choice depends on ship size and weather. Lengthwise storage offers better protection against high winds, crashing waves and rough seas.
Use of corner posts to stack containers
Stacking similar-sized containers requires the corner posts to be aligned to ensure even distribution of weight. The corner posts should project a little below the container’s floor and a little above the top. Stacking should be ordered from small underneath to large at the top. This ensures the corner posts of the bigger container sits on those of the one below.
Container stacking method for ships
When stacking containers on ships, the bottom layer is locked down to the deck with twist-locks. Banding and couplings are added to further secure the containers. The containers on the bottom underpin those to be stacked above.
The median or higher-level containers are secured by automatic twist-lock. These are connected when lifting the container from dock to ship. When a container is placed on another, the auto twist-lock clamps the two in place. On unloading, a unique key opens the twist-locks. These ensure containers do not move or slide.
Container stacking on ships
Stacking shipping containers on land
There are various factors, including wind, weather and stack height, container storage operators need to consider.
Container stacking, while possible, should be minimized and avoided when possible. The smaller container will be less secure than the larger, particularly the farther up the stack it is placed. Better to place two 20-ft units below, with a 40-ft unit above, thus maximizing the strength of the additional corner posts.
Eight containers can safely be stacked one above the other. Stacks can, however, be made as high as twelve without jeopardizing stability and risking safety. Other factors, such as neighboring structures and weather, will affect the layout, position and height of stacks within the container park.
Block Stacking
Block stacks of full containers no more than three high and no less than two deep may be placed along the boundaries. These act as bulwarks for unfilled containers or smaller groups in the area. These perimeter groupings should be securely roped together or fastened down in windy conditions.
Boundary stacking
Each perimeter stack needs to be leveled off to make sure there are no single containers at the top of the stack. Boundary full and empty containers must be placed in stacks set in the same direction as the wind. These stacks must be placed to allow adequate wind tunneling. Outlying stacks are positioned so they are shielded by a perimeter block stack. Tiered stacking is required on perimeters subject to high winds. Perimeter stacking of containers end-on requires appropriate spacing in areas subject to high winds. Barriers should be installed above boundaries where strong winds are common.
Stacking shipping containers on land
Reduced stacking
Shipping container park operators need to think about a way to lower container stack heights on a continual basis, especially where high winds are a problem. It would be sensible to think about creating buffer zones to provide seasonal protection within container holding parks.
Shipping container stacking regulations
A key factor that allows shipping container park operators to manage shipping container stacking are regulations that apply to the door markings. Cargo services, shipping depots and more in the supply chain can immediately authenticate container door signage. This is due to the sequence of information (owner and serial numbers) and other information that must align to a unique formula. The container number is the main entry on the container’s door and consists of seven numerals and four letters.
Owner code
This comprises a discrete group of three letters that form a code unique to the particular owner and registered with the Bureau of International Containers.
Equipment category
Comprises a single letter plus with freight containers denoted with “U”, trailers as “Z”and detachable container-related equipment. as “J.”
Serial number
A unique combination of six numerals.
Check digit
A single numeral following the serial number. This numeral is critical, as it is used to determine whether the container number is valid.
ISO code
Every container is provided a discrete size and type code to ensure there is no confusion. The height and width. The third and fourth characters specify the container type.
Container Safety Convention plate
All international shipping containers must have a Container Safety Convention plate on the left-hand door.
Shipping container ISO door markings
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