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If the container is loaded to its maximum capacity, the weight of the cargo must be distributed as evenly as possible over the floor area. The load-carrying components of the container floor structure are the bottom side rails, which absorb the load of a cargo via the container floor. For safe transport, it is important for as many as possible of the bottom rails to be loaded. Point loads are to be avoided, since they can damage the container floor. Point loads always occur when relatively high weights are concentrated on a small bearing area.

The maximum line load is also of importance when packing containers. [46] states a maximum line load of 4.5 metric tons/m for a 20' container and 3.0 metric tons/m for a 40' container. Line loads are calculated as follows:

Example 1: A cargo weighing 15 t extends over a length of 6 m. This gives a line load of

15 t / 6 m = 2.5 t/m

This cargo could therefore be transported in either a 20' or a 40' container.

Example 2: A cargo weighing 16 t extends over a length of 4 m. This gives a line load of

16 t / 4 m = 4.0 t/m

This cargo could only be transported in a 20' container. In a 40' container the maximum line load would be exceeded and transport would therefore be disallowed. However, if the cargo has nonetheless to be packed in a 40' container, the bearing area must be enlarged to ensure safe transport. This may be achieved for example by laying Dunnage  or a "sled" under the cargo. Where dunnage is used, it must be ensured that the lower wooden planks lie lengthwise and are thus supported on several container bottom cross members. Similarly, the skids of a "sled" must also lie in the lengthwise direction of the container.

 Figure 1

[46] gives the following details about minimum plank width and the minimum distance of the planks from the center line of the container:

Container type 20' 40'
A. Minimum plank width 0.10 m 0.15 m
B. Minimum distance of planks to either side of the center line of the container 0.4 m 0.4 m

 Figure 2

When packing a container with a forklift truck, care must also be taken to ensure that the container floor is not too heavily loaded and thus damaged. ISO 1496 (International Standards Organization) sets the following limit values for forklift truck access into containers:

Designation Limit value
Minimum wheel bearing area 142 cm2
Wheel width approx. 180 mm
Track width approx. 760 mm

Forklift trucks with a very high unladen weight may only enter the container when they are not laden to capacity and the maximum axle load of 5460 kg is thus not exceeded. It must additionally be noted that the load-carrying capacity of forklifts is reduced when add-ons are used. Add-ons are used whenever required by the nature (shape) of the cargo, e.g. hydraulic roll clamps in the case of roll transport.

 Figure 3

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The permissible loading capacity of the container walls and the container roof is laid down in ISO 1496 and the Container Safety Convention (CSC).

Containers must withstand a load in the lengthwise direction which corresponds to external acceleration of 2 g acting horizontally on the floor fastening elements. These stresses are transmitted by vehicles to the container via the twist locks (turn-lock fasteners for locking containers).

 Figure 4

The CSC stipulates that end walls must be designed to withstand loads of 0.4 g. This corresponds to 40% of the maximum payload of the container when the end wall is evenly loaded.

According to the CSC, the side walls must withstand loads of 0.6 g, which corresponds to an evenly distributed load amounting to 60% of maximum payload.

 Figure 5

The values apply only to large-area loads. As with the container floor, point loads very readily result in damage to the container walls. Where point loads are expected or are unavoidable because of the nature and shape of the cargo, appropriate precautions must be taken.

The permissible loading capacity of the container roof is only very slight. The CSC stipulates that it withstand a 200 kg load over an area of 600 x 300 mm; cargo must therefore never be put on the roof. When several containers are stacked on top of one another, the forces are conducted into the corner posts, thereby relieving the roof.

Each container must have a valid safety approval plate (CSC plate). If the end or side walls have a lower or higher permissible loading capacity than the above-stated values, this must be marked on the CSC plate.

 Figure 6

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Weight distribution in container

Especially where cargo is to be handled by spreader, forklift truck or crane, it is important for the center of gravity of the cargo to lie as close as possible to the center of the container. If the center of gravity is too far from the center, increased effort is required and there is greater potential risk during transport and handling of the container. To ensure safe handling, the following guidelines should be complied with:

Position of center of gravity 20' 40'
Lengthwise maximum of 0.60 m away from center of container maximum of 0.90 m away from center of container
Crosswise at center of container
Height beneath geometric center of gravity of container

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