Episode 20: Dunnage bags load securing – Part 1

Cargo must be secured in the container.

It is not a question of “whether to secure”, but rather “how to secure a load”. Even experts do not always agree on the “how”. Some say: “nothing has ever happened” and others: “it can’t work like that”.

One of many securing methods is the use of dunnage bags. Some of you may be familiar with these air bags that fill the empty spaces in parcels and packages and prevent the contents from moving. This is based on the same principle as the container.

The set of rules for securing cargo in containers is the CTU Code 2015. Appendix 7, Annex 4 devotes an entire chapter to the securing method with dunnage bags.

It states that dunnage bags should “block” the movement of the load. Conversely, this means that it is a positive locking method.

Determine the size and design of dunnage bags

Three pieces of information are required to determine the size of the stowage cushion:

• Weight of the load
• Length, width and height of the load
• Size of the gap

In principle, a distinction can be made between two types.

The main objective must always be to prevent cargo movement. This means compensating for the force generated by the movement of the load with the stowage cushion.

In principle, the considerations can be approached from different angles:

1. The dimensions and weight of the load in conjunction with the expected accelerations are used to determine the force from the load and thus the size of the stowage cushion or
2. The maximum load weight to be secured is calculated from the existing size of the stowage cushion.
3. A suitable pad is used depending on the surface area resulting from the load. In most cases, no calculation is made.

In practice, in my experience, the third variant is most frequently used.

For example, the classic block of five in a container. There are two options here. Either one large cushion or two small ones.

The large one is useful if the load is essentially always the same.

The little ones are more versatile. The pad size and force absorption can be precisely determined by the height of the load unit.

Understanding and using dunnage bags correctly

It is also important to know the relationship between pressure and surface area.

The greater the pressure and the smaller the area, the greater the acting force. This connection is the most frequent cause of faults or damage.

The aim should be for approx. 80% of the surface of the stowage pad to be in contact with the load, as this enables optimum force transmission/force absorption.

Situations such as the example below pose a considerable risk to the functionality of the traffic jam cushion. In the event of an inspection, the container would first be shut down and improvements would have to be made. This is usually time-consuming and cost-intensive.

Accumulation cushion calculation

The CTU Code describes the topic very comprehensively and theoretically. Many shippers are overwhelmed because all the formulas look complicated, but on closer inspection they are not.

A distinction is made between loads that slide/slide and those that can tip because they are of different heights. Before solving the problem of loads of different heights with the stowage cushion calculation, it is often easier to compensate for the different heights, e.g. by using a pallet substructure.

Only simple solutions are effective in the long term.

Of course, it is important that no technical mistakes are made, such as positioning a dunnage pad where the container is damaged and the pad could be worn through.

The formulas

It is important to note that in the following calculations according to the CTU Code, the cargo mass is given in tons (to).

The result is then converted into deca-Newtons (daN), because most shippers are more familiar with this.

Calculation of the load force when sliding

The following formula calculates the force generated by a sliding/sliding load.

It should be noted that the coefficient of sliding friction _µstatic is not known in most cases or is only estimated. It is therefore reduced by a factor of 0.75.

During sea transportation, the vertical acceleration _Cz can be between 0.2 g and 1.8 g when the ship is pitching. So there are considerable differences.

The acceleration when rolling the ship can be up to 0.8 g. This corresponds to a roll angle of 38º. A rolling period of between 2-3 times per minute is not unusual.

Calculation of the load force for tipping

When calculating the load force, taking into account the risk of tipping, the length/width(_bp) of the package is set in relation to the height(_hp). This results in the tilt factor. It increases the force generated by the load when tipping.

The figures on the right show the difference when a Euro pallet is positioned lengthways or crossways to the direction of acceleration under the same conditions.

A stowage plan helps to make it easier to assess the situation and decide how to stow.

Yours, Sigurd Ehringer

Tobias Kreft