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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.
Three pieces of information are required to determine the size of the stowage cushion:
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:
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.
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.
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.
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.
Glide:
FLAG = m * g * (cx,y – μstatic * 0.75 * cz ) [kN]
Tipping:
CHARGE = m * g * (cx,y – bp /hp * cz ) [kN]
Below are a few excerpts from the CTU code with explanations.
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.
Glide:
FLAG = m * g * (cx,y – μstatic * 0.75 * cz ) [kN]
FL= 1to * 9.81 m/s2 * (0.8 – 0.3 x 0.75 x 0.2 g)
FL = 9.81 kN * (0.8 – 0.045)
FL = 9.81 kN * 0.755 = 7.406 kN = 740.6 daN
FL = 1 to * 9.81 m/s2 * (0.8 – 0.3 * 0.75 * 1.0 g)
FL = 9.81 kN * (0.8 – 0.225)
FL = 9.81 kN * 0.575 = 5.641 kN = 564.1 daN
FL = 1 to * 9.81 m/s2 * (0.8 – 0.3 * 0.75 * 1.8 g)
FL = 9.81 kN * (0.8 – 0.405)
FL = 9.81 kN * 0.324 = 3.178 kN = 317.8 daN
In the worst case, with a vertical acceleration of only 0.2 g, the force from the load is more than twice as great as under normal circumstances. The load calculation should therefore be based on the first example.
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.
Tipping:
CHARGE = m * g * (cx,y – bp /hp * cz ) [kN]
FL = 1to * 9.81 m/s2 * (0.8 – 0.80 m/ 1.90 m * 1.0 g)
FL = 9.81 kN * (0.8 – 0.42)
FL = 9.81 kN * 0.38 = 3.728 kN = 372.8 daN
FL = 1 to * 9.81 m/s2 * (0.8 – 1.20 m/1.90 m x 1.0 g)
FL = 9.81 kN * (0.8 – 0.63)
FL = 9.81 kN * 0.17 = 1.668 kN = 166.8 daN
Yours, Sigurd Ehringer
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G&H GmbH Rothschenk
Industriestrasse 5 & 7-10
97239 Aub
Phone: +49 9335 97 15 – 79
Fax: +49 9335 97 15 – 15
E-mail: info@rothschenk.de
Collection/delivery
Mon – Fri: 8:00 – 15:00
Office hours
Mon – Thu: 8:00 – 17:00
Fri: 8:00 am – 1:00 pm
G&H GmbH Rothschenk
Industriestrasse 5 & 7-10
97239 Aub
Phone: +49 9335 97 15 – 79
Fax: +49 9335 97 15 – 15
E-mail: info@rothschenk.de
Collection/delivery
Mon – Fri: 8:00 – 15:00
Office hours
Mon – Thu: 8:00 – 17:00
Fri: 8:00 am – 1:00 pm
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