Episode 45: Are dunnage bags used transversely and longitudinally to the direction of travel CTU Code compliant?

Securing cargo in containers using dunnage bags is a common method, but there are still repeated discussions about the type and use of this securing method. Two episodes on this topic have already been published and can be referred to:

Episode 20 Cushioning part 1
Episode 21 Cushioning part 2

The regulations for securing loads in containers are set out in 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.

For the following considerations, it is important to define the perspective, as there are certainly differences here and incorrect interpretations should be avoided.

Viewpoint 1: The ship is in the center with the directions longitudinal=direction of travel and transverse to the direction of travel. All data, e.g. accelerations, refer to this. Thus we have the main load transverse to the direction of travel with 0.8g when the ship is rolling and only 0.4g in the longitudinal direction.

Viewpoint 2: The securing devices are at the center. They are positioned in the container according to their proportions. A dunnage pad, which is inserted with its broad side parallel to the longitudinal direction of the container, has been stowed lengthwise. Turned through 90°, it is stowed crosswise.

The same situation under

Viewing angle 1:
the cushions close gaps across the direction of travel

Viewing angle 2:
The upholstery is positioned lengthways.

The following descriptions are provided from perspective 1.

The CTU Code does not specify for which acceleration directions the bumpers are permissible or not permissible. However, the graphics in the versions always show situations in which the bumpers compensate for gaps at right angles to the direction of travel.

In this case, they must compensate for an acceleration of 0.8g. Congestion situations in which the bumpers are placed in gaps along the direction of travel are not described. In this case, they would “only” be exposed to an acceleration of 0.4g.

It is also not described that the dynamic pressure exerted by the cushioning on the cargo generates a frictional force between the cargo items and the container walls, which additionally secures the cargo. The ISO unit “1 bar” pressure results in a load of 1 ^kg/cm2.

Let’s assume we have a congestion situation like the classic block of 5 and the pallets are 165cm high. The left-hand pallet would have a side area (without Euro pallet) of 120x150cm = 18,^000cm2.

If the dunnage bag (level 1) is filled with 0.15 bar, a force of 2,700 daN would develop. With a friction of µ=0.2 to the container wall, this would result in a securing force of 540daN. The effect of the corrugations is not taken into account.

When the ship rolls, an even higher securing force would develop, limited in time, because the weight of the entire row of loads also presses against the wall. The weight of the right-hand pallet is therefore added, resulting in the increased frictional force.

Each of these paper rolls weighs approx. 770 kg.

Tipping tests with this load, which was only secured by a dunnage bag, resulted in tipping angles of approximately 45°.

The load was only held in place by the increased friction between the load and the container wall. This friction was generated by the pressure exerted by the cushion.

At this angle, the tangent has the value 1, which corresponds to an acceleration of 1g. The acceleration specified for the container in the longitudinal direction is only 0.4g.

These tests show that the friction between the load elements and the container walls results in values that should not be neglected in load securing.

Quite a few people are worried that the airbags will burst. As a rule, they do not. They are designed and manufactured in such a way that they can withstand several times the pressure caused by the weight of the load or the acceleration.

For example, a baffle with level 1, which should compensate for a gap of 30 cm, burst at a pressure of 0.55 bar in the test. However, the maximum working pressure is 0.2 bar. In practice, values between 0.1 and 0.15 bar have proven to be common, as more and more users are filling the dunnage bags with a blower instead of a compressor.

Many manufacturers print handling instructions on the dunnage bags. The extract in the picture shows the 0.20 bar mentioned.

Dunnage bags burst, if at all, due to incorrect handling. Sharp edges, nails, rust on container inner walls, chafe marks, sharp parts of the load, etc. These are the dangerous circumstances.

The stevedore has it in his hands and can exert influence.

A relatively frequent congestion situation occurs when the container is not completely full and there is a gap (> 15 cm) to the container door.

It is common practice to move this gap inwards and close it with dunnage bags so that the door can be used again for form-fit load securing. In other words, they close gaps that occur along the direction of travel.

Tests have shown that dunnage bags can be used to secure the load lengthways. They can either be used to close gaps or to correct the load distribution in the container so that the overall center of gravity is within the permissible ranges.

The load in this container consisted of several tons of chipboard, which were to be stowed at a distance from the end wall for load distribution reasons.

For the simulation, the 3D dunnage bag was placed against the container door so that the container could be tilted.

To prevent damage, a protective plate was inserted between the stowage cushion and the door.

The tipping process was carried out with a smooth hydraulic movement.

The achieved tilt angle was 39°, which corresponds to an acceleration of 0.81g.

In the longitudinal direction, only 0.4g is required for sea transportation.

The stowage cushion remained undamaged.

It is therefore perfectly conceivable to use dunnage bags in the container in this way. If a form-fit stowage method is achieved and the container including the load behaves like “one piece”, then the risk is comparatively low. It is only increased by incorrect handling.

It is therefore important that all parties involved explore the available options and implement them accordingly. It is not possible to present all possible solutions for load securing in the CTU Code.

VDI-2700 “Load securing on road vehicles” describes this as follows:

“The variety of goods and load securing options cannot always be taken into account in the examples shown. Equivalent load securing devices or those that achieve better securing can also be used as an alternative to those described.

The suitability of load securing aids/methods not regulated by these guidelines can be determined and verified by representative tests in individual cases with documentation by independent experts. The suitability verified in this way must be confirmed by technical information from the manufacturer with reference to the type, date of manufacture/serial number.”

This approach could also be applied to the CTU code, although it is not explicitly mentioned.

The shipper/stevedore is therefore required to check his solutions against the framework conditions specified in the CTU Code and, if necessary, to prove by appropriate tests that the specifications are met.

Let’s get to work!

Yours, Sigurd Ehringer

Thomas Bauer

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