Episode 48: Calculating dunnage bag to match the goods – what is correct?

The use of dunnage bags in containers is a common method and has become an integral part of the repertoire of securing methods. Nevertheless, it is a method whose effectiveness is obvious, but which is also associated with risks.

The CTU Code devotes an entire chapter to stowage cushions in Appendix 7, Annex 4, and is packed with formulas that at first glance seem to overwhelm the normal shipper/consignor. It therefore seems appropriate to shed some light on the subject and explain the logic contained therein.
In general, dunnage bag serves to close existing gaps between cargo units in the container and to transfer the resulting acceleration forces of the cargo to the container walls without interruption.

Two approaches are described:
1. How large must a dunnage bag be in order to prevent a load of known weight from moving under defined accelerations and friction coefficients?
2. What load forces can a dunnage bag with known dimensions and filling pressure absorb?
Corresponding formulas for both questions can be found in the CTU Code.

The calculation can also be simplified for practitioners who are more familiar with the unit “daN”. The example calculation is intended to show this. The first line calculates with tons and the second with kilograms. To convert from kN to daN, the result must first be multiplied by 1,000 and then divided by 10. If you take the weight of the load in kg and calculate the acceleration due to gravity as 1, you have the result in daN in one calculation.

Result: The cushion must be able to absorb a load force of 1,812daN.

If we now compare the two calculated results, namely the 0.45 m2 required by the cargo weight with the area resulting from the cargo dimensions, namely 2.80 m2 , we obtain 6 times the safety factor.

The calculated figures show that a cushion that is dimensioned according to the load can generally absorb considerably more load force than is actually required. This also leads to the conclusion that a high filling pressure is not necessary. The use of dunnage bags requires a lot of air but little pressure. Filling via a compressed air system takes more time than with a blower. Provided that no other technical mistakes are made, you are always on the safe side.

Of course, it is always necessary to consult with the supplier of dunnage bag and request the specific handling data.

The above description also shows that, despite its many formulas, the CTU code is not as complicated as it appears at first glance. So don’t be afraid to take a closer look at the rules.

Yours, Sigurd Ehringer