Dunnage bags on top

The necessity of securing goods between cargo and the container ceiling is a much-debated topic. Experienced shippers have varying opinions, influenced by their geographical locations and practices handed down. In developed countries with good quality road networks, vertical securing is often deemed unnecessary, although applied due to regulations. However, in developing nations with poorly maintained roads, this practice is more necessary.  Experts consider not just road conditions, but also the fact that containers on ships face multiple acceleration forces simultaneously due to their six degrees of freedom. In addition, containers on the top deck face greater angular acceleration than those below deck.  To address this issue, we must understand the acceleration coefficients in the CTU Code and when to secure against vertical movement. Additionally, we need to determine which securing devices to use.

For example, if a shipper prepares pallets weighing 1,000 kg each for road transport to a port and then loads them into a sea container for a 20-day journey, the CTU Code, Annex 7, Chapter 2.2.2, provides a formula for calculating the “magnitude of the vertical friction forces”.

F_f = µ c_z  M * g [kN] mass of cargo [ton] and g = 9.81 [m/s²]

A comprehensive list of known frictional factors (µ) is available in the CTU Code Annex 7, Appendix 2.  For our example, we will use a friction factor of 30% or 0.3µ.

Furthermore, the CTU Code provides acceleration coefficients for all modes of transport in Chapter 5.3 – please see the enclosed table.

Ff = µ cz  M * g [kN] mass of cargo [ton] and g = 9.81 [m/s²]

Road transport:

F_f = µ c_z  M * g

F_f = 0.3 1 1 * 9.81

F_f = 2.9 kN or 290 daN

Sea transport:

F_f = µ c_z  M * g

F_f = 0.3 0.2 1 * 9.81

F_f = 0.59 kN or 59 daN

This clearly shows that the frictional force between the pallet and the container floor is significantly reduced during ocean shipping compared to road transport. Several questions remain. During road transport, drivers typically use tie-down ratchet belts over pallets for securing loads, often due to regulatory requirements. These top-over lashings help secure against vertical acceleration. However, sea freight presents a different scenario. Cargo on a vessel may experience simultaneously vertical (0.2g), longitudinal (0.4g), and transverse (0.8g) forces, during pitching and rolling.

Assuming the pallets have a footprint of 1.0x1.0m, there will be a 35cm gap between them. If this gap is filled with dunnage to create a form-fit stow, no movement will occur during the ship's rolling, and vertical securing may not be necessary even when the friction is reduced.

If a form-fit stow isn't possible, pallets will shift due to the ship's rolling, exacerbated by angular acceleration. 

In this case, measures like direct or top lashings must be employed, or the gap between the cargo and the container ceiling must be filled. Dunnage bags are commonly used, but they have limitations. 

The container roof isn't designed to support excessive external forces exerted by dunnage bags and may bulge under pressure, leading to permanent damage. Any repair cost as a result of container damages, is the responsibility of the shipper.  Additionally, the weight of the container stowed on top of the deformed roof, can increase forces downward on the dunnage bag, potentially damaging the cargo underneath it.  Care must be taken whilst using dunnage bags in this position.

I trust this explains the use of dunnage bags between the cargo and container ceiling, but hereby a summary:

1. A form-fit stow is preferable

2. Used this way, dunnage bags provide minimal securing force and mainly fill the top void

3. Obey manufactures recommendation regarding the maximum void a specific bag can fill 

4. Use caution when applying this method