The Proper Use of Dunnage Bags in Cargo Securing

Dunnage bags have been an essential tool for securing cargo in containers, trucks, and ships for decades. However, despite widespread use, there are numerous misconceptions regarding their correct application. In practice, incorrect usage is more common than proper implementation. Furthermore, securing solutions are often based on experience and passed-down knowledge rather than precise calculations. This outdated approach is flawed, but the CTU Code has made significant contributions toward standardizing best practices. By following these guidelines, shippers can ensure that the appropriate size and number of dunnage bags are used to secure cargo safely during transport.

Fundamental Principles of Dunnage Bag Usage

To ensure proper cargo securing, the following principles must be observed:

• Cargo securing is the shipper's responsibility. The cargo must be properly restrained within a Cargo Transport Unit (CTU).

• Cargo must be secured against sliding and tipping.

• Void spaces exceeding 15 cm in any horizontal direction must be filled. Dunnage bags are preferred due to their simplicity and effective blocking properties.

• Manufacturer specifications must be followed.

• The securing forces generated by the dunnage bag must be greater than the forces from the cargo acting on the dunnage bag. F_DB ≥ F_Cargo

Selecting the Correct Dunnage Bag

When choosing a dunnage bag, two key factors must be considered:

1. Functionality

Manufacturers offer dunnage bags constructed from different materials, woven polypropylene, laminated paper or multiple plies of kraft paper.

Additionally, dunnage bags come with either standard or fast-inflation valves and fitted on the corner or edge or mid-length.

The choice of material and valve type is based on individual preferences or supplier limitations and does not necessarily determine the bag's capability.

2. Capability

Dunnage bags function as dynamic blocking devices. Their effectiveness depends on:

• Bag dimensions (L x W)

• Cargo dimensions (L x W x H) and weight

• Bag level (which determines the recommended working pressure)

For optimal performance, a minimum of 80% uniform contact area between the dunnage bag and cargo is the accepted standard.  Using the variables above, one can calculate the required securing forces per application.

Calculating the Correct Dunnage Bag Usage

The CTU Code Annex 7, Appendix 4 details the theory behind dunnage bag calculations. While these formulas may seem complex, AtMet provides a user-friendly reference table to simplify the selection process for their experts.

To illustrate, consider the following example:

• Pallet Dimensions: 110 x 110 x 200 cm

• Pallet Weight: 900 kg

• Dunnage Bag: 90 x 180 cm, Level 1

• Mode of Transport: Sea Freight

Since the selected dunnage bag covers more than 80% of the pallet’s surface area, it meets the standard requirements for effective cargo securing.  According to AtMet’s commercial specification, it is fit to be used in the void size between pallets.

FDB ≥ FCargo

Securing forces of a dunnage bag: F_DB

F_DB = A x 10 x g x P_b x SF [kN]

but

A = (b_DB – π x d/2) x (h_DB – π x d/2) [m²]

Step 1: calculate void between pallets

d = 2.35 – (1.1+1.1) = 0.15 m

Step 2: calculate the contact area of the dunnage bag

A = (0.9 – π x 0.15/2) x (1.8 – π x 0.15/2) = 1.05 m²

Step 3: calculate the forces exert by the bag

F_DB = 1.05 x 10 x 9.81 x 0.6 x 0.75 = 46.35 kN = 4,635 daN

Forces on the dunnage bag from the cargo: F_CARGO

Sliding transverse:

F_CARGO = m x g x {c_y – (μ x 0.75 x c_z)} [kN] 

F_CARGO = 0.9 x 9.81 x {0.8 – (0.45 x 0.75 x 1.0)} 

F_CARGO = 4.08 kN = 408 daN

Tipping transverse:

F_CARGO = m x g x {c_y – (bp/hp)} x c_z) [kN]

F_CARGO = 0.9 x 9.81 x {0.8 – (1.1/2) x 1}

F_CARGO = 2.21 kN = 221 daN

Now that we calculated both sides of the equation, we can compare to understand if the chosen dunnage bag exert sufficient forces.

F_DB ≥ F_Cargo

4,635 daN > 408 daN – sliding

4,635 daN > 201 daN – tipping 

Since the answer is true, the securing forces are sufficient to prevent sliding, tipping or rotation.

By following these guidelines and calculations, shippers can ensure that dunnage bags are applied correctly, enhancing cargo safety and compliance with international transport regulations. For further assistance, AtMet’s team of experts is available to provide tailored solutions for your cargo-securing needs.