Appendix 4. Specific packing and securing calculations

1            Resistivity of transverse battens

              The attainable resistance forces F of an arrangement of battens may be determined by the formula (see also figure 7.57):

2            Bedding a concentrated load in a general purpose freight container or on a flatrack

              Bedding arrangements for concentrated loads in general purpose freight containers and on flatracks should be designed in consultation with the CTU operator.

3            Longitudinal position of the centre of gravity of cargo

              The longitudinal position of the centre of gravity of the cargo should be used in connection with specific load distribution rules and diagrams of CTUs ^[1] . The longitudinal position of the centre of gravity of the cargo within the inner length of a packed CTU is at the distance d from the front, obtained by the formula (see also figure 7.58):

d =	distance of common centre of gravity of the cargo from the front of stowage area [m]
mn =	mass of the individual packages or overpack [t]
dn =	distance of centre of gravity of mass mn from front of stowage area [m]
Figure 7.58 Determination of longitudinal centre of gravity

Example:

A 20-foot container is packed with five groups of cargo parcels as follows:

 m

4            Cargo securing with dunnage bags

4.1         Introduction

4.1.1       Accelerations in different directions during transport may cause movements of cargo, either sliding or tipping. Dunnage bags, or air bags, used as blocking devices may be able to prevent these movements.

4.1.2       The size and strength of the dunnage bag are to be adjusted to the cargo weight so that the permissible lashing capacity of the dunnage bag, without risk of breaking it, is larger than the force the cargo needs to be supported with:

F_{DUNNAGE BAG}   ≥ F_CARGO

4.2          Force on dunnage bag from cargo (F_CARGO)

4.2.1       The maximum force, with which rigid cargo may impact a dunnage bag, depends on the cargo’s mass, size and friction against the surface and the dimensioning accelerations according to the formulas below:

4.2.2       The load on the dunnage bag is determined by the movement (sliding or tipping) and the mode of transport that gives the largest force on the dunnage bag from the cargo.

4.2.3       Only the cargo mass that actually impacts the dunnage bag that should be used in the above formulas. If the dunnage bag is used to prevent movement forwards, when breaking for example, the mass of the cargo behind the dunnage bag should be used in the formulas.

4.2.4       If the dunnage bag instead is used to prevent movement sideways, the largest total mass of the cargo that either is on the right or left side of the dunnage bag should be used, that is, either the mass m₁ or m₂ (see figure 7.59).

4.2.5       In order to have some safety margin in the calculations, the lowest friction factor should be used, either the one between the cargo in the bottom layer and the platform or between the layers of cargo.

4.2.6       If the package on each side of the dunnage bag has different forms, when tipping the relationship between the cargo width and height of the cargo stack that has the smallest value of b_p / h_p is chosen.

4.2.7       However, in both cases the total mass of the cargo that is on the same side of the dunnage bag should be used, that is, either the mass m₁ or m₂ in figure 7.60.

4.3          Permissible load on the dunnage bag (F_DB)

4.3.1       The force that the dunnage bag is able to take up depends on the area of the dunnage bag which the cargo is resting against and the maximum allowable working pressure. The force of the dunnage bag is calculated from:

4.4          Contact area (A)

4.4.1       The contact area between the dunnage bag and the cargo depends on the size of the bag before it is inflated and the gap that the bag is filling. This area may be approximated by the following formula:

4.5          Pressure in the dunnage bag

4.5.1       Upon application of the dunnage bag it is filled to a slight overpressure. If this pressure is too low there is a risk that the dunnage bag may come loose if the ambient pressure is rising or if the air temperature drops. Conversely, if the filling pressure is too high there is a risk of the dunnage bag bursting or damaging the cargo if the ambient pressure decreases, or if the air temperature rises.

4.5.2       The bursting pressure (P_B) of a dunnage bag depends on the quality and size of the bag and the gap that it is filling. The pressure exerted on a dunnage by the cargo forces should never be allowed to approach bursting pressure of the bag because of the risk of failure. A safety factor should, therefore, be incorporated and, if necessary, a dunnage bag with a higher bursting pressure selected.