Episode 8: Positive locking in the container

Securing loads in containers is not really a problem, but inspection practice and the damage rate show that many shippers/stevedores have difficulties with “knowing how”.

In this episode, I will explain the principles of form-fit stowing/packing and the options for implementation. The difference between frictional locking and positive locking has already been explained in many places, which is why it will not be discussed here.

Positive locking in the container means that the cargo should/must not move relative to the container due to accelerations, which are mainly generated by the rolling and pitching of the ship.

The formula

The calculation of the load securing acc. CTU code 2015 is calculated using the following formula:

C _x,y * m * g ≤ r _x,y * P * g + µ * _Cz * m * g + _Fsec

The formula consists of the following elements:

• Force from the load = C _x,y * m * g
• Retention force of the container = r _x,y * P * g
• Force from friction = µ * _Cz * m * g
• Force from securing devices: = _Fsec

Note: The restraining force of the container is calculated using the drag coefficient r _x,y and the payload P (which can be found on the container door). See also LaSi blog episode 3.

To better understand the formula, it can be imagined as the contents of scales.

The left-hand pan contains the sum of the forces resulting from the load weight (m) multiplied by the direction of acceleration (C _x,y) and the gravitational force (g).

The right-hand pan contains three components:

• Securing forces resulting from the restraining force of the container superstructure,
• + Securing force from the friction between the load and the container floor
• + Securing forces resulting from securing devices (e.g. lashings)

Their sum must be equal to or greater than the sum in the left-hand pan.

If you compare the elements in the right-hand weighing pan with each other, you will see that the form-fit securing, in which the forces from the container superstructure are used, is the most effective.
As it is common practice in load securing to state the forces in daN, the weight is stated in kg instead of tons in the following example calculation and the result is converted to daN.

Calculation example

A load of 20,000 kg stowed in a 42G1 container. The form-fit stowed load is calculated in the longitudinal direction with an acceleration of _Cx=0.4g. The retention force(rx₎ of the container door is 0.4 * the payload (P) 26,700 kg.

If the distance between the container door and the load were greater than 15 cm, i.e. no positive fit to the door, whereby _rx is set to 0, the calculation would look like this:

If the securing forces _Fsec were at least 66,708 N, the scales would be balanced and the load sufficiently secured. More securing force would of course also mean more safety and reserve. So don’t sew too tightly.

The more elegant solution would be to change the stowage plan by moving the gap to the inside of the container and closing it with stowage aids (e.g. stowage cushions / dunnage / empty pallets).

The positive fit to the container door(_rx=0.4) is then given again and the calculation looks as follows:

It is advisable to take into account in the stowage plan that there should be a load at the container door that can transfer the forces well and over a large area. A single pallet (which is often seen) would not fulfill this purpose.

Even if we were to calculate the longitudinal acceleration instead of _Cx=0.4 g for the ship with 0.8 g for the full braking of the truck and take the vertical acceleration _Cz with 1 into account, the result would still be in the green area, as the calculation shows.

Creating the form fit

So there is a lot to be said for not giving up the form closure lightly for reasons of time or cost.

How can this be achieved?

1. Fill the container with cargo without gaps (e.g. bulk goods)
2. Stow/pack the load so that the total gap is less than 15 cm
3. Compensate gaps with dunnage
4. Compensate for gaps by form-fit securing with lashing straps

1. fill with piece goods

Filling the container with bulk material is a simple solution in principle, but it also has its pitfalls if the bulk material is very free-flowing (coffee/beans etc.). The filling level must then be as high as possible.

2. gap sum less than 15 cm

Stowing and packing the load in such a way that the total gap in all directions is less than 15 cm requires that the dimensions of the packages or loading units correspond to the container dimensions. If, as is often the case, Euro pallets are stowed, gaps inevitably occur. If gaps are unavoidable, special attention must be paid to securing the load unit. The load units must be dimensionally stable and resistant to tipping.

3. use dunnage

Compensating for gaps with stowage material is common practice. The technically elegant solution is to fill the gaps with air to create a tight fit. However, this method also has its pitfalls. The CTU Code 2015 dedicates a separate chapter(Appendix 7 – Annex 4) to the bumper pads with many formulas that are rather difficult for practitioners to implement. The main risks lie in the inadequate use of pads, e.g. incorrect size, contact with sharp edges, chafing, etc.

Another episode of the LaSi blog will deal with this topic in more detail.
Episode 20: Dunnage bags load securing – Part 1

Gaps can of course also be compensated for with dunnage. Here, it is particularly important to ensure that the forces are always transferred to the container walls over the entire surface and not as a point load. You can see a corresponding example of a correct and incorrect application of timber bracing in the drawing.

If dunnage is used, the rules for nailing and screwing must also be observed so as not to weaken the joints.
A good nail driven in the wrong way (e.g. into the end grain (red circle)) provides less securing force than expected. Wood with a tree edge (red arrow) can lead to quarantine in certain countries such as Australia.

Pressing dunnage into the corrugations of the container wall in this way causes more damage than it provides useful securing force. Unfortunately, this is a common application, even for professional stevedoring companies.

The actual securing force is undetermined and no reliable data can be found. Therefore, this method should not be used unless appropriate tests have produced repeatable results and the handling is clearly described.

Empty pallets are also a popular aid. However, if the impression could be created that mainly empty pallets are being transported, a better solution should be sought.

Styrofoam or rigid foam blocks are also suitable as gap fillers, especially if the same congestion situation occurs frequently.

4. form-fit securing with lashing straps

Compensating for gaps by form-fit securing with lashing straps is a common method, especially for machines and systems. However, the load-bearing capacity of the lashing eyes in the container and the lashing force of the securing equipment must be taken into account here.

Depending on the manufacturer, it is important to check whether the breaking load or the MSL (maximum securing load) is specified. Chapter 2.4.2 of Appendix 7 of the CTU Code 2015 specifies how the MSL is calculated for certain securing devices. For example, an MSL of 75% of the breaking load with a maximum elongation of 9% is permissible for single-use belts, while the MSL for reusable belts may only be calculated at 50%.

The MSL of the securing devices must harmonize with the MSL of the lashing or attachment points and the weakest link in the chain must be taken into account.

Of course, the lashing angles α and β must also be taken into account for each lashing and the resulting effective forces calculated.

Yours, Sigurd Ehringer

Tobias Kreft