3 Extended approaches
3.1 Cargo movement
Cargo movements which result in necessary deformation (usually changes in length) of securing devices, primarily involve sliding or slight tilting and many and varied changes in the shape of the unit itself which may be superimposed on the first two types of movement mentioned. Sliding is generally irreversible, while slight tilting is reversed once the external tipping moment has disappeared.
Changes in the shape of the cargo unit may be elastic, but there is usually a considerable plastic component to the change. Since extreme loads occur as individual events in road transport, permanent deformation of the loading arrangement is more readily acceptable because checking and remedying the securing arrangement is immediately and straightforwardly possible.
This assessment may be explained by making reference to maritime transport, where extreme loads are associated with storms and rough seas. These conditions may last for an extended period, as a result of which an unforeseeable succession of extreme loads may occur and checking and remedying cargo securing arrangements during this period can often only be carried out at risk to life.
Figures 13 and 14 show the basic cargo movements, while some further cases could certainly be added to the deformations shown in Figure 14.
Figure 13: Types of movement of rigid cargo units
Figure 14: Types of movement of flexible loading arrangements
Which kinds and amounts of movement of secured cargo can be tolerated in road transport has not previously been specified or recommended in any regulatory texts, guidelines or similar documents. Some influencing variables which can play a part in such considerations will thus firstly be mentioned.
The frequency of a load causing cargo movement could influence the tolerable extent such, that rare events, such as full braking or extreme centrifugal forces, are allowed larger movements than would be accepted in normal travel, since it is reasonable after an extreme event to drive into a parking place and check the securing arrangements.
The nature of the movement likewise has an influence on its tolerable extent. A sliding offset movement shifts the center of gravity of the cargo and may moreover exceed the loading area limit. Racking (shear deformation) of compact cargo units, on the other hand, may remain within the elastic deformation range and is therefore less critical. Racking of bundled units, however, is quasi-plastic and ought therefore to be limited in a similar way to sliding offset. Tilting of a cargo unit should be limited to very small angles due to the low damping of the tilting process.
The direction of movement is influential. Movements in the longitudinal direction are less critical than movements in the transverse direction, because the latter may exceed the admissible breadth of the vehicle and may also have a major effect on the transverse center of gravity.
In order to gain an impression of the magnitude of cargo movements which have not previously been scientifically accepted, it is worthwhile analyzing direct securing methods which have conventionally been regarded as "good". This does not, however, mean that the identified movements may thus generally be recommended as being tolerable.
The proper and rational way of defining and recommending tolerable cargo movements should ultimately proceed by applying numerical criteria:
If this is to be achieved, typical loading situations must be fully calculated. In order at this point to provide an initial impression of the order of magnitude of previously accepted cargo movement, one example of conventional direct lashing will be presented, using some formulae taken from section 3.2 below.
A cargo unit with the dimensions breadth = 2.1 m, height = 2.5 m is secured in the transverse direction onto the vehicle with diagonal synthetic fiber belts which are attached to the upper corners of the unit. The geometric components of the belts are X = 1.0 m, Y = 2.3 m, Z = 2.5 m, while the loaded length L = 3.54 m. The belts have an LC = 25 kN and an elongation of 3.75% once the LC is reached. The spring constant of the belt is thus:
The belts are pre-tensioned to 2.5 kN. The sliding balance (not shown here) shows that LC of the belts is just reached. The change in length of the belts must then be:
In order to achieve this elongation, the upper corner of the cargo unit must be deflected to the side by the amountD Y by undergoing a sliding or racking movement or by a combination of both movements:
The lateral movement of 0.18 m transverse relative to the vehicle under extreme loading would appear to be acceptable and, under certain circumstances, for example with elastic deformation, an even larger deformation could be accepted.
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