What are the minimum requirements that need to be met by regulations governing the securing of cargo? This question is of concern to a very wide range of people, who will not be listed here. The answers are multi-faceted and driven by varying expectations.
As far as the practical implementation of cargo-securing measures is concerned and when carrying out police inspections, it is necessary to rely on „accepted technical rules and regulations“. In addition to notes on how securing should be performed, these rules also include mathematical test criteria that assess the balance between defined loads and stresses that arise during transportation on the one hand, and the effectiveness of the selected manner of securing the cargo on the other. Such assessments and their results are based on vastly simplified mathematical models, and it must not be assumed that these models offer an entirely accurate or complete reflection of reality either with respect to the loads and stresses experienced or with respect to the effectiveness of the securing. This has already been dealt with comprehensively in the report „Securing cargo in road transport – Who knows the truth?“.
An exact representation of the physical reality using the mathematical models described is hardly feasible; indeed, the huge variety of factors that play a role means that it is not even an appropriate goal. Rather, the objective is to make the model used simple and universally applicable, while at the same time ensuring that the objectives and requirements listed above can be achieved without an excessive amount of effort. The wide range of factors that play a role means that it is only possible to determine whether a mathematical model meets these requirements after a period of several years and careful analysis of accidents. The simple fact that accidents still occur is not, on its own, an indicator of the model’s technical suitability, because accidents are often demonstrably the result of a failure to comply with the model.
The table below shows an overview of the mathematical models that are currently regarded as being necessary.
Up to now, calculations have taken virtually no account of compaction in the form of strapping or bundling individual cargo units or covering bulk cargo. The effectiveness of any combination of tie-down lashing or direct lashing with blocking measures is checked by simply adding together the securing effects. Checks with respect to the longitudinal axis make a distinction between two directions: in the direction of travel and against the direction of travel. This is because it is assumed that the corresponding loads experienced during transportation are different. Tie-down lashings are generally regarded as a number of lashings passed over the cargo transverse to the vehicle and which are attached to both sides of the vehicle, but are pre-tensioned on one side only.Interpretation of vastly simplified mathematical models sometimes results in misunderstandings. Thus, for example, the currently accepted technical rules and regulations regard a tie-down lashing as adequate to withstand a load in the direction of travel if the inertial force of the cargo FX = 0.8 × m × g is equal to or less than the friction between the cargo and the vehicle FR = μ × (m × g + ∑FV).
0.8 × m × g ≤ μ × (m × g + ∑FV)At this point it is uncertain whether the coefficient of friction μ for static friction or dynamic friction should be used and how the total of all the vertical pre-tensioning forces ∑FV of the tie-down lashings should be arrived at. This is purely a question of calibrating the model so that the result is able to stand for the complex physical reality. It is therefore wrong to use the mathematical model to conclude that a tie-down lashing checked against it will necessarily be able to withstand an emergency braking manoeuvre involving a deceleration force measured at 0.8 g without the cargo sliding (referred to here as „displacement“) or becoming deformed. In the same way, the fact that the cargo is expected or observed to slide during an emergency braking manoeuvre despite being lashed down should not be taken to mean that the coefficient of dynamic friction should necessarily be used in the mathematical model.
Simplified mathematical models can be calibrated by means of statistical analysis of a considerable number of large-scale, systematic trials. However, this is an expensive undertaking that has never been put into practice. An alternative approach is to carry out comprehensive physical analysis of the load mechanisms on the basis of a small number of trials. It is possible that there is already a significant amount of material available. In fact, however, the preferred approach in the past appears to have been to assess proposed mathematical models on the basis of „previous practical experience“ and it is unavoidable that economic considerations will also have played a role. Nevertheless, after a number of years have elapsed, it is somehow possible to assess the suitability of a mathematical model retrospectively.
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