5  Summary, outlook and objectives

5.1  Load assumptions

The considerations and investigations into load assumptions have shown that it is inadmissible to interpret the previously conventional values of 0.8 g forward and in each case 0.5 g rearward and to the sides directly as vehicle accelerations in the form of braking deceleration, starting up acceleration or centrifugal force of cornering. In particular, direct reference to maximum forces which can be transferred by the vehicle tires is inadequate. The forces acting on the cargo are significantly boosted by inclination of the loading area (pitching and rolling angles) and by tangential inertial forces from superimposed pitching and rolling oscillations. At the same time, the normal force which is of importance to friction and inherent stableness of cargo units is reduced, despite always being inserted in conventional securing balances with the full weight of the units.

These findings may mean, for forward load assumptions, that for a vehicle equipped with tires and a braking system capable of delivering braking deceleration of 0.8 g, the acceleration assumed for cargo securing must be 1.0 g.

5.2  Rolling factor

In this context, the rolling factor of 0.2 g set out in VDI Guidelines 2700, Sheet 2 was also investigated, which has previously been intended for use as an allowance for addition to the assumed transverse acceleration of 0.5 g for securing cargo units which are not inherently stable against tipping. It is correct that such an allowance is necessary due to the rotational inertia of units, since the turning moment to be derived there from is not covered by the conventional tipping moment composed of transverse force and lever relative to tipping axis. However, at 0.2 g, the allowance is set too high. An allowance of 0.1 g, as stated in the 2009 draft standard DIN EN 12195-1, would seem entirely adequate. Moreover, a corresponding allowance should also be used for cargo units which are at risk of tipping in the longitudinal direction of the vehicle.

If, including the allowance of 0.1 g, the stableness of a cargo unit is ensured by the inherent stableness, a tipping balance need not be calculated. Further investigations should, however, be carried out to clarify the extent to which this test for inherent stableness should take the actual moment of rotational inertia of the cargo unit and the decrease in normal force into account.

5.3  Conventional balance calculation methods

The analysis of conventional calculation methods compares and comments upon the following sources: VDI 2700, Sheet 2, November 2002, draft DIN EN 12195-1, April 2004, and draft DIN EN 12195-1, January 2009.

Some shortcomings and also errors were identified. The investigated methods are substantially limited, in the case of direct securing, to introducing the maximum loading capacity of the securing devices into the balance, while in the case of frictional securing (tie-down lashing) the vertical components of the nominal pretension of the lashings are used. Shortcomings in direct securing are that the cargo movements necessary for developing the force in the securing devices do not appear in any form, not even as a warning, in the regulatory texts and the shortfalls in securing arising from the different load-deformation behavior of securing devices arranged in parallel are also not mentioned.

With few exceptions, the horizontal components of tie-down lashings arising from frictional engagement are ignored. They are introduced statically into the test of securing against tipping in draft DIN EN 12195-1 of April 2004, in order to take account of the k-factor which had in the meantime been recognized as important. This k-factor takes account of the known circumstance of friction of a lashing at the cargo edges and thus of impaired pre-tensioning if, as is customary, only one side is pre-tensioned. However, for reasons which are not publicly known, the k-factor was dropped again from the later draft of DIN EN 12195-1 of January 2009 and would seem to have been replaced by a half-hearted safety factor.

The k-factor is certainly justified and important as an expression of general weakening of the tie-down lashing principle in the case of a one-sided tensioning device. The stated sources make no meaningful interpretation and use of the underlying causes because there was a desire for simple formulae and there was therefore no willingness to take account of the laws of force and deformation.

5.4  Cargo movement and deformation of securing devices

Direct securing, which is justifiably regarded as highly effective, inevitably involves movement and/or deformation of the cargo. However, tolerable limits for such movement or deformation are not specified anywhere. They nevertheless exist and agreement should be reached. It would then, however, be consistent to allow the same movement latitude to tied down cargo units. The potential of frictional securing, which is itself associated with drawbacks, could be further exploited as a consequence.

The deformation brought about by development of force in portable securing devices may straightforwardly be calculated with sufficient reliability. Obtaining comparable data for fixed fittings and attachments on loading areas, such as sidewalls, end walls and stanchions is problematic. Enquires may be made of the vehicle manufacturers.

Taking account of cargo movement and deformation of securing means, the calculation of which has been demonstrated in a number of examples, demonstrates the worrying order of magnitude of the above-stated shortcomings in conventional calculation methods in both the positive and the negative direction.

5.5  Calculation methods

The objective cannot be to replace the conventional, relatively simple formulae which can be presented in tabular form for dimensioning sufficient securing effort with more complicated calculations, at least not for day to day use. The obvious conclusions must, however, be drawn. In so doing, all the advantages of the extended approach should be used. It must thus already be acknowledged that the tie-down lashing principle will benefit. Its reputation is enhanced and lashing requirements may be reduced to what is physically justifiable. It is as yet unclear which new formulae and associated constraints may be used to achieve this objective.

A similar situation may apply to direct securing, if certain physical laws are more effectively applied than in the past. However, the homogeneity of securing arrangements, i.e. uniform load-deformation behavior and limitation of cargo movement may also give rise to restrictions.

Similar approaches to calculation which are yet to be developed may also be applied to compaction, i.e. bundling and strapping, and enable economically attractive securing systems.

Ultimately, there must be simple, practical and legally reliable rules and guidelines, which may be applied while taking the fullest possible account of the underlying physical phenomena.