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.
Top of page
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.
Top of page
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,
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.
Top of page
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.
Top of page
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
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.