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would seem to have been replaced by a half-hearted safety factor.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.