8. Homogeneity of securing arrangements
The Annex 13 assigns strength capacity values to securing elements and materials by declaring the „maximum securing load“ (MSL) as a percentage of the nominal breaking strength. However, these MSL figures shall be reduced by a safety factor, in order to get the so-called „calculation strength“ (CS) when used in a balance calculation. The reason for this reduction is, in the first place, the risk of inhomogeneous distribution of forces among securing devices when it comes to the assumed extreme load case. This is an effective safeguard against overloading individual securing devices.
It appears nowadays, that increasingly fibre belts are used to secure heavy cargo units. These fibre belts have an allowable elastic elongation of 7% when loaded to their MSL. This is much more than the elastic elongation of wire rope lashings (about 1.6%), long link chains (about 1%) or short link chains (about 1,5%). The elastic deformation of welded stoppers or steel stanchions for the carriage of pipes on deck would be even less than that.
The combination of fibre belts with chains or moreover, the combination of fibre belts with welded stoppers may create a securing arrangement that is extremely inhomogeneous, in that the more stiff securing devices have to take the „lion-share“ of the load. This will no longer be compensated by the above mentioned safety factor. The Annex 13 should therefore be amended by an appropriate warning under its paragraph 6 as follows:
If securing devices of diverse elasticity are used in parallel, e.g. welded stoppers and fibre belts or steel stanchions and long wire lashings, the stiff devices, i.e. the stoppers or the stanchions in the above examples, must be able to keep the cargo in place alone. The more flexible securing devices in such an arrangement should be excluded in a sliding balance calculation or be taken into account in the tipping balance only.
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