|5.3 Securing balance|
|A securing balance may give an indication on whether a planned or implemented cargo securing arrangement has a sufficient level of strength. However, a balance cannot reveal deficiencies of workmanship. Hence, the application of a balance and the confidence placed into it are only justified if all securing elements considered, here including the container walls, corner posts and securing points, are able to transfer the forces appointed to them in the balance.
The balance is carried out in accordance with the Annex 13 to the IMO CSS-Code. The approach of the balance reads:
External force ≤ friction + sum of forces from lashing and shoring
The external forces in Kilo-Newton (kN) result from a longitudinal acceleration of 1 g and a transverse acceleration of 0.8 g, multiplied with the cargo mass m in tonnes.
The friction force in kN is the product of the applicable friction coefficient and the cargo weight, also in kN, calculated from mass multiplied with gravity acceleration m · g.
The securing forces from lashing and shoring in kN are the sum of MSL-values of the lashings and shores, divided by 1.5. The latter is requested by the CSS-Code as an extra safety factor, because it cannot be supposed that all securing devices carry with their MSL simultaneously.
The MSL of lashings in a container is generally limited by the securing points to 10 kN. Longitudinal or transverse lashings should not deviate from their assigned effective direction by more than 30°. Lashing running steeper than 60° should not be counted or at least only partially included in the calculation. The MSL of timber shores for bracing is 0.3 kN per cm² cross-section transverse to the timber grain and 1 kN per cm² along the grain (see Container Handbook, Vol. I, chapters 4.3 and 4.4).
Example: A coil of 19 t mass on a skid is placed into a container. Longitudinal securing is accomplished by two half loops to fore and aft with two loaded ends each (similar to Figure 18), and by three timber shores fore and aft of 12 x 12 cm (similar to Figure 20). Transverse securing is achieved by four timber shores of 10 x 10 cm to each side. Care has been taken that all timber shores can transfer the desired forces evenly into the container walls respectively corner posts. The balance in kN then reads:
Notice: Other than in the Annex 13 to the CSS-Code, this balance does not presume that the friction in the bottom of the container is reduced by vertical accelerations, simultaneously appearing with maximum longitudinal forces. This phenomenon is only encountered at sea, where, however, the longitudinal acceleration is only about 0.3 g. In the above balance the acceleration applicable for road or rail transport of 1 g is used, requiring greater securing forces. In this way, also the Annex 13 requirement for sea transport is satisfied.
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