6.5 Design criteria for different securing arrangements

  6.5.5 Top-over lashing alone is a frictional lashing method and the effect of the lashing is to apply vertical pressure increasing the friction force between the outer stows of deck cargo and the ship's deck/hatch cover.

Figure 6.3 Principles for top-over lashing

  6.5.6 For pure top-over lashing arrangements the friction alone will have to counteract the transverse forces so that the following equilibrium of forces is satisfied:

  6.5.7 In practice, sliding between the layers is often prevented due to slightly different heights of the timber packages. Alternatively it may be prevented by inserting vertical sturdy battens of proper dimensions between the columns.

Figure 6.4 Sliding of upper layer prevented by vertical sturdy battens

  6.5.8 If sliding between layers is not prevented, sliding between each individual layer should be considered by the following equilibrium of forces:

Units denoted with _a consider cargo units above the sliding level only.

  6.5.9 To prevent the packages in the bottom layer from collapsing due to racking, the weight of the cargo stowed on top of the bottom layer should be limited so that the following equilibrium of forces is satisfied:

Units denoted with _a consider cargo units above the bottom layer only.

  6.5.10 Lashings used should comply with 6.5.20 and 6.5.21. It is extremely important to keep the lashings tight when a top-over lashing arrangement is used as the arrangement is based on the vertical pressure from the lashings.

  6.5.11 When top-over lashings are used as the only means of securing longitudinally stowed packages of sawn wood, adequate friction against the hatch covers should be sought and/or the transverse accelerations should if possible be limited.

  6.5.12 Loop lashings are always applied in pairs as shown in the figure below. The lashings are drawn from one side of the cargo, under the cargo to the other side, up over the cargo and back to the same side. Alternatively, the lower part of the lashing may be fastened to a securing point on top of the hatch cover underneath the cargo.

Figure 6.5 Principals of loop lashing alternative 1 (be aware of chafing where lashings are lead around ship's structure as shown in the above figure, see section 2.10.10)

Figure 6.6 Principles for loop lashing alternative 2. The shorter length of the lashing compared to alternative 1 reduces the movement of the cargo due to elongation of the lashing

  6.5.13 The number and strength of the lashings are to be chosen so that the following equilibrium is satisfied:

  6.5.14 Sliding between the layers should be prevented (see 6.5.7).

  6.5.15 To prevent the packages in the bottom layer from racking, the weight of the cargo stowed on top of the bottom layer should be limited so that the following equilibrium is satisfied:

Units denoted with _a consider cargo units above the bottom layer only.

  6.5.16 The transverse movement of the deck cargo due to elongation of the lashings is calculated according to the following formula:

The elongation factor ∊ should be taken as 2% for chain and wire lashings and 7% for web lashings unless otherwise specified by certificate from the manufacturer.

 The maximum heeling angle of the vessel due to a small transverse movement of the cargo should in no case be more than 5°, based on the full timber deck load condition of the vessel calculated according to the following formula:

  6.5.17 Blocking means that the cargo is stowed against a blocking structure or fixture on the ship. If the cargo consists of packages with large racking capacity, bottom blocking should be sufficient in combination with top-over lashings.

Figure 6.7 Example of uprights for bottom blocking

  6.5.18 The required strength, MSL, of the bottom blocking devices is calculated by satisfying the following equilibrium:

  6.5.19 The spacing between top-over lashings in a longitudinal direction should be maximum 3 m for stowage heights below 2.5 m and maximum 1.5 m for stowage heights above 2.5 m.

  6.5.20 The pretension PT_V in the vertical part of the lashings should be not less than 16 kN and the pretension PT_H in the horizontal part of the lashing should not be less than 27 kN.

  6.5.21 All lashings and components used for securing in combination with bottom blocking should:

• .1 possess a breaking strength MBL of not less than 133 kN;

• .2 after initial stressing, show an elongation of not more than 5% at 80% of their breaking strength; and

• .3 show no permanent deformation after having been subjected to a proof load of not less than 40% of their original breaking strength.

  6.5.22 The bottom blocking devices are to be placed on both sides of the deck cargo equally spaced. Two blocking device per side should be used per cargo section and the height should extend to a height of at least 200 mm.

  6.5.23 Sliding between the layers should be prevented (see 6.5.7). If no such measures are taken, sliding between layers should be checked by the calculation for equilibrium of forces in 6.5.8.

  6.5.24 To prevent the packages in the bottom layer from racking, the weight of the cargo stowed on top of the bottom layer should be limited so that the following equilibrium of forces is satisfied:

Units denoted with _a consider cargo units above the bottom layer only.

  6.5.25 Longitudinally stowed sawn wood packages, loose sawn wood or round wood may be supported by uprights in combination depending on trading pattern with or without top-over lashings or hog wires.

  6.5.26 The uprights should be designed in accordance with chapter 7.

  6.5.27 The uprights should be placed on both sides of the cargo, equally spaced. Each cargo block of the stow should be supported by at least two uprights per side.

  6.5.28 The spacing of top-over lashings should for packaged sawn wood be a maximum of 3 m for stowage heights below 2.5 m and maximum 1.5 m for stowage heights above 2.5 m for round wood the spacing should be 1.5 m irrespective of the height.

  6.5.29 The pretension PT_V in the vertical part of the lashings should be not less than 16 kN and the pretension PT_H in the horizontal part of the lashing should not be less than 27 kN.

  6.5.30 All lashings and components used for securing in combination with bottom blocking should:

• .1 possess a breaking strength MBL of not less than 133 kN;

• .2 after initial stressing, show an elongation of not more than 5% at 80% of their breaking strength; and

• .3 show no permanent deformation after having been subjected to a proof load of not less than 40% of their original breaking strength.

  6.5.31 In restricted sea areas, round wood may be transversely stowed and secured by bottom blocking and/or friction between tiers only. This may be done only if the friction between layers is sufficient and the expected transverse accelerations are limited. When the friction is sufficient between bottom layers and deck/hatch, then the bottom blocking may not be required. If friction only is to be used, information on the maximum heel angle assumed should be included in the Cargo Securing Manual.

Figure 6.8 Principles for friction securing of round wood in restricted sea areas

  6.5.32 The required strength, MSL, of the bottom blocking devices is calculated by satisfying the following equilibrium:

  6.5.33 The required friction between the layers can be calculated by satisfying the following equilibrium: