9.1.1 The loads acting on grillages, cribbing, dunnage, seafastening and components of the cargo shall be derived from the loads acting on the cargo, according to Sections 6, 7 and 8, as applicable.
9.1.2 The loads shall include components due to the distribution of mass and rotational inertia of the cargo.
This is of particular importance in the calculation of shear forces and bending moments in the legs of self-elevating units and similar tall structures.
9.1.3 If the computed loads are less than the “Minimum allowable seafastening force” shown in Table 9-1, then the values in the Table shall apply.
9.1.4 Care should be taken in cases where the cargo may be designed for service loads in the floating condition, but is being dry-transported. Its centre of gravity may be higher above the roll centre in the dry-transportation condition than in any of its floating service conditions. Even though the transportation motions may appear to be less than the service motions, the loads on cargo components and ship-loose items may be greater.
9.2 FRICTION
9.2.1 For certain cargo weights, cargo overhangs and arrangements of cribbing and seafastenings, the effects of friction may be used, as shown in the following Table 9-1 and subject to Section 9.2.2, to resist part of the computed loadings on the cribbing and seafastenings. This shows the maximum coefficient of friction which may be considered, and the minimum required seafastening force, expressed as a percentage of cargo weight, below which the actual seafastening design capability shall not be allowed to fall.
Table 9-1 Maximum allowable coefficients of friction & minimum seafastening forces Cargo weight, W, tonnes
1. For 20,000 ≤ W < 40,000 tonnes, the minimum required seafastening force, transversely, shall be not less than 15 - W/4,000 (%W)
2. For 10,000 ≤ W < 20,000 tonnes, the minimum required seafastening force, longitudinally, shall be not less than 7.5 - W/4,000 (%W)
3. For 20,000 ≤ W < 40,000 tonnes, the minimum required seafastening force, longitudinally, shall be not less than 3.5 - W/20,000 (%W)
4. For transport of pipes and similar tubular goods, the above table does not apply. See Section 9.6.
5. The friction coefficient may be interpolated as a function of overhang using the maximum cargo overhang.
9.2.2 Friction is allowed as a contribution to seafastening restraint subject to the following:
a. Loadings are computed in accordance with Sections 7.2 though 7.8 and 8.2 above. Friction may not be used if the loadings are computed in accordance with the default criteria in Sections 7.9 and 8.3, except as allowed by Section 9.6.
b. Friction forces shall be computed using the normal reaction between the vessel and cargo compatible with the direction of the heave.sin(theta) term used in computing the forces parallel to the deck in Section 8.2.2. Thus, when heave.sin(theta) increases the force parallel to the deck, it also increases the normal reaction and vice-versa.
c. The cargo is supported by wood dunnage or cribbing – friction is not allowed for steel to steel interfaces.
d. The overhang is the distance from the side of the vessel to the extreme outer edge of the cargo.
e. For wood cribbing less than 600 mm high, with a width not less than 300 mm, the friction force due to the friction coefficient permitted in Table 9-1 may be assumed to act in any direction relative to the cribbing provided that:
(i) the cribbing is reasonably well balanced in terms of the proportion in the fore-aft and transverse directions, AND
(ii) each of these groups is reasonably well balanced about the cargo CoG in plan.
f. Provided that the conditions in (e) above are met, for cribbing heights between 600 and 900 mm, with a width not less than 300 mm, then the percentage computed friction force at right angles to the longitudinal axis of a cribbing beam shall not exceed (900 - H)/3 %, where H = the height of cribbing above deck, in mm. In the direction of the longitudinal axis of a cribbing
beam, the full friction force can be used. 4
g. For wood cribbing over 900 mm high, or with a width less than 300 mm, no friction force is assumed to act in a direction at right angles to the longitudinal axis of a cribbing beam.
h. If greater cribbing friction is required than available according to (f) and (g) above, stanchions may be fitted to provide transverse cribbing restraint. Where such stanchions are fitted, they should be designed to carry loads due to a friction coefficient of 0.5 (to ensure they are able to carry loads due to upper-bound friction assumptions).
i. The underlying assumption in the approach given above is that the seafastenings have sufficient flexibility to deflect in the order of at least 2mm without failing. This will be reasonable in most cases, but when this is not the case the more detailed approach given in (j) below shall be used.
j. As an alternative to (e) through (h) above, a more detailed approach may be used. In such cases, the friction permitted in Table 9-1 can be doubled, but the relative flexibility of the cribbing and seafastenings shall be taken into account. The arrangements shall be such as to ensure that the required lateral load can be carried by the combination of friction & seafastening reactions BEFORE the seafastenings are overstressed. Where stanchions are used, they shall comply with (h) above.
k. The “Minimum allowable seafastening force” is the minimum allowable value of seafastening restraint, expressed as a percentage of cargo weight, in the event that the total required seafastening force, as computed, is less than this value.