Section 6 Double bottom

6.1.1 A double bottom is to be fitted in accordance with Pt 4, Ch 1, 8 Double bottom structure.

6.1.2 Where podded drive systems are to be employed, adequate floors and girders are to be arranged in order to efficiently integrate the unit into the aft structure. The adequacy of the hull structure supporting the unit is to be verified using the maximum external forces and moments stated by the manufacturer. Due account is to be taken of additional forces induced by ship motion accelerations.

6.2.1 In ships where the deck centreline supports are widely spaced, transmission of the pillar loads to the double bottom structure will be specially considered, and additional reinforcement may be required if high shear and bending stresses are induced by the concentrated loads. The reinforcement should take the form of additional floors, and fore and aft girders. The final reinforcement is to be confirmed by direct calculation.

6.2.2 Where, in multi-decked passenger ships, the Rule deck loadings from Table 2.3.1 Design deck loadings (ferries and passenger ships only) have been used to determine the primary deck supporting structure, the cumulative pillar load P _p may be taken as:

6.2.3 Pillars are to be provided with suitable pads at their heels. Long pillars, and those terminating at the inner bottom are to be bracketed. At pillar heads, the free edges of deck primary structure face plates are to be at least 20 mm clear of the pillar head attachment weld. Where necessary, gusset plates are to be fitted at primary member intersections in way of pillars. These gussets may be applied as doublers onto the primary member face plates and should be at least equal in thickness to the pillar or the face plate, whichever is greater. Where pillars act in tension, the gussets are to be integral with the primary member face plates. The axial stress in tensile pillars is not to exceed 110/k N/mm² and full penetration welds are to be arranged in way of the end connections.

6.3.1 The thickness of double bottom centre girders may be reduced by 10 per cent, and the thickness of double bottom side girders and floors may be reduced by five per cent, from the values required by Pt 4, Ch 1, 8 Double bottom structure, but is in no case to be less than 6 mm.