Section 7 Double bottom structure

7.1.1 The double bottom is, in general, to be longitudinally framed for ships where L ≥ 100 m.

7.1.2 Longitudinally framed double bottoms are to comply with Pt 4, Ch 1, 8.2 General and the contents of this Section.

7.1.3 Transversely framed double bottoms are to comply with Pt 4, Ch 1, 8 Double bottom structure.

7.1.4 The ShipRight FDA Procedure, Structural Detail Design Guide (SDDG), indicates recommended examples of structural design configurations in critical areas of the double bottom.

7.2.1 The primary supporting structure formed by the double bottom comprises inner bottom plating, floors, longitudinal girders and bottom shell plating.

7.2.2 The scantlings of this primary structure are to be verified by direct calculations in accordance with Pt 4, Ch 8, 14.2 Procedures for verification of primary structure scantlings.

7.2.3 Where, towards the end of the ship, the depth of double bottom structure webs is reduced from that assumed in the direct calculations, the thickness may require to be increased locally.

7.2.4 Where mid-hold or quarter-length-of-hold supports for the double bottom structure are arranged, these are to take the form of an efficiently stiffened transverse box or open section structure, see Pt 4, Ch 8, 9.4 Transverse non-watertight mid-hold bulkheads.

7.3.1 The scantlings of the inner bottom are to satisfy the requirements of Pt 4, Ch 8, 3 Longitudinal strength and Pt 4, Ch 8, 7.2 Double bottom primary supporting structure and are to be not less than required by Pt 4, Ch 1, 8.4 Inner bottom plating and stiffening as modified by this sub-Section.

7.3.2 The requirements of Pt 4, Ch 1, 8.4 Inner bottom plating and stiffening 8.4.2 need not be applied to container ships.

7.3.3 In applying Pt 4, Ch 1, 8.4 Inner bottom plating and stiffening 8.4.5, the Rule value of bottom longitudinals may be calculated assuming F _sb = 1,05.

7.4.1 Girders are, in general, to be arranged under container corner seatings.

7.4.2 The scantlings of watertight centreline/side girders are to satisfy the requirements of Pt 4, Ch 8, 3 Longitudinal strength and Pt 4, Ch 8, 7.2 Double bottom primary supporting structure and are to be not less than required by Pt 4, Ch 1, 8.3 Girders.

7.4.3 The scantlings of non-watertight centreline/side girders are to satisfy the requirements of Pt 4, Ch 8, 3 Longitudinal strength and Pt 4, Ch 8, 7.2 Double bottom primary supporting structure.

7.4.4 For double bottoms having a depth greater than 1,6 m, additional longitudinal stiffening may have to be introduced in order to ensure the buckling stability of the girders.

7.5.1 Plate floors are to be fitted under watertight bulkheads, non-watertight bulkheads/mid-hold supports, under container corners at hold quarter length locations and at other locations to ensure that the maximum spacing does not, in general, exceed 3,8 m. Proposals for floor spacings greater than 3,8 m are to be supported by direct calculations agreed with LR.

7.5.2 The scantlings of watertight floors are to satisfy the requirements of Pt 4, Ch 8, 7.2 Double bottom primary supporting structure and are to be not less than required by Pt 4, Ch 1, 8.5 Floors.

7.5.3 The thickness, t, of non-watertight floors is to satisfy the requirements of 7.2 and is to be not less than:

7.5.4 Non-watertight floor stiffeners are to be fitted at approximately the same spacing as the bottom longitudinals. The scantlings are to satisfy the requirements of Pt 3, Ch 10 Welding and Structural Details.

7.5.5 Docking brackets, or equivalent, are to be fitted in accordance with Pt 4, Ch 1, 8.5 Floors 8.5.3.

7.6.1 In general, local stiffening is to be fitted to double bottom floors or girders under container corner seatings in order to ensure the effective transmission of load.

7.6.2 Such stiffening normally takes the form of additional brackets with suitable extensions to adjacent stiffening members. The scantlings of the adjacent stiffening members may require to be increased depending on the arrangements proposed.

7.6.3 Attention is drawn to the benefit of direct support in order to minimise the effect of eccentric loading on the support brackets.

7.6.4 The scantlings of these arrangements may be determined utilising simple beam models to verify the shear and bending strength. Based on static container loads, the stresses induced in the structure are not to exceed the permissible values stated in Table 8.7.1 Permissible stress values. Alternative more complex assessment methods are to be agreed with LR.

7.6.5 In general, doubling members or equivalent structures are to be attached to the inner bottom to distribute the load from container corners into the supporting structure. Doubler plates are to have well-rounded corners.