Section 7 Double bottom structure in way of cargo compartment space

7.1.1 This Section covers the arrangements and requirements for transversely and longitudinally framed double bottoms.

7.1.2 The depth of the double bottom is to be at least as required by Table 5.7.1 Double bottom structure in way of cargo compartment space, and is to be sufficient to ensure the the double bottom spaces are accessible for inspection, surveys, etc. see also Pt 4, Ch 6, 3 Hull envelope framing.

Table 5.7.1 Double bottom structure in way of cargo compartment space

Item	Parameter	Requirements
Longitudinal and transverse framing system
(1) Centreline girder, side girders and partial side girders	Minimum depth	d f = 40B mm
	Thickness	t = (0,01d f + 2,5) mm
(2) Inner bottom plating	Thickness	The greater of:
		t = 12s mm
		t = 6 mm
(3) Inner bottom plating in way of tank support structure	Thickness	As bottom shell plating in way
Transverse framing system
(4) Floors	Thickness	t = (0,007d f + 3) mm
(5) Watertight floors	Thickness	t = (0,007d f + 3,5) mm
(6) Combination of floors in conjunction with bottom and inner bottom plating
	Modulus	Z = 8,5klf (W – 0,45lf 2T) cm3
(7) Floors and watertight floors	Thickness	t = (0,0085d f + 3,5) mm
(8) Combination of floors in conjunction with bottom and inner bottom plating
	Modulus	Z = 8,5klf (W – 0,45lf 2T) cm3
(9) Bottom longitudinals	Modulus	Z = (3,95 + 0,04L1) D1 ksle 2 cm3
(10) Inner bottom longitudinals	Modulus
Symbols
L, B, D, T, s, le, k, Z and t are as defined in Pt 4, Ch 5, 1.10 Symbols and definitions 1.10.1 D = depth of girders or floors, in mm L = span of floor, in metres, measured at the top of floor between side shell or between longitudinal bulkheads according to Pt 4, Ch 5, 1.2 Ship arrangement 1.2.4.(b) D1 = D, but need not be taken greater than T + 0,4 m for Zone 3, T + 0,7 m for Zone 2, T + 1,0 m for Zone 1 L1 = L but is to be taken not less than 40 m, nor more than 100 m W = weight of tanks with maximum weight of cargo supported by the combination of floors, in tonnes

7.1.3 Provision is to be made for free passage of air to the ventilation pipes and where the double bottom spaces are to be used as ballast tanks, also for free passage of water to the tank suctions, account being taken of the pumping rates required. Where access openings are cut in the floors, girders or transverses, the height of the openings is not, in general, to exceed 50 per cent of the double bottom depth. Openings are to be avoided in ends of floors.

7.1.4 The depth of the double bottom in way of the support for the cargo tanks may require to be increased in height to meet the strength requirements in way.

7.1.5 For ADN related structural requirements regarding cargo tank supports, see Pt 4, Ch 4, 3.5 Special requirements for Type G tankers.

7.2.1 A centreline girder is generally required in ships with a breadth, B, of more than 6 m. A side girder is to be fitted on each side of the centreline in ships having a breadth, B, or more than 12 m and transversely framed bottom construction. Proposals to omit the centreline girder and/or side girders will be specially considered, but adequate support must be provided on the centreline for docking purposes.

7.3.1 Plate floors are generally to be fitted at every frame and are to comply with Table 5.7.1 Double bottom structure in way of cargo compartment space. Plate floors may be cut at the centreline with the centerline girder web plate continuous, provided the transverse strength of the floors is maintained.

7.3.2 As an alternative to Pt 4, Ch 5, 7.3 Transversely framed double bottoms 7.3.1, bracket floors may be fitted in association with plate floors spaced not more than 2,5 m apart. Scantlings and arrangements of bracket floors are to comply with the requirements for such floors in double bottoms in the forward end and aft end as indicated in Pt 3, Ch 5, 3 Bottom structure.

7.3.3 The bottom structure supporting the cargo tanks is to comply with the requirements of Table 5.7.1 Double bottom structure in way of cargo compartment space. The structure is to consist of three to five successive plate floors in conjunction with inner bottom plating of increased thickness from side to side or between the longitudinal bulkheads according to Pt 4, Ch 5, 1.2 Ship arrangement 1.2.4.(b) in association with continuous side girders according to Pt 4, Ch 5, 7.2 Girders 7.2.1 and at least two partial girders port and starboard, each extending over a length of six to ten frame spaces, see Figure 5.7.1 Double bottom structure in way of cargo tank support. Where no continuous side girders are provided, at least three partial girders are to be fitted on each side.

Figure 5.7.1 Double bottom structure in way of cargo tank support

7.3.4 The actual arrangement required depends on the total weight of cargo tanks with maximum cargo load. Verification of the structure by direct calculation may be required.

7.3.5 Bottom structures with an arrangement differing from that assumed in Pt 4, Ch 5, 7.3 Transversely framed double bottoms 7.3.3, will, in general, be required to be assessed by direct calculation.

7.4.1 Plate floors are to be fitted at a spacing not exceeding 3,5 m.

7.4.2 The bottom structure supporting the cargo tanks is to comply with the requirements of Table 5.7.1 Double bottom structure in way of cargo compartment space. The structure is, in general, to be identical to that required by Pt 4, Ch 5, 7.3 Transversely framed double bottoms 7.3.3. The plate floors are to have about the same spacing as the longitudinal framing and the partial girders are to extend forward and aft of the support structure over a full bay.

7.4.3 The actual arrangement required depends on the total weight of cargo tanks with maximum cargo load. Verification of the structure by direct calculation may be required.

7.4.4 Bottom structures with an arrangement differing from that assumed in Pt 4, Ch 5, 7.4 Longitudinally framed double bottoms 7.4.2, will in general be required to be assessed by direct calculation.