Section 7 Double bottom structure
Clasification Society 2024 - Version 9.40
Clasifications Register Rules and Regulations - Rules and Regulations for the Classification of Inland Waterways Ships, July 2022 - Part 4 Ship Structures (Ship Types) - Chapter 1 Dry Cargo Ships - Section 7 Double bottom structure

Section 7 Double bottom structure

Parent topic: Chapter 1 Dry Cargo Ships

7.1 General

7.1.1 Requirements are given in this Section for a double bottom construction with a transverse or a longitudinal framing system. The scantlings of girders, floors, inner bottom plating and longitudinals are to be not less than required in Table 1.7.1 Double bottom structure. See also Pt 4, Ch 1, 8 Additional requirements for double bottom structures in bulk carriers and Pt 4, Ch 1, 9 Additional requirements for container ships for additional requirements for bottom structures in Bulk Carriers and Container ships.

Table 1.7.1 Double bottom structure

Item Parameter Requirement
(1) Double bottom at centreline Minimum depth d f = 35B mm
(2) Centreline girder and side girder Thickness The greater of:
t = (0,008d f + 3,0) mm
t = 8,0 mm
(3) Floors in a transverse framing system Thickness The greater of:
  t = (0,0085d f + 2,0) mm
  t = 7,0 mm
Modulus Z = CkTslb 2 cm3
(4) Floors in a longitudinal framing system Thickness The greater of:
  t = (0,009 d f + 2,0) mm
  t = 8,0 mm
Modulus Z = CKTslb 2
(5) Watertight floors Thickness The greater of:
t = (0,0085 d f + 3,0) mm
t = 8,0 mm
(6) Inner bottom plating Thickness The greater of:
t = 12 s mm
t = 6,0 mm
(7) Inner bottom longitudinals Modulus Z = 4,85kHcsle2 cm3
(8) Bottom longitudinals Modulus Z = (3,95 + 0,04L 1) D 1ksle 2 cm3
Symbols
L, B, D, T, S, s, t and Z are as defined in Pt 4, Ch 1, 1.5 Symbols and definitions 1.5.1
lb = the width of the double bottom, in metres, and is normally the breadth of the ship. If longitudinal bulkheads or equivalent supports are provided, an equivalent breadth may be used, but this is to be taken as not less than 0,8B
le = effective length of stiffening members, in metres, as defined in Pt 3, Ch 3, 3.3 Determination of span point, but is to be taken not less than 1,5 m
ti = thickness of inner bottom plating or bottom plating, whichever is the lesser, in mm
As = minimum web sectional area at ends of floors (at toes of frames), in cm2
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
Hc = height from inner bottom to underside of deck or to the top of hatch coaming, in metres, as defined in Pt 3, Ch 3, 4.2 Symbols 4.2.1
L1 = L but is to be taken not less than 65 m, nor greater than 110 m
C = 6 for general dry cargo ships and container ships
C = 7 for bulk carriers

7.1.2 The depth of the double bottom is to comply with the requirements of Table 1.7.1 Double bottom structure, but it is recommended that the double bottom space be accessible for inspection and surveys, see Pt 1, Ch 3, 4.2 Examination and testing 4.2.6

7.1.3 Provision is to be made for free passage of air and water from all parts of the double bottom compartments to the air pipes and suctions, account being taken of the pumping rates required. Where access openings are cut in the floors and girders, the size of openings should not, in general, exceed 50 per cent of the double bottom depth. Openings are to be avoided in way of ends of floors and girders, and in way of concentrated loads.

7.2 Girders

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 with a breadth, B, of 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 Floors

7.3.1 In transversely framed double bottoms, floors are to be fitted at every frame.

7.3.2 In longitudinally framed double bottoms, floors are to be fitted at a spacing not exceeding 2,5 m. Vertical stiffeners having a depth not less than 50 mm are to be fitted to the floors at every fourth longitudinal. In between the floors, brackets are to be fitted in the double bottom in line with the transverse side frames, connected to tank top and shell plating and extending to the nearest bottom and inner bottom longitudinal. Midway between floors, brackets are to be fitted on either side of the centreline, extending to the nearest bottom and inner bottom longitudinals. The free edges of the brackets are to be suitably stiffened.

7.3.3  Plate floors are also to be arranged under bulkheads, and in line with web frames fitted in the side structure. Floors subjected to concentrated loads will be specially considered.

7.4 Inner bottom plating

7.4.1 The thickness of the inner bottom plating is to comply with the requirements of Table 1.7.1 Double bottom structure. Where no ceiling is laid on the inner bottom plating, the thickness of the plating is to be increased by 2 mm. The inner bottom plating may be attached by slot fillet welds to the face bars or flanges of the floors.

7.5 Longitudinals

7.5.1 The scantlings of bottom longitudinals and inner bottom longitudinals are to comply with the requirements of Table 1.7.1 Double bottom structure and are based on end connections in accordance with Pt 3, Ch 10, 3 Secondary member end connections.

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