Section 8 Double bottom structures

8.1.1 The requirements of this Section, unless specified otherwise, are applicable to all ship types, NS1, NS2 and NS3.

8.1.2 The basic structural scantlings of NS1 ships are to be determined in accordance with Vol 1, Pt 6, Ch 3, 3 NS1 scantling determination The basic structural scantlings of NS2 and NS3 ships are to be determined in accordance with Vol 1, Pt 6, Ch 3, 4 NS2 and NS3 scantling determination. The scantlings of the double bottom structure are also to comply with the appropriate minimum requirements given in Vol 1, Pt 6, Ch 3, 2 Minimum structural requirements The arrangements of double bottoms are to comply with the requirements of Vol 1, Pt 3, Ch 3, 2.6 Rudder force

8.1.3 Where a double bottom is required to be fitted, its depth at the centreline, d _DB, is to be in accordance with Vol 1, Pt 6, Ch 3, 2 Minimum structural requirements

8.1.4 This Section provides for longitudinal or transverse framing in the double bottom, but for NS1 and NS2 ships longitudinal framing is in general to be adopted. See Vol 1, Pt 3, Ch 2, 3.1 General

8.1.5 Where appropriate all ship types are to comply with the requirements of Vol 1, Pt 6, Ch 3, 14 Strengthening for bottom slamming for bottom slamming.

8.1.6 Where a floor, girder or inner bottom plating forms the boundary of a tank or part of the watertight subdivision, the requirements of Vol 1, Pt 6, Ch 3, 9 Bulkheads and deep tanks for deep tanks and watertight bulkheads are to be complied with. For corrosion margins, see Vol 1, Pt 6, Ch 6, 3.8 Corrosion margin

8.2.1  A centreline girder is to be fitted throughout the length of the ship. The web thickness, t _w, is not to be less than that required by Vol 1, Pt 6, Ch 3, 2 Minimum structural requirements

8.2.2 The geometric properties of the girder section are to be in accordance with Vol 1, Pt 6, Ch 2, 2.9 Proportions of stiffener sections. The buckling requirements of Vol 1, Pt 6, Ch 2, 3 Buckling are to be satisfied.

8.2.3 The depth of the double bottom and centreline girder is to be not less than 630 mm and it is to be sufficient to give access to all of the double bottom.

8.2.4 Where appropriate, vertical stiffeners are to be fitted at every bracket floor. They are to have a depth not less than the depth of the tank top frame or 150 mm, whichever is the greater. For NS2 and NS3 ships stiffeners are to have a depth of not less than 1,65L _R mm with a minimum of 50 mm. The thickness is to be as required for the girder.

8.3.1 The thickness of the side girders is not to be less than that of the plate floors. The buckling requirements of Vol 1, Pt 6, Ch 2, 3 Buckling are to be satisfied.

8.3.2 Vertical stiffeners are to be fitted at every bracket floors fitted and if midway between floors in accordance with Vol 1, Pt 6, Ch 3, 8.2 Centreline girder 8.2.4

8.4.1 The thickness need not be greater than 15 mm but without suitable stiffening the ratio of depth of floor at the centre to thickness must be less than 130.

8.4.2 Vertical stiffeners are to be fitted to all plate floors at each longitudinal. Each stiffener is to have a depth of not less than 10t _w or 50 mm and a thickness of not less than t _w, where t _w is the thickness of the plate floor as calculated in Vol 1, Pt 6, Ch 3, 2 Minimum structural requirements. For NS1 ships, the depth of stiffeners is not to be less than 150 mm.

8.5.1 Between plate floors, the shell and inner bottom plating centreline girders and side girders may be supported by bracket floors. The brackets are to have the same thickness as plate floors and are to be stiffened on the unsupported edge.

8.5.2 Where struts are fitted to reduce the unsupported span of the frames, reverse frames and longitudinals, they are to have a cross-sectional area of not less than:

1. or

2. 

where Z _BF is the modulus, in cm³, of the frame or longitudinal based on the effective length between floors as defined in Vol 1, Pt 6, Ch 2, 2.6 Determination of span length

8.6.1 The scantlings of watertight floors are to comply with the requirements for plate floors as given in Vol 1, Pt 6, Ch 3, 8.5 Bracket floors

8.7.1 Where the inner bottom also acts as the plating of a vehicle deck, stores deck or similar, the requirements of Vol 1, Pt 6, Ch 3, 10 Deck structures are to be complied with.

8.8.1 Where the double bottom tanks are interconnected with side tanks or cofferdams, the scantlings are to be not less than those required for deep tanks, see Vol 1, Pt 6, Ch 3, 9 Bulkheads and deep tanks.

8.8.2 Higher tensile steel inner bottom longitudinals are to be continuous as far as practicable throughout the length of the ship.

8.9.1 The scantlings of double bottom tanks are to comply with the requirements for deep tanks given in Vol 1, Pt 6, Ch 3, 9 Bulkheads and deep tanks

8.10.1 A margin plate, if fitted, is to have a thickness as required for inner bottom plating.

8.11.1 The arrangements of the structure in machinery spaces are to be in accordance with Vol 1, Pt 3, Ch 2, 6 Machinery space arrangements

8.11.2 The scantlings of floors clear of the main engine seatings, are generally to be as required in Vol 1, Pt 6, Ch 3, 8.4 Plate floors. In way of engine seatings the floors are to be increased in thickness, see Vol 1, Pt 6, Ch 3, 7.5 Single bottom structure in machinery spaces and Vol 1, Pt 6, Ch 3, 13 Machinery and raft seatings

8.11.3 Where the double bottom is longitudinally framed and transverse floors are fitted in way of the engine seatings as required by Vol 1, Pt 3, Ch 2, 6.6 Double and single bottom structure 6.6.4, no additional longitudinal stiffening is required in way of the engines other than the main engine girders, provided that the spacing of girders does not exceed 1,5 times the normal spacing of longitudinals. Where this spacing of girders is exceeded, shell longitudinals are to be fitted. These are to scarf into the longitudinal framing clear of the machinery spaces. The scantlings of the longitudinals are to be determined in accordance with the appropriate requirements for bottom shell longituduals using a minimum span of 1,3 m.