Section 3 Bottom structure

3.1.1 Requirements are given in this Section for both transversely and longitudinally framed bottoms.

3.1.2 Additional requirements for bottom structure in way of the machinery space are given in Pt 3, Ch 6 Machinery Spaces.

3.1.3 Provision is to be made for free passage of water, oil and air from all parts of single or double bottoms, and account being taken of the pumping rates required.

3.2.1 A centreline girder is to be arranged in ships having a breadth of more than 6 m, and is to be carried as far forward and aft as practicable, and is to comply with the requirements of Table 5.3.1 Single bottom construction forward and aft or Table 5.3.2 Double bottom construction forward and aft.

Table 5.3.1 Single bottom construction forward and aft

Item	Parameter	Requirements
Transverse framing system
(1) Floors	Web depth at centreline	d f = 40lf mm
	Modulus	Z = 6 × k × D 1 × s × lf 2 cm3
	Web thickness	mm
(2) Centreline girder	Web and face plate thickness	mm
	Face plate width	w = 140s mm
Longitudinal framing system
(3) Centreline girder	Modulus	Z = 8,5 × k × D1 × S × le 2 cm3
	Web thickness
(4) Bottom transverses	Modulus	Z = 7 × k × D 1 × S × le 2 cm3
	Web thickness	t = as centreline girder
Symbols
B, D, T, S, s, le, Z and t are as defined in Pt 3, Ch 5, 1.4 Symbols and definitions 1.4.1 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 df = depth of floor or bottom transverse at centreline, in mm lf = span of the floor, and is normally the breadth of the ship measured on the top of the floor under consideration, in metres. If longitudinal bulkheads or equivalent floor supports are provided an equivalent breadth may be used, but this should be not less than 0,4B w = width of face plate of a member, in mm k = higher tensile steel factor, see Pt 3, Ch 2, 1.3 Steel 1.3.3
Note The thickness of plates forming the single bottom structure is to be not less than 5 mm.

Table 5.3.2 Double bottom construction forward and aft

Item	Parameter	Requirements
Transverse and longitudinal framing system
(1) Centre girder	Least depth	The greater of: d f = mm D f = 300 mm
	Thickness
(2) Inner bottom plating	Thickness for dry spaces	t = 10s
	Thickness for tanks	The greater of: t = 12s mm t = as required by Pt 3, Ch 7, 2 Scantlings of bulkheads
(3) Struts	Cross-sectional area	Area = 2 × le × D1 × s cm2
(4) Watertight floors	Thickness	The thickness of plate floors + 0,5 mm
Transverse framing system
(5) Plate floors and brackets of bracket floors	Thickness	mm
(6) Bottom frames in bracket floors	Modulus	Z = 6 × k × s × D 1 × le 2 + 1,5 + 0,05L cm3
(7) Reverse frames in bracket floors	Modulus	Z = 5,5 × k × s × h × le 2 + 1,5 + 0,05L cm3
Longitudinal framing system
(8) Plate floors	Thickness	mm
(9) Tank top longitudinal	Modulus	Z = 5,5 × k × s × h × le 2 + 1,5 + 0,05L cm3
Symbols
L, D, T, S, s, t, and Z are as defined in Pt 3, Ch 5, 1.4 Symbols and definitions 1.4.1 D1 = D, but need not be taken greater than T + 0,4 m or Zone 3, T + 0,7 m for Zone 2, T + 1,0 m for Zone 1 df = depth of bottom in way of centre girder, in mm h = load head, which is to be taken as the greater of h 4 or h 5, in metres h4 = tank head, in metres, as defined in Pt 3, Ch 3, 4 Design loading h5 = distance, in metres, from mid point of span to the deck at side or to a point T + 0,4 m above the baseline, whichever is the lesser, but is to be taken as not less than 1 m le = as defined in Pt 3, Ch 5, 1.4 Symbols and definitions 1.4.1, but is to be taken as not less than 1,5 m. Struts are not considered as effective supports for the definition of le lb = the width of the double bottom, in metres, and is normally the breadth of the ship. If longitudinal bulkheads or equivalent support is provided, an equivalent breadth may be used, but this should be not less than 0,8B t1 = thickness of inner bottom plating or bottom plating, whichever is the lesser, in mm k = higher tensilbe steel factor, see Pt 3, Ch 2, 1.3 Steel 1.3.3
Note The thickness of plating forming the double bottom structure is to be not less than: 5 mm for dry tanks and oil tanks 5,5 mm for water ballast tanks

3.3.1 Plate floors are to be fitted at every frame and the scantlings are to comply with the requirements of Table 5.3.1 Single bottom construction forward and aft. The depth of floors at the centreline is to be as required in Table 5.3.1 Single bottom construction forward and aft, but in ships having considerable rise of floor towards the ends, the depth of floors may require to be increased, or the top edge sloped upwards towards the outboard end, see Figure 5.3.1 Basic floor arrangements. If required floors may be cut at the centreline, with the girder web plate continuous, but the strength of the floors is to be maintained in way of the centre girder connection. Notwithstanding the requirements for the fitting of a centreline girder as per Pt 3, Ch 5, 3.2 Girders 3.2.1, suitable arrangements to prevent tripping of the floors, e.g. riders or tripping brackets, are to be provided, where the unsupported length of the top edge of the floors exceeds 20b m;

where

b is the width in metres of the face plate of the floor.

Figure 5.3.1 Basic floor arrangements

3.3.2 The centre girder is to have the same depth as the floors.

3.4.1 Longitudinals are to be supported by transverses spaced not more than 3,5 m apart. The transverses are to be supported by a primary centreline girder or a centreline bulkhead. Tripping brackets connecting transverse to longitudinal are to be fitted between the centreline girder and shell at intervals not exceeding 3,5 m. The centreline girder may be omitted, provided the scantlings of the transverses are suitable for a span from side to side of the ship, and tripping brackets are fitted about 3,5 m or 20b m apart, whichever is the lesser, where b is the width in metres of the face plate of the transverse. Longitudinals are to be determined from Table 5.4.1 Shell frames and longitudinals forward and aft. The scantlings of the centreline girder and bottom transverses are to be determined from Table 5.3.1 Single bottom construction forward and aft.

3.5.1 Where a double bottom is fitted, the space should be accessible. The minimum depth of the centre girder and the thickness of the centre girder and inner bottom plating are to comply withTable 5.3.2 Double bottom construction forward and aft.

3.5.2 Plate floors in a transverse framing system are generally to be fitted at every frame and the scantlings are to comply with the requirements of Table 5.3.2 Double bottom construction forward and aft. Vertical stiffeners having a depth of 80d _f mm are to be fitted to the floors and spaced not more than 2,5 m apart. The ends of these stiffeners may be sniped. Alternatively, bracket floors may be fitted, see Figure 5.3.2 Bracket floor, in which case the spacing of the plate floors is not to exceed 2,5 m.

Figure 5.3.2 Bracket floor

3.5.3 Where bracket floors are fitted the unsupported span of bottom frames and reverse frames is not to exceed 2,5 m. Struts may be fitted to reduce the unsupported span. If a strut is fitted at approximately half-span, the modulus of bottom and reverse frames may be reduced by 50 per cent. Struts are to comply with the requirements of Table 5.3.2 Double bottom construction forward and aft. Watertight or plate floors are to be fitted below or in the vicinity of watertight bulkheads.

3.5.4 Where a longitudinal framing system is adopted in a double bottom, the scantlings of tank top longitudinals and plate floors are to comply with the requirements given in Table 5.3.2 Double bottom construction forward and aft. Vertical stiffeners having a depth not less than 50 mm are to be fitted to the floors at every fourth longitudinal. The plate floors are to be spaced not more than 3,5 m apart. The centre girder may require to be stiffened when the height of the bottom is excessive. If a strut is fitted at approximately half-span the modulus of bottom and tank top longitudinals may be reduced by 50 per cent. Struts are to comply with the requirements of Table 5.3.2 Double bottom construction forward and aft.

3.6.1 Where a longitudinal framing system is adopted, the transverses or floors supporting longitudinals are to be spaced not more than 2,5 m apart, and the moduli of longitudinals and transverses are to be increased by 40 per cent.

3.6.2 Where a transverse framing system is adopted in swim ends, the modulus of plate floors is to be increased by 40 per cent.

3.6.3 The draught for calculation of the structural members may be taken as the actual draught in way of the respective member.

3.6.4 For rake plating, see Pt 3, Ch 5, 2.4 Shell plating 2.4.6.