Section 14 Strengthening for bottom slamming
Clasification Society 2024 - Version 9.40
Clasifications Register Rules and Regulations - Rules and Regulations for the Classification of Naval Ships, January 2023 - Volume 1 Ship Structures - Part 6 Hull Construction in Steel - Chapter 3 Scantling Determination - Section 14 Strengthening for bottom slamming

Section 14 Strengthening for bottom slamming

Parent topic: Chapter 3 Scantling Determination

14.1 General

14.1.1 This section may be used to determine the additional scantlings for strengthening in respect of the bottom structure forward for NS1 and NS2 type ships.

14.1.2 The additional scantlings for strengthening in respect of the bottom structure forward for NS3 type ships will be specially considered on the basis of this Section.

14.2 Strengthening of bottom forward

14.2.1 The bottom forward is to be additionally strengthened where the ship has a draught forward of less than 0,045LR in any operational loading condition. The minimum draught, TFB, see Vol 1, Pt 6, Ch 3, 14.2 Strengthening of bottom forward 14.2.7 (NS1) or Vol 1, Pt 5, Ch 3, 1.3 Symbols and definitions 1.3.1 (NS2 or NS3), is to be indicated on the shell expansion plan, the plan showing the internal strengthening, the Loading Manual and loading instrument, where fitted.

14.2.2 The requirements for the additional strengthening apply to ships where LR is greater than 65 m.

14.2.3 The scantling requirements outside the areas which have been strengthened for bottom slamming are to be suitably tapered to maintain adequate continuity of strength in both longitudinal and transverse directions.

Table 3.14.1 Additional strengthening of bottom forward

Item Requirements
(1) Longitudinally framed bottom shell plating (including keel)
(see Notes 1 and 2) 		t = 0,003s f
(2) Bottom longitudinals – other than flat bars



(3) Bottom longitudinals – flat bars Will be specially considered
  Transverse framing Longitudinal framing
(4) Primary structure in way of single bottoms (a) Centreline girder:

Scantlings as required by Table 3.3.12 Single bottom construction forward, minimum requirements as appropriate, except that in determining Z in way of a deep tank forward of 0,8LR the value of h5 is to be increased by the following percentages:
where TFB ≤ 0,03L 2, 30 per cent
where TFB ≥ 0,04L2, 0 per cent
The increase in h5 for intermediate values of TFB to be obtained by interpolation

(a) Ships having one or more longitudinal bulkheads:
(i) Centreline girder
Scantlings as required by Table 3.3.12 Single bottom construction forward, minimum requirements as appropriate
(ii) Bottom transverses
Maximum spacing as for midships region, scantlings as required by Section 2, as appropriate
(iii) For horizontally stiffened longitudinal bulkheads and girders the depth to thickness ratio of the panel attached to the bottom shell plate is not to exceed 55
(iv) Where TFB <0,025L2 the scantlings and arrangements will receive individual consideration
(b) Floors:

Scantlings as required by Table 3.3.12 as appropriate, except that in way of dry spaces the minimum face area is to be increased by the following percentages:
where T FB ≤ 0,03L2, 50 per cent
where TFB ≥ 0,04L2, 0 per cent
The increase of minimum face area for intermediate values of TFB to be obtained by interpolation


(b) Other ship arrangements will receive individual consideration
(c) Side girders:

Arrangement and scantlings as required by Table 3.3.12 Single bottom construction forward, minimum requirements as appropriate, with the addition of intermediate half-height girders or equivalent fore and aft stiffening


   
(5) Primary structure in way of double bottoms (see Note 3) (a) Plate floors:

Maximum spacing, every frame
Scantlings as required by Vol 1, Pt 6, Ch 3, 2 Minimum structural requirements

(a) Plate floors:

Maximum spacing:
0,002sF m for T FB < 0,04L 2
0,003sF m for T FB ≥ 0,04L 2
but not to exceed that required by Table 3.3.12 Single bottom construction forward, minimum requirements as appropriate

(b) Centreline and side girders:

Maximum spacing, 0,003sF m
Scantlings as required by Vol 1, Pt 6, Ch 3, 2 Minimum structural requirements

(b) Centreline and side girders:

Maximum spacing:
0,003s L m for T FB < 0,04L 2
0,004s L m for T FB ≥ 0,04L 2
but not to exceed that required by Table 3.3.12 Single bottom construction forward, minimum requirements, as appropriate

(c) Intermediate half-height girders to be arranged midway between side girders: Scantlings as required by Table 3.3.12 Single bottom construction forward, minimum requirements, as appropriate
Scantlings as required for non-watertight side girders by Section 2
(6) Primary structure in way of double bottoms supported by longitudinal bulkheads — The scantlings and arrangements will receive individual consideration on the basis of direct calculations using, if necessary, a suitably defined two-dimensional grillage model
Symbols
L R, T as defined in Vol 1, Pt 3, Ch 1, 5.2 Principal particulars
L 2, S, s, k s as defined in Vol 1, Pt 6, Ch 3, 3.2 Symbols 3.2.1
c = 1,0 for S ≤ 2,5 m
= (0,87 + 0,16S) c 1 for S > 2,5 m
c 1 = 1,0 for S ≤ 1,0 m
= (1,14 – 0,14S) for 1,0 m < S ≤ 4,0 m
= for S > 4,0 m
d w = web depth, in mm, see Figure 2.2.1 Dimensions of longitudinals
f = but not greater than 1,0
s = e, in metres, as defined in Vol 1, Pt 6, Ch 2, 2.6 Determination of span length where in way of a double bottom
= S, in metres, where in way of a single bottom
p = 9,81h s sc 1 x 10-3 kN
s F = spacing of transverse frames, in mm, for longitudinally framed side and bottom construction s F may be taken as s L
s L = spacing of bottom longitudinals, in mm
t w = web thickness, in mm
A f = cross-sectional area of primary member web stiffener, in cm2
A fc = effective area of primary member web stiffener in way of butted end connection to the longitudinal, in cm2
A L = area of weld of lapped connection, in cm2, calculated as total length of weld, in cm x throat thickness, in cm
A w = area of weld of lug and web connection to the longitudinal, in cm2, calculated as total length of weld in cm x throat thickness, in cm
A 1 = effective total cross-sectional area of the lug and web connection to the longitudinal, in cm2
T FB = draught, in metres, at the F.P., as defined in Vol 1, Pt 6, Ch 3, 14.2 Strengthening of bottom forward 14.2.1
α = A f for the web stiffeners
= A fc for a butted connection to the longitudinals
= A L for a lapped connection
= permissible direct stress, in N/mm2, given in Table 3.14.2 Permissible stresses
= permissible shear stress, in N/mm2, given in Table 3.14.2 Permissible stresses

Note 1. If intermediate stiffening is fitted the thickness of the bottom shell plating may be 80 per cent of that required by (1) but is to be not less than the normal taper thickness.

Note 2. For transverse framing the bottom shell plating is to be specially considered.

Note 3. Particular care is to be taken to limit the size and number of openings in way of the ends of floors or girders or to fit suitable reinforcement where such openings are essential.

Note 4. The welding requirements of Vol 1, Pt 6, Ch 6, 4 Requirements for welded construction are also to be complied with.

Table 3.14.2 Permissible stresses

Item Direct stress,
in N/mm2 see Note 		Shear stress,
in N/mm2
Primary member web stiffener
on area A f 		a) Flat bars
(see Note)


—
(b) Bulb plates
(see Note)


—
(c) Inverted
angles


—
Primary member web stiffener on area A fc


—
Primary member web stiffener lapped to secondary
member on area A L 		—


Lug or web connection on
area A1 		Single —


Double —


Symbols
A f, A L, A 1 are as defined in Table 3.14.1 Additional strengthening of bottom forward
d = stiffener depth, in mm

k s as defined in Vol 1, Pt 6, Ch 3, 3.2 Symbols 3.2.1

t = stiffener web thickness, in mm

Note is to be taken no greater than.

14.2.4 The scantling requirements for the additional strengthening are given in Vol 1, Pt 6, Ch 3, 3.14 Single and double bottom structures 3.14.1, or may be obtained by direct calculation. Where T FB is less than 0,01L R, the additional strengthening is to be specially considered.

14.2.5 Bottom longitudinals are to pass through and be supported by the webs of primary members. The vertical web stiffeners are to be connected to the bottom longitudinals. The cross-sectional area of the connections is to comply with the requirements given in Table 3.14.1 Additional strengthening of bottom forward

14.2.6 The scantlings required by this Section must in no case be less than those required by other Sections in Vol 1, Pt 6, Ch 3, 3 NS1 scantling determination

14.2.7 For NS1 ships with a block coefficient, Cb, greater than 0,6, the equivalent slamming pressure expressed as a head of water, hs, is to be obtained from Figure 3.14.1 Pressure heads where hmax is calculated from the following expressions:

where

is the minimum draught, in metres, at the location under consideration for all operational loading conditions, and is to be measured from the underside of the keel to the operational waterline

LR is as defined in Vol 1, Pt 3, Ch 1, 5.2 Principal particulars

Figure 3.14.1 Pressure heads

14.2.8 For NS1 ships where Cb < 0,6 and for NS2 and NS3 ships, the equivalent bottom impact pressure head, hs, is to be derived in accordance with the following:

where

IPbi = instantaneous bottom impact pressure as given in Vol 1, Pt 5, Ch 3, 4.2 Bottom impact pressure, IPbi
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