Section 3 Re-assessment of double bottom structure

3.1 Application and definitions

3.1.1 Requirements for application of this procedure are given in Ch 2, 1.1 Application.

3.1.2 In the context of the following requirements, a double bottom is defined as the structure bounded by the transverse bulkhead lower stools (or bulkhead plating if no lower stools are fitted) and the hopper sides. The floors and girders immediately in way of these structures are excluded.

3.1.3 The shear capacity is defined as the sum of the shear strengths for:

all the floors adjacent to both hoppers, less one half the strength of the floors adjacent to each lower stool (or transverse bulkhead if no lower stool is fitted) (see Figure 2.3.1 Double bottom structure); and
all the girders adjacent to the lower stools (or transverse bulkheads if no lower stool is fitted).

Where a girder or floor terminates without direct attachment to the boundary stool or hopper side girder, its shear capacity is to include only that for the effectively connected end.

3.1.4 Shear strength calculations are to be performed using the net plate thickness, t _net, for the floors and girders:

where

t	=	as built thickness, in mm
t _c	=	thickness deduction for corrosion, in mm, generally to be taken as 2 mm.

Figure 2.3.1 Double bottom structure

3.1.5 Where unusual structural arrangements are involved, direct calculations may be required.

3.2 Loading

3.2.1 The most severe design loading conditions are to be considered when re-assessing the double bottom strength for hold flooding. The ship is to be assumed immersed to the draught, T _F, in metres, as defined in Ch 2, 3.3 Strength assessment 3.3.3, in way of the flooded cargo hold under consideration.

3.2.2 The flooding head, h _f, (see Figure 2.3.2 Loading) is the distance, in metres, measured vertically with the ship in the upright position, from the inner bottom to position, d _f, in metres, from the baseline given by:

In general:

d _f

D for the foremost hold

d _f

0,9D for other holds

For ships less than 50 000 tonnes deadweight with Type B freeboard:

d _f

0,95D for the foremost hold

d _f

0,85D for other holds

where

distance, in metres, from the baseline to the freeboard deck at side at the section under consideration.

Figure 2.3.2 Loading

3.3 Strength assessment

3.3.1 The shear strengths, S_f1, of floors adjacent to hoppers and, S_f2, of floors in way of openings in bays closest to the hoppers, are as follows:

where

A_f	=	net sectional area, in mm², of floor panel adjacent to hoppers
A_f,h	=	net sectional area, in mm², of floor panel in way of opening in the bay closest to hopper
η₁	=	1,10
η₂	=	1,20 generally
η₂	=	1,10 where appropriate reinforcement is fitted in way of the opening
σ₀	=	specified minimum yield stress, in N/mm² (kgf/mm²)
τ_p	=	permissible shear stress, to be taken equal to τ₀, in N/mm² (kgf/mm²)
	=

3.3.2 The shear strengths S_g1, of girders adjacent to stools (or transverse bulkheads if no lower stools are fitted) and, S_g2, of girders in way of the largest openings in bays closest to the lower stools (or transverse bulkheads if no lower stools are fitted), are as follows:

where

A_g	=	net sectional area, in mm², of the girder adjacent to transverse bulkhead lower stools (or transverse bulkhead, if no lower stools are fitted)
A_g,h	=	net sectional area, in mm², of the girder in way of the largest openings in the bays closest to the lower stools (or transverse bulkheads if no lower stools are fitted)
η₁	=	1,10
η₂	=	1,15 generally
η₂	=	1,10 where appropriate reinforcement is fitted in way of the opening.

3.3.3 The permissible cargo hold loading, W_p, is given by:

where

d_f, D	=	as defined in Ch 2, 3.2 Loading 3.2.2
g	=	gravitational constant, 9,81 m/sec²
h _f	=	flooding head, in metres, as defined in Ch 2, 3.2 Loading 3.2.2
	=
	=
n	=	number of floors between bulkhead lower stools or transverse bulkheads, if no lower stools are fitted
s	=	spacing, in metres, of double bottom longitudinals adjacent to hoppers
	=
	=
B _DB,i	=	(B_DB - s) for floors where shear strength is given by S_f1
B _DB,i	=	B_DB,h for floors where shear strength is given by S_f2
B_DB	=	breadth of double bottom, in metres, between hoppers, see Figure 2.3.3 Double bottom breadth
B_DB,h	=	distance, in metres, between openings, see Figure 2.3.3 Double bottom breadth
C_e	=	shear capacity of the double bottom, in kN (tonne-f), as defined in Ch 2, 3.1 Application and definitions 3.1.3, considering for each floor, the shear strength S_f1 (see Ch 2, 3.3 Strength assessment 3.3.1) and for each girder, the lesser of the shear strengths S_g1 and S_g2, (see Ch 2, 3.3 Strength assessment 3.3.2).
C_h	=	shear capacity of the double bottom, in kN (tonne-f), as defined in Ch 2, 3.1 Application and definitions 3.1.3, considering for each floor, the lesser of the shear strengths S_f1 and S_f2 (see Ch 2, 3.3 Strength assessment 3.3.1) and for each girder, the lesser of the shear strengths S_g1 and S_g2, (see Ch 2, 3.3 Strength assessment 3.3.2).
T_F	=	d_f - 0,1D
F_c	=	1,05 in general
F_c	=	1,00 for steel mill products
S_i	=	spacing of ith floor, in metres
V	=	volume, in m³, occupied by cargo at a level h₁
X	=	the lesser of X₁ and X₂ for bulk cargoes and
X	=	X ₁ for steel mill products
	=	where
	=
	=
X₂	=	Y + ρ g(T_F - h_fμ) where Y is in kN/m²
(X ₂	=	Y + ρ (T_F - h_fμ) where Y is in tonne-f/m²)
Y	=	the lesser of Y₁ and Y₂ given by:
	=
	=
μ	=	permeability of cargo
	=	0,3 for ore, coal cargoes
	=	0,0 for steel mill products
ρ	=	density of sea water, 1,025 tonne/m³
ρ_c	=	cargo density, in tonne/m³ (bulk density for bulk cargoes and actual cargo density for steel mill products).

Figure 2.3.3 Double bottom breadth

3.4 Retrospective action

3.4.1 Where the hold loading exceeds the calculated permissible value, W _p, suitable strengthening and/or loading restrictions will be required.

3.4.2 Where loading restrictions are imposed, the requirements of Ch 2, 2.4 Retrospective action 2.4.9 will apply.