1 Primary Supporting Members in Cargo Hold Region

1.1 General

1.1.1 The following requirements relate to the determination of scantlings of the primary supporting members within 0.4 L amidships and those outside 0.4 L amidships provided that the geometry and fixation of primary supporting member is similar with those amidships.

1.1.3 Floors, horizontal stringers, side transverses and vertical webs adjacent to transverse bulkheads which get additional supports by horizontal stringers or buttresses are excluded from the application of this section.

1.1.4 Webs of the primary supporting members are to be stiffened in accordance with Pt 1, Ch 8, Sec 2, [4].

1.1.5 Webs of the primary supporting members are to have a depth of not less than given by the requirements of [1.5.1], [1.7.1] and [1.8.1], as applicable.

1.1.6 In any case, primary supporting members that have open slots for stiffeners are to have a depth not less than 2.5 times the depth of the slots if slots are not closed.

1.1.7 Where it is impracticable to fit a primary supporting member with the required web depth, then it is permissible to fit a member with reduced depth provided that the fitted member has equivalent moment of inertia or deflection to the required member. The required equivalent moment of inertia is to be based on an equivalent section given by the effective width of plating at mid span with required plate thickness, web of required depth and thickness and face plate of sufficient width and thickness to satisfy the required mild steel section modulus.

The equivalent moment of inertia may be also demonstrated by an equivalent member having the same deflection as the required member.

All other rule requirements, such as minimum thicknesses, slenderness ratio, section modulus and shear area, are to be satisfied for the member of reduced depth.

1.2 Design load sets

1.2.1 The design load sets for the evaluation of primary supporting members are given in Table 1.

1.3 Floors in double bottom

1.3.1 Structural arrangement

Plate floors are to be arranged in way of transverse bulkheads and bulkhead stools.

1.3.2 Net shear area

The net shear area, A_shr-n50 in cm², of the floors at any position in the floor is not to be less than:

where:

f_shr-i : Shear force distribution factor at the end of the span, ℓ_shr, as given in Table 2.

ℓ_shr : Effective shear span, of the double bottom floor, in m, as shown in Figure 2. In way of bracket ends, the effective shear span is measured to the toes of the effective end bracket, as defined in Pt 1, Ch 3, Sec 7, [1.1.7]. Where the floor ends on a girder at a hopper or stool structure, the effective shear span is measured to a point that is one-half of the distance from the girder to the adjacent bottom and inner-bottom longitudinal, as shown in Figure 2.

y_i : Distance from the considered cross section of the floor to the nearest end of the effective shear span, ℓ_shr in m.

P : Design pressure given in Table 1 for the design load set being considered, calculated at mid point of effective shear span, ℓ_shr of a floor located midway between transverse bulkheads or transverse bulkhead and wash bulkhead, where fitted, in kN/m².

Figure 1 : Shear force distribution factors of floors

Figure 2 : Effective shear span of floors

1.4 Girders in double bottom

1.4.1 Structural arrangement

Continuous double bottom girders are to be arranged at the centreline or duct keel, at the hopper side and in way of longitudinal bulkheads and bulkhead stools.

1.4.2 Net shear area of centre girders

For double bottom centre girders where no longitudinal bulkhead is fitted above, the net shear area, A_shr-n50 in cm², of the double bottom centre girder in way of the first bay from each transverse bulkhead and wash bulkhead, where fitted, is not to be less than:

where:

ℓ_shr : Effective shear span as defined in [1.3.2].

P : Design pressure, in kN/m², as defined in [1.3.2].

S : Double bottom floor spacing, in m, as defined in Pt 1 Ch 3 Sec 7 [1.2.2]

1.4.3 Net shear area of side girders

For double bottom side girders where no longitudinal bulkhead is fitted above, the net shear area, A_shr-n50 in cm², of the double bottom side girder in way of the first bay from each transverse bulkhead and wash bulkhead, where fitted, is not to be less than:

where:

S: Double bottom floor spacing, in m, as defined in Pt 1 Ch 3 Sec 7 [1.2.2]

ℓ_shr : Effective shear span as defined in [1.3.2].

P : Design pressure, in kN/m², as defined in [1.3.2].

1.5 Deck transverses

1.5.1 Web depth

The web depth of under deck transverses is not to be less than:

• 0.20 ℓ_bdg-dt for deck transverses in the wing cargo tanks of ships with two longitudinal bulkheads.
• 0.13 ℓ_bdg-dt for deck transverses in the centre cargo tanks of ships with two longitudinal bulkheads. The web depth of deck transverses in the centre cargo tank is not to be less than 90% of that of the deck transverses in the wing cargo tank.
• 0.10 ℓ_bdg-dt for the deck transverses of ships with a centreline longitudinal bulkhead.
• The web height required in [1.1.6].

The web depth of above deck transverses is not to be less than:

• 0.10 ℓ_bdg-dt
• The web height required in [1.1.6].

where:

ℓ_bdg-dt : Effective bending span, in m, as defined in [1.5.2].

1.5.2 Net section modulus of deck transverses fitted below the upper deck

The net section modulus of deck transverses fitted below the upper deck, in cm³, is not to be less than Z_in-n50 and Z_ex-n50 as given by the following formulae.

The net section modulus of the deck transverses fitted below the upper deck in the wing cargo tanks is also not to be less than required for the deck transverses fitted below the upper deck in the centre tanks.

where:

M_in : Design bending moment due to cargo pressure, in kNm, taken as:

• For deck transverses in wing cargo tanks of ships with two longitudinal bulkheads, and for deck transverses in cargo tanks of ships with a centreline longitudinal bulkhead:
  • M_in = 0.042ϕ_t P_{in – dt} S + M_st but is not to be taken as less than M₀.
• For deck transverses in centre cargo tank of ships with two longitudinal bulkheads:
  • M_in = 0.042ϕ_t P_{in – dt} S + M_vw but is not to be taken as less than M₀.

P_in-dt : Design cargo pressure given in Table 1 for the design load set being considered, calculated at midpoint of effective bending span, ℓ_bdg-dt of the deck transverse located at mid tank, in kN/m².

P_in-st : Corresponding design cargo pressure in wing cargo tank given in Table 1 for the design load set being considered, calculated at the mid-point of effective bending span, ℓ_bdg-st of the side transverse located at mid-tank, in kN/m².

P_in-vw : Corresponding design cargo pressure in the centre cargo tank of ships with two longitudinal bulkheads given in Table 1 for the design load set being considered, calculated at mid-point of effective bending span, ℓ_bdg-vw of the vertical web frame on the longitudinal bulkhead located at midtank, in kN/m².

P_ex-dt : Design green sea pressure given in Table 1 for the design load set being considered, calculated at mid-point of effective bending span, ℓ_bdg-dt of the deck transverse located at mid tank, in kN/m².

y_toe : Distance from the end of effective bending span, ℓ_bdg-dt to the toe of the end bracket of the deck transverse, in m.

ℓ_bdg-dt : Effective bending span of the deck transverse, in m, see Pt 1, Ch 3, Sec 7, [1.1.6] and Figure 3, but is not to be taken as less than 60% of the breadth of the tank at the location being considered.

ℓ_bdg-st : Effective bending span of the side transverse, in m, between the deck transverse and the bilge hopper, see Pt 1, Ch 3, Sec 7, [1.1.6] and Figure 3.

ℓ_bdg-st-ct : Effective bending span of the side transverse, in m, between the deck transverse and the mid depth of the cross tie, where fitted in wing cargo tank, see Pt 1, Ch 3, Sec 7, [1.1.6].

ℓ_bdg-vw : Effective bending span of the vertical web frame on the longitudinal bulkhead, in m, between the deck transverse and the bottom structure, see Pt 1, Ch 3, Sec 7, [1.1.6] and Figure 3.

ℓ_bdg-vw-ct : Effective bending span of the vertical web frame on longitudinal bulkhead, in m, between the deck transverse and the mid depth of the cross tie, see Pt 1, Ch 3, Sec 7, [1.1.6].

I_dt-n50 : Net moment of inertia of the deck transverse at mid-span with an effective breadth of attached plating specified in Pt 1, Ch 3, Sec 7, [1.3.2], in cm⁴.

I_st-n50 : Net moment of inertia of the side transverse at mid-span with an effective breadth of attached plating specified in Pt 1, Ch 3, Sec 7, [1.3.2], in cm⁴.

I_vw-n50 : Net moment of inertia of the longitudinal bulkhead vertical web frame at mid-span with an effective breadth of attached plating specified in Pt 1, Ch 3, Sec 7, [1.3.2], in cm⁴.

c_st : Coefficient given in Table 3.

c_vw : Coefficient given in Table 3.

1.5.3 Net shear area of deck transverses fitted below the upper deck

The net shear area of deck transverses fitted below the upper deck, in cm², is not to be less than A_shr-in-n50 and A_shr-ex-n50 as given by:

where:

P_in-dt : Design pressure in kN/m², defined in [1.5.2].

P_ex-dt : Design pressure in kN/m², defined in [1.5.2].

ℓ_bdg-dt : Effective span, in m, defined in [1.5.2].

ℓ_shr : Effective shear span, of the deck transverse, in m, see Pt 1, Ch 3, Sec 7, [1.1.7].

c₁ : Coefficient taken as:

• c₁ = 0.04 in way of wing cargo tanks of ships with two longitudinal bulkheads.
• c₁ = 0.00 in way of centre tank of ships with two longitudinal bulkheads.
• c₁ = 0.00 for ships with a centreline longitudinal bulkhead.

b_ctr : Breadth of the centre tank, in m.

Figure 3 : Definition of spans of deck, side transverses, vertical web frames on longitudinal bulkheads and horizontal stringers on transverse bulkheads

1.5.4 Deck transverses fitted above the upper deck

When deck transverses are fitted above the upper deck, the net section modulus and shear area of deck transverses are not to be less than Z_n50 and A_shr-n50, in cm³ and cm² respectively, as given by the following formulae. The required section modulus and shear area are to be maintained over the full length of span.

where:

P : Design pressure given in Table 1 for the design load set being considered, calculated at midpoint of effective bending span, ℓ_bdg of the deck transverse located at mid tank, in kN/m².

l_bdg : Effective bending span of the deck transverse fitted above upper deck, in m, measured from inner hull welded to deck to longitudinal bulkhead, or upper stool plating where upper stool is fitted

l_shr : Effective shear span of the deck transverse fitted above upper deck, in m, measured from inner hull welded to deck to longitudinal bulkhead, or upper stool plating where upper stool is fitted

As an alternative, the required section modulus and shear area may be obtained by finite element method in accordance with Pt 1, Ch 7 and with in consideration of loading patterns A1, A2 or B1, B2 as defined in Pt 1, Ch 4, Sec 8, [3.2.9] with draught equal to T_sc and cargo density of 1.025 t/m³.

1.5.5 Deck transverse adjacent to transverse bulkhead

The scantling of deck transverse adjacent to the transverse bulkhead is to comply with the requirements of [1.5.2] to [1.5.4] for design green sea pressure only.

1.6 Side transverses

1.6.1 Net shear area

The net shear area, A_shr-n50, in cm², of side transverses is not to be less than:

where:

P_u : Design pressure given in Table 1 for the design load set being considered, in kN/m², calculated at mid length of tank and at mid height of h_u.

P_l : Design pressure given in Table 1 for the design load set being considered, calculated at mid height h_l located at mid length of tank, in kN/m².

ℓ_st : Length of the side transverse, in m, taken as follows:

• Where deck transverses are fitted below deck, ℓ_st is the length between the flange of the deck transverse and the inner bottom, see Figure 3.
• Where deck transverses are fitted above deck, ℓ_st is the length between the elevation of the deck at side and the inner bottom.

ℓ_st-ct : Length of the side transverse, in m, taken as follows:

• Where deck transverses are fitted below deck, ℓ_st-ct is the length between the flange of the deck transverse and mid depth of cross tie, where fitted in wing cargo tank.
• Where deck transverses are fitted above deck, ℓ_st-ct is the length between the elevation of the deck at side and mid depth of the cross tie, where fitted in wing cargo tank.

h_u : Effective length of upper bracket of the side transverse, in m, taken as follows:

• Where deck transverses are fitted below deck, h_u is as shown in Figure 3.
• Where deck transverses are fitted above deck:
  • When an inner hull longitudinal bulkhead is arranged with a top wing structure as follows, hu is taken as the distance between the deck at side and the lower end of slope plate of the top wing structure:
    • The breadth at top of the wing structure is greater than 1.5 times the breadth of the double side and.
    • The angle along a line between the point at base of the slope plate at its intersection with the inner hull longitudinal bulkhead and the point at the intersection of top wing structure and deck is 30 deg or more to vertical.
  • In the other cases: h_u is taken as 0.

h_l : Height of bilge hopper, in m, as shown in Figure 3.

c_u : Coefficient defined in Table 4.

c_l : Coefficient defined in Table 4.

1.6.2 Shear area over the length of the side transverse

where:

ℓ_st , ℓ_{st -ct},h_u, h_l, Q_u, Q_l: Parameters defined in [1.6.1].

1.7 Vertical web frames on longitudinal bulkhead

1.7.1 Web depth

The web depth of the vertical web frame on the longitudinal bulkhead is not to be less than:

• 0.14 ℓ_bdg-vw for ships with a centreline longitudinal bulkhead.
• 0.09 ℓ_bdg-vw for ships with two longitudinal bulkheads.
• The web height required in [1.1.6].

where:

ℓ_bdg-vw : Effective bending span, in m, defined in [1.7.2].

1.7.2 Net section modulus

The net section modulus, Z_n50 in cm³, of the vertical web frame is not to be less than:

where:

P : Design pressure given in Table 1 for the design load set being considered, calculated at mid-point of the effective bending span, ℓ_bdg-vw of the vertical web frame located at mid tank, in kN/m².

ℓ_bdg-vw : Effective bending span of the vertical web frame on the longitudinal bulkhead, between the deck transverse and the bottom structure, in m, see Figure 3.

ℓ_bdg-vw-ct : Effective bending span of the vertical web frame on longitudinal bulkhead, between the deck transverse and mid-depth of the cross tie on ships with two longitudinal bulkheads, in m.

c_u : Coefficient defined in Table 5.

c_l : Coefficient defined in Table 5.

1.7.3 Section modulus over the length of the vertical web frame

ℓ_bdg-vw, ℓ_bdg-vw-ct : Effective bending span, in m, defined in [1.7.2].

1.7.4 Net shear area

The net shear area, A_shr-n50 in cm², of the vertical web frame is not to be less than:

where:

Q_l : Shear force, in kN, taken as the greater of the following:

• S [c_l ℓ_vw(P_u + P_l) – h_l P_l]
• c_w S c_l ℓ_vw(P_u + P_l)
• 1.2 Q_u

where a cross tie is fitted in a centre or wing cargo tank and ℓ_vw-ct is greater than 0.7 ℓ_vw, then ℓ_vw in the above formula is to be taken as ℓ_vw-ct.

P_u : Design pressure given in Table 1 for the design load set being considered, calculated at mid-height of upper bracket of the vertical web frame, h_u located at mid tank, in kN/m².

P_l : Design pressure given in Table 1 for the design load set being considered, calculated at mid-height of lower bracket of the vertical web frame, h_l located at mid tank, in kN/m².

ℓ_vw : Length of the vertical web frame, in m, between the flange of the deck transverse and the inner bottom, see Figure 3.

ℓ_vw-ct : Length of the vertical web frame, in m, between the flange of the deck transverse and mid-depth of the cross tie, where fitted.

h_u : Effective length of upper bracket of the vertical web frame, in m, as shown in Figure 3.

h_l : Effective length of lower bracket of the vertical web frame, in m, as shown in Figure 3.

c_u : Coefficient defined in Table 6.

c_l : Coefficient defined in Table 6.

c_w : Coefficient taken as:

• c_w = 0.57 for ships with a centreline longitudinal bulkhead,
• c_w = 0.50 for ships with two longitudinal bulkheads.

1.7.5 Shear area over the length of the vertical web frame

where:

ℓ_vw, ℓ_vw-ct, h_u, h_l, Q_u, Q_l : Parameters defined in [1.7.4].

1.8 Horizontal stringers on transverse bulkheads

1.8.1 Web depth

The web depth of horizontal stringers on transverse bulkhead is not to be less than:

• 0.28 ℓ_bdg-hs for horizontal stringers in wing cargo tanks of ships with two longitudinal bulkheads.
• 0.20 ℓ_bdg-hs for horizontal stringers in centre tanks of ships with two longitudinal bulkheads, but the web depth of horizontal stringers in centre tank is not to be less than required depth for a horizontal stringer in wing cargo tanks.
• 0.20 ℓ_bdg-hs for horizontal stringers of ships with a centreline longitudinal bulkhead.
• The web height required in [1.1.6].

where:

ℓ_bdg-hs : Effective bending span, in m, defined in [1.8.2].

1.8.2 Net section modulus

The net section modulus, Z_n50 in cm³, of the horizontal stringer over the end 0.2 ℓ_bdg-hs is not to be less than:

where:

P : Design pressure given in Table 1 for the design load set being considered, calculated at mid-point of effective bending span, ℓ_bdg-hs and at mid-point of the spacing, S of the horizontal stringer, in kN/m².

ℓ_bdg-hs : Effective bending span of the horizontal stringer, in m, but is not to be taken as less than 50% of the breadth of the tank at the location being considered, see Figure 3.

c : Coefficient taken as:

• c = 0.073 for horizontal stringers in cargo tanks of ships with a centreline bulkhead.
• c = 0.083 for horizontal stringers in wing cargo tanks of ships with two longitudinal bulkheads.
• c = 0.063 for horizontal stringers in the centre tank of ships with two longitudinal bulkheads.

1.8.3 Section modulus over the length of horizontal stringers

The required section modulus at mid effective bending span is to be taken as 70% of that required at the ends, intermediate values are to be obtained by linear interpolation. When materials of different yield stress are employed, appropriate adjustments are to be made to account for differences in material yield stress.

1.8.4 Net shear area

The net shear area, A_shr-n50 in cm², of the horizontal stringer over the end 0.2 ℓ_shr is not to be less than:

P : Design pressure given in Table 1 for the design load set being considered, calculated at mid-point of effective bending span, ℓ_bdg-hs and at mid-point of the spacing, S of the horizontal stringer, in kN/m².

S_hs : Spacing, in m, defined in [1.8.2].

ℓ_shr : Effective shear span of the horizontal stringer, in m.

1.8.5 Shear area at mid effective shear span

The required shear area at mid effective shear span is to be taken as 50% of that required in the ends, intermediate values are to be obtained by linear interpolation. When materials of different yield stress are employed, appropriate adjustments are to be made to account for differences in material yield stress.

1.9 Cross ties

1.9.1 Maximum applied design axial load

The maximum applied design axial load on cross ties, W_ct is to be less than or equal to the permissible load, W_ct-perm as given by:

W_ct ≤ W_{ct – perm}

where:

W_ct-perm : Permissible load, in kN.

• W_{ct – perm} = 0.12 A_{ct – n50} η_all σ_cr

P : Maximum design pressure for all the applicable design load sets being considered given in Table 1, calculated at centre of the area supported by the cross tie located at mid tank, in kN/m².

b_ct : Span, in m, taken as:

• Cross tie fitted in centre cargo tank: b_ct = 0.5 ℓ_bdg-vw
• Cross ties fitted in wing cargo tanks:
  • b_ct = 0.5 ℓ_bdg-vw for design cargo pressure from the centre cargo tank.
  • b_ct = 0.5 ℓ_bdg-st for design sea pressure.

ℓ_bdg-vw : Effective bending span, in m, defined in [1.5.2].

ℓ _bdg-st : Effective bending span, in m, defined in [1.5.2].

η_all : Allowable buckling utilisation factor as defined in Pt 1, Ch 8, Sec 1, [3.3].

σ_cr : Critical buckling stress in compression of the cross tie, in N/mm², as calculated using the net sectional properties in accordance with Pt 1, Ch 8, Sec 5, [3.1.1].

A_ct-n50 : Net cross sectional area of the cross tie, in cm².

1.9.2 Welded connections

Special attention is to be paid to the adequacy of the welded connections for the transmission of the forces, and also to the stiffening arrangements, in order to provide effective means for transmission of the compressive forces into the webs.

Particular attention is to be paid to the welding at the toes of all end brackets of the cross ties.

1.9.3 Horizontal stiffeners

Horizontal stiffeners are to be located in line with, and attached to, the longitudinals at the ends of the cross ties.