3 Transverse Vertically Corrugated Watertight Bulkheads Separating Cargo Holds in Flooded Condition

3.1 Net thickness of corrugation

3.1.1 Cold formed corrugation

The net plate thickness t, in mm, of transverse vertically corrugated watertight bulkheads separating cargo holds is not to be taken less than:

The net thicknesses is also to comply with the requirements given in Pt 1, Ch 6, Sec 4, [1.2.1].

3.1.2 Built-up corrugation

Where the thicknesses of the flange and web of built-up corrugations of transverse vertically corrugated watertight bulkheads separating cargo holds are different, the net plate thicknesses are not to be taken less than that obtained from the following formula.

The net thickness t_N, in mm, of the narrower plating is not to be taken less than:

S_N : Plate width, in mm, of the narrower plating.

The net thickness t_W, in mm, of the wider plating is not to be taken less than the greater of the following formulae:

where:

t_NO : Net offered thickness of the narrower plating, in mm, not to be taken greater than:

The net thicknesses is also to comply with the requirements given in Pt 1, Ch 6, Sec 4, [1.2.2].

3.1.3 Lower part of corrugation

The net thickness of the lower part of corrugations is to be maintained for a distance of not less than 0.15 ℓ_C measured from the top of the lower stool, or from the inner bottom where no lower stool is fitted. The span of the corrugations ℓ_C, in m, is to be taken as given in Pt 1, Ch 3, Sec 6, [10.4.5].

3.1.4 Middle part of corrugation

The net thickness of the middle part of corrugations is to be maintained for a distance not greater than 0.3 ℓ_C from the bottom of the upper stool, or from the deck if no upper stool is fitted. The net thickness is also to comply with the requirements in [3.2.1] and Pt 1, Ch 6, Sec 4, [1.2].

3.2 Bending, shear and buckling check

3.2.1 Bending capacity and shear capacity

The bending capacity and the shear capacity of the corrugations of transverse watertight corrugated bulkheads separating cargo holds are to comply with the following formulae:

where:

M : Bending moment in a corrugation, in kNm, taken as:

F_R : Resultant force, in kN, given in Pt 1, Ch 4, Sec 6, [3.1.7].

ℓ_C : Span of the corrugations, in m, as given in Pt 1, Ch 3, Sec 6, [10.4.5].

W_LE : Net section modulus, in cm³, of one half pitch corrugation, to be calculated at the lower end of the corrugations according to [3.3], not to be taken greater than:

W_G : Net section modulus, in cm³, of one half pitch corrugation, to be calculated in way of the upper end of shedder or gusset plates, as applicable, according to [3.3].

h_G : Height, in m, of shedders or gusset plates, as applicable as shown in Figure 4 to Figure 6.

P_R : Resultant pressure, in kN/m², to be calculated in way of the middle of the shedders or gusset plates, as applicable, according to Pt 1, Ch 4, Sec 6, [3.1.7].

W_M : Net section modulus, in cm³, of one half pitch corrugation, to be calculated at the mid-span of corrugations according to [3.3] without being taken greater than 1.15 W _LE.

τ : Shear stress, in N/mm², in the corrugation to be taken as:

A_shr : Net shear area, in cm², of one half pitch corrugation. The calculated net shear area is to consider possible reduced shear efficiency due to non-straight angles between the corrugation webs and flanges. In general, the reduced shear area may be obtained by multiplying the web sectional area by sin ϕ.

ϕ : Angle between the web and the flange, see Pt 1, Ch 3, Sec 6, Figure 21.

The net section modulus of the corrugations in the upper part of the bulkhead, as defined in Figure 3, is not to be taken less than 75% of that of the middle part complying with this requirement and Pt 1, Ch 6, Sec 4, [1.2], corrected for different minimum yield stresses.

Figure 3 : Parts of corrugation

3.2.2 Shear buckling check of the bulkhead corrugation webs

The shear stress τ, calculated according to [3.2.1], is to comply with the following formula:

τ ≤ τ_C

where:

τ_C : Critical shear buckling stress, in N/mm², to be taken as:

for

for

τ_E : Euler shear buckling stress, in N/mm², to be taken as:

k_t : Coefficient, to be taken equal to 6.34.

t_w : Net thickness, in mm, of the corrugation webs.

c : Width, in mm, of the corrugation webs as shown in Pt 1, Ch 3, Sec 6, Figure 21.

3.3 Net section modulus at the lower end of the corrugations

3.3.1 Effective flange width

The net section modulus at the lower end of the corrugations is to be calculated with the compression flange having an effective flange width b_eff not larger than the following formula:

where:

C_E : Coefficient to be taken equal to:

for β > 1.25

C_E = 1.0 for β ≤ 1.25

β : Coefficient to be taken equal to:

β =

a : Width, in mm, of the corrugation flange as shown in Pt 1, Ch 3, Sec 6, Figure 21.

t_f : Net flange thickness, in mm.

3.3.2 Webs not supported by local brackets

Unless welded to a sloping stool top plate as defined in [3.3.5], if the corrugation webs are not supported by local brackets below the stool top plate (or below the inner bottom) in the lower part, the section modulus of the corrugations is to be calculated considering the corrugation webs 30% effective.

3.3.3 Effective shedder plates

Provided that effective shedder plates are fitted as shown in Figure 4, when calculating the section modulus at the lower end of the corrugations (Sections ’1’ in Figure 4), the net area, in cm², of flange plates may be increased by I_SH to be taken as:

without being taken greater than 2.5 a t_f 10^-3

where:

a : Width, in mm, of the corrugation flange as shown in Pt 1, Ch 3, Sec 6, Figure 21.

t_SH : Net shedder plate thickness, in mm.

t_f : Net flange thickness, in mm.

Effective shedder plates are those which:

• are not knuckled,
• are welded to the corrugations and the lower stool top plate according to Pt 1, Ch 12,
• are fitted with a minimum slope of 45°, their lower edge being in line with the lower stool side plating,
• have net thickness not less than 75% of the net required for the corrugation flanges,
• have material properties not less than those required for the flanges.

Figure 4 : Symmetrical and unsymmetrical shedder plates

Figure 5 : Symmetrical and unsymmetrical gusset / shedder plates

Figure 6 : Asymmetrical gusset / shedder plates

3.3.4 Effective gusset plates

Provided that effective gusset plates are fitted, when calculating the section modulus at the lower end of the corrugations (Sections ‘1’ in Figure 5 and Figure 6), the net area, in cm², of flange plates may be increased by I_G to be taken as:

I_G = 7 h_G t_f

where:

h_G : Height, in m, of gusset plates as shown in Figure 5 and Figure 6 but not to be taken greater than:

S_GU : Width, in m, of gusset plates.

t_f : Net flange thickness, in mm.

Effective gusset plates are those which:

• are in combination with shedder plates having thickness, material properties and welded connections as requested for shedder plates in [3.3.3],
• have a height not less than half of the flange width,
• are fitted in line with the lower stool side plating,
• are welded to the lower stool top plate, corrugations and shedder plates according to Pt 1, Ch 12, Sec 3, [2.4.6],
• have net thickness and material properties not less than those net required for the flanges.

3.3.5 Sloping stool top plate

Where the corrugation webs are welded to a sloping stool top plate which has an angle not less than 45º with the horizontal plane, the section modulus at the lower end of the corrugations may be calculated considering the corrugation webs fully effective. For angles less than 45º, the effectiveness of the web may be obtained by linear interpolation between 30% efficient for 0º and 100% efficient for 45º.

Where effective gusset plates are fitted, when calculating the net section modulus of corrugations, the net area of flange plates may be increased as specified in [3.3.4] above. No credit may be given to shedder plates only.