1 Strength Characteristics of Hull Girder Transverse Sections

1.1 General

1.1.1 This section specifies the criteria for calculating the hull girder strength characteristics to be used for the checks in [2] to [3], in association with the hull girder loads specified in Ch 4, Sec 4.

1.2 Hull girder transverse sections

1.2.1 General

Hull girder transverse sections are to be considered as being constituted by the members contributing to the hull girder longitudinal strength, i.e. all continuous longitudinal members below and including the strength deck defined in [1.3], taking into account the requirements in [1.2.2] to [1.2.13].

1.2.2 Net scantling

The members contributing to the hull girder longitudinal strength are to be considered using the net offered scantlings based on gross offered thickness reduced by 0.5 t_c, as defined in Ch 3, Sec 3, when the hull girder strength characteristics are used for the hull girder yielding check according to[2][3].

1.2.3 Structural members not contributing to hull girder sectional area

The following members are not to be considered in the calculation as they are considered not contributing to the hull girder sectional area:

Superstructures which do not form a strength deck.
Deckhouses.
Vertically corrugated bulkheads, according to [1.2.7].
Bulwarks and gutter plates.
Bilge keels.
Sniped or non-continuous longitudinal stiffeners.
Non-continuous hatch coaming.

1.2.4 Continuous trunks and longitudinal continuous hatch coamings

Continuous trunks and longitudinal continuous hatch coamings may be included in the hull girder transverse sections, provided that they are effectively supported by longitudinal bulkheads or primary supporting members.

1.2.5 Longitudinal stiffeners or girders welded above the strength deck

Longitudinal stiffeners or girders welded above the strength deck, including the deck of any trunk fitted as specified in [1.2.4], are to be included in the hull girder transverse sections.

1.2.6 Longitudinal girders between hatchways, supported by longitudinal bulkheads

Where longitudinal girders, effectively supported by longitudinal bulkheads, are fitted between hatchways, the sectional area of these longitudinal girders are to be included in the hull girder transverse section.

1.2.7 Longitudinal bulkheads with vertical corrugations

For longitudinal bulkheads with vertical corrugations, the vertical corrugations are not to be included in the hull girder transverse section. Longitudinal bulkheads with vertical corrugations are not effective for hull girder bending, but they are effective for hull girder shear force.

1.2.8 Members in materials other than steel

Where a member contributing to the longitudinal strength is made in material other than steel with a Young’s modulus, E equal to 2.06×10⁵ N/mm², the steel equivalent sectional area that may be included in hull girder transverse section is obtained, in m², from the following formula:

where:

A_M-n50 : Sectional area, in m², of the member under consideration.

1.2.9 Definitions of openings

The following definitions of opening are to be applied:

a) Large openings are:

Elliptical openings exceeding 2.5 m in length or 1.2 m in breadth.
Circular openings exceeding 0.9 m in diameter.

b) Small openings (i.e. drain holes, etc) are openings that are not large ones.
c) Manholes.
d) Isolated openings are openings spaced not less than 1 m apart in the ship’s transverse/vertical direction.

1.2.10 Large openings, manholes and nearby small openings

Large openings and manholes are to be deducted from the sectional area used in hull girder moment of inertia and section modulus. When small openings are spaced less than 1 m apart in the ship’s transverse/vertical direction to large openings or manholes, the total breadth of them is to be deducted from the sectional area.

Additionally, isolated small openings which do not comply with the arrangement requirements given in Ch 3, Sec 6, [6.3.2] are to be deducted from the sectional areas included in the hull girder transverse sections.

1.2.11 Isolated small openings

Isolated small openings in one transverse section in the strength deck or bottom area need not be deducted from the sectional areas included in the hull girder transverse sections, provided that:

Σb_S ≤ 0.06(B – Σb)

Σb_S : Total breadth of isolated small openings, in m, in the strength deck or bottom area at the transverse section considered, determined as indicated in Figure 1, not deducted from the section area as per [1.2.10].

Σb : Total breadth of large openings, in m, at the transverse section considered, determined as indicated in Figure 1, deducted from the section area as defined in [1.2.10].

Where the total breadth of isolated small openings Σb_s does not fulfil the above criteria, only the excess of breadth is to be deducted from the sectional areas included in the hull girder transverse sections.

Figure 1 : Calculation of Σ_b and Σ_bs

1.2.12 Lightening holes, draining holes and single scallops

Lightening holes, draining holes and single scallops in longitudinals need not be deducted if their height is less than 0.25 h_w, where h_w is the web height of the longitudinals, in mm. Otherwise, the excess is to be deducted from the sectional area or compensated.

1.2.13 Non-continuous decks and longitudinal bulkheads

When calculating the effective area in way of non-continuous decks and longitudinal bulkheads, the effective area is to be taken as shown in Figure 2. The shadow area, which indicates the ineffective area, is obtained by drawing two tangent lines with an angle of 15 deg to the longitudinal axis of the ship.

Figure 2 : Effective area in way of non-continuous decks and bulkheads

1.3 Strength deck

1.3.1 The strength deck is, in general, the uppermost continuous deck. In the case of a superstructure or deckhouses contributing to the longitudinal strength, the strength deck is the deck of the superstructure or the deck of the uppermost deckhouse.

1.4 Section modulus

1.4.1 Section modulus at any point

The section modulus at any point of a hull transverse section is obtained, in m³, from the following formula:

1.4.2 Section modulus at bottom

The section modulus at bottom is obtained, in m³, from the following formula:

1.4.3 Section modulus at deck

The section modulus at equivalent deck line is obtained, in m³, from the following formula:

where:

V_D : Vertical distance of the equivalent deck line, in m, taken equal to:

When no effective longitudinal members specified in [1.2.4] and [1.2.5] are positioned above a line extending from strength deck at side to a position (z_D– z_n)/0.9 from the neutral axis at the centreline

V_D = z_D – z_n

When effective longitudinal members as specified in [1.2.4] and [1.2.5] are positioned above a line extending from strength deck at side to a position (z_D– z_n)/0.9 from the neutral axis at the centreline

z_D : Z coordinate, in m, of strength deck at side, defined in [1.3].

y_T, z_T : Y and Z coordinates, in m, of the top of continuous trunk, hatch coaming, longitudinal stiffeners or girders, to be measured for the point which maximises the value of V_D.

1.5 Moments of inertia

1.5.1 The net moment of inertia, I_y-n50 and I_z-n50, in m⁴, are those, calculated about the horizontal and vertical neutral axes, respectively, of the hull transverse sections defined in [1.2].