Section 3 Structural idealisation

3.1.1 For derivation of scantlings of stiffeners, beams, girders, etc. the formulae in the Rules are normally based on elastic or plastic theory using simple beam models supported at one or more points and with varying degrees of fixity at the ends, associated with an appropriate concentrated or distributed load.

3.1.2 The properties of stiffener, beam or girder are defined by a section modulus and a moment of inertia. Apart from local requirements for web thickness, the minimum thickness in relation to the plating to which it is connected is to comply with Table 3.3.1 Minimum web plating thickness in relation to the connected plating. Primary members are also to comply with the requirements of Pt 3, Ch 10, 4 Construction details for primary members.

3.1.3 For flat bar stiffeners, the ratio of depth to thickness should not exceed 16.

3.2.1 The symbols used in this sub-Section are defined as follows:

Values of this factor are given in Table 3.3.2 Values of f as a function of l/b .

3.2.2 The effective geometric properties of rolled or built sections may be calculated directly from the dimensions of the section and associated effective area of attached plating. Alternatively, the geometric properties may be taken from Pt 3, Ch 3, 5 Geometric properties of rolled sections. Where the web of the section is not normal to the attached plating, the angle is not normal to the attached plating, and exceeds 20, the properties of the section are to be determined about an axis parallel to the attached plating, alternatively, the required section modulus is to be multiplied by a factor:

α is the angle between stiffener and plating.

3.2.3 The geometric properties of rolled or built stiffener sections are to be calculated in association with an effective area of attached load-bearing plating of thickness t _p in mm and of width 500 mm; for swedges, the width of plating is to be taken as the actual width of flat plating between the swedges. The thickness, t _p, is the actual thickness of the attached plating. Where this varies, the mean thickness over the appropriate span is to be used.

3.2.4 The effective section modulus of a corrugation over a spacing, s, is to be calculated from the dimensions, and for symmetrical corrugations, may be taken as:

where d _w, b, t _p, c and t _w are measured, in mm, and are as shown in Figure 3.3.3 Dimensions and symbols for corrugated bulkheads. The value of b is to be taken not greater than 50t _p in this calculation, and θ is to be not less than 40°. The moment of inertia is to be calculated from:

3.2.5 For symmetrical corrugations the following additional requirements are also to be complied with:

1. the ratio b/t _p should not exceed

2. the ratio c/t _p should not exceed

   k = see Pt 3, Ch 2, 1.3 Steel 1.3.3

3. d _w is to be not less than 39l_e mm (for deep tank bulkheads only),

4. the plating thickness at the middle of span l_e of corrugated bulkheads is to extend not less than 0,2l_e above mid span, (for definition of l_e, see Pt 3, Ch 3, 3.3 Determination of span point).

3.2.6 The effective section modulus of a built section may be taken as:

Rolled or built sections fitted on top of supported stiffening members, see Figure 3.3.4 Rolled or built sections fitted on top of supported stiffening members, are to have a modulus not less than two thirds of the modulus required for the primary member in the same position. This section should be attached at both ends of the plating for at least one stiffener spacing and should also be properly attached to the supported stiffeners.

3.2.7 The geometric properties of primary support members, i.e. girders, transverses, webs, stringers, etc. are to be calculated in association with an effective area of attached load-bearing plating, A, determined as follows:

1. For a member attached to plane plating:

   A

   =

   10fbt p cm2 (for f, see Table 3.3.2 Values of f as a function of l/b )

2. For a member attached to corrugated plating and parallel to the corrugations:

   A

   =

   10bt p cm2

   See Figure 3.3.3 Dimensions and symbols for corrugated bulkheads

3. For a member attached to corrugated plating and at right angles to the corrugations:

A is to be taken as equivalent to the area of the face plate of the member.

3.3.1 The effective span, l _e, of a stiffening member is generally less than the overall length, l, by an amount which depends on the design of the end connections. The span points, between which the value of l _e is measured, are to be determined as follows:

1. For rolled or built stiffener sections, swedges and corrugations:

   The span point is to be taken at the point where the depth of the end bracket, measured from the face of the stiffener is equal to the depth of the stiffener. Where there is no Rule end bracket, the span point is to be taken at the end of the stiffener.

2. For primary support members, i.e. girders, transverses, webs, stringers, etc.:

   The span point is to be taken at a point distant b e from the end of the member,

   where:

   

   See also Pt 3, Ch 3, 3.2 Geometric properties of section 3.2.1.

3.3.2 It is assumed that the ends of stiffening members are substantially fixed against rotation and displacement. If the arrangement of supporting structure is such that this condition is not achieved, consideration will be given to the effective span to be used for the stiffener.

3.4.1 All continuous longitudinal structural material is to be included in the calculation of the inertia of the hull midship section, and the lever z is, except where otherwise specified for particular ship types, to be measured vertically from the neutral axis to the top of keel and to the moulded strength deck line at the side. The strength deck is to be taken as follows:

1. Where there is a complete upper deck and no effective superstructure, the strength deck is the upper deck.

2. Where there is an effective superstructure or a stepped deck the position of the strength deck will be specially considered.

3.4.2 An effective superstructure is a superstructure extending over the full breadth of the ship and which exceeds 0,20L in length or 10 m whichever is the greater and is situated within the 0,5L region.

3.4.3 Lightening holes in girders need not be deducted, provided that their depth does not exceed 20 per cent of the web depth.

3.4.4 Isolated weld scallops, drain and air holes in longitudinals need not be deducted, provided that their depth does not exceed 65 mm. In no case is the opening to be greater than 25 per cent of the web depth. Such openings are considered isolated if they are spaced not less than 1 m apart.

3.4.5 In general, isolated deck openings need not be deducted, but compensation may be required. See individual ship type Chapters.

3.4.6 Where trunk decks or continuous hatch coamings are effectively supported by longitudinal bulkheads or deep girders, they are to be included in the longitudinal sectional area when calculating the hull section modulus. The lever z_t is to be taken as:

z_c and y are to be measured to the point giving the largest value of z_t