Section 2 Structural design

2.1.1 This Section gives the basic principles to be adopted in determining the Rule structural requirements given in Vol 1, Pt 6, Ch 3 Global Strength Requirements and Vol 1, Pt 6, Ch 4 Local Scantling Requirements.

2.1.2 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 distributed load.

2.1.3 The stiffener, beam or girder strength is defined by a section modulus and moments of inertia requirements. In addition there are local requirements for web thickness and flange thickness.

2.2.1 For stiffening members, the geometric properties of rolled or built sections are to be calculated in association with an effective area of attached load bearing plating of thickness, t _p, in mm and an effective breadth, t _p in mm.

2.2.2 The effective breadth of attached plating to secondary stiffener members b _e, in mm, is to be taken as the lesser of:

1. The actual spacing of the stiffeners, b, and

2. The greater of 40t _p and 600 mm

3. The effective breadth of attached plating to primary support members (girders, transverses, webs, etc.) is to be taken as follows:

   

but is not to exceed S

S and l are defined in Vol 1, Pt 6, Ch 1, 1.3 Symbols and definitions.

2.3.1 The effective geometric properties of rolled or built sections are to be calculated directly from the dimensions of the section and associated effective area of attached plating. Where the web of the section is not normal to the actual plating, and the angle exceeds 20°, the properties of the section are to be determined about an axis parallel to the attached plating.

2.3.2 If applicable, idealised section properties may be calculated as described in the Complementary Rules.

2.4.1 The thickness of plating as determined by the Rules may be reduced where significant curvature exists between the supporting members. In such cases a plate curvature correction factor may be applied:

see Figure 2.2.1 Convex curvature

2.4.2 The required section modulus of transverse main and ‘tween deck frames, which have reasonably constant convex curvature over their entire length, may be corrected for curvature as follows:

2.5.1 The thickness of plating as determined by the Rules may be reduced when the panel aspect ratio is taken into consideration. In such cases a panel aspect ratio correction factor may be applied:

2.6.1 The effective length, 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 secondary stiffening members: The span point is to be taken at the point where the depth of the end bracket, measured from the face of the member, is equal to the depth of the member. Where there is no end bracket, the span point is to be measured between primary member webs. For double bottom construction the span may be reduced by the depth of primary member web stiffener, see Figure 2.2.2 Definition of span points.

2. For primary support members: The span point is to be taken at a point distant from the end of the member,

2.7.1 Requirements for the proportions of stiffener sections are to be calculated according to the following sections of the Complementary Rules:

1. Vol 1, Pt 6, Ch 2, 2.9 Proportions of stiffener sections of the Rules for Naval Ships

2. Pt 6, Ch 3, 1.18 Geometric properties and proportions of stiffener sections of the Rules for Special Service Craft

3. Pt 3, Ch 10, 4.5 Web stability of the Rules for Ships.

2.8.1 For complex girder systems, a complete structural analysis using numerical methods may have to be performed to demonstrate that the stress levels are acceptable when subjected to the most severe and realistic combination of loading conditions intended.

2.8.2 General or special purpose computer programs or other analytical techniques may be used provided that the effects of bending, shear, axial load and torsion are properly accounted for and the theory and idealisation used can be justified.

2.8.3 In general, grillages consisting of slender girders may be idealised as frames based on beam theory provided proper account of the variations of geometric properties is taken. For cases where such an assumption in not applicable, finite element analysis or equivalent methods may have to be used.

2.9.1 Items such as details of end connections and scantlings of end brackets are to be in compliance with the Complementary Rules.