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 Scantling Determination

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 or 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 or flange thickness.

2.1.4 Some of the details given in this section will be specially considered for ships with a military distinction notation MD.

2.2.1 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.

2.2.2 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 a breadth b_e, in mm.

2.2.3 The effective breadth of attached plating to secondary stiffener members b_e, is to be taken as:

or 600 mm, whichever is the greater

or the actual spacing of stiffeners in mm, whichever is the lesser.

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

where

S and are as defined in Vol 1, Pt 6, Ch 2, 1.3 Symbols and definitions 1.3.1

2.3.1 The dimensions of rolled and built stiffeners are illustrated in Figure 2.2.1 Dimensions of longitudinals The section properties of stiffeners can be based on the illustrated dimensions if manufacturer’s information is not available.

2.3.2 The effective section properties of a corrugation over a spacing b, see Figure 2.2.2 Corrugated section, is to be calculated from the dimensions and, for symmetrical corrugations, may be taken as:

Section modulus

Moment of inertia

Shear area

where

d _w, b _f, t _p, c and t _w are measured, in mm, and are as shown in Figure 2.2.2 Corrugated section. The value ofb _e is to be taken not greater than b _f or:

The value of θ is to be not less than 40°.

2.3.3 The section properties of a double skin primary member over a spacing b, see Figure 2.2.3 Double skin section, may be calculated as:
Section modulus

Moment of inertia

Shear area

where

d _w, b, t _p and t _w are measured, in mm, and are as shown in Figure 2.2.2 Corrugated section

NOTE

If the plate flanges of the double bulkhead are of unequal thicknesses, then the equations in Vol 1, Pt 6, Ch 2, 2.3 Section properties 2.3.4 may be used with b _e = b _f = f b.

2.3.4 The effective section properties of a built section, see Vol 1, Pt 6, Ch 2, 2.2 Effective width of attached plating, be 2.2.1, may be taken as:

Section modulus of flange

Neutral axis of section above plating

Moment of inertia about neutral axis

Section modulus at plate

Shear area

where

b _f, t _f, d _w, t _w and t _p are in mm and are illustrated in Figure 2.2.1 Dimensions of longitudinals.

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

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

   A _p = b _f t _p/100 cm²

   (See Figure 2.2.2 Corrugated section).

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

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

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:

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

1. For rolled or built-up 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 secondary stiffening member, is equal to the depth of the member, see Figure 2.2.5 Definition of span points. Where there is no end bracket, the span point is to be measured between primary member webs.

2. For primary support members:

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

   

where b_s, b_b, d_w and d_b are as shown in Figure 2.2.5 Definition of span points

2.6.2 Where the stiffening member is inclined to a vertical or horizontal axis and the inclination exceeds 10°, the span is to be measured along the member.

2.6.3 Where the stiffening member is curved then the span is to be taken as the effective chord length between span points.

2.6.4 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.

2.7.1 The equation given in this sub Section is to be used to determine the thickness of plating for NS2 and NS3 ship types. The design pressure, p, is given in the Tables in Vol 1, Pt 6, Ch 3, 4 NS2 and NS3 scantling determination for each structural component and is to be used with the limiting stress coefficient, see Vol 1, Pt 6, Ch 5, 3.1 Design criteria 3.1.1, to determine the required plate thickness.

2.7.2 The requirements for the thickness of plating, t _p, is, in general, to be in accordance with the following:

where

s, γ, β, σ _o are as defined in Vol 1, Pt 6, Ch 2, 1.3 Symbols and definitions 1.3.1

2.8.1 The equations given in this sub Section are to be used to derive the scantling requirements for stiffeners. The design pressure, p, is given in the Tables in Vol 1, Pt 6, Ch 3, 4 NS2 and NS3 scantling determination for each structural component and is to be used with the limiting stress coefficient, see Vol 1, Pt 6, Ch 5, 3.1 Design criteria 3.1.1 to determine the required section modulus, web area and inertia of the stiffeners.

2.8.2 The requirements for section modulus, inertia and web area of stiffening members subjected to pressure loads are, in general, to be in accordance with the following:

1. For secondary members:
   Section modulus:

   

   Inertia:

   

   Web area:

   

2. For primary members:
   Section modulus:

   

   Inertia:

   

   Web area:

   

where

E, S, s, _e, σ_o and τ_o are as defined in Vol 1, Pt 6, Ch 2, 1.3 Symbols and definitions 1.3.1.

2.8.3 The requirements for section modulus, inertia and web area of stiffening members subjected to point loads are, in general, to be in accordance with the following:

1. For primary and secondary members:

   Section modulus:

   

   Inertia

   

   Web area

   

where

E, _e, and σ _o are as defined in Vol 1, Pt 6, Ch 2, 1.3 Symbols and definitions 1.3.1

2.8.4 Where a stiffener is subjected to a combination of loads, then the requirements are to be based on the linear supposition of those loads onto the stiffener. For example the section modulus requirement for a UDL load and a point load will be as follows:

2.9.1 From structural stability and local buckling considerations, the proportions of stiffening members are, in general, to be in accordance with Table 2.2.2 Stiffener proportions For primary member minimum thickness see Table 6.6.1 Minimum thickness of primary members in Vol 1, Pt 6, Ch 6 Material and Welding Requirements.

2.9.2 Primary members are to be supported by tripping brackets. The tripping brackets supporting asymmetrical sections are to be spaced no more than two secondary frames apart. The tripping brackets supporting symmetrical sections are to be spaced no more than four secondary frames apart.

2.9.3 Tripping brackets are in general required to be fitted at the toes of end brackets and in way of heavy or concentrated loads such as the heels of pillars.

2.9.4 Where the ratio of unsupported width of face plate (or flange) to its thickness exceeds 16:1, the tripping brackets are to be connected to the face plate and on members of symmetrical section, the brackets are to be fitted on both sides of the web.

2.9.5 Intermediate secondary members may be welded directly to the web or connected by lugs in accordance with Vol 1, Pt 6, Ch 6, 6.3 Secondary member end connections.

2.10.1 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.

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

2.10.3 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 is not applicable, finite element analysis or equivalent methods may have to be used.