Section 2 Structural idealization for pontoons

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

2.1.2 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. Where the web of the section is not normal to the attached plating, and the angle exceeds 20°, the properties of the section are to be determined about an axis parallel to the attached plating.

2.1.3 The geometric properties of rolled or built stiffener sections and of swedges are to be calculated in association with the effective area of attached load bearing plating of thickness t _p and of width 600 mm or 40t _p whichever is the greater. In no case, however, is the width of plating to be taken as greater than either the spacing of the stiffeners or the width of the flat plating between swedges, whichever is appropriate. 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.

2.1.4 The effective section modulus of a corrugation over a spacing p 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 3.2.1 Corrugated section The value of b is to be taken not greater than:

The moment of inertia is to be calculated from:

2.1.5 The section modulus of a double plate bulkhead over a spacing b may be calculated as:

where d _w, b, t _p and t _w are measured, in mm, and are as shown in Figure 3.2.2 Double plate bulkhead section

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

2.1.7 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

   where b is as defined in Pt 3, Ch 3, 2.1 Geometric properties of section 2.1.1.

2. For a member attached to corrugated plating and parallel to the corrugations (where b is as defined in Pt 3, Ch 3, 2.1 Geometric properties of section 2.1.4, see also Figure 3.2.1 Corrugated section):

   A

   =

   0,01bt p cm2

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.

2.2.1 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 secondary stiffening 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 skin construction the span may be reduced by the depth of the primary member web stiffener, see Figure 3.2.3 Span points

2. For primary support members:

   The span point is to be taken at a point distant b _e from the end of the member (see Figure 3.2.3 Span points), where:

   

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

2.2.3 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.