Section
3 Structural idealisation
3.1 General
3.1.1 In general, the special and primary structure of a unit is to be analysed
by a three-dimensional finite plate element method. Only if it can be demonstrated
that other methods are adequate will they be considered.
3.1.2 The complexity of the mathematical model together with the associated
computer element types used must be sufficiently representative of all the parts of
the primary structure to enable accurate internal stress distributions to be
obtained.
3.1.3 When requested, LR can perform an independent structural analysis of the
unit.
3.1.4 For derivation of local 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.
3.1.5 Apart from local requirement for web thickness or flange thicknesses, the
stiffener, beam or girder strength is defined by a section modulus and moment of
inertia requirement.
3.2 Geometric properties of section
3.2.1 The symbols used in this sub-Section are defined as follows:
|
b |
= |
actual width, in metres, of the load-bearing plating, i.e.,
one-half of the sum of spacings between parallel adjacent members or
equivalent supports |
 |
= |
thickness, in mm, of the attached plating. Where this
varies, the mean thickness over the appropriate span is to be used. |
Table 3.3.1 Effective width
factor
|
l
|
f
|
l
|
f
|
| 0,5
|
0,19
|
3,5
|
0,69
|
| 1,0
|
0,30
|
4,0
|
0,76
|
| 1,5
|
0,39
|
4,5
|
0,82
|
| 2,0
|
0,48
|
5,0
|
0,88
|
| 2,5
|
0,55
|
5,5
|
0,94
|
| 3,0
|
0,62
|
6 and
above
|
1,00
|
| NOTE
|
| Intermediate values to be obtained by linear
interpolation.
|
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. 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.
3.2.4 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
 , b,  , c and  are measured, in mm, and are as shown in Figure 3.3.1 Corrugation geometry. The value of b is to
be taken not greater than:
-
 for welded corrugations
-
 for cold formed corrugations
The value of θ is to be taken not less than 40°. The moment of inertia
is to be calculated from:
Figure 3.3.1 Corrugation geometry
3.2.6 The effective section modulus of a built section may be taken as:
where
|
a |
= |
area of the face plate of the member, in cm2
|
 |
= |
depth, in mm, of the web between the inside of the face plate
and the attached plating. Where the member is at right angles to a line of
corrugations, the minimum depth is to be taken |
 |
= |
thickness of the web of the section, in mm |
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:
- For a member attached to plane plating:
- For a member attached to corrugated plating and parallel to the
corrugations:
SeeFigure 3.3.1 Corrugation geometry.
- 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 Determination of span point
3.3.1 The effective length,  , 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  is measured, are to be determined as follows:
- 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 primary member web stiffener, see
Figure 3.3.3 Span points
- For primary support members: the span point is to be taken at a
point distant
 from the end of the member, where
See also
Figure 3.3.3 Span points.
3.3.2 Where the end connections of longitudinals are designed with brackets to
achieve compliance with the ShipRight FDA Procedure, no reduction in span is
permitted for such brackets unless the fatigue life is subsequently reassessed and
shown to be adequate for the resulting reduced scantlings.
3.3.3 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.
3.3.4 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.
Figure 3.3.3 Span points
3.4 Grouped stiffeners
3.4.1 Where stiffeners are equally spaced and are arranged in groups of the
same scantling, the section modulus requirement of each group is to be based on the
greater of:
- the mean value of the section modulus required for individual
stiffeners within the group; and
- 90 per cent of the maximum section modulus required for
individual stiffeners within the group.
|