Clasification Society Rulefinder 2016 - Version 9.25
Clasification Society Rules and Regulations - Rules and Regulations for the Classification of Offshore Units, January 2016 - Part 4 STEEL UNIT STRUCTURES - Chapter 3 Structural Design - Section 3 Structural idealisation |
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![]() Section 3 Structural idealisation3.1 General3.1.1 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 section3.2.1 The symbols used in this sub-Section are defined as follows:
Table 3.3.1 Effective width factor
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. 3.2.3 The geometric properties of rolled or built stiffener sections and of
swedges are to be calculated in association with effective area of attached load
bearing plating of thickness 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 ![]() ![]() ![]()
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 geometry3.2.5 The section modulus of a double plate bulkhead over a spacing b may be calculated as: where
![]() Figure 3.3.2 Double plate bulkhead geometry3.2.6 The effective section modulus of a built section may be taken as: where
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:
3.3 Determination of span point3.3.1 The effective length,
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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 points3.4 Grouped stiffeners3.4.1 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:
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