Section 4 Buckling strength of primary members
Clasification Society 2024 - Version 9.40
Clasifications Register Rules and Regulations - Rules and Regulations for the Classification of Offshore Units, July 2022 - Part 4 Steel Unit Structures - Chapter 5 Primary Hull Strength - Section 4 Buckling strength of primary members

Section 4 Buckling strength of primary members

Parent topic: Chapter 5 Primary Hull Strength

4.1 Application

4.1.1 The requirements of this Section are applicable to individual primary structural members which are subjected to axial compression or combined axial compression and bending due to overall loading.

4.2 Symbols

4.2.1 The symbols used in this Section are defined as follows:

,E as defined in Pt 4, Ch 5, 3.2 Symbols

= computed axial compressive stress, in N/mm2 (kgf/mm2)
= computed compressive stress due to bending, in N/mm2 (kgf/mm2)
= factor of safety for compression, in accordance with Pt 4, Ch 5, 2.1 General
= factor of safety for bending, in accordance with Pt 4, Ch 5, 2.1 General
= factor of safety for overall member buckling, as determined from Table 5.4.2 Factors of safety for overall member buckling
= critical overall member buckling stress, in N/mm2 (kgf/mm2), as determined fromTable 5.4.1 Overall member critical buckling stress
= local member critical buckling stress, in N/mm2 (kgf/mm2)
= permissible axial compressive stress, in N/mm2 (kgf/mm2)
= whichever is the lesser
= permissible compressive stress due to bending, in N/mm2 (kgf/mm2)
= whichever is the lesser
D = mean diameter of cylindrical shell, in mm
t = thickness of cylindrical shell, in mm.

4.3 Elastic critical buckling stress

4.3.1 Where the elastic critical buckling stress exceeds 50 per cent of the specified minimum yield stress of the material, the calculated critical buckling stresses are to be corrected for yielding effects and are given by:

N/mm2 (kgf/mm2) in compression.

4.4 Scantling criteria

4.4.1 Individual members are to be investigated for overall critical buckling in accordance with an agreed Code or Standard or Table 5.4.1 Overall member critical buckling stress and Table 5.4.2 Factors of safety for overall member buckling and also for local buckling.

Table 5.4.1 Overall member critical buckling stress

Condition Member critical buckling stress
, N/mm2 (kgf/mm2)
(a) When λ <
(b) When λ ≥
Symbols and parameters
, E as defined in Pt 4, Ch 5, 3.2 Symbols
l = unsupported length of member, in metres
K = effective length factor to be generally taken as unity but will be specially considered in association with end conditions
= Kl = unsupported effective length of member, in metres
r = least radius of gyration of member cross-section, in mm, and may be taken as:
A = cross-sectional area of member, in cm2
I = least moment of inertia of member cross-section, in cm4
λ = slenderness ratio and may be taken as:

Table 5.4.2 Factors of safety for overall member buckling

Condition Factor of safety,
(1) For case (a) as defined in Pt 4, Ch 5, 2.1 General:  
(a) When λ < 1,67 +
(b) When λ ≥ 1,92
(2) For cases (b) and (c) as defined in Pt 4, Ch 5, 2.1 General:  
(a) When λ < 1,25 +
(b) When λ ≥ 1,44
(3) For case (d) as defined in Pt 4, Ch 5, 2.1 General:  
(a) When λ < 1,0 +
(b) When λ ≥ 1,15
Symbols and parameters
as defined in Pt 4, Ch 5, 4.2 Symbols
λ and η as defined in Pt 4, Ch 5, 4.4 Scantling criteria 4.4.1

4.4.2 The local buckling of cylindrical shells, either unstiffened or ring-stiffened, is to be investigated if the proportions of the shell conform to the following:

4.4.3 When individual primary structural members are subjected to axial compression or combined axial compression and bending, the computed design stresses are to satisfy the following requirement:

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