Determination of Scantlings of Stiffeners

5.2.1 Determination of scantlings of stiffeners - idealisation of stiffeners

5.2.1.1 Scantlings of individual stiffeners are to be derived based on the idealisation of the as-built structure as a series of stiffened panels.

5.2.1.2 A stiffened panel consists of a single idealised stiffener profile and effective plate flange supporting a boundary of one or more elementary plate panels. The arrangement of stiffened panels is based on the idealisation of the structure according to the elementary plate panel definition in 5.1.1.

5.2.1.3 Scantlings of stiffeners based on requirements in Section 8 may be decided based on the concept of grouping designated sequentially placed stiffeners of equal scantlings.

The scantling of the group is to be taken as the greater of the following:

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(a) the average of the required scantling of all stiffeners within a group

(b) 90% of the maximum scantling required for any one stiffener within the group.

The concept of grouping is not applicable to fatigue requirements as given in Section 9/3 and Appendix C.

5.2.2 Determination of scantlings of stiffened panels for scantling requirements and fatigue

5.2.2.1 The required scantling of a stiffened panel is to be based on a pressure load calculation point defined as:

(a) mid point of the overall span, lfull, of the stiffener between primary support members, see Figure 3.5.4

(b) at the connection of the stiffener to the plating.

Figure 3.5.4

Definition of Overall Span of Stiffeners, lfull

Double Skin Construction

l_full

Single Skin Construction

l_full

5.2.2.2 For longitudinal and horizontal framing the design pressure is to be taken as the pressure at the mid point of the overall span.

5.2.2.3 For transverse and vertical framing the design pressure is to be taken as the greater of the following:

Pms kN/m² 2

) (P_end−₁+P_end−₂

kN/m² Where:

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Pms calculated pressure at mid point of overall span, lfull, in kN/m²

Pend-1 calculated pressure at 1^st end of overall span, in kN/m²

Pend-2 calculated pressure at 2^nd end of overall span, in kN/m²

lfull overall span, in m, see Figure 3.5.4

5.2.2.4 The section modulus requirements given in these Rules relate to the reference point giving the minimum section modulus. In general, this will be on the outer surface of the faceplate. The reference point for calculation of section modulus for typical profiles is shown in Figure 3.5.5.

5.2.2.5 The hull girder stress used for calculation of local scantling requirements for stiffeners is to be taken at the reference point as shown in Figure 3.5.5

Figure 3.5.5

Reference Point for Calculation of Section Modulus and Hull Girder Stress for Local Scantling Assessment

Reference point Reference point

Reference point

Reference point Reference point

Reference point

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5.2.3 Determination of scantlings of stiffened panels for hull girder buckling strength

5.2.3.1 The required scantling of a stiffened panel, with respect to buckling, is to be based on the axial stress calculated at the attachment point of the stiffener to the plate and at the mid length of the stiffener measured along the global x-axis.

5.2.3.2 The required scantling as given in 5.2.3.1 applies to stiffeners outside of a distance s from the support, where s is the stiffener spacing.

5.2.4 Determination of scantlings of stiffened panels for FEM strength assessment

5.2.4.1 The required scantlings of the stiffened panel are to be based on the derivation of applied stresses in accordance with Section 9/2.

5.2.5 Shear area requirements of stiffeners

5.2.5.1 Requirements for the shear area and/or web thickness of stiffeners are given in Section 8.

5.2.5.2 The requirements in Section 8 are to be calculated based on the load point defined in 5.2.2 and the effective span as given in Section 4/2.1.2.

5.2.5.3 The requirements in Section 8 are to be evaluated against the actual shear area of the stiffener, based on the effective shear height of the stiffener as given in Section 4/2.4.2 and based on the specified minimum yield of the stiffener.

5.2.5.4 The effect of brackets may be included in the calculation of the effective span, but no part of the bracket is to be included in the calculation of the actual shear area.

5.2.6 Bending requirements of stiffeners

5.2.6.1 Requirements for the section modulus and moment of inertia of stiffeners are given in Section 8.

5.2.6.2 The requirements in Section 8 are to be calculated based on the load point defined in 5.2.1 and the effective span as given in Section 4/2.1.1.

5.2.6.3 The requirements in Section 8 are to be evaluated against the actual section modulus/moment of inertia of the stiffener. The stiffener web and flanges are to be included in the calculation of actual sectional properties.

5.2.6.4 The effect of brackets may be included in the calculation of the effective span, but no part of the bracket is to be included in the calculation of section modulus/moment of inertia.

5.2.6.5 When the stiffener is of a higher strength material than the attached plate, the yield stress used for the calculation of the section modulus requirements in Section 8 is in general not to be greater than 1.35 times the minimum specified yield stress of the attached plate. If the yield stress of the stiffener exceeds this limitation the following criterion is to be satisfied:

( )

σ_yd-stf specified minimum yield stress of the material of the stiffener, in N/mm²

σyd-plt specified minimum yield stress of the material of the attached plate, in

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N/mm²

σhg maximum hull girder stress of sagging and hogging (S+D), in N/mm², as defined in Table 8.2.5 and Table 8.4.3 for stiffeners in cargo tank region and machinery spaces respectively and not to be taken as less than 0.4 σyd-plt

Znet net section modulus, in way of face plate/free edge of the stiffener, in cm³ Znet-plt net section modulus, in way of the attached plate of stiffener, in cm³

5.2.7 Evaluation of slanted stiffeners

5.2.7.1 The shear area and section modulus requirements for local support members are valid about an axis parallel to the plate flange. If the angle ϕw between the stiffener web and the attached plating is less than 75 degrees, see Figure 4.2.14, then the actual shear area and section modulus is to be adjusted in accordance with Sections 4/2.4.2 and 2.4.3. The angle between the stiffener web, ϕw, and the attached plating is not to be less than 50 degrees.

5.3 Calculation and Evaluation of Scantling Requirements for Primary Support