Section 1 General

1.1.1 The requirements for longitudinal and transverse global strength are contained within this Chapter.

1.1.2 This Chapter contains Sections detailing the analysis requirements for the following topics:

• Hull girder strength.
• Extreme strength assessment.
• Residual strength assessment

1.1.3  Section 2, Hull girder strength. This Section specifies the hull girder strength requirements based on the conventional elastic design and buckling analyses. All ships are to comply fully with the requirements of this Section.

1.1.4  Section 3, Extreme strength assessment. This Section specifies the requirements for the assessment of the extreme hull girder strength to withstand wave loads that have a low probability of occurring during the life of the ship. This is an optional assessment and ships which comply with the extreme strength requirements can apply for the notation ESA1 or ESA2, see Vol 1, Pt 1, Ch 2, 3.7 Hull strength notations 3.7.1

1.1.5  Section 4, Residual strength assessment. This Section specifies the requirements for the assessment of the residual hull girder strength after the ship has sustained structural damage. This is an optional assessment and ships which comply with the residual strength requirements can apply for the notation RSA1, RSA2 or RSA3, see Vol 1, Pt 1, Ch 2, 3.7 Hull strength notations 3.7.1.

1.2.1 The following notations are available for all ships with regard to global hull girder strength aspects:

• ESA1, ESA2 Extreme strength assessment.
• RSA1, RSA2, RSA3 Residual strength assessment.

A distinction is made between levels of performance and levels of assessment; the numeral in the notation reflects the level of assessment. Levels of performance are denoted by the letters A, B, C for collision or grounding damage and numerals I, II and III for damage from military threats and are confidential to the Owner. See Vol 1, Pt 6, Ch 4, 4.2 Extent of damage and analysis for non-military threats and Vol 1, Pt 4, Ch 2, 7 Residual strength for military threats.

1.2.2 The performance of the ship with respect to extreme hull girder strength aspects may be evaluated at two levels. ESA1, the lowest level offers a basic assessment of the ship’s capability. ESA2, the higher level is a much more rigorous assessment of the hull’s capability to withstand extreme sea states.

1.2.3 It is recommended that ships of groups NS1 and NS2 should comply with ESA1. However, it is the responsibility of the Owner to specify the level of extreme strength assessment required.

1.2.4 The extreme strength assessment level adopted must reflect the performance level required by other notations.

1.2.5 The two levels of assessment available for the extreme strength assessment notation are summarised as follows:

1.2.6 The performance of the ship with respect to residual strength aspects may be evaluated at several levels. The lowest level offers a basic assessment of the ship’s capability to survive. Higher residual strength levels are designed to show that the ship has an improved performance with respect to hull’s capability to withstand increased damage extents and scenarios.

1.2.7 Three assessment and performance levels are available for the residual strength assessment notation. RSA1, the assessment Level 1 residual strength assessment, and performance level A are recommended as a minimum for all ships of groups NS1 and NS2. However, it is the responsibility of the Owner to specify the level of residual strength assessment required.

1.2.8 The residual strength assessment level adopted must reflect the performance level required by other notations.

1.2.9 The three levels of assessment available for the residual strength assessment notation are summarised as follows:

1.3.1 The symbols and definitions applicable to this Chapter are defined below or in the appropriate sub-Section.

τ ο

=

1.3.2 The strength deck is to be taken as follows:

1. Where there is a complete upper deck, the strength deck is the upper deck.

2. Where the upper deck is stepped, as in the case of ships with a raised quarterdeck, the strength deck is also stepped, see Vol 1, Pt 6, Ch 4, 1.4 Calculation of hull section modulus. Adequate provision should be made for the transfer of load between the stepped decks.

1.4.1 In general, the effective sectional area of continuous longitudinal strength members, after deduction of openings, is to be used for the calculation of midship section modulus.

1.4.2  Structural members which contribute to the overall hull girder strength are to be carefully aligned so as to avoid discontinuities resulting in abrupt variations of stresses and are to be kept clear of any form of openings which may affect their structural performances.

1.4.3 In general, short superstructures or deckhouses will not be accepted as contributing to the global longitudinal or transverse strength of the ship. However, where it is proposed to include substantial, continuous stiffening members, special consideration will be given to their inclusion on submission of the designer’s/builder’s calculations, see also Vol 1, Pt 6, Ch 4, 2.5 Superstructures global strength

1.4.4 A transition zone, see Figure 4.1.1 Strength deck stepping, is to be assumed at the ends of stepped decks where the longitudinal structural material cannot be included in the calculation of the section modulus of hull sections. The length of the zone is to be taken as four times the deck height. Local insert plates are to be fitted in the side shell as appropriate with a thickness at least 25 per cent greater than the adjacent plating. They are to extend a minimum of one primary frame spacing forward and aft, but are to be not less than 1500 mm each way from the end of the transition zone. For vessels with complex arrangements or geometries, shadow area plans are to be submitted.

1.4.5 A superstructure deck can only be considered as a strength deck and included in the calculation of the section modulus of hull sections if it extends over the full 0,4L_R midship region and out to the side shell with the transition zones being located outside the 0,4L_R midship region, see Figure 4.1.2 Strength deck stepping, superstructure deck.

Figure 4.1.1 Strength deck stepping

Figure 4.1.2 Strength deck stepping, superstructure deck

1.4.6 Where continuous deck longitudinals or deck girders are arranged above the strength deck, special consideration may be given to the inclusion of their sectional area in the calculation of the hull section modulus, Z. The lever is to be taken to a position corresponding to the height of the longitudinal member above the moulded deck line at side amidships. Each such case will be individually considered.

1.4.7 Adequate transition arrangements are to be fitted at the ends of effective continuous longitudinal strength members in the deck and bottom structures.

1.4.8 Scantlings of all continuous longitudinal members of the hull girder based on the minimum section stiffness requirements determined from 2.2 are to be maintained within 0,4L _R amidships. However, in special cases, based on consideration of type of ship, hull form and loading conditions, the scantlings may be gradually reduced towards the ends of the 0,4L _R part, bearing in mind the desire not to inhibit the ship’s loading and operational flexibility.

1.4.9 Structural material which is longitudinally continuous but which is not considered to be fully effective for longitudinal strength purposes will need to be specially considered. The global longitudinal strength assessment must take into account the presence of such material when it can be considered effective. The consequences of failure of such structural material and subsequent redistribution of stresses into or additional loads imposed on the remaining structure must be considered.

1.4.10 In particular, all longitudinally continuous material will be fully effective in tension whereas this may be not be so in compression due to a low buckling capability. In this case, it may be necessary to derive and apply different hull girder section moduli to the hogging and sagging bending moment cases.

1.4.11 Openings in decks, longitudinal bulkheads and other longitudinal effective material having a length in the fore and aft directions exceeding 0,1B m or 2,5 m or a breadth exceeding 1,2 m or 0,04B m, whichever is the lesser, are in all cases to be deducted from the sectional areas used in the section modulus calculation.

1.4.12 Openings smaller than stated in Vol 1, Pt 6, Ch 4, 1.4 Calculation of hull section modulus 1.4.11, including manholes, need not be deducted provided they are isolated and the sum of their breadths or shadow area breadths, see Vol 1, Pt 6, Ch 4, 1.4 Calculation of hull section modulus 1.4.15, in one transverse section does reduce the section modulus at deck or bottom by more than 3 per cent.

1.4.13 The expression 0,06 (B ₁ – Σb ₁ ), where B ₁ equals the breadth of the ship at the section considered and Σb ₁ equals the sum of breadths of deductible openings, may be used for deck openings in lieu of the 3 per cent limitation of reduction of section modulus in Vol 1, Pt 6, Ch 4, 1.4 Calculation of hull section modulus 1.4.12.

Figure 4.1.3 Isolated openings

1.4.14 Where a large number of openings are proposed in any transverse space, special consideration will be required.

1.4.15 Where calculating deduction-free openings, the openings are assumed to have longitudinal extensions as shown by the shaded areas in Figure 4.1.3 Isolated openings The shadow area is obtained by drawing two tangent lines to an opening angle of 30°. The section to be considered is to be perpendicular to the centreline of the ship and is to result in the maximum deduction in each transverse space.

1.4.16 Isolated openings in longitudinals or longitudinal girders need not be deducted if their depth does not exceed 25 per cent of the web depth or 75 mm, whichever is the lesser.

1.4.17 Openings are considered isolated if they are spaced more than 1 m apart.

1.4.18 A reduction for drainage holes and scallops in beams and girders, etc. is not necessary so long as the global section modulus at deck or keel is reduced by no more than 0,5 per cent.

1.5.1 The Level 1 assessment procedures specified in Clasifications Register’s (hereinafter referred to as 'LR') Structural Detail Design Guide for fatigue design assessment, FDA, are to be generally applied to the construction details of all ships.

1.6.1 Direct calculations using finite element analysis may be necessary for ships with complicated longitudinal structural arrangements such as ships:

• of novel design;
• with significant discontinuous longitudinal material;
• with large deck openings, or where warping stresses in excess of 14,7 N/mm² are likely to occur;
• with large openings in the side shell, especially in way of the sheerstrake.