Section 2 Longitudinal strength

2.1.1 Longitudinal strength calculations are to be made in accordance with the requirements given in Pt 3, Ch 4 Longitudinal Strength and the additional notes contained in this Section.

2.1.2 The design vertical wave bending moments and design wave shear forces to be used in Pt 3, Ch 4 Longitudinal Strength are to be determined in accordance with Pt 4, Ch 2, 2.5 Design wave shear force and Pt 4, Ch 2, 2.4 Design vertical wave bending moments below. For ships of unusual hullform or where their design parameters are outwith the applicability of the Rules, see Pt 3, Ch 4 Longitudinal Strength, special consideration will be given to the values and distributions of the wave induced global loads.

2.1.3 The still water bending moment and shear force envelopes are to take into account the requirements of Pt 4, Ch 2, 2.3 Still water bending moments and shear forces.

2.1.4 For ships where the side shell or side casings contain large openings or where the effectiveness of the superstructures in resisting hull girder bending loads is expected to be reduced by the presence of large numbers of windows or openings, the combined hull and superstructure response may require to be verified using direct calculation techniques.

2.1.5 The requirements of Pt 3, Ch 4, 8.3 Loading instrument regarding loading instruments are not applicable to passenger ferries, roll on-roll off passenger ferries and passenger vehicle ferries without a ShipRight SDA notation.

2.2.1 The calculation of section modulus is to be in accordance with Pt 3, Ch 3, 3.4 Calculation of hull section modulus and the additional notes in this Section. 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. For ships where the effectiveness of the superstructure is only partial due to the presence of large or numerous shell openings or discontinuities in the shell envelope, an equivalent section modulus for the purposes of this Section may be derived using direct calculations in accordance with the SDA procedure relevant to the ship type.

2.2.2 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.

2.2.3 In general, short superstructures, see also Pt 3, Ch 3, 3.4 Calculation of hull section modulus 3.4.2, 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 Pt 4, Ch 2, 2.6 Buckling strength.

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

2.2.5 Scantlings of all continuous longitudinal members of the hull girder based on the minimum section stiffness requirements determined from Pt 4, Ch 4, 5 Lifting appliances, equipment integration and foundations are to be maintained within 0,4L 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 part, bearing in mind the desire not to inhibit the ship's loading and operational flexibility.

2.2.6 Structural material which is longitudinally continuous but which is not considered to be fully effective for longitudinal strength purposes may 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 is to be considered.

2.2.7 In particular, all longitudinally continuous material will be fully effective in tension whereas this may 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.

2.3.1 The design still water hogging and sagging bending moment distribution envelope, M _S, is to be taken as the maximum sagging (negative) and maximum hogging (positive) still water bending moments, calculated at each position along the ship. The maximum moments from all loading conditions are to be used to define the still water bending moment distribution envelope.

2.3.2 It is normal for ships which have a low deadweight requirement or a uniform loading rate in association with a low block coefficient to have a hogging still water bending moment in all conditions of loading. For these ships, the maximum design sagging still water bending moment may be taken as the minimum actual hogging bending moment.

2.3.3 The design still water shear force distribution envelope, Q _S, is to be taken as the maximum positive and negative shear force values, calculated at each position along the ship. The maximum shear forces from all loading conditions are to be used to define the still water shear force distribution envelope.

2.4.1 The minimum value of vertical wave bending moment, M_w at any position along the ship may be taken as follows:

2.4.2 The area ratio factor, R_A, for the combined stern and bow shape is to be derived as follows:

2.4.3 The bow flare area, A_BF, is illustrated in Pt 4, Ch 2, 2.4 Design vertical wave bending moments 2.4.3 and may be derived as follows:

Alternatively the following formula may be used:

2.4.4 The stern flare area, A_SF, is illustrated in Pt 4, Ch 2, 2.4 Design vertical wave bending moments 2.4.3 and is to be derived as follows:

Where half the water plane area A_US is less than any half water plane area below the waterline T_C,Ufor the longitudinal extents illustrated in Figure 2.2.1 Derivation of bow and stern flare areas, for example, in the case of ships with tumblehome in the stern region, the maximum half waterplane area for similar longitudinal extents at any waterline less than T_C,U is to be considered as A_US.

The effects of appendages including bossings are to be ignored in the calculation of A_LS.

Alternately, the impact of stern flare on the design vertical wave bending moment may be specially considered, see Pt 4, Ch 2, 2.4 Design vertical wave bending moments 2.4.6.

2.4.5 Direct calculation methods may be used to derive the vertical wave bending moments, see Pt 3, Ch 4, 2.5 Direct calculation procedures.

2.4.6 The sagging correction factor, f_fS, in the vertical wave bending moment formulation in Pt 4, Ch 2, 2.3 Still water bending moments and shear forces 2.3.1 may be derived by direct calculation methods. Appropriate direct calculation methods include a combination of long-term ship motion analysis, non linear ship motion analysis and static balance on a wave crest or trough.

2.5.1 The design vertical wave shear force, Q _w, at any position along the ship is given by:

where

Q_wo and K₂ are given in Pt 3, Ch 4, 6.3 Design wave shear force

Intermediate values of K_f to be obtained by linear interpolation.

f_fS and f_fH are defined in Pt 4, Ch 2, 2.4 Design vertical wave bending moments 2.4.1.

2.6.1 The buckling requirements in Pt 3, Ch 4, 7 Hull buckling strength are to be applied to plate panels and longitudinals subject to hull girder compression and shear stresses. The design stresses are to be based on the design values of still water and wave bending moments and shear forces and are given in Pt 4, Ch 2, 2.4 Design vertical wave bending moments 2.4.1 and Pt 4, Ch 2, 2.5 Design wave shear force 2.5.1.

2.6.2 The standard deduction for corrosion, d _t, to be applied to plating and longitudinals is to be taken in accordance with Table 4.7.1 Standard deduction for corrosion, d t in Pt 3, Ch 4 Longitudinal Strength.

2.6.3 The buckling factors of safety, λ, to be applied to the corrected critical buckling stress, σ_CRB, of plate panels and longitudinals subjected to hull girder compression are given in Table 2.2.1 Buckling factors of safety, λ, where the corrected critical buckling stress is to be determined in accordance with Pt 3, Ch 4, 7.3 Elastic critical buckling stress.

2.6.4 The shear buckling requirements of Pt 3, Ch 4, 7.3 Elastic critical buckling stress are to be applied.