Section 3 Additional hull girder strength requirements for multi-hull craft

3.1.1 Except as otherwise specified within this Section, the global strength requirements for multi-hull craft are to comply with Pt 6, Ch 6, 2 Hull girder strength for mono-hull craft.

3.1.2 Longitudinal strength calculations are to be submitted for all craft with a length, L _R, exceeding 40 m covering the range of load and ballast conditions proposed, in order to determine the required hull girder strength. Still water, static wave and dynamic bending moments and shear forces are to be calculated for both departure and arrival conditions.

3.1.3 For craft of ordinary hull form length with a Rule length, L _R, less than 40 metres, the minimum hull girder strength requirements are generally satisfied by scantlings obtained from local strength requirements. However longitudinal strength calculations may be required at LR's discretion, dependent upon the proposed loading.

3.1.4 Where the length, L _R, of the craft exceeds 60 m, or for new designs of large, structurally complicated craft, the design loads and scantling determination formulae in this Chapter are to be supplemented by direct calculation and structural analysis by 3-D finite element methods. These supplementary calculations are to include the results of model tests and full scale measurement where available or required by LR. Full details of such methods and all assumptions and calculations, which are to be based on generally accepted theories, are to be submitted for appraisal.

3.1.5 The strength deck plating in way of the cross-deck structure, the wet-deck plating, longitudinal bulkheads and girders, and other continuous members may be included in the determination of the midship section stiffness.

3.1.6 Special consideration will be given to the global strength requirements for craft with more than two hulls linked by cross-deck structure.

3.2.1 The requirements of Pt 6, Ch 6, 2.2 Bending strength are in general to be complied with, using the appropriate design bending moment and effective pressure loadings applicable to multi-hull craft, as determined from Pt 5, Ch 5, 5 Design criteria and load combinations.

3.3.1 The requirements of Pt 6, Ch 6, 2.3 Minimum hull section modulus are to be complied with so far as they are applicable.

3.4.1 Where a craft is of unusual form or novel construction, or at the discretion of LR, the torsional stress is to be determined by direct calculation methods using the twin hull torsional connecting moment as defined in Pt 5, Ch 5, 5 Design criteria and load combinations. Such calculations are to be submitted in accordance with Pt 6, Ch 6, 1.5 Direct calculation procedure.

3.5.1 Design loads to be applied for scantling calculations are transverse vertical bending moment and shear force, twin hull torsional connecting moment, external pressure load and appropriate internal loads as defined in Pt 5 Design and Load Criteria.

3.5.2 The primary stiffening members of the cross-deck structure are to provide sufficient strength to satisfy the stress criteria given in Table 6.3.1 Primary member stress criteria.

Table 6.3.1 Primary member stress criteria

Stress type	Component stresses	Allowable stress level (N/mm2)
Total direct stress, σP	σP = σMB + σMT + σd	f σgVσs
Total shear stress, τP	τP = τT + τMBT+ τMT	f τgV s
Equivalent stress, σeq		1,2 f σegσs
Symbols and definitions
σMB, σMT, τT, τMBTσd and τMT are component stresses, in N/mm2, to be taken from Table 6.3.2 Cross-deck component stresses for designs complying with 3.5.3. f σgV, f τgV and f σeg are limiting stress coefficients for cross-deck structures to be taken from Table 7.3.2 Limiting stress coefficients for global loading in Chapter 7. σS and τS are defined inPt 6, Ch 6, 1.2 Symbols and definitions.

3.5.3 The component nominal stresses may be determined in accordance with Table 6.3.2 Cross-deck component stresses for designs complying with 3.5.3 in the case where the cross-deck is formed by transverse primary stiffeners or bulkheads and the following assumptions are taken:

1. The cross-deck is symmetrical forward and aft of a transverse axis at its half length.

2. Primary stiffeners having the same scantlings and spacing.

Table 6.3.2 Cross-deck component stresses for designs complying with 3.5.3

Component stress type	Nominal stress (N/mm2)
Hull girder bending stress at strength deck amidships, see Table 6.2.1 Longitudinal component stresses
Stress induced by the transverse bending moment M B, as defined in Pt 5, Ch 5, 5 Design criteria and load combinations
Stress induced by the torsional moment M T, as defined in Pt 5, Ch 5, 5 Design criteria and load combinations
Shear stress induced by the vertical shear force Q T, as defined in Pt 5, Ch 5, 5 Design criteria and load combinations
Bending shear stress induced by the torsional moment M T, as defined in Pt 5, Ch 5, 5 Design criteria and load combinations
Shear stress induced by the torsional moment M T, as defined in Pt 5, Ch 5, 5 Design criteria and load combinations
Symbols and definitions
Q T = vertical shear force, in kN, as determined from Pt 5, Ch 5, 5 Design criteria and load combinations M B = transverse bending moment in kNm, as determined from Pt 5, Ch 5, 5 Design criteria and load combinations M T = torsional moment in kNm, as determined from Pt 5, Ch 5, 5 Design criteria and load combinations n = total number of transverse primary stiffeners or bulkheads A W = stiffener web area, cm2 Z = primary stiffeners sections section modulus, in cm3 s p = stiffener spacing, in metres y = moment of inertia of stiffener, cm4 x H = transverse distance between the centre of the two hulls, in metres κ = t f, for symmetrical l-section, in mm = b b h/(b b+ h), for constant thickness box sections, in mm σMB = stress induced by the transverse bending moment M B, as defined in Pt 5, Ch 5, 5 Design criteria and load combinations, in N/mm2 σ MT = stress induced by the torsional moment M T, as defined in Pt 5, Ch 5, 5 Design criteria and load combinations, in N/mm2 τ T = shear stress induced by the vertical shear force Q T, as defined in Pt 5, Ch 5, 5 Design criteria and load combinations, in N/mm2 τMBT = bending shear stress induced by the torsional moment M T, as defined in Pt 5, Ch 5, 5 Design criteria and load combinations, in N/mm2 τMT = shear stress induced by the torsional moment M T, as defined in Pt 5, Ch 5, 5 Design criteria and load combinations, in N/mm2 t f = face plate thickness, in mm b b = breadth of box section, in mm h b = height of box section, in mm f MR, f MB and f MT are load combination factors reflecting the portions of each component global design load, M R, Q T, M Band M T, corresponding to the most severe load combinations. The most severe load combinations are the combinations of loads resulting in the maximum bending, shear and effective stress, respectively. The assessment of these load combinations need to take due consideration for the component load magnitude variation with wave heading and also the phasing in time between them. Generally, f MR, f MBand f MT are to be taken as indicated in Table 6.3.3 Load combination factors.

3.5.4 Other cross-deck designs subjected to global transverse loads will require a two-dimensional grillage analysis to be performed to demonstrate compliance with Pt 6, Ch 6, 3.5 Strength of cross-deck structures 3.5.2.

3.5.5 Section properties are to be calculated using an effective breadth of plating to be determined in accordance with Pt 6, Ch 3, 1.11 Other materials.

3.5.6 Where primary stiffening members support areas of plating of the extruded plank type, or the floating frame system is used, the effect of the plating attached to the secondary stiffening members is to be ignored when determining the global section modulus requirements.

3.6.1 For complex girder systems, a complete structural analysis using numerical methods may be required to be performed to demonstrate that the stress levels are acceptable when subjected to the most severe and realistic combination of loading conditions intended.

3.6.2 In general, the transverse and vertical girders, bottom and side structures, bridge structure, deck structures and any other parts of the craft which LR considers critical to the craft's structural integrity are to be included in the numerical modelling of the craft.

3.7.1 General or special purpose computer programs or any other analytical techniques may be used provided that the effects of bending, shear, axial and torsion are properly accounted for and the theory and idealisation used can be justified.

3.7.2 In general, grillages consisting of slender girders may be idealised as frames based on beam theory provided proper account of the variations of geometric properties is taken. For cases where such an assumption is not applicable, finite element analysis or equivalent methods may have to be used.

3.7.3 Analysis of the cross-deck structures with regard to impact loads due to slamming may have to be carried out using advanced structural analysis techniques.