Section 3 Hull girder strength 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 8, 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 Rule length, L _R, exceeding 35 m, covering the range of load 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 and for any special mid-voyage conditions caused by changes in ballast distribution.

3.1.3 For craft of ordinary hull form with Rule length, L _R, less than 35 m, 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 hull form, constructional arrangement and proposed loading.

3.1.4 Where the Rule length, L _R, of the craft exceeds 50 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 8, Ch 6, 2.2 Bending strength are to be complied with, using the appropriate design bending moment applicable to multi-hull craft, as determined from Pt 5, Ch 5, 5 Design criteria and load combinations.

3.2.2 The allowable tensile and compressive stress limits indicated in Table 7.3.2 Limiting stress criteria for global loading are to be complied with.

3.3.1 The requirements of Pt 8, Ch 6, 2.3 Shear strength are to be complied with in so far as they are applicable.

3.3.2 The allowable shear stress limits indicated in Table 7.3.2 Limiting stress criteria for global loading are to be complied with.

3.4.1 At the discretion of LR or where a craft is of unusual form or novel construction, 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 Global Load and Design Criteria. Such calculations are to be submitted in accordance with Pt 8, Ch 6, 1.5 Direct calculation procedure.

3.5.1 Cross-deck structures are to have adequate transverse strength in relation to the design loads and moments. Generally the net areas with effective flange, after deductions of openings, are to be used for the calculations of the total stiffness of the longitudinal section of the cross-deck structures. The effective shear area of transverse strength members is the net web area after deduction of openings.

3.5.2 The twin hull transverse bending strength of the craft at any position along the length is to be examined.

3.5.3 The twin hull transverse bending stresses for both the compressive and tensile cases are to be determined by direct calculation methods, or on the basis of Pt 8, Ch 6, 2.2 Bending strength 2.2.3 and Pt 8, Ch 6, 2.2 Bending strength 2.2.4 respectively. The stresses are to be determined in conjunction with the twin hull transverse bending moment, M _R, as defined in Pt 5, Ch 5, 5 Design criteria and load combinations.

3.5.4 Due consideration is to be given to the increased bending moments which may arise due to local point loadings from pillars, fuel bunkers, heavy items of machinery, stores, etc.

3.5.5 The shear strength of the cross-deck structure is to be examined by applying the appropriate vertical shear force at the centreline of the cross-deck structure between the twin hulls. The shear stress, τ_v, is to be determined from:

3.5.6 The allowable shear stress limits indicated in Table 7.3.2 Limiting stress criteria for global loading are to be complied with.

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, see also Pt 8, Ch 3, 4.15 Grillage structures.

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.