Section 1 General

1.1.1 The requirements for longitudinal and transverse global strength for mono-hull and multi-hull craft of steel construction, are contained within this Chapter. Due consideration is taken of the dynamic effects, where appropriate, in both the crest and trough wave landing conditions.

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

1.2.2 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 raised quarterdeck craft, the strength deck is stepped as shown in Figure 6.1.1 Strength deck.

1.3.1 The additional pressures arising from the influence of the global loading are considered in the determination of the longitudinal strength requirements for local and secondary stiffening and bottom shell plating.

1.3.2 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.

1.3.3 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.

1.3.4 In general, superstructures or deckhouses will not be accepted as contributing to the global longitudinal or transverse strength of the craft. 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.

1.3.5 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 depth of the longitudinal member above the moulded deckline at side amidships. Each such case will be individually considered.

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

1.3.7 Scantlings of all continuous longitudinal members of the hull girder based on the minimum section stiffness requirements determined from Pt 6, Ch 6, 2.2 Bending strength 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 craft's loading and operational flexibility.

1.4.1 Deck openings having a length in the fore and aft directions exceeding 0,1B m or a breadth exceeding 0,05B m are in all cases to be deducted from the sectional areas used in the section modulus calculation.

1.4.2 Deck openings smaller than stated in 1.4.1, including manholes, need not be deducted provided they are isolated and the sum of their breadths or shadow area breadths (see Pt 6, Ch 6, 1.4 Openings 1.4.3) in one transverse section does not exceed 0,06 (B _o - Σ b _o).

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

1.4.3 Where calculating deduction-free openings, the openings are assumed to have longitudinal extensions as shown by the shaded areas in Figure 6.1.2 Isolated openings. The shadow area is obtained by drawing two tangent lines to an opening angle of 30^o. 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.

1.4.4 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.

1.4.5 Openings are considered isolated if they are spaced not less than 1 m apart.

1.4.6 A reduction for drainage holes and scallops in beams and girders, etc. is not necessary so long as the original section stiffness at deck or keel is reduced by no more than 3,0 per cent.

1.5.1 In direct calculation procedures capable of deriving the wave induced loads on the craft, and hence the required modulus, account is to be taken of the craft's actual form and weight distribution.

1.5.2 Clasifications Register's (hereinafter referred to as 'LR') direct calculation method involves derivation of response to regular waves by strip theory, short-term response to irregular waves using the sea spectrum concept, and long-term response predictions using statistical distributions of sea states. Other direct calculation methods submitted for approval are normally to contain these three elements and produce similar and consistent results when compared with LR's methods.

1.6.1 Where the assumptions, method and procedures of a longitudinal strength calculation system have received general approval from LR, calculations using the system for a particular craft may be submitted.

1.7.1 In order that an assessment of the longitudinal strength requirements can be made, the following information is to be submitted, in LR's standard format where appropriate:

1. General arrangement and capacity plan or list, showing details of the volume and position of centre of gravity of all tanks and compartments.

2. Details of the calculated lightweight and its distribution.

3. Details of the weights, fore and aft extents, and centres of gravity of all deadweight items for each of the main loading conditions. It is recommended that this information be submitted in the form of a preliminary Loading Manual.

4. Still water bending moments and shear forces for each of the main loading conditions. It is recommended that in the preliminary Loading Manual, a suitable margin be applied to the still water bending moments and shear forces to allow for changes between the preliminary and final loading manuals. The responsibility for the correctness of the submitted still water bending moments and shear forces rests with the designer.

1.8.1 Sufficient information is to be supplied to the Master of every craft to enable him to arrange loading in such a way as to avoid the creation of unacceptable stresses in the craft's structure.