Section 3 Longitudinal strength

3.1.1 Longitudinal strength calculations are to be made in accordance with the requirements given in Pt 3, Ch 4 Longitudinal Strength.

3.1.2 The requirements of Pt 3, Ch 4, 8.3 Loading instrument regarding loading instruments are not applicable to general cargo ships under 120 m with the exception of ships with the notation Hatch Covers may be omitted in Hold (No(s) ...).

3.2.1 The hull section modulus for ships of length, L, between 120 m, and 170 m, and maximum service speed greater than 17,5 knots in association with a bow shape factor, ψ, of more than 0,15, is to comply with the requirements of this sub-Section.

3.2.2 The bow shape factor is defined as:

See also Figure 1.3.1 Derivation of bow shape factor.

3.2.3 For longitudinal strength requirements, the Rule minimum hull midship section modulus and the distribution of longitudinal material in the forward half-length will be considered. In general, the following requirements are to be complied with:

1. The vertical hull midship section modulus, about the horizontal neutral axis, at deck is to be not less than 331LkΣA _b cm³, or that required by Pt 3, Ch 4, 5 Hull bending strength, whichever is the greater. ΣA _b is defined in Pt 4, Ch 1, 3.2 Fast cargo ships 3.2.2.

2. The horizontal hull midship section modulus, about a vertical axis through the ship centreline, is to be not less than 32,5 L ² D cm³.

3. In the forward half-length, the hull section modulus is not to be a lesser percentage of the midship value than that shown in Table 1.3.1 Fast cargo ships.

4. Any load or ballast condition resulting in a sagging still water bending moment, or a hogging moment less than 80 per cent of the Rule value of still water bending moment, will be specially considered with a view to minimising the compressive stresses in the deck in waves.

3.2.4 For local strength, in general the following requirements are to be complied with:

1. Longitudinal deck stiffening is to be carried forward to the fore peak bulkhead or as far forward as practical. Where a long forecastle is fitted, the buckling strength of the proposed structure will be specially considered.

2. Substantial web frames in way of deck transverses are to be fitted in the forward half-length. Scantlings of webs and frames are to be based on actual lengths, not 'tween deck heights, and collars are to be fitted at ends of members in way of high shear.

3. Scantlings of bottom structure in forward part are to be specially considered.

4. Deck and side shell panels forward of 0,5L from F.P. are to be examined to establish the critical buckling stress from the following formula:

   
   where

   s	=	length of shorter edge, in mm
   t	=	thickness of plating, in mm
   E	=	Young's modulus, in N/mm2
   K c	=	a factor depending on aspect ratio and boundary restraint
   	=	4 for longitudinally stiffened plating or as shown in Figure 1.3.2 Plate factor, K c for transversely stiffened plating
   ν	=	Poisson's ratio (0,3 for steel and aluminium alloy).

   Where the buckling stresses, as evaluated, exceed 50 per cent of yield stress, the actual critical buckling stress is given by:

   
   where

   σac	=	corrected critical buckling stress, in N/mm2
   σo	=	yield stress, in N/mm2

   The critical buckling stress from the above formulae must be not less than 176,6 N/mm² within 0,4L amidships, nor less than 147,2 N/mm² for the deck forward of this, nor less than 117,7 N/mm² for the side shell between the first and second deck forward of 0,5L from F.P. For higher tensile steel plating, the above permissible stresses are to be divided by k.

5. In order to obtain the necessary critical buckling strength, either of the following is to be applied:

   1. plate thickness to be increased, or

   2. panel aspect ratio to be altered by the fitting of additional panel stiffening.