Section 2 Longitudinal strength

2.1.1 The longitudinal strength of pontoons with L/D > 6 is to be considered for the worst combination of imposed loadings. (See Pt 3, Ch 3, 3 Design loading).

2.2.1 Section modulus calculations are required on all pontoons with an L/D > 6.

2.2.2 All continuous longitudinal structural components are to be included in the calculation of the inertia of the pontoon midship section, where appropriate, and the lever z is to be measured vertically from the neutral axis to the top of bottom plating and to the moulded strength deck line at the side. Where the deck is stepped the hull section modulus for sections between the deck overlaps may be calculated using a mean height h _mean, as indicated in Figure 4.2.1 Stepped decks. If the deck thickness varies between the stepped decks, the lesser value is to be used in the calculation of section modulus.

Figure 4.2.1 Stepped decks

2.2.3 Deck openings having a length in the fore and aft directions exceeding 2,5 m or a breadth exceeding 1,2 m or 0,04B m, whichever is the lesser, are always to be deducted from the sectional areas used in the section modulus calculation.

2.2.4 Deck openings smaller than those referred to in Pt 3, Ch 4, 2.2 Calculation of pontoon section modulus 2.2.3 including manholes, need not be deducted provided they are isolated and the sum of their breadths or shadow area breadths, see Pt 3, Ch 4, 2.2 Calculation of pontoon section modulus 2.2.5, in one transverse section does not exceed 0,06(B ₁ - Σb ₁)

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

2.2.5 In the calculation of deduction-free openings, the openings are assumed to have longitudinal extensions as shown by the shaded areas in Figure 4.2.2 Deduction-free openings. The shadow area is obtained by drawing two tangent lines to an opening angle of 30°. The section to be considered should be perpendicular to the centreline of the pontoon and should result in the maximum deduction in each transverse space.

2.2.6 Isolated openings in longitudinals or longitudinal girders need not be deducted if their depth does not exceed 25 per cent of the web depth with a maximum depth for scallops of 75 mm.

2.2.7 Openings are considered isolated if they are spaced more than 1 m apart in either the transverse or longitudinal direction.

Figure 4.2.2 Deduction-free openings

2.2.8 Deck openings should be avoided in areas of high stress.

2.3.1 The permissible stress for hull vertical bending, σ, is given by:

2.3.2 The permissible shear stress, τ, is given by: