Section 5 Hull bending strength

5.1.1 The symbols used in this Section are defined as follows:

5.2.1 The appropriate hogging or sagging design hull vertical wave bending moment at amidships is given by the following:

Consideration will be given to direct calculations of long-term vertical wave bending moments, see Pt 3, Ch 4, 2.6 Approved calculation systems.

5.2.2 The longitudinal distribution factor, C ₂, of wave bending moment is to be taken as follows:

• 0 at the aft end of L
• 1,0 between 0,4L and 0,65L from aft
• 0 at the forward end of L

Intermediate values are to be determined by linear interpolation.

5.2.3 For operation in sheltered water or short voyages, a higher permissible still water bending moment can be assigned based on a reduced vertical wave bending moment given by:

1. For operating in sheltered water:

   M w

   =

   0,5f 2 M wo

2. For short voyages:

   M w

   =

   0,8f 2 M wo

These expressions can only be used in the expression for permissible still water bending moment, see Pt 3, Ch 4, 5.4 Minimum hull section modulus, and the relevant loading conditions are to be included in the Loading Manual, see Pt 3, Ch 4, 8.1 General.

5.2.4 `Short voyages' are defined as voyages of limited duration in reasonable weather. `Reasonable weather' and `sheltered water' are defined in Pt 1, Ch 2, 2 Character of classification and class notations.

5.3.1 The design still water bending moment, M _s, hogging and sagging is the maximum moment calculated from the loading conditions, given in Pt 3, Ch 4, 5.3 Design still water bending moments 5.3.3, and is to satisfy the following relationship:

5.3.2 Still water bending moments are to be calculated along the ship length. For these calculations, downward loads are to be taken as positive values and are to be integrated in the forward direction from the aft end of L. Hogging bending moments are positive.

5.3.3 In general, the following loading conditions, based on amount of bunkers, fresh water and stores at departure and arrival, are to be considered.

1. General cargo ships, container ships, passenger ships, roll on-roll off ships and refrigerated cargo carriers:

   1. Homogeneous loading conditions, at maximum draught.

   2. Ballast conditions.

   3. Special loading conditions, e.g. container or light load conditions at less than the maximum draught, heavy cargo, empty holds or non-homogeneous cargo conditions, deck cargo conditions, etc. where applicable.

2. Bulk carriers (see Pt 3, Ch 4, 3.2 General 3.2.2), ore carriers and combination carriers

   1. For ships of length, L, less than 150 m:

      Alternate hold loading conditions at maximum draught, where applicable.

      Homogeneous loading conditions at maximum draught.

      Ballast conditions, including intermediate conditions associated with ballast exchange at sea.

      Special conditions, e.g. deck cargo conditions.

      For combination carriers, the conditions as specified in (c) for oil tankers are also to be considered.

   2. For ships of length, L, 150 m or above:

      Alternate light and heavy cargo loading conditions at maximum draught, where applicable.

      Homogeneous light and heavy cargo loading conditions at maximum draught.

      Ballast conditions. Where vessels are designed with a ballast hold adjacent to topside wing, hopper and double bottom tanks, the structure design is to be such that the ballast hold can be filled with all adjacent tanks empty;

      Short voyage conditions where the ship is loaded to maximum draught with reduced bunkers, where applicable.

      Multiple port loading/unloading conditions, where applicable.

      Deck cargo conditions, where applicable.

      Typical loading and discharging sequences from commencement to end of cargo operation, for homogeneous, alternate and part load conditions, where applicable.

      Typical sequences for exchange of ballast at sea, where applicable.

      For combination carriers, the conditions as specified in (c) for oil tankers are also to be considered.

      For bulk carriers, the conditions as specified in Pt 3, Ch 4, 5.4 Minimum hull section modulus for the relevant notation are also to be considered.

3. Oil tankers (see Pt 3, Ch 4, 3.2 General 3.2.2):

   1. Homogeneous loading conditions (excluding dry and clean ballast tanks) and ballast or part loaded conditions.

   2. Any specified non-uniform distribution of loading.

   3. Mid-voyage conditions relating to tank cleaning or other operations where these differ significantly from the ballast conditions.

4. Chemical tankers:

   1. Conditions as specified for oil tankers.

   2. Conditions for high density or segregated cargo.

5. Liquefied gas carriers:

   1. Homogeneous loading conditions for all approved cargoes.

   2. Ballast conditions.

   3. Cargo conditions where one or more tanks are empty or partially filled or where more than one type of cargo having significantly different densities is carried.

6. All ships:

   1. Any other loading condition likely to result in high bending moments and/or shear forces (including docking conditions, as appropriate).

5.3.4 Where the amount and disposition of consumables at any intermediate stage of the voyage are considered more severe, calculations for such intermediate conditions are to be submitted in addition to those for departure and arrival conditions. Also where any ballasting and/or de-ballasting is intended during voyage, calculations of the intermediate condition just before and just after ballasting and/or de-ballasting any tank are to be submitted and, where approved, included in the loading manual for guidance. The specified percentage consumables for a given condition is based on the overall percentage filling for a consumable type, e.g. for 50 per cent consumables, the loading for each consumable type is to be 50 per cent, i.e. 50 per cent fresh water, 50 per cent fuel oil etc. Individual tanks within consumable groups can have filling levels greater than the overall specified percentage filling level

5.3.5 Ballast loading conditions involving partially filled peak and/or other ballast tanks at departure, arrival or during intermediate conditions are not permitted as design conditions unless the design stress limits are satisfied for all filling levels between empty and full, and for bulk carriers the requirements of Pt 4, Ch 7, 3 Longitudinal strength, as applicable, are to be complied with for all filling levels between empty and full. To demonstrate compliance with all filling levels between empty and full, it will be acceptable if, in each condition at departure, arrival and where required by Pt 3, Ch 4, 5.3 Design still water bending moments 5.3.3, any intermediate condition, the tanks intended to be partially filled are assumed to be:

• empty
• full
• partially filled at intended level.

Where multiple tanks are intended to be partially filled, all combinations of empty, full or partially filled at intended level for those tanks are to be investigated.

See also Pt 3, Ch 4, 5.3 Design still water bending moments 5.3.7 and Pt 3, Ch 4, 5.3 Design still water bending moments 5.3.8 for ore carriers and Pt 3, Ch 4, 5.3 Design still water bending moments 5.3.6 for cargo ships in general except container ships.

5.3.6 Cargo ships which might have one ballast water tank (or one pair of ballast water tanks) partially filled in ballast loading conditions are to be considered as either Case A or Case B as appropriate where;

For Case B ships, clear operational guidance for partial filling of ballast tanks, in association with the consumption level is to be given in the loading manual.

The maximum and minimum filling levels of the above mentioned pairs of side ballast tanks are to be indicated in the loading manual.

5.3.9 When considering cargo loading conditions, the requirements of Pt 3, Ch 4, 5.3 Design still water bending moments 5.3.5 apply to peak tanks only.

5.3.10 When considering ballast water exchange using the sequential method, the requirements of Pt 3, Ch 4, 5.3 Design still water bending moments 5.3.5 and Pt 3, Ch 4, 5.3 Design still water bending moments 5.3.9 do not apply. However, bending moment and shear force calculations for each de-ballasting or ballasting stage in the ballast water exchange sequence are to be included in the loading manual or ballast water management plan of any vessel that intends to employ the sequential ballast water exchange method.

5.4.1 The hull midship section modulus about the transverse neutral axis, at the deck or the keel, is to be not less than:

5.4.2 For materials to be included in the calculation of actual hull section properties, see Pt 3, Ch 3, 3 Structural idealisation.

5.4.3 The midship section modulus for ships with a service restriction notation is to be not less than half the minimum value required for unrestricted service.

5.4.4 Scantlings of all continuous longitudinal members of the hull girder based on the minimum section modulus requirements given in Pt 3, Ch 4, 5.4 Minimum hull section modulus 5.4.1 are to be maintained within 0,4L 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 part, bearing in mind the desire not to inhibit the vessel's loading flexibility.

5.4.5 Outside 0,4L amidships, as a minimum, hull girder bending strength checks are to be carried out at the following locations:

1. In way of the forward end of the engine room.

2. In way of the forward end of the foremost cargo hold.

3. At any locations where there are significant changes in hull cross-section.

4. At any locations where there are changes in the framing system.

5.5.1 The permissible still water bending moments sagging and hogging are to be taken as the lesser of the following:

1. =

   F DσZ D x 103 − |M w | kN m

2. =

   F BσZ B x 103 − |M w | kN m

5.6.1 The permissible combined (still water plus wave) stress for hull vertical bending, σ, is given by:

1. within 0,4L amidships

   

2. for continuous longitudinal structural members outside 0,4L amidships

where d is the distance, in metres, from the F.P. (for the fore end region) or from the A.P. (for the aft end region), as appropriate, to the location under consideration.

Special consideration will be given to increasing the permissible stress outside 0,4L amidships to

provided that sufficient buckling checks are carried out.

5.6.2 The requirements for ships of special or unusual design and for the carriage of special cargoes will be individually considered.

5.7.1 The maximum hull vertical bending stresses at deck, σ_D, and keel, σ_B, are given by the following, using the appropriate combination of bending moments to give sagging and hogging stresses:

Where the ship is always in the hogging condition, the sagging bending moment is to be specially considered.

5.7.2 Where the maximum hull vertical bending stress at deck or keel is less than the permissible combined stress, σ, reductions in local scantlings within 0,4L amidships may be permitted. The reduction factors applicable in Pt 4 Ship Structures (Ship Types) are defined as follows:

1. For hull members above the neutral axis

   

2. For hull members below the neutral axis

In general, the values of σ_D and σ_Bto be used are the greater of the sagging or hogging stresses, and F _D and F _B are not to be taken less than 0,67 for plating and 0,75 for longitudinal stiffeners.

5.7.3 Where higher tensile steel is used in the hull structure, the values of F _D and F _B for the mild steel part are to be taken as not less than

5.8.1 The hull midship section moment of inertia about the transverse neutral axis is to be not less than the following using the maximum total bending moment, sagging or hogging:

where values of σ are given in Pt 3, Ch 4, 5.6 Permissible hull vertical bending stresses 5.6.1.

In addition for vessels with L ≥ 90 m the hull midship section moment of inertia is not to be less than the following:

where C ₁is given in Pt 3, Ch 4, 5.2 Design vertical wave bending moments 5.2.1.

5.9.1 Where trunk decks or continuous hatch coamings are effectively supported or deck longitudinals or girders are fitted above the strength deck, the modulus Z _c is to be not less than Z _min. The scantling reduction factor, F _D, referred to strength deck at side, is applicable and, in addition to the requirement given in Pt 3, Ch 4, 5.5 Permissible still water bending moments 5.5.1, the permissible still water bending moments, sagging and hogging, are not to exceed: