Section 4 Shell envelope framing

4.1.1 The requirements in this Section apply to longitudinally and transversely framed shell envelopes.

4.2.1 To determine the required scantlings, the formulae indicated in Pt 8, Ch 3, 1.15 Stiffeners general are, in general, to be used in conjunction with the design loadings specified in Pt 5 Design and Load Criteria.

4.3.1 Symbols and definitions for use throughout this Chapter are as given in Pt 8, Ch 3, 1.5 Symbols and definitions 1.5.1 or specified in the appropriate Section.

4.4.1 The bottom longitudinals are to be supported by bottom transverse web frames, floors, bulkheads, or other primary structure, generally spaced not more than 2 m apart.

4.4.2 Bottom longitudinals are to be continuous through the supporting structures.

4.4.3 Where it is impracticable to comply with the requirements of Pt 8, Ch 3, 4.4 Bottom longitudinal stiffeners 4.4.2, or where it is desired to terminate the bottom longitudinals in way of the transom, bulkheads or integral tank boundaries, they are to be bracketed in way of their end connections to maintain the continuity of structural strength. Particular attention is to be taken to ensure accurate alignment of the brackets.

4.4.4 The Rule requirements for bending moment, shear force, shear stress and deflection are to be determined from the general equations given in Pt 8, Ch 3, 1.15 Stiffeners general, using the design pressure from Pt 5, Ch 3, 3.1 Hull structures or Pt 5, Ch 4, 3.1 Hull structures for non-displacement or displacement type craft as appropriate, and the coefficients Φ_M, Φ_S and Φ_δ as indicated in Table 3.1.10 Shear force, bending moment and deflection coefficients for the load model (b).

4.4.5 The allowable tensile, compressive and shear stress limits indicated in Table 7.3.1 Limiting stress criteria for local loading , and the span/deflection ratios indicated in Table 7.2.1 Limiting span/deflection ratio are to be complied with.

4.5.1 Bottom longitudinal primary stiffeners are to be supported by bottom deep transverse web frames, floors, bulkheads, or other primary structure, generally spaced not more than 6 m apart.

4.5.2 Bottom longitudinal primary stiffeners are to maintain their continuity through the supporting structures.

4.5.3 Where it is impracticable to comply with the requirements of Pt 8, Ch 3, 4.5 Bottom longitudinal primary stiffeners 4.5.2, or where it is desired to terminate the bottom longitudinal primary stiffeners in way of the transom, bulkheads or integral tank boundaries, they are to be bracketed in way of their end connections to maintain the continuity of structural strength. Particular attention is to be taken to ensure accurate alignment of the brackets. All brackets are to be `soft toed' and are to terminate on suitable supporting structure capable of carrying the transmitted bending moment.

4.5.4 The Rule requirements for bending moment, shear force, shear stress and deflection are to be determined from the general equations given in Pt 8, Ch 3, 1.15 Stiffeners general, using the design pressure from Pt 5, Ch 3, 3.1 Hull structures or Pt 5, Ch 4, 3.1 Hull structures for non-displacement or displacement type craft as appropriate, and the coefficients Φ_M, Φ_S and Φ_δ as indicated in Table 3.1.10 Shear force, bending moment and deflection coefficients for the load model (a).

4.5.5 The allowable tensile, compressive and shear stress limits indicated in Table 7.3.1 Limiting stress criteria for local loading , and the span/deflection ratios indicated in Table 7.2.1 Limiting span/deflection ratio are to be complied with.

4.6.1 Bottom transverse stiffeners are defined as local stiffening members which support the bottom shell, and which may be continuous or intercostal.

4.6.2 The Rule requirements for bending moment, shear force, shear stress and deflection are to be determined from the general equations given in Pt 8, Ch 3, 1.15 Stiffeners general, using the design pressure from Pt 5, Ch 3, 3.1 Hull structures or Pt 5, Ch 4, 3.1 Hull structures for non-displacement or displacement type craft as appropriate, and the coefficients Φ_M, Φ_S and Φ_δ as indicated in Table 3.1.10 Shear force, bending moment and deflection coefficients for the load model (b).

4.6.3 The allowable tensile, compressive and shear stress limits indicated in Table 7.3.1 Limiting stress criteria for local loading , and the span/deflection ratios indicated in Table 7.2.1 Limiting span/deflection ratio are to be complied with.

4.7.1 Bottom transverse frames are defined as stiffening members which support the bottom shell, they are to be effectively continuous and be bracketed at their end connections to side frames and bottom floors as appropriate.

4.7.2 The Rule requirements for bending moment, shear force, shear stress and deflection are to be determined from the general equations given in Pt 8, Ch 3, 1.15 Stiffeners general, using the design pressure from Pt 5, Ch 3, 3.1 Hull structures or Pt 5, Ch 4, 3.1 Hull structures for non-displacement or displacement type craft as appropriate, and the coefficients Φ_M, Φ_S and Φ_δ as indicated in Table 3.1.10 Shear force, bending moment and deflection coefficients for the load model (a).

4.7.3 The allowable tensile, compressive and shear stress limits indicated in Table 7.3.1 Limiting stress criteria for local loading , and the span/deflection ratios indicated in Table 7.2.1 Limiting span/deflection ratio are to be complied with.

4.8.1 Bottom transverse web frames are defined as primary stiffening members which support bottom shell longitudinals, they are to be continuous and be substantially bracketed at their end connections to side web frames and bottom floors.

4.8.2 Where it is impracticable to comply with the requirements of Pt 8, Ch 3, 4.8 Bottom transverse web frames 4.8.1, or where it is desired to terminate the bottom transverse web frames in way of bulkheads or integral tank boundaries, etc. they are to be bracketed in way of their end connections, to maintain the continuity of structural strength. Particular attention is to be taken to ensure accurate alignment of the brackets. All brackets are to be `soft toed', see Figure 3.4.1 `Soft-toe' and are to terminate on suitable supporting structure capable of carrying the transmitted bending moment.

4.8.3 The Rule requirements for bending moment, shear force, shear stress and deflection are to be determined from the general equations given in Pt 8, Ch 3, 1.15 Stiffeners general, using the design pressure from Pt 5, Ch 3, 3.1 Hull structures or Pt 5, Ch 4, 3.1 Hull structures for non-displacement or displacement type craft as appropriate, and the coefficients Φ_M, Φ_S and Φ_δ as indicated in Table 3.1.10 Shear force, bending moment and deflection coefficients for the load model (a).

4.8.4 The allowable tensile, compressive and shear stress limits indicated in Table 7.3.1 Limiting stress criteria for local loading , and the span/deflection ratios indicated in Table 7.2.1 Limiting span/deflection ratio are to be complied with.

4.9.1 The side longitudinals are to be supported by side transverse web frames, bulkheads, or other primary structure, generally spaced not more than 2 m apart.

4.9.2 Side longitudinals are to be continuous through the supporting structures.

4.9.3 Where it is impracticable to comply with the requirements of Pt 8, Ch 3, 4.9 Side longitudinal stiffeners 4.9.2, or where it is desired to terminate the side longitudinals in way of the transom, bulkheads or integral tank boundaries, they are to be bracketed in way of their end connections to maintain the continuity of structural strength. Particular attention is to be taken to ensure accurate alignment of the brackets.

4.9.4 The Rule requirements for bending moment, shear force, shear stress and deflection are to be determined from the general equations given in Pt 8, Ch 3, 1.15 Stiffeners general, using the design pressure from Pt 5, Ch 3, 3.1 Hull structures or Pt 5, Ch 4, 3.1 Hull structures for non-displacement or displacement type craft as appropriate, and the coefficients Φ_M, Φ_S and Φ_δ as indicated in Table 3.1.10 Shear force, bending moment and deflection coefficients for the load model (b).

4.9.5 The allowable tensile, compressive and shear stress limits indicated in Table 7.3.1 Limiting stress criteria for local loading , and the span/deflection ratios indicated in Table 7.2.1 Limiting span/deflection ratio are to be complied with.

4.10.1 Side longitudinal primary stiffeners are to be supported by side transverse web frames, bulkheads, or other primary structure, generally spaced not more than 6 m apart.

4.10.2 Side longitudinal primary stiffeners are to maintain their continuity through the transverse bulkheads and other supporting structures.

4.10.3 Where it is impracticable to comply with the requirements of Pt 8, Ch 3, 4.10 Side longitudinal primary stiffeners 4.10.2, or where it is desired to terminate the side longitudinal primary stiffeners in way of the transom, bulkheads or integral tank boundaries, they are to be bracketed in way of their end connections to maintain the continuity of structural strength. Particular attention is to be taken to ensure accurate alignment of the brackets. All brackets are to be `soft toed' and are to terminate on suitable supporting structure capable of carrying the transmitted bending moment.

4.10.4 The Rule requirements for bending moment, shear force, shear stress and deflection are to be determined from the general equations given in Pt 8, Ch 3, 1.15 Stiffeners general, using the design pressure from Pt 5, Ch 3, 3.1 Hull structures or Pt 5, Ch 4, 3.1 Hull structures for non-displacement or displacement type craft as appropriate, and the coefficients Φ_M, Φ_S and Φ_δ as indicated in Table 3.1.10 Shear force, bending moment and deflection coefficients for the load model (a).

4.10.5 The allowable tensile, compressive and shear stress limits indicated in Table 7.3.1 Limiting stress criteria for local loading , and the span/deflection ratios indicated in Table 7.2.1 Limiting span/deflection ratio are to be complied with.

4.11.1 Side transverse stiffeners are defined as local stiffening members which support the side shell, and which may be continuous or intercostal.

4.11.2 The Rule requirements for bending moment, shear force, shear stress and deflection are to be determined from the general equations given in Pt 8, Ch 3, 1.15 Stiffeners general, using the design pressure from Pt 5, Ch 3, 3.1 Hull structures or Pt 5, Ch 4, 3.1 Hull structures for non-displacement or displacement type craft as appropriate, and the coefficients Φ_M, Φ_S and Φ_δ as indicated in Table 3.1.10 Shear force, bending moment and deflection coefficients for the load model (b).

4.11.3 The allowable tensile, compressive and shear stress limits indicated in Table 7.3.1 Limiting stress criteria for local loading , and the span/deflection ratios indicated in Table 7.2.1 Limiting span/deflection ratio are to be complied with

4.12.1 Side transverse frames are defined as stiffening members supporting the side shell and spanning continuously between bottom floors/frames and decks. They are to be effectively constrained against rotation at their end connections.

4.12.2 The Rule requirements for bending moment, shear force, shear stress and deflection are to be determined from the general equations given in Pt 8, Ch 3, 1.15 Stiffeners general, using the design pressure from Pt 5, Ch 3, 3.1 Hull structures or Pt 5, Ch 4, 3.1 Hull structures for non-displacement or displacement type craft as appropriate, and the coefficients Φ_M, Φ_S and Φ_δ as indicated in Table 3.1.10 Shear force, bending moment and deflection coefficients for the load model (a).

4.12.3 The allowable tensile, compressive and shear stress limits indicated in Table 7.3.1 Limiting stress criteria for local loading , and the span/deflection ratios indicated in Table 7.2.1 Limiting span/deflection ratio are to be complied with.

4.13.1 Side transverse web frames are defined as primary stiffening members which support side shell longitudinals, they are to be continuous and be substantially bracketed at their head and heel connections to deck beams and bottom web frames respectively.

4.13.2 Where it is impracticable to comply with the requirements of Pt 8, Ch 3, 4.13 Side transverse web frames 4.13.1, or where it is desired to terminate the side transverse web frames in way of side longitudinal primary stiffeners, bulkheads or integral tank boundaries, they are to be bracketed in way of their end connections, to maintain the continuity of structural strength. Particular attention is to be taken to ensure accurate alignment of the brackets. All brackets are to be `soft toed' and are to terminate on suitable supporting structure capable of carrying the transmitted bending moment.

4.13.3 The Rule requirements for bending moment, shear force, shear stress and deflection are to be determined from the general equations given in Pt 8, Ch 3, 1.15 Stiffeners general, using the design pressure from Pt 5, Ch 3, 3.1 Hull structures or Pt 5, Ch 4, 3.1 Hull structures for non-displacement or displacement type craft as appropriate, and the coefficients Φ_M, Φ_S and Φ_δ as indicated in Table 3.1.10 Shear force, bending moment and deflection coefficients for the load model (a).

4.13.4 The allowable tensile, compressive and shear stress limits indicated in Table 7.3.1 Limiting stress criteria for local loading , and the span/deflection ratios indicated in Table 7.2.1 Limiting span/deflection ratio are to be complied with.

4.14.1 For the purposes of satisfying Rule requirements, frames may, subject to agreement by LR, be grouped. The number of frames in any group shall not in general exceed five. The summation of section stiffness, E , for the group of frames is not to be less than the summation of the Rule requirements for the individual framing members. In addition, in no case is the proposed scantlings of an individual framing member within the group to be less than ninety per cent of the Rule value for that member.

4.15.1 For complex girder systems, a complete structural analysis using numerical methods may have 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, 1.3 Direct calculations.

4.15.2 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.

4.15.3 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.

4.16.1 Where longitudinal and transverse primary stiffeners form grillage structures the scantlings may be derived in accordance with Pt 8, Ch 3, 4.15 Grillage structures.

4.17.1 Where the floating frame system is used, the effect of the plating attached to the stiffening members is to be ignored when calculating the required section stiffness, E , of the primary stiffening members, i.e. the full section stiffness, E , is to be provided by the primary stiffening member only.

4.18.1 Where struts are fitted to side shell transverse web frames or longitudinal primary stiffeners to carry axial loads the strut cross-sectional area is to be derived as for pillars in Pt 8, Ch 3, 10 Pillars and pillar bulkheads. If fitted at the stiffener half span point the stiffener section modulus may be taken as half the modulus derived from the general equations for the stiffening member being considered.

4.18.2 Design of end connections is to be such that the strut loads can be efficiently transmitted into the supporting structure.

4.19.1 The design of and responsibility for the fendering on any craft rests with the designer and prospective Owner and are outside the scope of classification scantling approval requirements. The arrangement for fendering fitted should not be detrimental to the general working of the structure and therefore the requirements indicated in Pt 8, Ch 3, 4.19 Fenders and reinforcement in way 4.19.2 are provided as recommendations of the areas requiring special consideration by the designer and Builder.

4.19.2 Wood belting and fenders, which may be subject to considerable impact load, are to be bedded down on a flexible sealing compound or a neoprene type gasket to ensure watertightness. The bolts are to be both adequate in number and size and, where practicable, reeled to prevent perforation of the laminate. Substantial plate washers or, where practicable, a continuous backing plate are to be provided. The arrangement for the attachment of the fender should, in general, be arranged so that where sections of the fender are damaged or torn, the watertight integrity of the hull is not impaired.

4.19.3 The laminate in way of such fittings is to be substantially increased in thickness to prevent overloading, and depending on the position, a back-up block of wood, plastic or metal may be required.

4.19.4 For craft such as pilot craft, fishing craft, etc. which may be subject to repeated impact loadings from contact with other craft whilst in service, due consideration is to be given to increasing the scantlings of stiffening members in way of the fenders. Details of these increased scantlings, anticipated loadings and calculations, are to be indicated on the submitted plans, see also Pt 8, Ch 3, 3.6 Sheerstrake 3.6.3 and Pt 8, Ch 3, 3.6 Sheerstrake 3.6.4.

4.19.5  Pilot craft are, in general, to be fitted with large knees in way of the sheerstrake in areas as indicated in Pt 8, Ch 3, 3.6 Sheerstrake 3.6.5. The knees are to be aligned between the transverse frames and the deck beams. The thickness of the webs for these knees is to be twice that required by Pt 8, Ch 3, 1.17 Stiffener proportions or 6 mm at a fibre content by weight, of 0,5. Where the fibre content is less than 0,5 the minimum thickness is be increased by the factor k _c as follows:

f _c is as defined in Pt 8, Ch 3, 1.5 Symbols and definitions 1.5.1.

In the case of longitudinally framed craft, web frames with knees are to be fitted at a spacing of generally no greater than 500 mm. A side longitudinal with a section modulus of, in general, twice that of the Rule longitudinal for the web frame spacing is to be positioned just below the lower fendering to carry the load associated with the dynamic loading from pitching and rolling. Consideration is also to be given to the termination of such brackets by use of a ‘soft-toe’ in way of the deck.

4.19.6  Fishing craft engaged in pair trawling and other modes of fishing, and which may be subject to repeated impact loading from contact with the other craft, are to have additional stiffening fitted in way of the impact areas. This may be in the form of large knees or intermediate knees, substantial fendering/rubbing strakes. Additionally, the shell and deck in way of all working areas are to be suitably sheathed.