Section 4 Shell envelope framing

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

4.1.2 For each stiffening member an assumed load model is stated. Where the proposed stiffener arrangement differs from that assumed, consideration will be given to an alternative load model.

4.1.3 The geometric properties of stiffener sections are to be in accordance with Pt 6, Ch 3, 1.18 Geometric properties and proportions of stiffener sections

4.2.1 Bottom longitudinal stiffeners are to be supported by bottom transverse web frames, floors, bulkheads, or other primary structure, generally spaced not more than 2 m apart.

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

4.2.3 Where it is impracticable to comply with the requirements of Pt 6, Ch 3, 4.2 Bottom longitudinal stiffeners 4.2.2, or where it is proposed 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 care is to be taken to ensure accurate alignment of the brackets.

4.2.4 The requirements for section modulus, inertia and web area are to be determined from the general equations given in Pt 6, Ch 3, 1.17 Stiffening general, using the design pressures from Pt 5, Ch 3, 3.1 Hull structures or Pt 5, Ch 4, 3 Hull envelope design criteria for non-displacement or displacement craft as appropriate, and the coefficients Φ_Z, Φ_I, and Φ_A as detailed in Table 3.1.1 Section modulus, inertia and web area coefficients for the load model (b).

4.3.1 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.

4.3.2 Bottom longitudinal primary stiffeners are to be continuous through transverse bulkheads and supporting structures.

4.3.3 Where it is impracticable to comply with the requirements of Pt 6, Ch 3, 4.3 Bottom longitudinal primary stiffeners 4.3.2, or where it is proposed to terminate the 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 care 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.

4.3.4 The requirements for section modulus, inertia and web area are to be determined from the general equations given in Pt 6, Ch 3, 1.17 Stiffening general, using the design pressures from Pt 5, Ch 3, 3.1 Hull structures or Pt 5, Ch 4, 3.1 Hull structures for non-displacement or displacement craft as appropriate, and the coefficients Φ_Z, Φ_I, and Φ_A as detailed in Table 3.1.1 Section modulus, inertia and web area coefficients for the load model (a).

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

4.4.2 The requirements for section modulus, inertia and web area are to be determined from the general equations given in Pt 6, Ch 3, 1.17 Stiffening general, using the design pressures from Pt 5, Ch 3, 3.1 Hull structures or Pt 5, Ch 4, 3.1 Hull structures for non-displacement or displacement craft as appropriate, and the coefficients Φ_Z, Φ_I, and Φ_A as detailed in Table 3.1.1 Section modulus, inertia and web area coefficients for the load model (b).

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

4.5.2 The requirements for section modulus, inertia and web area are to be determined from the general equations given in Pt 6, Ch 3, 1.17 Stiffening general, using the design pressures from Pt 5, Ch 3, 3.1 Hull structures or Pt 5, Ch 4, 3.1 Hull structures for non-displacement or displacement craft as appropriate, and the coefficients Φ_Z, Φ_I, and Φ_A as detailed in Table 3.1.1 Section modulus, inertia and web area coefficients for the load model (a).

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

4.6.2 Where it is impracticable to comply with the requirements of Pt 6, Ch 3, 4.6 Bottom transverse web frames 4.6.1, or where it is proposed to terminate the bottom transverse web frames in way of longitudinal primary girders, bulkheads or integral tank boundaries, they are to be bracketed in way of their end connections to maintain the continuity of structural strength. Particular care 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.

4.6.3 The requirements for section modulus, inertia and web area are to be determined from the general equations given in Pt 6, Ch 3, 1.17 Stiffening general, using the design pressures from Pt 5, Ch 3, 3.1 Hull structures or Pt 5, Ch 4, 3.1 Hull structures for non-displacement or displacement craft as appropriate, and the coefficients Φ_Z, Φ_I, and Φ_A as detailed in Table 3.1.1 Section modulus, inertia and web area coefficients for the load model (a).

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

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

4.7.3 Where it is impracticable to comply with the requirements of Pt 6, Ch 3, 4.7 Side longitudinal stiffeners 4.7.2, or where it is proposed to terminate the side longitudinal 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 care is to be taken to ensure accurate alignment of the brackets.

4.7.4 The requirements for section modulus, inertia and web area are to be determined from the general equations given in Pt 6, Ch 3, 1.17 Stiffening general, using the design pressures from Pt 5, Ch 3, 3.1 Hull structures or Pt 5, Ch 4, 3.1 Hull structures for non-displacement or displacement craft as appropriate, and the coefficients Φ_Z, Φ_I, and Φ_A as detailed in Table 3.1.1 Section modulus, inertia and web area coefficients for the load model (b).

4.8.1 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.

4.8.2 Side longitudinal primary stiffeners are to be continuous through transverse bulkheads and supporting structures.

4.8.3 Where it is impracticable to comply with the requirements of Pt 6, Ch 3, 4.8 Side longitudinal primary stiffeners 4.8.2, or where it is proposed to terminate the side longitudinal 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 care 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.

4.8.4 The requirements for section modulus, inertia and web area are to be determined from the general equations given in Pt 6, Ch 3, 1.17 Stiffening general, using the design pressures from Pt 5, Ch 3, 3.1 Hull structures or Pt 5, Ch 4, 3.1 Hull structures for non-displacement or displacement craft as appropriate, and the coefficients Φ_Z, Φ_I, and Φ_A as detailed in Table 3.1.1 Section modulus, inertia and web area coefficients for the load model (a).

4.9.1 Side transverse stiffeners are defined as local stiffening members supporting the side shell and may be continuous or intercostal.

4.9.2 The requirements for section modulus, inertia and web area are to be determined from the general equations given in Pt 6, Ch 3, 1.17 Stiffening general, using the design pressures from Pt 5, Ch 3, 3.1 Hull structures or Pt 5, Ch 4, 3.1 Hull structures for non-displacement or displacement craft as appropriate, and the coefficients Φ_Z, Φ_I, and Φ_A as detailed in Table 3.1.1 Section modulus, inertia and web area coefficients for the load model (b).

4.10.1 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.

4.10.2 The requirements for section modulus, inertia and web area are to be determined from the general equations given in Pt 6, Ch 3, 1.17 Stiffening general, using the design pressures from Pt 5, Ch 3, 3.1 Hull structures or Pt 5, Ch 4, 3.1 Hull structures for non-displacement or displacement craft as appropriate, and the coefficients Φ_Z, Φ_I, and Φ_A as detailed in Table 3.1.1 Section modulus, inertia and web area coefficients for the load model (a).

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

4.11.2 Where it is impracticable to comply with the requirements of Pt 6, Ch 3, 4.11 Side transverse web frames 4.11.1, or where it is proposed to terminate the 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 care 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.

4.11.3 The requirements for section modulus, inertia and web area are to be determined from the general equations given in Pt 6, Ch 3, 1.17 Stiffening general, using the design pressures from Pt 5, Ch 3, 3.1 Hull structures or Pt 5, Ch 4, 3.1 Hull structures for non-displacement or displacement craft as appropriate, and the coefficients Φ_Z, Φ_I, and Φ_A as detailed in Table 3.1.1 Section modulus, inertia and web area coefficients for the load model (a).

4.12.1 For the purposes of satisfying Rule scantling 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 the section modulii and inertia for the group of frames is not to be less than the summation of the Rule requirement for the individual framing members. In addition, in no case is the proposed scantling of an individual framing member within the group to be less than ninety per cent of the Rule value for that member.

4.13.1 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.

4.13.2 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.

4.13.3 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.

4.14.1 Where longitudinal and transverse primary stiffeners form grillage structures the scantlings may be derived in accordance with Pt 6, Ch 3, 4.13 Grillage structures.

4.15.1 Floating framing systems, where proposed, will be subject to special consideration.

4.16.1 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 6, 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 above.

4.16.2 Design of end connections is to be such that the area of the welding is to be not less than the minimum cross-sectional area of the strut derived in Pt 6, Ch 3, 4.16 Frame struts 4.16.1. To achieve this full penetration welding may be required. The weld connections between the face flats and webs of the pillar supporting structure are to be welded using double continuous welding of an equivalent area to that derived by Pt 6, Ch 3, 4.16 Frame struts 4.16.1.

4.17.1 The arrangement of the connection between the stiffener and the bracket is to be such that at no point in the connection are the section modulus and inertia reduced to less than that of the stiffener with associated plating.

4.17.2 The web stability, openings in the web and continuity and alignment are to be in accordance with Pt 6, Ch 3, 1.24 Web stability, Pt 6, Ch 3, 1.25 Openings in the web and Pt 6, Ch 3, 1.26 Continuity and alignment, respectively.

4.17.3 Secondary and primary end connections and arrangements at intersection of continuous secondary and primary members are to be in accordance with Pt 6, Ch 3, 1.20 Secondary member end connections, Pt 6, Ch 3, 1.22 Primary member end connections and Pt 6, Ch 3, 1.28 Arrangements at intersection of continuous secondary and primary members, respectively.

4.17.4 Stiffeners in slamming areas are to be lugged or bracketed.

4.18.1 For craft, including pilot boats and fishing vessels, 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 fenders. Details of anticipated loadings and calculations for the required increased scantlings are to be submitted, see also Pt 6, Ch 3, 3.6 Sheerstrake 3.6.3 and Pt 6, Ch 3, 3.6 Sheerstrake 3.6.4.

4.18.2  Pilot craft are to be fitted with large knees in way of the sheerstrake in areas as indicated in Pt 6, Ch 3, 3.6 Sheerstrake. The knees are to be aligned between the transverse frames and the deck beams. In the case of longitudinally framed craft, intermediate knees are to be fitted with a spacing in general not greater than 500 mm. Where such intermediate brackets are fitted they are to terminate on a side longitudinal with a section modulus of, in general, twice that of the Rule longitudinal for the web frame spacing, and a deck longitudinal. The side longitudinal is to be positioned below any fendering to carry the heel of the knee. Consideration will given to the termination of such brackets by use of a `soft-toe' in way of the deck. The thickness of the webs for these knees is to be twice that required by Pt 6, Ch 3, 1.21 Scantlings of end brackets.

4.18.3  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, intermediate knees, substantial fendering/rubbing strakes.

4.19.1 The scantlings are to be determined by direct calculation where the shell framing is of unusual design, form or proportions.