Section 1 General

1.1.1 The requirements of this Chapter are applicable to mono-hull craft of composite construction as defined in Ch 1, 1 Background.

1.2.1 The scantlings of motor and sailing, mono-hull craft of conventional form and proportions are to be determined from the formulae contained within this Chapter.

1.2.2 The mechanical properties to be used for scantling calculation purposes are to be 90 per cent of the mean first ply/resin cracking failure values determined from accepted mechanical tests, or the mean values minus twice times the standard deviation for the five samples, whichever is the lesser. All test pieces are to be representative of the product to be manufactured and details submitted for consideration.

1.2.3 In the absence of suitable test data, the mechanical properties of the materials is to be estimated from the appropriate procedures and formulae contained within this Part. The acceptable design values for glass reinforced polyester resin laminates are, in general, not to be taken greater than those determined from Table 3.1.1 Mechanical properties for chopped strand mat (CSM) glass reinforced polyester resin laminates to Table 3.1.3 Mechanical properties for uni-directional glass reinforced polyester resin laminates at 0/90° degree orientation. Additional information on the application of the various formulae is given in Clasifications Register's (hereinafter referred to as 'LR') Guidance Notes for Calculation Procedures for Composite Construction.

1.2.4 In the absence of suitable test data, the mechanical properties of aramid and carbon reinforced epoxy resin laminates are, in general, not to be taken greater than those determined from Table 3.1.3 Mechanical properties for uni-directional glass reinforced polyester resin laminates at 0/90° degree orientation.

1.2.5 The various formulae referred to in Pt 8, Ch 3, 1.2 General 1.2.3 and Pt 8, Ch 3, 1.2 General 1.2.4 require that sufficient input data be available which relates to each of the proposed materials. The designers and/or Builders are to, in general, agree the values for use in the scantling analysis with LR at the design stage and prior to the submission of plans and data for appraisal.

1.2.6 Typical acceptable values for the various fibre properties of materials commonly in use are given in Table 3.1.8 Typical minimum fibre properties.

1.2.7 Typical acceptable values for the various resin properties of materials commonly in use are given in Table 3.1.9 Typical minimum resin properties.

1.3.1 The scantlings are to be determined by direct calculation where the craft is of unusual design, form or proportions, or where the speed of the craft exceeds 60 knots.

1.3.2 The requirements of this Section may be modified where direct calculation procedures are adopted to analyse the stress distribution in the primary structure.

1.4.1 LR will consider direct calculations for the derivation of scantlings as an alternative and equivalent to those derived by Rule requirements in accordance with Ch 2, 3 Impact testsof the Rules for the Manufacture, Testing and Certification of Materials (hereinafter referred to as the Rules for Materials).

1.5.1 The symbols used in this Chapter, unless specified otherwise, are defined as follows:

1.5.2 The side shell is defined as the portion of the hull between the bottom shell and the deck at side.

1.6.1 The nominal thickness of an individual ply, t _i, may be determined from:

where f _ci, t _i, m _Fi, ζ_Fi and ζ_Ri are as defined in Pt 8, Ch 3, 1.5 Symbols and definitions 1.5.1

1.7.1 The geometric properties of stiffening sections are to be calculated in accordance with Pt 8, Ch 3, 1.16 Geometric properties stiffener sections using an effective width, 2b ₁, of attached load bearing plating determined as follows:

1. Single skin construction:

   b 1

   =

   0,5b w + 10 t ap

2. Sandwich skin construction:

   Generally:

   b 1

   =

   0,5b w + 10(t outer + t inner)

   Where a plywood core is used:

   b 1

   =

   0,5b w + 10(t outer + t inner + 0,5 t ply)

   where

   b 1	=	effective width of attached load bearing plating, in mm, and is not to be taken as greater than one half the spacing between the centres of adjacent stiffeners
   b w	=	base width of the stiffener section, in mm
   t ap	=	thickness, or mean thickness of attached plate laminate, in mm
   t inner	=	thickness, or mean thickness of inner skin laminate, in mm
   t outer	=	thickness, or mean thickness of outer skin laminate, in mm
   t ply	=	thickness of plywood core, in mm

1.7.2 The geometric properties of primary support members (i.e. girders, stringers, web frames, etc.) are to be calculated in accordance with Pt 8, Ch 3, 1.16 Geometric properties stiffener sections using an effective area of attached load bearing plate laminate of nominal thickness, t mm, and of width equal to one-half the sum of spacings between parallel adjacent members or equivalent support.

1.8.1 Strength calculations for all advanced fibre composites are to be based on the results of testing of truly representative sections of the proposed design. In general the sections are to be manufactured under typical production conditions using the same materials, fibre contents, methods of lay-up and time delays.

1.8.2 Mechanical testing is, in general, to be based upon the requirements of Ch 14, 3 Testing procedures of the Rules for Materials.

1.8.3 Where test data is not available for standard glass fibre laminates, the following theoretical approach is to be used to estimate the tensile modulus and the shear modulus of a laminate:

The tensile modulus of a uni-directional reinforcement at angle θ to the axis of the fibres is to be determined from:

E _0i, the longitudinal tensile modulus of individual ply, i, for an unfilled resin system is determined from:

E _F, V _F and E _R are as defined in Pt 8, Ch 3, 1.5 Symbols and definitions 1.5.1

V _F, the volume fraction of the fibres of individual ply, i, is determined from:

W _F, ζ_F and ζ_R are as indicated in Pt 8, Ch 3, 1.5 Symbols and definitions 1.5.1

E _90i, the transverse tensile modulus of individual ply, i, is determined from:

E _F, E _R and V _F are as indicated in Pt 8, Ch 3, 1.5 Symbols and definitions 1.5.1.

G _0/90i, the shear modulus of individual ply, i, is determined from:

Where the shear modulus of the resin, G _R is determined from:

Where the shear modulus of the fibre, G _F is determined from:

E _F, E _R, ν_R and ν_F are as indicated in Pt 8, Ch 3, 1.5 Symbols and definitions.

The longitudinal Poisson's ratio, ν_0/90, of individual ply, i, is determined as follows:

V _F, ν_F and ν_R are as indicated in Pt 8, Ch 3, 1.5 Symbols and definitions 1.5.1.

1.8.4 Where specific test data is not available for glass fibre reinforced polyester laminates, the mechanical properties for design are to be the values determined from the formulae given in Table 3.1.1 Mechanical properties for chopped strand mat (CSM) glass reinforced polyester resin laminates and Table 3.1.2 Mechanical properties for woven roving (WR) and cross-plied (CP) glass reinforced polyester resin laminates at 0/90° degree orientation.

1.9.1 Unless otherwise specified in this Part, the bending moments, M _b and M _c, to be applied to a 1 cm length of panel, for both plate and sandwich laminates, subjected to lateral pressure are to be determined from:

1. Bending moment at panel boundary and under base of stiffener, M _b:

   

2. Bending moment at centre of panel, M _c:

   

1.10.1 The Rule bending moments, M _b and M _c, to be applied to plate laminates as determined by Pt 8, Ch 3, 1.9 Plate and sandwich laminates 1.9.1, may be reduced when the panel aspect ratio is taken into consideration. For panels with aspect ratio less than two the following factor, K _AR, may be applied:

1.11.1 The Rule bending moments, M _b and M _c, as determined by Pt 8, Ch 3, 1.9 Plate and sandwich laminates 1.9.1, may be reduced where significant curvature exists between the support members. For such panels the following factor, K _c, may be applied:

1.12.1 The Rule bending moments, M _b and M _c, as determined by Pt 8, Ch 3, 1.9 Plate and sandwich laminates 1.9.1, may be reduced for panels subject to impact pressure, P _dI, in crafts operating in the non-displacement mode. For such panels, the following factor, K _i, may be applied:

but is not to be taken greater than 1 or less than 0,7

1.13.1 An estimate of the thickness of single skin plating required to carry the bending moment given in Pt 8, Ch 3, 1.9 Plate and sandwich laminates 1.9.1, is to be determined from:

where b, p and E _tp are as defined in Pt 8, Ch 3, 1.5 Symbols and definitions 1.5.1.

1.13.2 The distance of the neutral axis, x _L, from the surface of the plate laminate is to be determined from the following:

where E _i, t _i and x _i are as defined in Pt 8, Ch 3, 1.5 Symbols and definitions 1.5.1.

1.13.3 The resultant tensile stress, σ_ti, at the extreme outer fibre of an individual ply, , is to be determined from:

where σ_ti, E _ti, y _i, M, E _i and _i are as defined in Pt 8, Ch 3, 1.5 Symbols and definitions 1.5.1.

1.13.4 The resultant compressive stress, σ_ci, at the extreme outer fibre of an individual ply, i, is to be determined from:

where σ_ci, E _ci, y _i, M, E _i and _i are as defined in Pt 8, Ch 3, 1.5 Symbols and definitions 1.5.1.

1.13.5 The effective flexural modulus of elasticity in bending, E _fp, for the plate laminate is to be determined from:

where E _ti, _i and _P are as defined in Pt 8, Ch 3, 1.5 Symbols and definitions 1.5.1.

1.13.6 The apparent flexural strength, σ_f, of a plate laminate is to be determined from:

where E _fp and e _f are as defined in Pt 8, Ch 3, 1.5 Symbols and definitions 1.5.1.

1.14.1 For the application of the various formulae relating to the use of sandwich construction, the following assumptions have been made:

1. the sandwich skins carry the majority of the bending load,

2. the core carries the majority of the shear load,

3. the initial estimate of the skin thickness from Pt 8, Ch 3, 1.14 Mechanical properties sandwich laminates 1.14.2 is based upon the limiting condition for thin skin theory:

   

4. the sandwich skins are of approximately equal thickness (i.e. the panel is of balanced or approximately balanced construction), with the thickness of the outer sandwich facing not greater than:

   t OUTER

   =

   1,33 t INNER (excluding gel coat and non-structural materials).

1.14.2 An estimate of the thicknesses of the sandwich skins and core required to carry the Rule bending moment may be determined from the following formula. The subsequent design is then to be tested against the other criteria required by the Rules.

k _S, E _tps, b and p are as defined in Pt 8, Ch 3, 1.5 Symbols and definitions 1.5.1.

1.14.3 Where it is proposed to use a thicker core than assumed in Pt 8, Ch 3, 1.13 Determination of properties and stresses for single skin plate laminates 1.13.2, the required skin thickness, t _s, is to be calculated from:

k _S, E _tps, b and p are as defined in Pt 8, Ch 3, 1.5 Symbols and definitions 1.5.1.

1.14.4 The tensile modulus, E _tp, of a plate laminate which forms a skin of a sandwich laminate subject to tensile loading is to be determined from:

where E _tps, E _ti and t _i are as defined in Pt 8, Ch 3, 1.5 Symbols and definitions 1.5.1.

1.14.5 The compressive modulus, E _cp, of a plate laminate which forms a skin of a sandwich laminate subject to compressive loading is to be determined from:

where E _cps, E _ci and t _i are as defined in Pt 8, Ch 3, 1.5 Symbols and definitions 1.5.1.

1.14.6 The distance of the neutral axis, x _S, from the outer surface of the sandwich laminate is to be determined from:

where E _i, t _i and x _i are as defined in Pt 8, Ch 3, 1.5 Symbols and definitions 1.5.1.

1.14.7 The resultant tensile stress, σ_ti, at the extreme outer fibre of an individual ply, i, is to be determined from:

where σ_ti, E _ti, y _i, M, E _i and _i are as defined in Pt 8, Ch 3, 1.5 Symbols and definitions 1.5.1.

The allowable tensile stress limits indicated in Table 7.3.1 Limiting stress criteria for local loading in Chapter 7, are to be complied with.

1.14.8 The resultant compressive stress, σ_ci, at the extreme outer fibre of an individual ply, i, is to be determined from:

where σ_ci, E _ci, y _i, M, E _i and _i are as defined in Pt 8, Ch 3, 1.5 Symbols and definitions 1.5.1.

The allowable compressive stress limits indicated in Table 7.3.1 Limiting stress criteria for local loading in Chapter 7, are to be complied with.

1.14.9 The direct core shear stress, τ_c, at the edges of a sandwich panel subjected to lateral pressure is to be determined from:

t _c and t _s are as defined in Pt 8, Ch 3, 1.5 Symbols and definitions 1.5.1.

The allowable shear stress limits against core shear failure indicated in Pt 8, Ch 7, 3.5 Core shear stress 3.5.1 are to be complied with. For the purposes of this comparison it is assumed that the stated shear properties of the proposed core material have been determined by use of the four point sandwich beam bending test ASTM C393 or equivalent.

1.14.10 Where the core shear stress, τ_c, determined from Pt 8, Ch 3, 1.14 Mechanical properties sandwich laminates 1.14.9 is in excess of the limiting stress for a particular core material, the effective shear strength of the core material in the direction of the panel breadth, may be increased by the addition of shear ties. The effective shear strength, τ_eff, of the core material is to be determined from:

1.14.11 Where the Poisson's ratio, υ_f, for a particular facing laminate is known, the deflection, δ, of a flat sandwich panel with all edges assumed to be fully fixed, and subjected to a uniform lateral pressure is to be determined from:

where

E_pi is the lesser of E _tps or E _cps of the inner skin

E _po is the lesser of E _tps or E _cps of the outer skin

ν _f, p, b, t _c, t _s, E _tps, E _cps and G are as defined in Pt 8, Ch 3, 1.5 Symbols and definitions 1.5.1 t _inner and t _outer are as defined in Pt 8, Ch 3, 1.7 Effective width of attached plating 1.7.1.

1.14.12 Where the Poisson's ratio, υ_f, for a particular facing laminate is not known, the deflection, δ, of a flat sandwich panel with all edges assumed to be fully fixed, and subjected to a uniform lateral pressure is to be estimated from:

where

δ, p, b, t _c, and G are as defined in Pt 8, Ch 3, 1.5 Symbols and definitions 1.5.1

D _s, k _db, k _ds are as defined in Pt 8, Ch 3, 1.14 Mechanical properties sandwich laminates 1.14.11.

1.14.13 The deflection determined from Pt 8, Ch 3, 1.14 Mechanical properties sandwich laminates 1.14.11 or Pt 8, Ch 3, 1.14 Mechanical properties sandwich laminates 1.14.12, as appropriate, is not to exceed the limiting deflection for the structural element under consideration, as indicated in Table 7.2.1 Limiting span/deflection ratio in Chapter 7.

1.15.1 Unless otherwise specified elsewhere in this Part, the Rule bending moment, M _s, to be applied to all stiffening members subjected to uniform lateral pressure is to be determined from:

1.15.2 Unless otherwise specified elsewhere in this Part, the Rule shear force, F _s, to be applied to all stiffening members subjected to uniform lateral pressure is to be determined from:

1.15.3 The shear stress, τ_S, in the webs of stiffening members of `top-hat' type section is to be determined from:

The maximum allowable shear stress is not to exceed that determined from Table 7.3.1 Limiting stress criteria for local loading , for the stiffener member under consideration.

1.15.4 The shear stress, τ_S, in the webs of stiffening members of inverted angle or `T bar' type section is to be determined from:

where F _S, t _W and d _W are as defined in Pt 8, Ch 3, 1.15 Stiffeners general 1.15.3.

The maximum allowable shear stress is not to exceed that determined from Table 7.3.1 Limiting stress criteria for local loading in Chapter 7, for the stiffener member under consideration.

1.15.5 Unless otherwise specified elsewhere in this Part, the deflection, δ_s, of stiffening members, subjected to uniform lateral pressure is to be determined from:

s, _e, E, and p are as defined in Pt 8, Ch 3, 1.5 Symbols and definitions 1.5.1.

1.15.6 The maximum allowable deflection is not, in general, to exceed that determined from Table 7.2.1 Limiting span/deflection ratio for the stiffener member under consideration.

1.16.1 The effective geometric properties of the stiffener sections are to be calculated directly from the dimensions of the section and associated effective width of attached plating in accordance with Pt 8, Ch 3, 1.7 Effective width of attached plating. Where the mean line of the stiffener webs is not normal to the attached laminate, and the angle exceeds 20^o, the properties of the section are to be determined about an axis parallel to the attached plate laminate. Where plywood, solid timber, aluminium alloy, steel or other materials are integrated into a stiffening member, the effectiveness of the material is to be determined in accordance with Pt 8, Ch 3, 1.21 Plywood 1.21.3. The stress in the individual material is to be limited to the allowable strain associated with the constituent material.

1.16.2 The distance of the neutral axis, x _S, from the outer surface of the plate laminate is to be determined from:

where E _i, t _i, b _i and x _i are as defined in Pt 8, Ch 3, 1.5 Symbols and definitions 1.5.1.

1.16.3 The resultant extreme fibre tensile stress for an individual ply, σ_ti, is to be determined from:

where σ_ti, E _ti, y _i, M, E _i and _i are as defined in Pt 8, Ch 3, 1.5 Symbols and definitions 1.5.1.

The term (E )_s refers to the whole stiffener section, i.e. including the attached plating in accordance with Pt 8, Ch 3, 1.7 Effective width of attached plating. See also LR's Guidance Notes for Calculation Procedures for Composite Construction.

1.16.4 The resultant extreme fibre compressive stress for an individual ply, σ_ci, is to be determined from:

where σ_ci, E _ci, y _i, M, E _i and _i are as defined in Pt 8, Ch 3, 1.5 Symbols and definitions 1.5.1.

The term (E )_s refers to the whole stiffener section, i.e. including the attached plating in accordance with Pt 8, Ch 3, 1.7 Effective width of attached plating. See also LR's Guidance Notes for Calculation Procedures for Composite Construction.

1.17.1 From structural stability and local buckling considerations, the proportions of stiffening members are, in general, to be in accordance with the requirements of this Section.

1.17.2 The thickness of the web for `top-hat' type stiffeners, t _w, is to be not less than that required to satisfy the web shear from Pt 8, Ch 3, 1.15 Stiffeners general 1.15.3 and Pt 8, Ch 3, 1.15 Stiffeners general 1.15.4, and in no case is to be taken as less than that determined from the following formula:

1.17.3 The thickness of the web of an inverted angle or `T' bar stiffener section is to be twice the web thickness determined from Pt 8, Ch 3, 1.17 Stiffener proportions 1.17.2.

1.18.1 The effective span, _e, of a stiffening member is generally less than the overall length, , by an amount which depends on the design of the end connections. The span points, between which the value of _e is measured, are to be determined from:

1. For secondary stiffening members of top-hat type section as shown in Figure 3.1.3 Span points the span point is to be taken at the point where the depth of the end bracket, measured from the face of the secondary stiffening member is equal to the depth of the member. Where there is no end bracket, the span point is to be measured between primary member webs.

2. For primary stiffening members of top-hat type section as shown in Figure 3.1.4 Span points the span point is to be taken at the point where the depth of the end bracket, measured from the face of the primary stiffening member is equal to the half depth of the member. Where there is no end bracket, the span point is to be measured between primary member webs.

1.18.2 Where the stiffener member is inclined to a vertical or horizontal axis and the inclination exceeds 10°, the span is to be measured along the member.

1.18.3 Where the stiffening member is curved then the span is to be taken as the effective chord length.

1.18.4 Where there is a pronounced turn of bilge, chine or the structure is significantly pitched, the span is to be measured as in Figure 3.1.5 Span points, Figure 3.1.6 Span points, Figure 3.1.7 Span points and Figure 3.1.8 Span points.

1.18.5 The determined effective span assumes that the ends of stiffening members are substantially fixed against rotation and displacement. If the arrangement of supporting structure is such that this condition is not achieved, the span is to be determined excluding any effect from the end brackets.

1.19.1 The connection of the various laminates into assemblies and the connection of units to the main structure is generally to be made by means of single or double angles of the type shown in Figure 3.1.9 Bonding angles.

1.19.2 These matting-in angles are to be formed by layers of reinforcements, laid-up in situ, and normally secondary bonded to the structure before the laminates are advanced in cure. Where the laminating schedule is such that this cannot be achieved then suitable peel plies and secondary bonding techniques, as recommended by the resin manufacturer, see Pt 8, Ch 2, 5.9 Secondary bonding and peel ply, are to be arranged in way of the surfaces to be connected.

1.19.3 All surfaces to be bonded are to be clean and suitably prepared prior to the application of the bonding angles. Suitable fillets of compliant resin are to be arranged as shown in Figure 3.1.10 Resin fillets and Figure 3.1.11 Width of bonding angle.

1.19.4 Where floors, bulkheads, tank boundaries, etc. are manufactured from plate laminate the weight of the laminate forming each angle is to be not less than 50 per cent of the weight of the lighter member being connected, or 900g/m² chopped fibre reinforcement or equivalent, whichever is the greater.

1.19.5 Double angles are normally to be used, but when this is not possible, such as where satisfactory access cannot be achieved on the reverse side, a single angle can be used provided it is suitably increased in width and weight. The weight of a single bonding angle is to be determined by direct calculation, and in no case to be taken as less than two thirds the weight of the lighter laminate being connected or 900g/m² chopped fibre reinforcement or equivalent, whichever is the greater.

1.19.6 Where frames and stiffeners are of the `top-hat' type, the width of the flange connection to the plate laminate is to be as shown in Figure 3.1.11 Width of bonding angle. The width of bonding angle is to be 25 mm for the first layer + 15mm per each additional layer, but not less than 50 mm.

1.19.7 Where sandwich panels are to be connected the weight of bonding is to be not less than the weight of the appropriate skin. The inner and outer skins of primary sandwich structures such as bulkheads are to be effectively `tied' by a suitable weight of reinforcement or by use of fillets and wedges of suitable compliant resin, as shown in Figure 3.1.12 Bonding ties.

1.19.8 Where the floors, bulkheads, etc. are manufactured from plywood the weight of the laminate forming each angle is to be not less than 50 per cent of the weight of the equivalent thickness of bulkhead in the material used for the bonding angle or the lighter member being connected.

1.19.9 In no case is the thickness of the double bonding angle to be less than 2 mm at a glass content, by weight, of 0,5. Where a glass content is less than 0,5, the thickness is to be not less than that required to resist the same shear force using the formulae in Table 3.1.1 Mechanical properties for chopped strand mat (CSM) glass reinforced polyester resin laminates and Table 3.1.2 Mechanical properties for woven roving (WR) and cross-plied (CP) glass reinforced polyester resin laminates at 0/90° degree orientation.

1.19.10 Alternative bonding arrangements incorporating epoxy fillets, bonded wedges, bolting, etc. may be specially considered. It is however the responsibility of the Builder to demonstrate their suitability and equivalence to the Rule requirements.

1.20.1 It is presumed that, in the selection of the species of timber for a particular application, the designers will relate the known characteristics, strength, density, bending and working capabilities of the particular species to the constructional design. The mechanical properties of timbers and assumptions used for design purposes are to be clearly indicated on the submitted construction plans, see also Pt 8, Ch 2, 2.17 Plywood and Pt 8, Ch 2, 1.15 Scaffolding 1.15.1.

1.20.2 All timbers are to be identified by their botanical name.

1.20.3 The moisture content of timber which is to be glued, bonded or overlaminated is to be about 15 per cent, see also Pt 8, Ch 2, 2.17 Plywood.

1.21.1 Structural plywoods are to comply with Pt 8, Ch 2, 2.17 Plywood, see also Pt 8, Ch 2, 2.16 Materials for integrated structural members 2.16.3.

1.21.2 The mechanical properties of the plywood proposed for use in structural applications is to be obtained from the plywood manufacturer and submitted for consideration. In the absence of such data the mechanical properties can be determined from Table 3.1.11 Mechanical properties for plywood panels and Table 3.1.12 Mechanical properties for plywood on edge.

1.21.3 Where stiffeners incorporate encapsulated plywood structurally bonded to the plate laminate in accordance with Pt 8, Ch 3, 1.19 Boundary bonding 1.19.8, its effective E _{i i} is to be incorporated into the Σ (E _{i i}) as indicated in Pt 8, Ch 3, 1.16 Geometric properties stiffener sections, with the basic thickness and tensile/compressive moduli of the plywood being taken as those corresponding to the least effective over the span of the stiffener. Directional considerations for structural plywood incorporated in stiffening members are to be indicated on construction plans submitted for appraisal.

1.22.1 The use of aluminium alloy is permitted for craft in accordance with Pt 7 Hull Construction in Aluminium. Where this material is to be integrated structurally, with the fibre composite structure, see Pt 8, Ch 2, 2.15 Adhesives and Pt 8, Ch 2, 1.15 Scaffolding 1.15.1.

1.23.1 The use of steel is permitted for craft in accordance with Pt 6 Hull Construction in Steel. Where this material is to be integrated structurally, with the fibre composite structure, see Pt 8, Ch 2, 2.15 Adhesives and Pt 8, Ch 2, 1.15 Scaffolding 1.15.1.

1.24.1 Special consideration will be given to the use of other types of materials. Details of the type of material, the specification to which it was manufactured and its mechanical properties are to be submitted for appraisal, see also Pt 8, Ch 3, 1.16 Geometric properties stiffener sections 1.16.1.

1.25.1 Secondary members, i.e. longitudinals, beams, frames and bulkhead stiffeners forming part of the hull structure are, in general, to be connected at their ends in accordance with the requirements of this Section. Where it is desired to adopt bracketless connections, the proposed arrangements will be individually considered on the basis of Pt 8, Ch 3, 1.18 Determination of span points 1.18.5.

1.25.2 Where end connections are fitted in accordance with these requirements, they may be taken into account in determining the effective span of the member.

1.25.3 Where a longitudinal strength member is cut at a primary support and the continuity of strength is provided by brackets, the scantlings of the brackets are to be such that their section properties and effective cross-sectional area are not less than those of the member. Care is to be taken to ensure correct alignment of the brackets on each side of the primary member.

1.25.4 The thickness of the bracket webs is to be not less than that required for the webs of the stiffening member. See Pt 8, Ch 3, 1.15 Stiffeners general.

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

1.25.6 The design of end connections and their supporting structure is to be such as to provide adequate resistance to rotation and displacement of the joint.

1.25.7 Hard spots are to be avoided in way of end connections.

1.26.1 Secondary members, i.e. longitudinals, beams, frames and bulkhead stiffeners forming part of the hull structure, are generally to be connected at their ends in accordance with the requirements of this Section. Where it is desired to adopt bracketless connections, the proposed arrangements will be individually considered.

1.26.2 Where end connections are fitted in accordance with these requirements, they may be taken into account in determining the effective span of the member.

1.26.3 The symbols used in this sub-Section are defined as follows:

1.26.4 Typical arrangements of stiffener end brackets are shown diagrammatically in Figure 3.1.13 Arrangement of end brackets.

1.26.5 The section stiffness, (EI), in way of the bracket at the point to which the effective span of the stiffener, l_e, is measured is to be not less than two times the section stiffness of the basic stiffener.

1.26.6 The web thickness, t _w, and face width of end brackets are to be not less than that of the connecting stiffeners. Additionally the requirements of Pt 8, Ch 3, 1.17 Stiffener proportions are to be complied with.

1.26.7 Where brackets are of the inverted angle or `T' bar stiffener section, their free edge is to be suitably stiffened by a flange or other equivalent means. The dimensions of the flange are to be such that the requirements of Pt 8, Ch 3, 1.17 Stiffener proportions are complied with.

1.26.8 Where the free edge of the bracket is hollowed out to form a `soft-toe', the dimensions of the bracket arms and throat depth are to be increased such that the stiffness requirements of Pt 8, Ch 3, 1.17 Stiffener proportions are complied with.

1.27.1 Primary members are to be so arranged as to ensure effective continuity of strength, and abrupt changes of depth or section are to be avoided. Where members abut on both sides of a bulkhead, or on other members, arrangements are to be made to ensure that they are in alignment. Primary members in tanks are to form a continuous line of support and wherever possible, a complete ring system.

1.27.2 The members are to have adequate lateral stability and web stiffening and the structure is to be so arranged as to minimize hard spots and other sources of stress concentration.

1.27.3 Primary members are to be provided with adequate end fixity by end brackets or equivalent structure. The design of end connections and their supporting structure is to be such as to provide adequate resistance to rotation and displacement of the joint and effective distribution of the load from the member.

1.27.4 Where the primary member is supported by structure which provides only a low degree of restraint against rotation, the member is generally to be extended for at least two frame spaces, or equivalent, beyond the point of support before being tapered.

1.27.5 Where primary members are subject to concentrated loads, particularly if these are out of line with the member web, additional strengthening will, in general, be required.

1.27.6 The thicknesses of the bracket webs are, in general, to be not less than those of the primary member webs. Where brackets are of the plate type, the free edge of the bracket is to be adequately stiffened and the plate positioned to limit any hard spot.

1.27.7 Where a deck girder or transverse is connected to a vertical member on the shell or bulkhead, the scantlings of the latter may be required to be increased to provide adequate stiffness to resist rotation of the joint.

1.27.8 Where a member is continued over a point of support, such as a pillar or pillar bulkhead stiffener, the design of the end connection is to be such as to ensure the effective distribution of the load into the support. Proposals to fit brackets of reduced scantlings, or alternative arrangements, will be considered.

1.27.9 Connections between primary members forming a ring system are to minimize stress concentrations at the junctions. Integral brackets are generally to be radiused or well rounded at their toes. The arm length of the bracket, measured from the face of the member, is to be not less than the depth of the smaller member forming the connection.

1.28.1 The arrangement of the connection between the stiffener and the bracket is to be such that at no point in the connection is the section stiffness (E ), reduced to less than that of the stiffener with associated plating.

1.28.2 The design of end connections and their supporting structure is to be such as to provide adequate resistance to rotation and displacement of the joint.

1.29.1 The stability of composite beams, girders, stringers etc. is to be analysed with respect to global buckling due to compressive loads. The flanges and webs shall be analysed with respect to local buckling due to compressive and shear loads. Design calculations are to be submitted indicating the margin against failure.

1.30.1 Where openings are cut in the webs of stiffening members, the depth of the opening is not to exceed 50 per cent of the web depth, and the opening is to be so located that the edges are not less than 25 per cent of the web depth from the face laminate. The length of opening is not to exceed the web depth or 60 per cent of the secondary member spacing, whichever is the greater, and the ends of the openings are to be equidistant from the corners of cut-outs for secondary members. Where larger openings are proposed, the arrangements and compensation required will be specially considered.

1.30.2 Openings are to have smooth edges and well rounded corners. Exposed edges in way of cut-outs in single skin/plate laminate are to be suitably sealed with resin and/or be over laminated. Exposed edges in way of cut-outs in sandwich panels and top hat type stiffening members are to be overlaminated with a weight of laminate not less than the lower of the two skins which form the panel (or stiffener) or 2 mm in thickness whichever is the greater.

1.30.3 Cut-outs for the passage of secondary members are to be arranged so as to minimise the creation of stress concentrations. To avoid excessive use of filler material the breadth of cut-out is to be kept as small as necessary and the fit as accurate as practicable. Suitable fillets are to be arranged to ensure efficient bonding.

1.30.4 Consideration is to be given to the provision of adequate drainage and unimpeded flow of air and water when designing the cut-outs and connection details.

1.31.1 The arrangement of material is to be such as will ensure structural continuity. Abrupt changes of shape or section, sharp corners and points of stress concentration are to be avoided.

1.31.2 Where members abut on both sides of a bulkhead or similar structure, care is to be taken to ensure good alignment.

1.31.3 Pillars and pillar bulkheads are to be fitted in the same vertical line wherever possible, and elsewhere arrangements are to be made to transmit the out of line forces satisfactorily. The load at head and heel of pillars is to be effectively distributed and arrangements are to be made to ensure the adequacy and lateral stability of the supporting members.

1.31.4 Continuity is to be maintained where primary members intersect and where the members are of the same depth, see also LR's Guidance Notes for Structural Details.

1.31.5 End connections of structural members are to provide adequate end fixity and effective distribution of the load into the supporting structure.

1.31.6 The toes of brackets, etc. are not to land on unstiffened panels of plating. Special care is to be taken to avoid notch effects at the toes of brackets, by making the toe concave or otherwise tapering it off in accordance with Figure 3.4.1 `Soft-toe' in Chapter 3.

1.32.1 Cut-outs for the passage of secondary members through the webs of primary members, and the related bonding arrangements, are to be so designed as to minimize stress concentrations around the perimeter of the opening and in the attached hull envelope or bulkhead plating. The critical shear buckling stress of the panel in which the cut-out is made is to be examined. Longitudinals will be required to have double bonding angles which may require to be locally increased in weight in areas of high stress, such as under bulkheads, machinery seating, mast steps, etc. The increased shear stresses in these areas are to be examined.

1.32.2 It is recommended that the web plate connection to the hull envelope, or bulkhead end in a smooth tapered `soft toe'. Recommended shapes of cut-out are shown in Chapter 3, Figure 3.4.1 `Soft-toe', but consideration will be given to other shapes on the basis of maintaining equivalent strength and minimising stress concentration.

1.32.3 Alternative arrangements will be considered on the basis of their ability to transmit load with equivalent effectiveness. Details of the calculations made and testing procedures are to be submitted.