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General Hull Requirements for Fibre Composite and Sandwich Constructions

In document Indian Coast Guard Ships (Page 121-127)

Chapter 6

General Hull Requirements for Fibre Composite and

p = design pressure [kN/m2] as per 1.1.3

Gc = the glass content by weight of the reinforcement within the laminate G = shear modulus of sandwich core material [N/mm2]

σu = ultimate tensile or compressive stress, as applicable, for the laminate under consideration [N/mm2]

Ei = tensile modulus of individual laminate, [N/mm2] tc = core thickness [mm]

τu = ultimate shear stress of the laminate under consideration [N/mm2].

1.3 Frame spacing

1.3.1 The normal frame spacing ‘sr’ may be taken as 350 + 5L [mm].

1.3.2 Where the actual spacing of stiffeners differs from the normal frame spacing sr, the minimum thickness requirements specified in this Chapter for various structural members is to be corrected in direct proportion to the frame spacing, however, any reduction in thickness is not to be more than 30%.

1.4 Stiffeners and girders

1.4.1 For girders and stiffeners, the span ‘l’ [m] is to be taken as the length of the girder or stiffener between the two supporting members less the depth of girder or stiffener on crossing panel if any, with correction for end brackets as shown in Fig. 1.4.1(a) and 1.4.1(b).

For curved or knuckled stiffeners, ‘l’ may be based on the chord length.

1.4.2 The area of the attached plating to be used in the calculation of sectional properties of the stiffeners and girders, is to be taken as the cross sectional area within the effective width of the attached plating as given in 1.4.3

1.4.3 In case of single-skin laminates, the effective width ‘w’ of plating attached to a stiffener or girder is to be taken as the mean stiffener or girder spacing in [mm] or ( 18 t + bw) [mm ], whichever is less where, t = thickness of plating [mm] and bw = net width of stiffener [mm]. See Fig. 1.4.3.

Where FRP sandwich laminate plating with an effective (balsa or marine plywood or plastic) core is used, “t” in the above equation is to be taken as the thickness of a single-skin laminate having the same moment of inertia per unit width as the two skins of the sandwich.

For a stiffener along an opening, the effective plating width is to be taken as either half the stiffener spacing in [mm] or ( 9t + b ) [mm], whichever is less.

1.4.4 The effective cross sectional area of the attached plating is not to be less than that of the faceplate.

1.4.5 The effective cross sectional area of the girder web Aw is to be taken as per Chapter 5, Section 1.4.8.






le/2 θ >150o

Fig. 1.4.1 (a) and (b)

9t 9t

t bw

w=18t+bw Fig. 1.4.3

1.4.6 The proportions of stiffeners and girders with hollow form or nonstructural cores or forms, including ineffective wood cores (softwoods used below the waterline are considered to be ineffective), are to conform with Fig.1.4.6. The widths and heights of the stiffeners are to be not greater than:

w = 20 t1 [mm] h = 30 tw [mm]


w = width of stiffener face [mm]

h = height of stiffener webs [mm]

t1 = thickness of stiffeners face [mm]

tw = thickness of stiffener webs and flanges [mm].

The thickness of the web of an inverted angle or T bar stiffener or girder is not to be less than twice the web thickness specified for hollow forms.

For encapsulated effective wood or plywood core, webs and faces are not subjected to the above mentioned thickness limitations, however, minimum thickness of the web and face is to be 3 [mm].




Hat section t

3t t

Minimum lap greater of 0.2h or 50 [mm]; may not be greater than 6t

Fig. 1.4.6 : Proportions and connections of stiffeners 1.4.7 Shear ties (stiffeners)

Where the total web depth to thickness ratio requirement in 1.4.6 for buckling of stiffener webs is not complied with, cross linking of the stiffener webs at the Rule depth to thickness ratio is to be provided by the use of shear ties, as indicated in Fig.1.4.7.

Alternative arrangements will be subject to individual consideration in conjunction with submitted direct calculations.

Web depth Unsupported 2nd tie

1st tie

Fig. 1.4.7 : Arrangement of shear ties (stiffeners)

1.4.8 Where openings are cut in the girder web, they are to be away from the girder ends and scallops for stiffeners, with their centre location as near to the neutral axis of the girder as practicable. Openings of depth exceeding 25% of the girder depth or 300 [mm] and of length exceeding the depth of the girder or 60% of the secondary stiffener spacing, are to be suitably reinforced all around at the edge

1.4.9 Girders are to be provided with adequate lateral stability by tripping brackets fitted generally at every alternate stiffener in case of asymmetrical section or at every fourth stiffener in case of symmetrical section. Tripping brackets are also to be fitted at the toes of end brackets and in way of concentrated loads such as heels of pillars or cross ties.

1.5 End attachments

1.5.1 The end attachment of stiffeners and girders are to ensure good structural continuity. As far as practicable the stiffeners and girders are to be continuous through and adequately supported at the supporting members. In case of connections side frame to deck beams or where it is not feasible to provide the continuity, the stiffeners and girders may be abutted provided brackets as required by 1.5.2 are fitted. Typical acceptable arrangements are shown in Fig.1.5.1.

Continuous reinforcementface

Beam knee Deck beam

Side frame


Longit- udinal Girder

Continu- ous face reinforc- ement Transverse


Supporting BHD

Taper 1:4 min.


Fig. 1.5.1 : End attachments

1.5.2 In general, the thickness of webs and flanges of the stiffeners or girders is to be continued over the bracket region. The bracket arm length (including the depth of the stiffener or girder) is not to be less than 1.5 times the depth of the stiffener or girder.

1.6 Bottom structures

1.6.1 In Ships of single skin construction, the bottom structures are normally to be longitudinally stiffened. In Ships with sandwich construction, adequate longitudinals or longitudinal girders are to be provided in way of shear ties see 7.2.3 and may also be provided to support the bottom panels.

The bottom longitudinals are to be supported by bottom transverses or bulkheads spaced not more than 2 metres apart.

1.6.2 Web frame rings continuous around the transverse cross section of the Ship are to be arranged in line with the bottom transverses. Where intermediate floors are fitted for additional bottom support, their ends are to be well tapered or connected to local panel stiffening.

1.6.3 In the engine room, floors are generally to be fitted at every frame. The floors are preferably to be carried continuously through the engine girders. In way of thrust bearings additional strengthening is to be provided.

1.6.4 Bottom longitudinal girders in general, are to spaced 2.5 m and are to be carried continuously through bulkheads.

A centre girder is to be fitted for docking purposes if the external keel or bottom shape does not give sufficient strength and stiffness.

1.6.5 Main engines are to be supported by longitudinal girders with suitable local reinforcement to take the engine and gear mounting bolts. Rigid core materials are to be applied in all through bolt connections. Also see 4.7 for inserts.

1.6.6 Adequate number of manholes are to be provided for easy access to all parts of the double bottom. Exposed edges of openings in sandwich constructions are to be sealed with resin impregnated mat see 4.8 for details. All openings are to have well-rounded corners.

1.7 Side structures

1.7.1 The Ship sides may be longitudinally or vertically stiffened.

1.7.2 In open deck Ships, the top ends of the stiffeners are to be efficiently connected to fore and aft gunwale at deck line.

1.8 Deck structure

1.8.1 Decks of single skin construction are normally to be longitudinally stiffened.

1.8.2 In areas subjected to high compressive stresses additional transverse intermediate stiffeners may have to be fitted to ensure adequate buckling strength.

1.8.3 Hull to deck connections details are to be as per 4.10.

1.9 Bulkhead structures

1.9.1 Number and location of transverse watertight bulkheads are to be in accordance with the requirements given in Chapter 4, Section 4.

1.9.2 Bulkheads are to be suitably strengthened at the ends of deck girders and where subjected to concentrated loads.

1.10 Bow protection

1.10.1 For Ship of composite sandwich construction the fore foot region upto atleast one frame spacing abaft of the stem, is to be so designed that in the event of local impact the effect of damage will be limited. This may be achieved by providing an additional sacrificial nose or sheathing or by arranging the individual plies of the laminate such that any delamination due to the impact will be directed to the outer surface of the laminate.

1.11 Superstructures and deckhouses

1.11.1 In superstructures and deckhouses, the front bulkhead is to be in line with a transverse bulkhead in the hull below or be supported by a combination of girders and pillars. The after end bulkhead is also to be effectively supported. As far as practicable, exposed sides and internal longitudinal and transverse bulkheads are to be located above girders and frames in the hull structure and are to be in line in the various tiers of accommodation. Where such structural arrangement in line is not possible other effective supports are to be arranged.

1.11.2 Sufficient transverse strength is to be provided by means of transverse bulkheads or girders supported by web frames.

1.11.3 At the break of superstructures, which have not set-in from the ship’s side, the side plating is to extend beyond the ends of the superstructure and is to be gradually reduced in height down to the deck or bulwark. The transition is to be smooth and without local discontinuities.

1.11.4 Openings in the sides of deckhouses are to have well-rounded corners. Large openings in sides of deckhouses are to be substantially stiffened along the edges.

1.11.5 Machinery casings supporting one or more decks above are to be adequately strengthened.

1.12 Bulwarks

1.12.1 Bulwark sides shall have the same scantlings as required for a superstructure in the same position.

1.12.2 A strong flange is to be made along the upper edge of the bulwark. Bulwark stays are to be arranged in line with transverse beams or local stiffening. The stays are to have sufficient width at deck level. If the deck is of sandwich construction solid core inserts are to be fitted at the foot of the bulwark stays. Stays of increased strength are to be fitted at ends of bulwark openings. Openings in bulwarks are not to be situated near the ends of superstructures.

1.12.3 Where bulwarks on exposed decks form wells, ample provision is to be made to facilitate freeing the decks of water.

Section 2

In document Indian Coast Guard Ships (Page 121-127)