Rules and Regulations for the Classification of Naval Ships. Volume 1 Part 6 Hull Construction in Steel January 2015

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1 Rules and Regulations for the Classification of Naval Ships Volume 1 Part 6 Hull Construction in Steel January 2015

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3 Chapter Contents Volume 1, Part 6 PART 1 REGULATIONS PART 2 RULES FOR THE MANUFACTURE, TESTING AND CERTIFICATION OF MATERIALS PART 3 DESIGN PRINCIPLES AND CONSTRUCTIONAL ARRANGEMENTS PART 4 MILITARY DESIGN AND SPECIAL FEATURES PART 5 ENVIRONMENTAL LOADS PART 6 HULL CONSTRUCTION IN STEEL Chapter 1 General 2 Design Tools 3 Scantling Determination 4 Hull Girder Strength 5 Structural Design Factors 6 Material and Welding Requirements PART 7 ENHANCED STRUCTURAL ASSESSMENT Lloyd's Register Group Limited All rights reserved. Except as permitted under current legislation no part of this work may be photocopied, stored in a retrieval system, published, performed in public, adapted, broadcast, transmitted, recorded or reproduced in any form or by any means, without the prior permission of the copyright owner. Enquiries should be addressed to Lloyd's Register Group Limited, 71 Fenchurch Street, London, EC3M 4BS. LLOYD S REGISTER 1

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5 Contents Volume 1, Part 6 CHAPTER 1 GENERAL Section 1 Application 1.1 General 1.2 Equivalents 1.3 Symbols and definitions Section 2 General requirements 2.1 General 2.2 Plans to be submitted 2.3 Plans to be supplied to the ship 2.4 Novel features 2.5 Enhanced scantlings 2.6 Direct calculations 2.7 Exceptions CHAPTER 2 DESIGN TOOLS Section 1 General 1.1 General 1.2 Equivalents 1.3 Symbols and definitions 1.4 Rounding policy for Rule plating thickness 1.5 Material properties 1.6 Higher tensile steel Section 2 Structural design 2.1 General 2.2 Effective width of attached plating, b e 2.3 Section properties 2.4 Convex curvature correction 2.5 Aspect ratio correction 2.6 Determination of span length 2.7 Plating general 2.8 Stiffening general 2.9 Proportions of stiffener sections 2.10 Grillage structures Section 3 Buckling 3.1 General 3.2 Symbols 3.3 Plate panel buckling requirements 3.4 Derivation of the buckling stress for plate panels 3.5 Additional requirements for plate panels which buckle elastically 3.6 Shear buckling of stiffened panels 3.7 Secondary stiffening in direction of compression 3.8 Secondary stiffening perpendicular to direction of compression 3.9 Buckling of primary members 3.10 Shear buckling of girder webs 3.11 Pillars and pillar bulkheads Section 4 Vibration control 4.1 General 4.2 Frequency band 4.3 Natural frequency of plate 4.4 Natural frequency of plate and stiffener combination 4.5 Effect of submergence Section 5 Dynamic loading 5.1 General 5.2 Gradually applied load 5.3 Instantaneous load 5.4 Triangular pulse load LLOYD S REGISTER 3

6 Contents Volume 1, Part 6 CHAPTER 3 SCANTLING DETERMINATION Section 1 General 1.1 Application 1.2 General 1.3 Direct calculations 1.4 Equivalents Section 2 Minimum structural requirements 2.1 General 2.2 Corrosion margin 2.3 Impact consideration 2.4 Sheathing 2.5 Special considerations 2.6 Direct calculations Section 3 NS1 scantling determination 3.1 General 3.2 Symbols 3.3 Hull girder strength 3.4 Minimum hull section modulus 3.5 Minimum hull moment of inertia 3.6 Local reduction factors 3.7 Taper requirements for hull envelope 3.8 Grouped stiffeners 3.9 Shell envelope plating 3.10 Shell envelope framing 3.11 Watertight bulkheads and deep tanks 3.12 Deck structures 3.13 Superstructures, deckhouses and bulwarks 3.14 Single and double bottom structures 3.15 Fore peak structure 3.16 Magazine structure Section 4 NS2 and NS3 scantling determination 4.1 General 4.2 Hull girder strength 4.3 Shell envelope plating 4.4 Shell envelope framing 4.5 Inner bottom structures 4.6 Watertight bulkheads and deep tanks 4.7 Deck structures 4.8 Superstructures, deckhouses and bulwarks Section 5 Shell envelope plating 5.1 General 5.2 Plate keel 5.3 Sheerstrake 5.4 Skegs 5.5 Transom 5.6 Shell openings 5.7 Sea inlet boxes 5.8 Local reinforcement/insert plates 5.9 Novel features Section 6 Shell envelope framing 6.1 General 6.2 Frame struts or cross ties 4 LLOYD S REGISTER

7 Contents Volume 1, Part 6 Section 7 Single bottom structures 7.1 General 7.2 Centreline girder 7.3 Side girders 7.4 Floors General 7.5 Single bottom structure in machinery spaces 7.6 Rudder horns 7.7 Forefoot and stem Section 8 Double bottom structures 8.1 General 8.2 Centreline girder 8.3 Side girders 8.4 Plate floors 8.5 Bracket floors 8.6 Additional requirements for watertight floors 8.7 Inner bottom plating 8.8 Inner bottom longitudinals 8.9 Double bottom tanks 8.10 Margin plates 8.11 Double bottom structure in machinery rooms Section 9 Bulkheads and deep tanks 9.1 General 9.2 Deep tank stiffening 9.3 Gastight bulkheads 9.4 Non-watertight or partial bulkheads 9.5 Corrugated bulkheads 9.6 Wash plates 9.7 Cofferdams 9.8 Testing Section 10 Deck structures 10.1 General 10.2 Deck plating 10.3 Deck stiffening 10.4 Deck openings 10.5 Sheathing 10.6 Decks used as ramps Section 11 Superstructures, deckhouses and bulwarks 11.1 General 11.2 Forecastle requirements 11.3 Superstructures formed by extending side structures 11.4 Mullions 11.5 Sheathing Section 12 Pillars and pillar bulkheads 12.1 Application 12.2 Determination of span length 12.3 Design loads 12.4 Scantling determination 12.5 Minimum slenderness ratio 12.6 Pillars in tanks 12.7 Pillar bulkheads 12.8 Direct calculations 12.9 Novel features Section 13 Machinery and raft seatings 13.1 General 13.2 Seats for oil engines 13.3 Seats for turbines 13.4 Seats for boilers 13.5 Seats for auxiliary machinery LLOYD S REGISTER 5

8 Contents Volume 1, Part 6 Section 14 Strengthening for bottom slamming 14.1 General 14.2 Strengthening of bottom forward Section 15 Strengthening for wave impact loads above waterline 15.1 General 15.2 Strengthening against bow flare wave impacts 15.3 Strengthening against wave impact loads Section 16 Masts 16.1 General 16.2 Pole masts 16.3 Unstayed masts 16.4 Stayed masts CHAPTER 4 HULL GIRDER STRENGTH Section 1 General 1.1 Application 1.2 Hull girder strength notations 1.3 Symbols and definitions 1.4 Calculation of hull section modulus 1.5 General 1.6 Direct calculations Section 2 Hull girder strength 2.1 General 2.2 Bending strength 2.3 Shear strength 2.4 Torsional strength 2.5 Superstructures global strength 2.6 Buckling strength Section 3 Extreme Strength Assessment, ESA 3.1 General 3.2 Determination of critical sections 3.3 Bending strength Simplified assessment method ESA1 3.4 Shear strength Simplified assessment method ESA1 3.5 Bending and shear strength Ultimate strength analysis method ESA2 Section 4 Residual Strength Assessment, RSA 4.1 Application 4.2 Extent of damage and analysis 4.3 Determination of critical sections 4.4 Bending strength Simplified assessment method RSA1 4.5 Shear strength Simplified assessment method RSA1 4.6 Bending and shear strength Ultimate strength analysis method RSA2 CHAPTER 5 STRUCTURAL DESIGN FACTORS Section 1 Structural design factors 1.1 Application 1.2 General 1.3 Higher tensile steel Section 2 Scantling determination for NS1 ships 2.1 Design criteria Section 3 Scantling determination for NS2 and NS3 ships 3.1 Design criteria 6 LLOYD S REGISTER

9 Contents Volume 1, Part 6 CHAPTER 6 MATERIAL AND WELDING REQUIREMENTS Section 1 General 1.1 Application 1.2 General 1.3 Symbols and definitions 1.4 Builder s facilities 1.5 Works inspection 1.6 Quality control 1.7 Building environment 1.8 Storage areas 1.9 Materials handling 1.10 Faults 1.11 New building inspection 1.12 Acceptance criteria 1.13 Repair Section 2 Materials 2.1 General 2.2 Grade of steel 2.3 Refrigerated spaces 2.4 Ships operating in cold weather conditions 2.5 Mechanical properties for design 2.6 Paints and coatings 2.7 Cathodic protection 2.8 Bimetallic connections 2.9 Deck coverings 2.10 Corrosion margin Section 3 Requirements for welded construction 3.1 General 3.2 Information to be submitted 3.3 Welding equipment 3.4 Welding consumables 3.5 Welding procedures 3.6 Inspection Section 4 Welded joints and connections 4.1 General 4.2 Weld symbols 4.3 Welding schedule 4.4 Butt welds 4.5 Fillet welds 4.6 Throat thickness limits 4.7 Single sided welding 4.8 Double continuous fillet welding 4.9 Intermittent and single sided fillet welding 4.10 Connections of primary structure 4.11 Primary and secondary member end connection welds 4.12 Tank boundary penetrations 4.13 Intersection of primary and secondary members LLOYD S REGISTER 7

10 Contents Volume 1, Part 6 Section 5 Construction details 5.1 Continuity and alignment 5.2 Primary end connections 5.3 Secondary member end connections 5.4 Scantlings of end brackets 5.5 Arrangement at intersection of primary and secondary members 5.6 Arrangement with offset stiffener 5.7 Watertight collars 5.8 Insert plates 5.9 Doubler plates 5.10 Other fittings and attachments 5.11 Bilge keels and ground bars 5.12 Rivetting of light structure 5.13 Adhesive bonding of structure 5.14 Triaxial stress considerations 5.15 Aluminium/steel transition joints 5.16 Steel/wood connection Section 6 Inspection and testing procedures 6.1 General 6.2 Definitions 6.3 Testing arrangements 6.4 Structural testing 6.5 Leak testing 6.6 Hose testing 6.7 Hydropneumatic testing 6.8 Gastight testing 8 LLOYD S REGISTER

11 General Volume 1, Part 6, Chapter 1 Section 1 Section 1 Application 2 General requirements Section 1 Application 1.1 General The Rules apply to sea-going monohull ships of normal form, proportions and speed. Although the Rules are, in general, for steel ships of all welded construction, other materials for use in hull construction will be specially considered on the basis of the Rules An overview of the structural design process is illustrated in Fig Equivalents Alternative scantlings and arrangements may be accepted as equivalent to the Rule requirements. Details of such proposals are to be submitted for consideration in accordance with Pt 3, Ch 1, Symbols and definitions The symbols and definitions for use throughout this Part are as defined within the appropriate Chapters and Sections. LLOYD S REGISTER 1

12 General Volume 1, Part 6, Chapter 1 Section 1 Operational requirements Required deck loads, deadweight and lightship weights Determine environmental parameters Pt 5, Ch 2 Loading conditions Minimum thicknesses Pt 6, Ch 3,2 Derive local loads Pt 5, Ch 3 Derive global loads SW BM and SF Wave BM and SF Pt 5, Ch 4 NS1 or NS2/3 Ship type? NS1 NS2/3 First estimate of scantlings based on empirical equations (assumed F D, F B factors) Pt 6, Ch 3,3 Scantlings based on local loads, design factors Pt 6, Ch 3,4 Derive global section modulus Pt 6, Ch 4,1.4 Derive global section modulus Pt 6, Ch 4,1.4 Additional requirements Determine stresses due to Determine stresses due to Determine stresses due to global loads global loads global loads Pt 6, Ch 4,2 Pt 6, Ch 4,2 Pt 6, Ch 4,2 Strengthening of bottom forward Pt 6, Ch 3,14 Empirical equations for scantlings (correct F D, F B factors) Pt 6, Ch 3,3 Strengthening against wave impact loads above the waterline Pt 6, Ch 3,15 Derive buckling capability of plating and stiffeners Pt 6, Ch 2,4 Requirements for Special features, Military design, etc Part 4 Revise scantlings No Satisfy the stress and buckling requirements? Yes Design scantlings Total Load Assessment (TLA) or Structural Design Assessment (SDA) Procedures? Fig Overview of the structural design process 2 LLOYD S REGISTER

13 General Volume 1, Part 6, Chapter 1 Section 2 Section 2 General requirements 2.1 General Limitations with regard to the application of these Rules are indicated in the various Chapters for differing ship types. 2.2 Plans to be submitted Plans are to be of sufficient detail for plan approval purposes. For all areas of structure listed below the submitted plans are to show all plating thickness, stiffeners sizes and spacings, bracket arrangements and connections. Where appropriate, the plans should clearly show the allowance for corrosion margin or enhanced scantlings. Welding, constructional arrangements and tolerances are also to be submitted and this may be in the form of a booklet In general all items of steel structure are to be shown except they are ineffective in supporting hull girder and local loads and do not impinge on such structure Equipment seating and supports are to be shown they require additional stiffening and support to be incorporated in items of hull structure. In such cases the loading on the seating is also to be supplied. Generally total combined equipment weights on seating less than 0,5 tonnes need not be considered Normally the plans for each item will be able to be contained on a few sheets. Unit or production drawings will not be accepted Plans covering the following items are to be submitted: Midship and other critical sections showing longitudinal and transverse material. Profile and decks. Shell expansion. Structural continuity plan showing shadow areas of openings, longitudinally effective material and primary structural continuity material. Oiltight and watertight bulkheads. Double bottom construction. Pillars and girders. Aft end construction. Engine room construction. Engine and thrust seatings. Fore end construction. Storing routes. Deckhouses and superstructures. Propeller brackets and appendages. Rudder, stock and tiller. Equipment. Loading Manuals and stability information booklets, preliminary and final ( applicable). Scheme of corrosion control ( applicable) including; location of anodes, method of attachment, details of cathodic protection system. Ice strengthening. Welding schedule. Hull penetration plans including scuppers, sea connections, overboard discharges, arrangements and fittings. Support structure for masts, derrick posts, cranes, RAS points, weapons handling/stowage or machinery lifting point and large items of equipment. Bilge keels showing material grades, welded connections and detail design. Flight deck arrangement and structure. Additionally, for in-water survey the following plans and information are to be submitted: Details showing how rudder pintle and bush clearances are to be measured and how the security of the pintles in their sockets are to be verified with the ship afloat. Details showing how stern bush clearances are to be measured with the ship afloat. Details of high resistant paint, for information only The following supporting documents are to be submitted: General arrangement. Capacity plan. Lines plan or equivalent. Dry-docking plan. Towing and mooring arrangements The following supporting calculations are to be submitted: Equipment number. Hull girder still water and dynamic bending moments and shear forces as applicable. Midship section and other critical section modulus. Structural items in the aft end, midship and fore end regions of the ship In the process of plan approval and from Fatigue Design Assessment and Structural Design Assess ment certain critical locations may be identified which require special attention. Plans and records of production are to be submitted in accordance with the construction monitoring procedures associated with the notation CM Where the specific notations are applied for additional plans will be required. 2.3 Plans to be supplied to the ship To facilitate the ordering of materials for repairs, plans listed in are to be carried on board the ship. They are to clearly indicate the disposition and grades (other than Grade A) of hull structural steel, the extent and location of higher tensile steel together with details of specification and mechanical properties, and recommendations for welding, working and treatment of these steels Similar information is to be provided aluminium alloy or other materials are used in the construction A copy of the final Loading Manual or Stability Information booklet, when approved, is to be placed on board the ship. LLOYD S REGISTER 3

14 General Volume 1, Part 6, Chapter 1 Section Details of any corrosion control system fitted are to be placed on board the ship Approved plans and information covering the items detailed in relating to in-water survey are to be placed on board the ship. 2.4 Novel features Where the proposed construction of any part of the hull or machinery is of novel design, or involves the use of unusual material, or experience, in the opinion of Lloyd s Register (hereinafter referred to as LR ), has not sufficiently justified the principle or mode of application involved, special tests or examinations before and during service may be required. In such cases a suitable notation may be entered in the Register Book. 2.5 Enhanced scantlings Where scantlings in excess of the approved Rule requirement are fitted at defined locations as corrosion margins or for other purposes, a notation ES, Enhanced Scantlings, will be assigned and it will be accompanied by a list giving items to which the enhancement has been applied and the increase in scantling. For example, the item bottom shell (strakes A, B, C, D) +2 will indicate that an extra 2 mm has been fitted to the bottom shell of the ship for the particular strakes listed, see also Ch 6,2.10. In addition the plans submitted for approval are to contain the enhanced scantling plan, together with the nominal thickness, less the enhancement adjacent, in brackets. 2.6 Direct calculations Direct calculations may be specifically required by the Rules and may be required for ships having novel design features or in support of alternative arrangements and scantlings. LR may, when requested, undertake calculations on behalf of designers and make recommendations with regard to suitability of any required model tests. 2.7 Exceptions Ships of unusual form, proportions or speed, with unusual features, or for special or restricted service, not covered specifically by the Rules, will receive individual consideration based on the general requirements of the Rules. 4 LLOYD S REGISTER

15 Design Tools Volume 1, Part 6, Chapter 2 Section 1 Section 1 General 2 Structural design 3 Buckling 4 Vibration control 5 Dynamic loading Section 1 General 1.1 General The guidance notes, information and formulae contained within this Chapter are to be used in the scantling determination (Chapter 3) and total load assessment. 1.2 Equivalents Lloyd s Register (hereinafter referred to as LR ) will consider direct calculations for the derivation of scantlings as an alternative and equivalent to those derived by Rule requirements in accordance with Pt 3, Ch 1, Symbols and definitions The symbols used in this Chapter are defined below and in the appropriate Section: Z = section modulus of stiffening member, in cm 3 I = moment of inertia, in cm 4 A w = shear area of stiffener web, in cm 2 l = overall length of stiffener or primary member, in metres l e = effective span length, in metres, as defined in 2.6 p = design pressure, in kn/m 2 s = secondary stiffener spacing, in mm S = primary stiffener spacing, in metres t p = plating thickness, in mm β = panel aspect ratio correction factor as defined in 2.5 γ = convex curvature correction factor as defined in 2.4 k s = higher tensile steel factor for local loads, see Ch 5,2.1.1 k L = higher tensile steel factor for global loads, see Ch 5,2.1.2 σ o = guaranteed minimum yield strength of the material, in N/mm 2 τ o = shear strength of the material in N/mm 2 σ = o 3 E = modulus of elasticity, in N/mm Rounding policy for Rule plating thickness Where plating thicknesses as determined by the Rules require to be rounded then this should be carried out to the nearest full or half millimetre, with thicknesses 0,75 and 0,25 being rounded up. 1.5 Material properties The basic grade of steel used in the determination of the Rule scantling requirements is taken as mild steel with the following mechanical properties: (a) Yield strength (minimum) σ o = 235 N/mm 2 (b) Tensile strength = N/mm 2 (c) Modulus of elasticity, E = 200 x 10 3 N/mm Higher tensile steel Steels having a yield stress not less than 265 N/mm 2 are regarded as higher tensile steels Where higher tensile steels are to be used, due allowance is given in the determination of the Rule requirement for plating thickness, stiffener section modulus, inertia and cross-sectional area by the use of higher tensile steel correction factors k s and k L or f hts. Normally, this allowance is included in the appropriate scantling requirements. Where this is not the case, the following correction factors may be applied: (a) Plating thickness factor = k s for local loads k L for global loads (b) Section modulus and cross sectional area factor = k s k s and k L are defined in f hts is defined in Ch 5, Higher tensile steel may be used for both deck and bottom structures or deck structure only. Where fitted for global strength purposes, it is to be used for the whole of the longitudinal continuous material for the following vertical distances: (a) z htd below the line of deck at side k z htd = ( 1 L ) z D m F D (b) z htb above the top of keel k z htb = ( 1 L ) z B m F B In the above formulae F D and F B are to be taken not less than k L F D and F B are defined in Ch 3,3.6. Note the F D and F B factors derived in Ch 3,3.6 for NS1 ships may also be applied to ship types NS2 and NS3 z D and z B are the vertical distances, in m, from the transverse neutral axis of the hull cross-section to the uppermost continuous longitudinally effective material and to the top of the keel respectively k L is defined in LLOYD S REGISTER 1

16 Design Tools Volume 1, Part 6, Chapter 2 Sections 1 & The designer should note that there is no increase in fatigue performance with the use of higher tensile steels. Section 2 Structural design 2.1 General This Section gives the basic principles to be adopted in determining the Rule structural requirements given in Chapter For derivation of scantlings of stiffeners, beams, girders, etc., the formulae in the Rules are normally based on elastic or plastic theory using simple beam models supported at one or more points and with varying degrees of fixity at the ends, associated with an appropriate concentrated or distributed load The stiffener, beam or girder strength is defined by a section modulus and moments of inertia requirements. In addition there are local requirements for web thickness or flange thickness Some of the details given in this Section will be specially considered for ships with a military distinction notation MD. 2.2 Effective width of attached plating, b e The effective geometric properties of rolled or built sections are to be calculated directly from the dimensions of the section and associated effective area of attached plating. Where the web of the section is not normal to the actual plating, and the angle exceeds 20, the properties of the section are to be determined about an axis parallel to the attached plating For stiffening members, the geometric properties of rolled or built sections are to be calculated in association with an effective area of attached load bearing plating of thickness t p, in mm, and a breadth b e, in mm The effective breadth of attached plating to secondary stiffener members b e, is to be taken as: b e = 40t p mm or 600 mm, whichever is the greater or the actual spacing of stiffeners in mm, whichever is the lesser The effective breadth of attached plating to primary support members (girders, transverses, webs, etc.), b e,is to be taken as: l b e = 300S( ) 2/3 mm but is not to exceed 1000S. S S and l are as defined in Section properties The dimensions of rolled and built stiffeners are illustrated in Fig The section properties of stiffeners can be based on the illustrated dimensions if manufacturer s information is not available The effective section properties of a corrugation over a spacing b, see Fig , is to be calculated from the dimensions and, for symmetrical corrugations, may be taken as: Section modulus d w Z = (3b e t p + c t w ) cm Moment of inertia Z d I = ( w ) cm Shear area A w = d w t w 100 cm 2 d w, b f, t p, c and t w are measured, in mm, and are as shown in Fig The value of b e is to be taken not greater than b f or: 50t p k s for welded corrugations 60t p k s for cold formed corrugations k s = local high strength steel factor, see The value of θ is to be not less than The section properties of a double skin primary member over a spacing b, see Fig , may be calculated as: Section modulus d w Z = (6f b t p + d w t w ) cm Moment of inertia Z d I = ( w ) cm Shear area A w = d w t w 100 cm 2 d w, b, t p and t w are measured, in mm, and are as shown in Fig f 1000l = 0,3 ( ) 2/3 b mm but is not to exceed 1,0 NOTE If the plate flanges of the double bulkhead are of unequal thicknesses, then the equations in may be used with b e = b f = f b. 2 LLOYD S REGISTER

17 Design Tools Volume 1, Part 6, Chapter 2 Section 2 Flat bar Built up t p t p Tee profile t p Built up L profile t w t w t w d w d w d w b f b f t f t f t p Rolled angle t p Bulb plate A = area of bulb plate, in mm 2 t w d w t w A d w h b f = c + t w t A h t w f = c c d w = h t f b f b f 3563/01 t f t f Fig Dimensions of longitudinals b f t p t p t w d w c t w 0,5bf θ d w t p b b Fig Double skin section Fig Corrugated section The effective section properties of a built section, see Fig , may be taken as: Section modulus of flange A f d w A w d w 2 (A Z f = + ( p A f ) 1 + ) cm A 3 p + A w Neutral axis of section above plating d w A x na = ( w + A f ) mm A 2 Moment of inertia about neutral axis I = z f (d w x na ) cm 10 4 Section modulus at plate Z p I = 10 x na cm 3 Shear area A w = d w t w 100 cm 2 LLOYD S REGISTER 3

18 Design Tools Volume 1, Part 6, Chapter 2 Section 2 A f = area of face plate of flange in cm 2 A w = area of web plating in cm 2 A p = area of attached plating in cm 2, see A d w b e = A f + A w + A p = the depth, in mm, of the web between the inside of the face plate and the attached plating. Where the member is at right angles to a line of corrugations, the minimum depth is to be taken = effective breadth of attached plating, in mm, see 2.2 b f, t f, d w, t w and t p are in mm and are illustrated in Fig The geometric properties of primary support members (i.e. girders, transverses, webs, stringers, etc.) attached to corrugated bulkheads, are to be calculated in association with an effective area of attached load bearing plating, A p, determined as follows: (a) For a member attached to corrugated plating and parallel to the corrugations: A p = b f t p /100 cm 2 (See Fig ). (b) For a member attached to corrugated plating and at right angles to the corrugations: A p is to be taken as equivalent to the area of the face plate of the member. 2.4 Convex curvature correction The thickness of plating as determined by the Rules may be reduced significant curvature exists between the supporting members. In such cases a plate curvature correction factor may be applied: γ = plate curvature factor = 1 d c /s c, and is not to be taken as less than 0,7 d c = the distance, in mm, measured perpendicularly from the chord length, s c, (i.e. spacing in mm) to the highest point of the curved plating arc between the two supports, see Fig Aspect ratio correction The thickness of plating as determined by the Rules may be reduced when the panel aspect ratio is taken into consideration. In such cases a panel aspect ratio correction factor may be applied: β = aspect ratio correction factor = A R (1 0,25A R ) for A R 2 = 1 for A R > 2 A R = panel aspect ratio = panel length/panel breadth. 2.6 Determination of span length The effective span length, l e, of a stiffening member is generally less than the overall length, l, by an amount which depends on the design of the end connections. The span points, between which the value of l e is measured, are to be determined as follows: (a) For rolled or built-up secondary stiffening members: The span point is to be taken at the point the depth of the end bracket, measured from the face of the secondary stiffening member, is equal to the depth of the member, see Fig Where there is no end bracket, the span point is to be measured between primary member webs. (b) For primary support members: The span point is to be taken at a point distant, b s, from the end of the member d b s = b b ( w 1 d b ) b s, b b, d w and d s are as shown in Fig Where the stiffening member is inclined to a vertical or horizontal axis and the inclination exceeds 10, the span is to be measured along the member Where the stiffening member is curved then the span is to be taken as the effective chord length between span points It is assumed 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, consideration will be given to the effective span to be used for the stiffener. d c 2.7 Plating general s c The equation given in this sub-section is to be used to determine the thickness of plating for NS2 and NS3 ship types. The design pressure, p, is given in the Tables in Ch 3,4 for each structural component and is to be used with the limiting stress coefficient, see Ch 5,3.1.1, to determine the required plate thickness. Fig Convex curvature 4 LLOYD S REGISTER

19 Design Tools Volume 1, Part 6, Chapter 2 Section 2 Span point Secondary support Span points Span point d s Span point d s d s Span point d s Primary support b s Span point Span point b s d b d w d b b b d w b b l l Fig Definition of span points The requirements for the thickness of plating, t p, is, in general, to be in accordance with the following: t p = 22,4s γ β p f σ σ o x 10 3 mm p is the design pressure, in kn/m 2, given in Ch 3,4 f σ = limiting stress coefficient for local plate bending for the plating area under consideration given in Ch 5,3, σ b column in Table 5.3.2, see also Ch 5,3.1.1 s, γ, β, σ o are as defined in Stiffening general The equations given in this sub-section are to be used to derive the scantling requirements for stiffeners. The design pressure, p, is given in the Tables in Ch 3,4 for each structural component and is to be used with the limiting stress coefficient, see Ch 5,3.1.1, to determine the required section modulus, web area and inertia of the stiffeners. LLOYD S REGISTER 5

20 Design Tools Volume 1, Part 6, Chapter 2 Section The requirements for section modulus, inertia and web area of stiffening members subjected to pressure loads are, in general, to be in accordance with the following: (a) For secondary members: Section modulus: φ z p s l 2 e Z = cm f 3 σ σ o Inertia: 100 φ I p s l 3 e I = cm f 4 δ E Web area: φ A p s l e A w = cm 100 f 2 τ τ o (b) For primary members: Section modulus: 10 3 φ z p S l 2 e Z = cm f 3 σ σ o Inertia: 10 5 φ I p S l 3 e I = cm f 4 δ E Web area: Aw = 10 φ A p S l e f τ τ o cm 2 p is the design pressure, in kn/m 2, given in Ch 3,4 φ Z = section modulus coefficient dependent on the loading model assumption taken from Table f σ = limiting local stiffener bending stress coefficient for stiffening member given in Ch 5,3.1.1 φ I = inertia coefficient dependent on the loading model assumption taken from Table f δ = limiting inertia coefficient for stiffener member given in Ch 5,3.1.1 φ A = web area coefficient dependent on the loading model assumption taken from Table f τ = limiting web area coefficient for stiffener member given in Ch 5,3.1.1 E, S, s, l e, σ o and τ o are as defined in The requirements for section modulus, inertia and web area of stiffening members subjected to point loads are, in general, to be in accordance with the following: For primary and secondary members: Section modulus: 10 Z = 3 φ z F l e cm f 3 σ σ o Inertia 10 5 φ I F l 2 e I = cm f 4 δ E Web area A w = 10φ A F f τ τ o cm 2 F is the design point load, in kn φ Z = section modulus coefficient dependent on the loading model assumption taken from Table f σ = limiting local stiffener bending stress coefficient for stiffening member given in Ch 5,3.1.1, column σ x in Table φ I = inertia coefficient dependent on the loading model assumption taken from Table f δ = limiting inertia coefficient for stiffener member given in Ch 5,3.1.1, column f δ in Table φ A = web area coefficient dependent on the loading model assumption taken from Table f τ = limiting web area coefficient for stiffener member given in Ch 5,3.1.1, column f τ in Table E, l e, and σ o are as defined in Where a stiffener is subjected to a combination of loads, then the requirements are to be based on the linear supposition of those loads onto the stiffener. For example the section modulus requirement for a UDL load and a point load will be as follows: φ z p s l 2 e 10 3 φ z F l e Z = + cm f 3 σ σ o f σ σ o 2.9 Proportions of stiffener sections From structural stability and local buckling considerations, the proportions of stiffening members are, in general, to be in accordance with Table For primary member minimum thickness see Table in Pt 6, Ch Primary members of asymmetrical section are to be supported by tripping brackets at alternate secondary members. If the section is symmetrical, the tripping brackets may be four spaces apart Tripping brackets are in general required to be fitted at the toes of end brackets and in way of heavy or concentrated loads such as the heels of pillars Where the ratio of unsupported width of face plate (or flange) to its thickness exceeds 16:1, the tripping brackets are to be connected to the face plate and on members of symmetrical section, the brackets are to be fitted on both sides of the web Intermediate secondary members may be welded directly to the web or connected by lugs in accordance with Ch 6, Grillage structures 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 General or special purpose computer programs or 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. 6 LLOYD S REGISTER

21 Design Tools Volume 1, Part 6, Chapter 2 Section 2 Table Section modulus, inertia and web area coefficients for different load models (see continuation) Position ( j) Web area Section Inertia Load (j) coefficient modulus coefficient model coefficient end midspan end φ A φ Z φ I Application (A) 1 1/2 1/12 Primary and other members p 2 1/24 1/384 the end fixity is 3 1/2 1/12 considered encastré Uniformly distributed pressure (B) 1 1/2 1/10 Local, secondary and other 2 1/10 1/288 members the end 3 1/2 1/10 fixity is considered to be partial Uniformly distributed pressure (C) p 1 7/20 1/20 Linearly varying distributed 2 1/764 pressure 3 3/20 1/30 Built in both ends (D) 1 1 1/2 1/8 Uniformly distributed 2 pressure cantilevered beam 3 (E) 1 1/2 Uniformly distributed pressure 2 1/8 5/384 Simply supported 3 1/2 Hatch covers, glazing and other members the ends are not fixed (F) 1 5/8 1/8 Uniformly distributed pressure 2 9/128 1/185 Cantilever plus simple 3 3/8 support (G) 1 1 1/3 0 Uniformly distributed pressure 2 0 Built in one end. Other end 3 1/3 1/24 free to deflect but slope restrained (H) δ Built in both ends with forced 2 deflection at one end (I) a F 1 (l a) 2 (l + 2a) a (l a) 2 Single point load, load l 3 l 3 any in the span 2 2a2 (l a) 2 2(l a) 2 a 3 Built in at both ends l 4 3(l + 2a) 2 l 3 l 3 a 2 (3l 2a) a 2 (l a) l 3 l 3 l/2 F 1 1/2 1/8 Single point load in the centre 2 1/8 1/192 of the span 3 1/2 1/8 Built in at both ends l LLOYD S REGISTER 7

22 Design Tools Volume 1, Part 6, Chapter 2 Section 2 Table Section modulus, inertia and web area coefficients for different load models (conclusion) Position (j) Web area Section Inertia Load (j) coefficient modulus coefficient model coefficient end midspan end φ A φ Z φ I Application (J) l F a a (3l 2 a 2 ) a (l 2 a 2 ) Single point load, load 2l 3 2l 3 any in the span a (l a) 2 (2l + a) a (l a) 2 a 1/2 Cantilever plus simple support 2l 4 6l 3 ( 2l + a ) (l a) 2 (2l + a) 2l 3 F l/2 1 11/16 3/16 Single point load in the centre 2 5/32 1/108 of the span 3 5/16 Cantilever plus simple support l (K) a F l 1 l a l 2 a (l a) a l 2 a 2 3/2 l ( 2 3l 4 3 ) Single point load, load any in the span a 3 Simply supported l l/2 F 1 1/2 Single point load in the centre 2 1/4 1/48 of the span 3 1/2 Simply supported l (L) F a 1 1 (l a) l 2 Single point load any in the span l 3 2l 3 3l 2 a + a 3 l 3 Cantilevered beam NOTE In all cases, the coefficient that results in the most pessimistic requirement is to be used in the stiffening equations in 2.8. Table Type of stiffener Stiffener proportions Requirement (1) Flat bar Minimum web thickness: continuous t w = d w /18 2,5 mm intercostal t w = d w /15 2,5 mm 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 such an assumption is not applicable, finite element analysis or equivalent methods may have to be used. (2) Rolled or built sections (a) Minimum web thickness: t w = d w /60 2,5 mm (b) Maximum unsupported face plate (or flange) width: b f = 16t f Symbols t w d w b f t f = web thickness of stiffener with unstiffened webs, in mm = web depth of stiffener, in mm = face plate (or flange) unsupported width, in mm = face plate (or flange) thickness, in mm 8 LLOYD S REGISTER

23 Design Tools Volume 1, Part 6, Chapter 2 Section 3 Section 3 Buckling 3.1 General This Section contains the requirements for buckling control of plate panels subject to in-plane compressive and/or shear stresses and buckling control of primary and secondary stiffening members subject to axial compressive and shear stresses The requirement for buckling control of plate panels is contained in 3.3 to 3.6. The requirements for secondary stiffening members are contained in 3.7 to 3.8. The requirements for primary members are contained in 3.9 and In general all areas of the structure are to meet the buckling strength requirements for the design stresses. The design stresses are to be taken as the global hull girder bending and shear stresses derived in accordance with Chapter 4. In addition, the structural member is subject to local compressive loads, then the design stresses are to be based on these loads The buckling requirements are to be met using the net scantlings, hence any additional thickness for corrosion margin or enhanced scantlings is not included in the scantlings used to assess the buckling performance. For enhanced scantlings, see Pt 3, Ch 6,6. For corrosion margins, see Pt 6, Ch 6, Symbols The symbols used in this Section are defined below and in the appropriate sub-section: A R = panel aspect ratio = a b a = panel length, i.e., parallel to direction of compressive stress being considered, in mm b = panel breadth, i.e. perpendicular to direction of compressive stress being considered, in mm S p = span of primary members, in metres σ e = elastic compressive buckling stress, in N/mm 2 σ c = critical compressive buckling stress, including the effects of plasticity appropriate, in N/mm 2 τ e = elastic shear buckling stress, in N/mm 2 τ c = critical shear buckling stress, in N/mm 2 b eb = lesser of 1,9t p E σ o or 0,8b mm A te = cross-sectional area of secondary stiffener, in cm 2, including an effective breadth of attached plating, b eb s = length of shorter edge of plate panel, in mm (typically the spacing of secondary stiffeners) l = length of longer edge of plate panel, in metres S = spacing of primary member, in metres (measured in direction of compression) 3.3 Plate panel buckling requirements The Section gives methods for evaluating the buckling strength of plate panels subjected to the following load fields: (a) uni-axial compressive loads; (b) shear loads; (c) bi-axial compressive loads; (d) uni-axial compressive loads and shear loads; (e) bi-axial compressive loads and shear loads The plate panel buckling requirements will be satisfied if the buckling interaction equations given in Table for the above load fields are complied with The critical compressive buckling stresses and critical shear buckling stresses required for Table are to be derived in accordance with The buckling factors of safety λ σ and λ τ required by Table are to be extracted from Chapter 5 for the structural member concerned For all structural members which contribute to the hull girder strength, the plate panel buckling requirements for uni-axial compressive loads, Table 2.3.2(a), and shear loads, Table 2.3.2(b) are to be complied with In addition to 3.3.5, structural members which are subjected to local compressive loads and/on shear loads are to be verified using the plate panel buckling requirements in Table 2.3.2(c) to (e) However, some members of the structure have been designed such that elastic buckling of the plate panel between the stiffeners is allowable, then the requirements of 3.5 must be applied to the buckling analysis of the stiffeners supporting the plating. In addition, panels which do not satisfy the panel buckling requirements must be indicated on the appropriate drawing and the effect of these panels not being effective in transmitting compressive loads taken into account for the hull girder strength calculation, see Ch 4,1.4.7 and In general the plate panel buckling requirements for more complex load fields, see 3.3.1(c), (d) and (e), are to be complied with. Where this is not possible, due to elastic buckling of the panel, then the critical buckling stress, σ c, may be based on the ultimate collapse strength of the plating, σ u from 3.5.4, instead of the elastic buckling stress, σ e, derived in In addition, the requirements of 3.5 are to be met for the supporting secondary stiffeners and primary members. 3.4 Derivation of the buckling stress for plate panels The critical compressive buckling stress, σ c, for a plate panel subjected to uni-axial in-plane compressive loads is to be derived in accordance with Table 2.3.1(a) The critical shear buckling stress, τ c, for a plate panel subjected to pure in-plane shear load is to be derived in accordance with Table 2.3.1(b). LLOYD S REGISTER 9

24 Design Tools Volume 1, Part 6, Chapter 2 Section 3 Table Buckling stress of plate panels Mode Elastic buckling stress, N/mm 2 see Note (a) Uni-axial compression: (i) Long narrow panels, loaded on the narrow edge A R 1 σ e = 3,62ϕ E ( ) t p b 2 σ d σ d a a σ d b σ d (ii) Short broad panels, loaded on the broad edge A R < 1 b a 2 t 2 σ e = 0,9C ϕ ( + ) E a b ( p b ) b (b) Pure shear: u ( ( )) ( ) τ e = 3,62 1, t 2 E p v u NOTE u is to be the minimum dimension τ d Stiffeners for C factor τ d u v NOTE The critical buckling stresses, in N/mm 2, are to be derived from the elastic buckling stresses as follows: σ o τ o σ c = σ e when σ e < τ c = τ e when τ e < 2 2 σ σ = σ o ( o 1 ) o τ when σ e = τ 4σ e 2 o ( o 1 ) when τ e 4τ e σ c is defined in σ o is defined in τ c is defined in τ o is defined in τ o 2 Symbols and definitions A R = panel aspect ratio, see σ e = elastic compressive buckling stress, in N/mm 2 τ e = elastic shear buckling stress, in N/mm 2 a and b are the panel dimensions in mm, see figures above t p = thickness of plating, in mm ϕ = stress distribution factor for linearly varying compressive stress across plate width = 0,47μ 2 1,4μ + 1,93 for μ 0 = 1 for constant stress μ = σ d1 σ d2 σ d1 and σ d2 are the smaller and larger average compressive stresses respectively E = Young s Modulus of elasticity of material, in N/mm 2 C = stiffener influence factor for panels with stiffeners perpendicular to compressive stress = 1,3 when plating stiffened by floors or deep girders = 1,21 when stiffeners are built up profiles or rolled angles = 1,10 when stiffeners are bulb flats = 1,05 when stiffeners are flat bars σ d and τ d are the design compressive and design shear stresses in the direction illustrated in the figures. With linearly varying stress across the plate panel, σ d is to be taken as σ d For welded plate panels with plating thicknesses below 8 mm, the critical compressive buckling stress is to be reduced to account for the presence of residual welding stresses. The critical buckling stress for plating is to be taken as the minimum of: σ cr = σ e σ r or σ c as derived using σ r = reduction in compressive buckling stress due to residual welding stresses 2 β = RS σ o b/t p β RS = residual stress coefficient dependent on type of weld (average value of β RS to be taken as 3) t p and σ o are defined in σ c is derived in b is defined in In general the effect of lateral loading on plate panels (for example hydrostatic pressure on bottom shell plating) may be neglected and the critical buckling stresses calculated considering the in-plane stresses only Unless indicated otherwise, the effect of initial deflection on the buckling strength of plate panels may be ignored. 10 LLOYD S REGISTER

25 Design Tools Volume 1, Part 6, Chapter 2 Section Additional requirements for plate panels which buckle elastically Elastic buckling of plate panels between stiffeners occurs when both the following conditions are satisfied: (a) The design compressive stress, σ d, is greater than the elastic buckling stress of the plating, σ e, σ d > σ e (b) The elastic buckling stress is less than half the yield stress σ σ e o 2 (c) Elastic buckling of local plating between stiffeners, including girders or floors etc, may be allowed if all of the following conditions are satisfied: (a) The critical buckling stress of the stiffeners in all buckling modes is greater than the axial stress in the stiffeners after redistribution of the load from the elastically buckled plating into the stiffeners, hence σ de 1 σ c (i) λ σ i = (a), (t), (w) or (f) (b) Maximum predicted loadings are used in the calculations. Functional requirements will allow a degree of plating deformation. σ de is the stiffener axial stress given in σ c(i) is given by Table i is a, t, w or f depending on the mode of buckling. λ σ is the buckling factor of safety given in Table in Chapter The effective breadth of attached plating for stiffeners, girder or beams that is to be used for the determination of the critical buckling stress of the stiffeners attached to plating which buckles elastically is to be taken as follows: b eu = b σ u σ o mm σ u = ultimate buckling strength of plating as given in σ o is defined in b eu = effective panel breadth perpendicular to direction of compressive stress being considered b is given in The ultimate buckling strength of plating, σ u, which buckles elastically, may be determined as follows: (a) shortest edge loaded, i.e. A R 1: 1,9 0,8 σ u = σ o N/mm 2 Ω Ω 2 ( ) (b) longest edge loaded i.e. A R < 1: 1,77σ o A 0,78 R σ u = N/mm Ω 2 s σ o Ω = t p E A R and s are defined in t p, E and σ o are defined in The axial stress in stiffeners attached to plating which is likely to buckle elastically is to be derived as follows: A t σ de = σ d A tb σ d is the axial stress in the stiffener when the plating can be considered fully effective A t bt = A s cm 2 A tb b eu t = A s cm 2 b and b eu are given in t is the plating thickness, in mm A s is the stiffener area in cm Shear buckling of stiffened panels The shear buckling capability of longitudinally stiffened panels between primary members is to satisfy the following condition: τ = d 1 τ c λ τ τ c is derived from τ d is the design shear stress λ τ is given in Table in Chapter The elastic shear buckling stress of longitudinally stiffened panels between primary members may be taken as: t 2 τ e = K s E for A R 1 () s s 2 1 N r K s = 4,5 (( ) + + ( 2 1 ω 1000l N )( ) ) 2 N ω N = number of subpanels 1000S p = s 10I se ω = l t 3 I se = moment of inertia of a section, in cm 4, consisting of the longitudinal stiffener and a plate flange of effective width s/2 r = 1 0,75 ( s 1000l ) s, l, E and S p are as defined in 3.2.1, see also Fig The critical shear buckling stress, τ c, may be determined from τ e, see Note in Table LLOYD S REGISTER 11

26 Design Tools Volume 1, Part 6, Chapter 2 Section 3 S p To ensure that overall buckling of the stiffened panel cannot occur before local buckling of the secondary stiffener, the critical overall buckling stress σ c(a), is to be greater than the critical torsional buckling stress, hence: σ c(a) > σ c(t) τ d 3.8 Secondary stiffening perpendicular to direction of compression l s τ d Where a stiffened panel of plating is subjected to a compressive load perpendicular to the direction of the stiffeners, see Fig , e.g., a transversely stiffened panel subject to longitudinal compressive load, the requirements of this Section are to be applied. Fig Longitudinally stiffened panels between primary members σ d 3.7 Secondary stiffening in direction of compression The buckling performance of stiffeners will be considered satisfactory if the following conditions are satisfied: σ d 1 σ d 1 σ c(a) λ σ σ c(t) λ σ l s S σ d 1 σ d 1 σ c(w) λ σ σ c(f) λ σ σ c(a), σ c(t), σ c(w) and σ c(f) are the critical buckling stress of the stiffener for each mode of failure, see σ d is the design compressive stress, see also and 3.5 λ σ is the buckling factor of safety given in Table in Chapter 5. The value of λ σ to be chosen depends on the buckling assessment of the attached plating, see Note 3, Table The critical buckling stresses for the overall, torsional, web and flange buckling modes of longitudinals and secondary stiffening members under axial compressive loads are to be determined in accordance with Table To prevent torsional buckling of secondary stiffeners from occurring before buckling of the plating, the critical torsional buckling stress, σ c(t), is to be greater than the critical buckling stress of the attached plating as detailed in The critical buckling stresses of the stiffener web, σ c(w), and flange, σ c(f), are to be greater than the critical torsional buckling stress, hence: σ c(w) > σ c(t) σ c(f) > σ c(t) σ d Fig Secondary stiffening perpendicular to direction of compression The minimum moment of inertia of each stiffener including attached effective plating of width, b eb, to ensure that overall panel buckling does not precede plate buckling is to be taken as: I s = D s (4N 2 L 1)((N 2 L 1) 2 2 (N 2 L 1) κ + κ 2 ) 2 (5N 2 L + 1 κ) Π 4 E mm 4 D = E t 3 p 12 (1 υ 2 ) κ = A 2 R Π 2 A R = plate panel aspect ratio = s 1000l Π = S l N L = number of plate panels N L 1 = number of stiffeners υ = 0,3 12 LLOYD S REGISTER