Section 6 Construction details

6.1.1 Continuity is to be maintained where primary members intersect and where the members are of the same depth, a suitable gusset plate or brackets are to be fitted, see Figure 6.6.1 Primary member intersection

6.1.2 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.

Figure 6.6.1 Primary member intersection

6.1.3 Particular care is to be paid to the design of the end bracket toes in order to minimise stress concentrations. Sniped face plates which are welded onto the edge of primary member brackets are to be carried well around the radiuses bracket toe and are to incorporate a taper not exceeding one in three. Where sniped face plates are welded adjacent to the edge of primary member brackets, adequate cross-sectional area is to be provided through the bracket toe at the end of the snipe. In general, this area measured perpendicular to the face plate, is to be not less than 60 per cent of the full cross-sectional area of the face plate, see Figure 6.6.2 Bracket toe construction

Figure 6.6.2 Bracket toe construction

6.2.1 The requirements for section modulus and inertia (if applicable) of primary members are given in the appropriate Chapter. The scantling requirements for primary member end connections in dry spaces and in tanks of all ship types are generally to comply with the requirements of Vol 1, Pt 6, Ch 2 Design Tools,Vol 1, Pt 6, Ch 3 Scantling Determination, taking Z as the section modulus of the primary member.

6.2.2 Guidance on the arrangement of primary stiffeners is given in Vol 1, Pt 3, Ch 2, 3.2 Primary members

6.2.3 Connections between primary members forming ring system are to minimise 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.

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

6.2.5 The geometric properties of the members are to be calculated in association with an effective width of attached plating determined in accordance with Vol 1, Pt 6, Ch 2, 2.2 Effective width of attached plating, be

6.2.6 The minimum thickness or area of material in each component part of the primary member is given in Table 6.6.1 Minimum thickness of primary members.

Table 6.6.1 Minimum thickness of primary members

Item		Requirement
(1)	Member web plate (see Note)	tw = 0,01Sw but not less than 6 mm in dry spaces and not less than 7 mm in tanks
(2)	Member face plate	Af not to exceed cm2
(3)	Deck plating forming the upper flange of underdeck girders	Plate thickness not less than mm
Symbols
dw = depth of member web, in mm ks = higher tensile steel factor, see Vol 1, Pt 6, Ch 2 Design Tools tw = thickness of member web, in mm Af = area of member face plate or flange, in cm2 Sw = spacing of stiffeners on member web, or depth of unstiffened web, in mm
Note For primary members having a web depth exceeding 1500 mm, the arrangement of stiffeners will be individually considered, and stiffening parallel to the member face plate may be required.

6.3.1 Secondary members, that is longitudinals, beams, frames and bulkhead stiffeners forming part of the hull structure, are to be effectively continuous and are to be suitably bracketed at their end connections. Where it is desired to adopt bracketless connections, the proposed arrangements will be individually considered, see also Table 6.5.3 Connections of primary structure.

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

6.3.3 The scantlings of secondary member end connections are to be in accordance with Vol 1, Pt 6, Ch 6, 6.4 Scantlings of end brackets.

6.4.1 For a naval ship, longitudinal strength members are to be continuous through primary supports. In exceptional cases for ships having a military distinction notation MD and in areas not subject to significant fatigue loading, longitudinal strength members may be cut at a primary support and the continuity of strength is to be provided by brackets. In such cases the scantlings of the brackets are to be such that their section modulus 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.

6.4.2 In other cases the scantlings of the bracket are to be based on the modulus as follows:

1. Bracket connecting stiffener to primary member – modulus of the stiffener.

2. Bracket at the head of a main transverse frame where frame terminates – modulus of the frame.

3. Brackets connecting lower deck beams or longitudinals to the main frame in the forward 0,5L _R – modulus of the frame.

4. Elsewhere – the lesser modulus of the members being connected by the bracket.

6.4.3 The web thickness and face flat area of end brackets are not in general to be less than those of the connecting stiffeners. Additionally, the stiffener proportion requirements of Vol 1, Pt 6, Ch 2, 2.9 Proportions of stiffener sections are to be satisfied.

6.4.4 Typical arrangements of stiffener end brackets are shown diagrammatically in Figure 6.6.3 Stiffener end brackets.

Figure 6.6.3 Stiffener end brackets

6.4.5 The lengths, d _a and b _a, of the arms are to be measured from the plating to the toe of the bracket and are to be such that:

1. d _a + b _a ≥ 2,0l_b

2. d _a ≥ 0,8l_b

3. b _a ≥ 0,8l_b

where

a and b are the actual lengths of the two arms of the bracket, in mm, measured from the plating to the toe of the bracket

lb

=

6.4.6 The scantlings of deep web frames are based on the inclusion of the standard brackets specified in Vol 1, Pt 6, Ch 6, 6.4 Scantlings of end brackets 6.4.5 at top and bottom, while the scantlings of side frames are normally to be based on a standard bracket at the top only. Where the actual arm lengths fitted, d _a1, and b _a1 (in mm) are smaller than Rule size above or the bracket is omitted then, for comparison purposes, an equivalent arm length, l_a, is to be derived from:

1. 

2. d _a1 ≥ 0,8l_a

3. b _a1 ≥ 0,8l_a

4. l_a = 0

   where

   1. bracket is omitted from the upper or lower ends of the frame, or

   2. lower frame bracket at bilge is at same level as the inner bottom, or

   3. lower frame is welded directly to the inner bottom.

6.4.7 The free edge of the bracket is to be stiffened where any of the following apply:

1. The section modulus, Z, exceeds 2000 cm³.

2. The length of free edge exceeds 50 times the bracket thickness.

3. The bracket is fitted at the lower end of main transverse side framing.

6.4.8 Where a face flat is fitted, its breadth, b _f, is to be not less than:

6.4.9 Where the edge is stiffened by a welded face flat, the cross-sectional area of the face flat is to be not less than:

1. 0,009k _s b _f t _b cm² for offset edge stiffening.

2. 0,014k _s b _f t _b cm² for symmetrically placed stiffening.

   b f

   =

   breadth of face flat, in mm

   t b

   =

   the thickness of the bracket, in mm k s is as defined in Vol 1, Pt 6, Ch 2, 1.3 Symbols and definitions 1.3.1.

6.4.10 Where the stiffening member is lapped on to the bracket, the length of overlap is to be adequate to provide for the required area of welding. In general, the length of overlap is not to be less than , or the depth of stiffener, whichever is the greater.

6.4.11 Where the free edge of the bracket is hollowed out, it is to be stiffened or increased in size to ensure that the modulus of the bracket through the throat is not less than that of the required straight edged bracket.

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

6.4.13 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.

6.4.14 The thickness of the bracket is to be not less than as required by Table 6.6.2 Thickness of end brackets.

Table 6.6.2 Thickness of end brackets

Bracket		Thickness, in mm	LImits
			Minimum, in mm	Maximum, in mm
With edge stiffened:
(a)	in dry spaces	3,5 + 0,25	6,5	12,5
(b)	in deep tanks	4,5 + 0,25	7,5	13,5
Unstiffened brackets:
(a)	in dry spaces		7,5
(b)	in deep tanks		8,5

6.5.1 Lugs or tripping brackets are to be fitted where shell longitudinals are continuous through web frames in way of highly stressed areas of the side shell (e.g. in way of equipment supports, bollards, fenders, etc.).

6.5.2 Lugs or tripping brackets are also to be fitted where continuous secondary stiffeners are greater than half the depth of the primary stiffeners.

6.5.3 Cut-outs in primary members are to comply with the requirements of Vol 1, Pt 3, Ch 2, 3.2 Primary members 3.2.11and Vol 1, Pt 3, Ch 2, 3.2 Primary members 3.2.12

6.5.4 Cut-outs for the passage of secondary members through the webs of primary members, and the related collaring arrangements, are to be designed to minimise 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 investigated. Cut-outs for longitudinals will be required to have double lugs in areas of high stress. Some typical lug connections are shown in Figure 6.6.4 Typical lug connections and Figure 6.6.5 Cut-out and connections, see Vol 1, Pt 6, Ch 6, 6.5 Arrangement at intersection of continuous secondary and primary members 6.5.12

6.5.5 The breadth of cut-outs is to be as small as practicable, with the top edge suitably radiused. Cut-outs are to have smooth edges, and the corner radii are to be as large as practicable. Where the web depth is greater than 100 mm the corner radii are to be a minimum of 20 per cent of the breadth of the cut-out or 20 mm, whichever is the greater, and for large cut-outs greater than 250 mm deep, the web plate connection to the hull envelope, or bulkhead, should end in a smooth tapered ‘soft toe’. Recommended shapes of cut-out are shown in Figure 6.6.4 Typical lug connections, but consideration will be given to other shapes on the basis of maintaining equivalent strength and minimising stress concentration.

6.5.6 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.

6.5.7 Asymmetrical secondary members are to be connected on the heel side to the primary member web plate. Additional connection by lugs on the opposite side may be required.

6.5.8 Symmetrical secondary members are to be connected by lugs on one or both sides, as necessary.

6.5.9 Where a bracket is fitted to the primary member web plate in addition to a connected stiffener it is to be arranged on the opposite side to, and in alignment with the stiffener. The arm length of the bracket is to be not less than the depth of the stiffener, and its cross-sectional area through the throat of the bracket is to be included in the calculation of A _f, see Vol 1, Pt 6, Ch 6, 5.13 Intersection of primary and secondary members 5.13.1.

6.5.10 In general where the primary member stiffener is connected to the secondary member it is to be aligned with the web of the secondary member, except where the face plate of the latter is offset and abutted to the web, in which case the stiffener connection is to be lapped. Lapped connections of primary member stiffeners to mild steel bulb plate or rolled angle secondary members may also be permitted. Where such lapped connections are fitted, particular care is to be taken to ensure that the primary member stiffener wrap around weld connection is free from undercut and notches.

Figure 6.6.4 Typical lug connections

Figure 6.6.5 Cut-out and connections

6.5.11 Fabricated longitudinals having the face plate welded to the underside of the web, leaving the edge of the web exposed, are not recommended for side shell and longitudinal bulkhead longitudinals. Where it is proposed to fit such sections, a symmetrical arrangement of connection to transverse members is to be incorporated. This can be achieved by fitting backing brackets on the opposite side of the transverse web or bulkhead. The primary member stiffener and backing brackets are to be lapped to the longitudinal web, see Vol 1, Pt 6, Ch 6, 6.5 Arrangement at intersection of continuous secondary and primary members 6.5.10.

6.5.12 The cross-sectional areas of the connections are to be determined from the proportion of load transmitted through each component in association with the appropriate allowable stress coefficient given in Table 6.6.3 Allowable stress coefficients.

6.5.13 The load transmitted through the intersection arrangement is to be determined using the design pressure for the structural element being assessed in accordance with Vol 1, Pt 5, Ch 3 Local Design Loads.

6.5.15 The arrangement of lug/collar/direct connection to the primary web stiffener determines the load apportioned to each component. The effect on each component of the intersection is to be assessed for shear and direct stress. Where the web stiffener is not connected to the secondary member, the load, P, is transmitted through the lug/collar/direct connection.

6.6.1 Where the stiffeners of the double bottom floors and transverse bulkheads are unconnected to the secondary members and offset from them (see Figure 6.6.6 Arrangement with offset stiffener) the collar arrangement for the secondary members are to satisfy the requirements of Vol 1, Pt 6, Ch 6, 6.5 Arrangement at intersection of continuous secondary and primary members 6.5.4. In addition, the fillet welds attaching the lugs to the secondary members are to be based on a weld factor of 0,44 for the throat thickness. To facilitate access for welding the offset stiffeners are to be located 50 mm from the slot edge furthest from the web of the secondary member. The ends of the offset stiffeners are to be suitably tapered and softened.

Figure 6.6.6 Arrangement with offset stiffener

6.6.2 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.

6.6.3 For ships with shock enhancement notation, see Vol 1, Pt 4, Ch 2, 4 Fragmentation protection

6.7.1 Watertight steel collars are to be fitted, where stiffeners are continuous through watertight or oiltight boundaries.

6.7.2  Watertight steel collars or equivalent are to be fitted at gastight boundary.

6.8.1 Where thick insert plates are butt welded to thin plates, the edge of the thick plate may require to be tapered. The slope of the taper is generally not to exceed one in three.

6.8.2 The corners of insert plates are generally to be suitably radiused.

6.8.3 For ships with shock enhancement, see Vol 1, Pt 4, Ch 2 Military Load Specification

6.9.1 Doubler plates are to be avoided and are not to be fitted in areas where corrosion may be a problem and access for inspection and maintenance is limited.

6.9.2 Where doubler plates are fitted, they are to have well radiused corners and the perimeter is to be continuously welded. Large doubler plates are also to be suitably slot welded, the details of which are to be submitted for consideration.

6.10.1 Gutterway bars and spurnwaters are not to be welded to boundary angles, or within 100 mm of the deck edge.

6.10.2 Minor attachments, such as pipe clips, staging lugs and supports, are generally to be kept clear of toes of end brackets, corners of openings and similar areas of high stress. Where connected to asymmetrical stiffeners, the attachments may be in line with the web providing the fillet weld leg length is clear of the offset face plate or flange edge. Where this cannot be achieved the attachments are to be connected to the web, and in the case of flanged stiffeners they are to be kept at least 25 mm clear of the flange edge. On symmetrical stiffeners, they may be connected to the web or to the centreline of the face plate in line with the web.

6.10.3 Where necessary in the construction of the ship, lifting lugs may be welded to the hull plating but they are not to be slotted through.

6.11.1 Bilge keel plating is to be attached to the shell plating as shown in Figure 6.6.7 Double plate bilge keel construction. Butt and seam welds in shell plating and bilge keels are to be staggered by at least 100 mm.

Figure 6.6.7 Double plate bilge keel construction

6.11.2 The shell plating in way of the bilge keel is to be at least grade D. Insert plates of 50 per cent greater thickness than the as fitted surrounding shell plate are to be fitted. They are to be greater than 300 x 300mm² with well rounded corners.

6.11.3 The thickness of the bilge keel is to be assessed using the appropriate scantling equation for shell envelope plating. To prevent possible damage to the shell, the bilge keel plate is not to be thicker than the adjacent shell plate. The material class, grade and quality of the bilge keel plating is to be the same as the adjacent shell plating, see Table 6.2.1 Material classes and grades.

6.11.4 Full continuous welding is to be used to connect the bilge keel to the shell.

6.11.5 The ends of the bilge keels are to have a 1 in 3 taper and terminate within 300 mm to 100 mm past an internal frame. Suitable internal framing is to be arranged in way of the ends of the bilge keels where, for hydrodynamic reasons, a steeper taper is necessary, the termination of the bilge keels will be especially considered.

6.11.6 For ships over 65 m in length, all welds are to be subject to non-destructive examination.

6.11.7 Bilge keels of an alternative design from that shown in Figure 6.6.6 Arrangement with offset stiffener with single plate construction or fitted with ground bars will be specially considered.

6.11.8 Internal stiffening is to be arranged in line with hull framing but is not to be attached to the shell plating.

6.11.9 Bilge keels are to be watertight and tested in accordance with Vol 1, Pt 6, Ch 6, 7 Inspection and testing procedures

6.11.10 The internal surfaces of the bilge keel and stiffening are to be suitably protected against corrosion.

6.12.1 Where it is proposed to adopt rivetted construction, full details of the rivets or similar fastenings, including mechanical test results, are to be indicated on the construction plans submitted for approval or a separate rivetting schedule is to be submitted.

6.12.2 Samples may be required of typical rivetted joints made by the Builder under representative construction conditions and tested to destruction in the presence of the Surveyor in shear, tension, compression or peel at LR’s discretion.

6.12.3 Where rivetting strength data sheets have been issued by a recognised Authority, the values quoted in these sheets will normally be accepted for design purposes.

6.12.4 Where two dissimilar metals are to be joined by rivetting, precautions are to be taken to eliminate electrolytic corrosion to LR’s satisfaction, and where practicable, the arrangements are to be such as to enable the joint to be kept under observation at each survey without undue removal of lining and other items.

6.12.5 Where a sealing compound is used to obtain an airtight or watertight joint, details are to be submitted of its proposed use and of any tests made or experience gained in its use for similar applications.

6.12.6 Sealing paints or compounds are not to be used with hot driven rivets.

6.13.1 Where adhesive bonding of any load-bearing structure is proposed, details of the materials and the processes to be used are to be submitted for approval. These details are to include test results of samples manufactured under LR survey under workshop conditions to verify the strength, ageing effects and moisture resistance of a typical joint.

6.13.2 The adhesive manufacturer’s recommendations in respect of the specified jointing system, comprising preparation of the surfaces to be adhered, the adhesive, bonding and curing processes, are to be strictly followed as variation of any step can severely affect the performance of the joint.

6.13.3 Meticulous preparation is essential where the joint is to be made by chemical bonding. The method of producing bonded joints is to be documented so that the process is repeatable after the procedure has been properly established.

6.13.4 Bonded joints are suitable for carrying shear loads, but are not, in general, to be used in tension or where the load causes peeling or other forces tending to open the joint. Loads are to be carried over as large an area as possible.

6.13.5 Bonded joints are to be suitably supported after assembly for the period necessary to allow the optimum bond strength of the adhesive to be developed. Air pockets are to be avoided.

6.13.6 The use of adhesive for main structural joints is not to be contemplated unless considerable testing has established its validity, including environmental testing and fatigue testing where considered necessary by LR.

6.14.1 Particular care is to be taken to avoid triaxial stresses which may result from poor joint design.

6.15.1 Provision is made in this Section for bi-metallic composite aluminium/steel transition joints used for connecting aluminium structures to steel plating. Such joints are to be used in accordance with the manufacturer’s requirements, see also Ch 8, 4 Aluminium/steel transition joints of the Rules for Materials.

6.15.2 Where a manufacturer is not approved, details of the materials to be used and the manufacturing procedures are to be submitted for approval before use.

6.15.3 Bimetallic joints where exposed to seawater or used internally within wet spaces are to be suitably protected to prevent galvanic corrosion.

6.15.4 Control of heat input is required when welding the transition joints to the steel structure in order to prevent disbondment.

6.16.1 To minimise corrosion of steel when in contact with wood in a damp or marine environment the timber is to be primed and painted in accordance with good practice. Alternatively the surface of the steel in contact with the timber is to be coated with a substantial thickness of a suitable sealant.