Section 5 Structural details

5.1.1 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.

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

5.1.3 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.

5.1.4 Continuity is to be maintained where primary members intersect and where the members are of the same depth, a suitable gusset plate is to be fitted.

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

5.1.6 The toes of brackets, etc. should not land on unstiffened panels of plating. Special care should be taken to avoid notch effects at the toes of brackets, by making the toe concave or otherwise tapering it off.

5.1.7 Where primary and/or secondary members are constructed of higher tensile steel, particular attention 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 radiused bracket toe and are to incorporate a taper not exceeding 1 in 3. 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 10.5.1 Bracket toe construction. See also Pt 4, Ch 1, 4.3 Deck stiffening, Pt 4, Ch 1, 6.1 General, Pt 4, Ch 9, 5.7 Connections of longitudinals and Pt 4, Ch 9, 10.13 Brackets connecting primary members.

Figure 10.5.1 Bracket toe construction

5.2.1 Cut-outs for the passage of secondary members through the web 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, e.g. in way of cross tie ends and floors under bulkhead stools in ore and ballast holds.

5.2.2 Cut-outs are to have smooth edges, and the corner radii are to be as large as practicable, with a minimum of 20 per cent of the breadth of the cut-out or 25 mm, whichever is the greater. It is recommended that 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 10.5.3 Cut-outs and connections, but consideration will be given to other shapes on the basis of maintaining equivalent strength and minimizing stress concentration. 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.

5.2.3 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.

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

5.2.5 The cross-sectional areas of the connections are to be determined from the proportion of load transmitted through each component in association with its appropriate permissible stress.

5.2.6 The total load P, transmitted to the primary member is to be derived in accordance with Table 10.5.1 Total load transmitted to connection of secondary members.

5.2.7 This load is to be apportioned between the connections as follows:

1. Transmitted through the collar arrangement:

   

   where A₁ is derived in accordance with Pt 3, Ch 10, 5.2 Arrangements at intersections of continuous secondary and primary members 5.2.8 and is not to be taken as greater than 0,25

   The collar load factor, C _f, is to be derived as follows:

   Symmetrical secondary members

   C f = 1,85	for A f ≤ 18
   C f = 1,85 - 0,0341 (A f -18)	for 18 < A f ≤ 40
   C f = 1,1 - 0,01 (A f - 40)	for A f > 40

   Asymmetrical secondary members

   Cf

   =

   0,68 + 0,0224

   where

   A f	=	the area, in cm2, of the primary member web stiffener in way of the connection including backing bracket, where fitted (see Pt 3, Ch 10, 5.2 Arrangements at intersections of continuous secondary and primary members 5.2.10)
   b l	=	the length of lug or direct connection, in mm, as shown in Figure 10.5.3 Cut-outs and connections. Where the lug or direct connections differ in length a mean value of b l is to be used.

2. Transmitted through the primary member web stiffener:

   

3. Where the web stiffener is not connected to the secondary member, P ₁ is to be taken equal to P.

Table 10.5.1 Total load transmitted to connection of secondary members

Ship type	Head, h 1, in metres		Total load, P, transmitted to connection
(1) Oil tankers, bulk chemical tankers and combination carriers, see Pt 3, Ch 10, 1.1 Application 1.1.3	h 1 = load height, in metres, derived in accordance with the following provisions, but to be taken as not less than or (0,01L 1 + 0,7) m whichever is the greater
	For shell framing members:
	(a) With mid-point of span at base line, h 1 = 0,8D 2
	(b) With mid-point of span at a distance 0,6D 2 above base line, h 1 = f D 2 B f
	(c) With mid-point of span intermediate between (a) and (b). The value of h 1 is to be obtained by linear interpolation between values from (a) and (b).		(a) In general P = 10,06 (S w - s 1/2) s 1 h 1 kN
	(d) With mid-point of span higher than 0,6D 2 above base line.		(b) For wash bulkheads
	The value of h 1 is to be obtained by linear interpolation between the values from (b) and the values at the following points:		P = 11,77 (S w - s 1/2) s 1 h 1 kN
	(i) For framing members located at and abaft 0,2L from the forward perpendicular (see Figure 10.5.2 Load height diagrams for framing members (a) at and abaft 0,2L from the forward perpendicular and (b) forward of cargo tank region for oil tankers, bulk chemical tankers and combination carriers (see Pt 3, Ch 10, 1.1 Application 1.1.3) and forward of 0,15L from the forward perpendicular for other ship types)	Zero value at the level of the deck edge amidships
	(ii) For framing members forward of cargo tank region (see Fig. 10.5.2(b))	Value of f D 2 (B f - 1) at the level 3 m above the minimum bow height determined from the Load Lines Convention
	(iii) Intermediate values between locations (i) and (ii) are to be determined by linear interpolation
	For secondary stiffening members of transverse and longitudinal bulkheads, and inner hull an inner bottom of double hull tankers, see Pt 3, Ch 10, 1.1 Application 1.1.3:
	h 1 = distance from the mid-point of span to top of tank but need not exceed 0,8D 2
	Side and bottom shell longitudinals
(2) Other ship types for which oil tanker (see Pt 3, Ch 10, 1.1 Application 1.1.3) requirements are not applicable	As for (1) except as follows: (a) h 1 to be derived in accordance with (1) above but to be taken as not less than m for type `B-60' and the greater of , or 1,20 m for Type 'B' ships		P = 10,06 (S w - s 1/2) s 1 h 1kN
	(b) h 1 for item (1)(d)(ii) above to extend forward of 0,15L from the forward perpendicular
(3) Other ship types for which oil tanker (see Pt 3, Ch 10, 1.1 Application 1.1.3) requirements are not applicable (continued)	Internal tank boundaries (a) Topside tank longitudinals h 1 = distance from the longitudinal under consideration to the highest point of the tank with the ship inclined 30° either way, or = the greater of the distance from the longitudinal under consideration to the top of the tank, or half the distance to the top of the overflow, or = 1,5 m = whichever is the greatest		P = 10,06 (S w - s 1/2) s 1 h 1kN
	(b) Inner bottom and hopper longitudinals		P = 9,81 (S w - s 1/2) s 1 h 1/C kN
			= but not to be taken less than the load derived from (b)(iv), (b)(v), (b)(vi) or (c) where applicable
	(i) For cargo ships and bulk carriers (see Pt 3, Ch 10, 1.1 Application 1.1.3) without the notation ‘strengthened for heavy cargoes’ h 1 = 1,39T
	(ii) For cargo ships and bulk carriers (see Pt 3, Ch 10, 1.1 Application 1.1.3) with the notation ‘strengthened for heavy cargoes’ h 1 = H
	(iii) For longitudinal bulkheads of ore carriers h 1 = H K c
	(iv) For bulk carriers (see Pt 3, Ch 10, 1.1 Application 1.1.3) where the topside wing tank is interconnected with hopper side and double bottom tanks h 1 = the distance from the longitudinal under consideration to the top of the topside tank with the ship inclined 25° either way		P = 10,06 (S w - s 1/2) s 1 h 1kN
	(v) For bulk carriers (see Pt 3, Ch 10, 1.1 Application 1.1.3) in way of ballast hold h 1 = the distance from the longitudinal under consideration to the top of the hatchway coaming		P = 10,06 (S w - s 1/2) s 1 h 1kN
	(vi) For cargo ships and bulk carriers (see Pt 3, Ch 10, 1.1 Application 1.1.3) with double hull where tank at side interconnected with double bottom h 1 = H		P = 10,06 (S w - s 1/2) s 1 h 1kN
	(c) Longitudinals of inner hull of double hull cargo ships and bulk carriers (see Pt 3, Ch 10, 1.1 Application 1.1.3)		P = 10,06 (S w - s 1/2) s 1 h 1kN
	h 1 = the distance from the longitudinal under consideration to the top of the tank, or half the distance to the top of the overflow, whichever is the greater
B f = bow fullness factor determined from Figure 5.4.3 Illustration of bow fullness factor determination in Chapter 5 to be considered. To be taken as 1 for framing members located at and abaft 0,2L from the forward perpendicular
f = load height factor at level 0,6D above base line, see Table 10.5.2 Load height factor, f
h 1 = load height, in metres, see also Figure 10.5.2 Load height diagrams for framing members (a) at and abaft 0,2L from the forward perpendicular and (b) forward of cargo tank region for oil tankers, bulk chemical tankers and combination carriers (see Pt 3, Ch 10, 1.1 Application 1.1.3) and forward of 0,15L from the forward perpendicular for other ship types
C = stowage rate, in m3 /tonne, as defined in Pt 3, Ch 3, 5.2 Symbols. For cargo ships without the notation ‘strengthened for heavy cargoes’, the value to be used is 1,39 m3/tonne. For cargo ships and bulk carriers (see Pt 3, Ch 10, 1.1 Application 1.1.3) with the notation ‘strengthened for heavy cargoes’, the actual stowage rate is to be used, but the value is not to be taken greater than 0,865 m3/tonne
H = height from inner bottom at position under consideration, to deck at side amidships, in metres, for inner bottom longitudinals = height from the longitudinal under consideration to the underside of the topside tank sloped bulkhead, in metres, for hopper longitudinals
S w = spacing of primary members, in metres
s 1 = spacing of secondary members, in metres
T = the summer draught, in metres, measured from top of keel
D 2 = D in metres, but need not be taken greater than 1,6T
L 1 = L but need not be taken as greater than 190 m
K c = as defined in Table 11.7.1 Longitudinal and transverse bulkhead scantlings for ore loading in Pt 4, Ch 11 Ore Carriers

Figure 10.5.2 Load height diagrams for framing members (a) at and abaft 0,2L from the forward perpendicular and (b) forward of cargo tank region for oil tankers, bulk chemical tankers and combination carriers (see Pt 3, Ch 10, 1.1 Application 1.1.3) and forward of 0,15L from the forward perpendicular for other ship types

5.2.8 The effective cross-sectional area A ₁ of the collar arrangements is to be taken as the sum of cross-sectional areas of the components of the connection as follows:

1. Direct connection:

   

2. Lug connection:

   
   where

   f 1	=	1,0 for symmetrical secondary member connections
   	=	but not greater than 1,0, for asymmetrical secondary member connections
   t w	=	thickness of primary member web, in mm
   t 2	=	thickness, in mm, of lug connection, and is to be taken not greater than the thickness of the adjacent primary member web plate
   W	=	overall width of the cut-out, in mm
   W 2	=	width for cut-out asymmetrical to secondary member web, in mm

   see Figure 10.5.3 Cut-outs and connections

5.2.9 The values of A _f and A ₁ are to be such that the stresses given in Table 10.5.3 Permissible stresses are not exceeded.

5.2.10 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.

5.2.11 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, see also Pt 3, Ch 10, 2.9 Inspection of welds.

Figure 10.5.3 Cut-outs and connections

5.2.12 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 Pt 3, Ch 10, 5.2 Arrangements at intersections of continuous secondary and primary members 5.2.11.

5.2.13 For ship types for which oil tanker (see Pt 3, Ch 10, 1.1 Application 1.1.3) requirements are not applicable, the collar arrangement is to satisfy the requirements of Pt 3, Ch 10, 5.2 Arrangements at intersections of continuous secondary and primary members 5.2.1 to Pt 3, Ch 10, 5.2 Arrangements at intersections of continuous secondary and primary members 5.2.12 inclusive. In addition the weld area of the connections is to be not less than the following:

1. Connection of primary member stiffener to the secondary member:

   A w

   =

   0,25 A f or 6,5 cm2, whichever is the greater, corresponding to a weld factor of 0,34 for the throat thickness

2. Connection of secondary member to the web of the primary member:

   A w

   =

   corresponding to a weld factor of 0,34 in tanks or 0,27 in dry spaces for the throat thickness

   where

   A w	=	weld area, in cm2, and is calculated as total length of weld, in cm, multiplied by throat thickness, in cm
   A f	=	cross-sectional area of the primary member web stiffener, in cm2, in way of connection
   Z	=	the section modulus, in cm3, of the secondary member.

5.2.14 Where the stiffeners of the double bottom floors, and the hopper primary members are unconnected to the secondary members and offset from them (see Figure 10.5.4 Arrangement with offset stiffener) the collar arrangement is to satisfy the requirements of Pt 3, Ch 10, 5.2 Arrangements at intersections of continuous secondary and primary members 5.2.1 to Pt 3, Ch 10, 5.2 Arrangements at intersections of continuous secondary and primary members 5.2.13 inclusive. 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 10.5.4 Arrangement with offset stiffener

5.2.15 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.

5.3.1 Manholes, lightening holes and other cut-outs are to be avoided in way of concentrated loads and areas of high shear. In particular, manholes and similar openings are not to be cut in vertical or horizontal diaphragm plates in narrow cofferdams or double plate bulkheads within one-third of their length from either end, nor in floors or double bottom girders close to their span ends, or below the heels of pillars, unless the stresses in the plating and the panel buckling characteristics have been calculated and found satisfactory.

5.3.2 Manholes, lightening holes and other openings are to be suitably framed and stiffened where necessary.

5.3.3 Air and drain holes, notches and scallops are to be kept at least 200 mm clear of the toes of end brackets and other areas of high stress. Openings are to be well rounded with smooth edges. Details of scalloped construction are shown in Figure 10.2.1 Weld dimensions and types. Closely spaced scallops are not permitted in higher tensile steel members. Widely spaced air or drain holes may be accepted, provided that they are of elliptical shape, or equivalent, to minimise stress concentration and are, in general, cut clear of the weld connection.

5.4.1 Where an angled gunwale is fitted, the top edge of the sheerstrake is to be kept free of all notches and isolated welded fittings. Bulwarks are not to be welded to the top of the sheerstrake within the 0,5L amidships.

5.4.2 Where a rounded gunwale is adopted, the welding of fairlead stools and other fittings to this plate is to be kept to the minimum, and the design of the fittings is to be such as to minimise stress concentration.

5.4.3 Arrangements are to ensure a smooth transition from rounded gunwale to angled gunwale towards the ends of the ship.

5.4.4 At the ends of superstructures where the side plating is extended and tapered to align with the bulwark plating, the transition plating is to be suitably stiffened and supported. Where freeing ports or other openings are essential in this plate, they are to be suitably framed and kept well clear of the free edge.

5.5.1 The quality of welding and general workmanship of fittings and attachments as given in Pt 3, Ch 10, 5.6 Bilge keels and ground bars and Pt 3, Ch 10, 5.7 Other fittings and attachments are to be equivalent to that of the main hull structure. Visual examination of all welds is to be supplemented by non-destructive testing as considered necessary by the Surveyor.

5.6.1 It is recommended that bilge keels should not be fitted in the forward 0,3L region on ships intended to navigate in severe ice conditions.

5.6.2 Bilge keels are to be attached to a continuous ground bar as shown in Figure 10.5.5 Bilge keel construction. Butt welds in shell plating, ground bar and bilge keels are to be staggered.

5.6.3 The minimum thickness of the ground bar is to be equal to the thickness of the bilge strake or 14 mm, whichever is the lesser.

5.6.4 The material class, grade and quality of the ground bar are to be in accordance with Table 2.2.1 Material classes and grades.

5.6.5 The ground bar is to be connected to the shell with a continuous fillet weld and the bilge keel to the ground bar with a light continuous fillet weld.

5.6.6 Direct connection between ground bar butt welds and shell plating, and between bilge keel butt welds and ground bar is to be avoided.

5.6.7 The design of single web bilge keels is to ensure that failure to the web occurs before failure of the ground bar. In general, this may be achieved by ensuring the web thickness of bilge keels does not exceed that of the ground bar.

Figure 10.5.5 Bilge keel construction

5.6.8 The end details of bilge keels and intermittent bilge keels, where adopted, are to be as shown in Figure 10.5.6 Bilge keel end design.

5.6.9 The ground bar and bilge keel ends are to be tapered or rounded. Where the ends are tapered, the tapers are to be gradual with ratios of at least 3:1, see Figure 10.5.6 Bilge keel end design. Where the ends are rounded, details are to be as shown in Figure 10.5.6 Bilge keel end design. Cut-outs on the bilge keel web within zone 'A' (see Figure 10.5.6 Bilge keel end design) are not permitted.

5.6.10 The end of the bilge keel web is to be between 50 mm and 100 mm from the end of the ground bar, see Figure 10.5.6 Bilge keel end design.

5.6.11 An internal transverse support is to be positioned as close as possible to halfway between the end of the bilge keel web and the end of the ground bar, see Figure 10.5.6 Bilge keel end design.

5.6.12 Where an internal longitudinal stiffener is fitted in line with the bilge keel web, the longitudinal stiffener is to extend to at least the nearest transverse member outside zone 'A', see Figure 10.5.6 Bilge keel end design. In this case, the requirement of Pt 3, Ch 10, 5.6 Bilge keels and ground bars 5.6.10 does not apply.

5.6.13 For ships over 65 m in length, holes are to be drilled in the bilge keel butt welds. The size and position of these holes are to be as illustrated in Figure 10.5.5 Bilge keel construction. Where the butt weld has been subject to non-destructive examination the stop hole may be omitted.

5.6.14 Bilge keels of a different design from that shown in Figure 10.5.5 Bilge keel construction and Figure 10.5.6 Bilge keel end design will be specially considered.

5.6.15 Within zone 'B', (see Figure 10.5.6 Bilge keel end design), welds at the end of the ground bar and bilge plating, and at the end of the bilge keel web and ground bar, are to have weld factors of 0,44 and 0,34 respectively. These welds are to be ground and to blend smoothly with the base materials.

Figure 10.5.6 Bilge keel end design

5.6.16 A plan of the bilge keels is to be submitted for approval of material grades, welded connections and detail design.

5.7.1 Gutterway bars at the upper deck are to be so arranged that the effect of main hull stresses on them is minimised.

5.7.2 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.

5.7.3 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. Where they are subsequently removed, this is to be done by flame or mechanical cutting close to the plate surface, and the remaining material and welding ground off. After removal the area is to be carefully examined to ensure freedom from cracks or other defects in the plate surface.