Section 10 Construction details and minimum thickness

10.1.1 The symbols used in this Section are defined as follows:

For the primary member:

For the primary member web stiffener:

For the primary member end bracket, see Figure 9.10.2 Primary member end brackets:

Other symbols are defined in Pt 4, Ch 9, 1.5 General definitions and symbols.

10.2.1 Within the cargo tank region, including wing ballast tanks and cofferdams at the ends of or between cargo tanks, the thickness of primary member webs and face plates, hull envelope and bulkhead plating is to be not less than:

t = 2,15L ^0,3 mm, or

t = 7,5 mm

whichever is the greater.

10.2.2 The minimum thickness of secondary members is to be determined as above, but need not exceed 11,0 mm.

10.2.3 In pump-rooms the minima apply to shell, deck, longitudinal bulkhead and associated longitudinals. For other items solely within the pump-room, including transverse bulkheads separating the adjacent machinery spaces from the pump-room, the minima may be reduced by 1,0 mm, subject to a lower limit of 7,5 mm.

10.2.4 Within the fore peak tank, minimum thicknesses are to be in accordance with Pt 4, Ch 9, 10.2 Compartment minimum thickness 10.2.1 and Pt 4, Ch 9, 10.2 Compartment minimum thickness 10.2.2 reduced by 1,0 mm but are to be not less than 7,5 mm.

10.3.1 The depth of the web of any primary member is to be not less than 2,5 times the depth of the cut-outs for the passage of secondary members, except where compensation is arranged to provide satisfactory resistance to deflection and shear buckling in the web.

10.3.2 The area of material in the face plate of any primary member structure is not to exceed:

0,00667d _w t _w cm²

nor is it to be less than:

0,00417s _t d _w cm².

10.3.3 The geometric properties of rolled stiffeners and built sections are to be calculated in association with an effective width of attached plating in accordance with Pt 3, Ch 3, 3 Structural idealisation.

10.4.1 Primary members are to be so arranged as to ensure effective continuity of strength throughout the range of tank structure. Abrupt changes of depth or section are to be avoided. Where members abut on both sides of a bulkhead or on other members, arrangements are to be made to ensure that they are in alignment.

10.4.2 The members are to have adequate end fixity, lateral support and web stiffening, and the structure is to be arranged to minimise hard spots or other sources of stress concentration. Openings are to have well rounded corners and smooth edges and are to be located having regard to the stress distribution and buckling strength of the plate panel.

10.5.1 The webs of primary members are to be supported and stiffened in accordance with the following requirements, which are designated as requirements `A', `B', `C', `D' and `E'. The application of these requirements is detailed in Pt 4, Ch 9, 10.7 Application of stiffening requirements, and the corresponding locations indicated in Figure 9.10.1 Key to application requirements. Where webs are slotted for the passage of secondary members, the web stiffeners are to be arranged to provide adequate support for the loads transmitted, see Pt 3, Ch 10, 5.2 Arrangements at intersections of continuous secondary and primary members. Where direct calculations are carried out in accordance with Pt 4, Ch 9, 1.1 General 1.1.8 and Pt 4, Ch 9, 14 Direct calculations, other stiffening arrangements will be accepted subject to compliance with the maximum permissible stress and plate panel buckling criteria given in the ShipRight SDA Procedure, Guidance Notes on Direct Calculations: Primary Structure of Tankers.

10.5.2 Where higher tensile steel is used for the primary members, the maximum spacing of stiffeners given in this Section is to be multiplied by .

10.5.3 In addition to these stiffeners, tripping brackets as required by Pt 4, Ch 9, 10.11 Lateral stability of primary members are also to be fitted.

10.5.4 For requirement `A' stiffening:

1. The thickness, t _w of the web is to be not less than

2. Stiffening is generally to be fitted normal to the face plate of the member, but the stiffeners parallel to the face plate will be required when the web depth, d _w, exceeds a value, d _max which is to be taken as:

   for s ≤ 55t _w

   for s > 55t _w

3. Where stiffening parallel to the face plate is required, the distance from the face plate of the member to the nearest stiffener is not to exceed 65t _w. Further stiffeners are to be fitted at similar spacing so that the distance between the last stiffener and the shell or bulkhead plating does not exceed d _max. In way of end brackets to transverse bulkhead primary structure, stiffeners are to be fitted normal to the face plate of the member so that web plate panel dimensions parallel to the face plate do not exceed 80t _w.

10.5.5 For requirement `B' stiffening:

1. The thickness, t _w of the web is to be not less than

2. Stiffening is generally to be fitted normal to the face plate of the member, but stiffeners parallel to the face plate will be required when the web depth, d _w, exceeds a value d _max, which is to be taken as:

   for s ≤ 70t _w

   for s > 70t _w

3. Where stiffening parallel to the face plate is required, the distance from the face plate of the member to the nearest stiffener is not to exceed 80t _w. Further stiffeners are to be fitted at similar spacing so that the distance between the last stiffener and the shell or bulkhead plating does not exceed d _max.

10.5.6 For requirement `C' stiffening:

1. Stiffening is generally to be fitted normal to the face plate of the member in line with alternate secondary members, but stiffeners parallel to the face plate will be required, when the web depth, d _w exceeds a value, d _max which is to be taken as:

   for s ≤ 76t _w

   for s > 76t _w

2. Where stiffening parallel to the face plate is required, the distance from the face plate of the member to the nearest stiffener is not to exceed 90t _w. Further stiffeners are to be fitted at similar spacing so that the distance between the last stiffener and the deck plating does not exceed d _max.

10.5.7 For requirement `D' stiffening:

1. Stiffening parallel to the face plate will be required such that the distance between the stiffener and face plate, or between two stiffeners, does not exceed:

   80t _w where L ≤ 90 m

   55t _w where L ≥ 190 m

   with intermediate values by interpolation.

2. Brackets are to be fitted to support the face plates and stiffeners.

10.5.8 For requirement `E' stiffening:

1. Stiffening parallel to the face plate will be required such that the distance between the stiffener and face plate, or between two stiffeners, does not exceed:

   85t _w where L ≤ 90 m

   60t _w where L ≥ 190 m

   with intermediate values by interpolation.

2. Brackets are to be fitted to support the face plates and stiffeners.

10.6.1 The moment of inertia is to be not less than:

1. For stiffeners normal to the primary member face plate:

   

   Where t _w need not be greater than the values in Table 9.10.1 Maximum web thickness for stiffener inertia and p is to be obtained from Table 9.10.2 Coefficients for stiffener inertia.

2. For stiffeners parallel to the primary member face plate:

   On transverses, webs and stringers

   

   On longitudinal deck, side and double bottom girders, see also Pt 3, Ch 4, 7 Hull buckling strength

   

10.6.2 Where stiffeners are fitted in both directions, the inertia of the stiffeners parallel to the face plate of the member is to be not less than that of the stiffeners fitted normally.

10.6.3 The depth of web stiffeners is to be not less than 75 mm.

10.6.4 Where flat bar stiffeners are used, the ratio of depth to thickness is not to exceed 18 .

10.7.1 The requirements as detailed in Pt 4, Ch 9, 10.5 Primary member web plate stiffening and Pt 4, Ch 9, 10.6 Inertia and dimensions of stiffeners are to be applied in the following locations, see also Figure 9.10.1 Key to application requirements.

1. For transverses at longitudinal bulkhead:

   Requirement `A' stiffening is to extend at least as far as the lower surface of the lower cross-tie. Elsewhere, requirement `B' stiffening is to be fitted.

2. For deck transverses:

   Requirement `C' stiffening is to be fitted.

3. For stringers and horizontal girders on bulkheads:

   Requirement `A' stiffening is to extend for a distance from each end of 20 per cent of the span of the stringer or girder, but at least beyond the toes of the end brackets. Elsewhere, requirement `B' stiffening is to be fitted.

4. For cross-ties:

   Cross-ties are to be suitably stiffened to prevent buckling and twisting. Requirement `D' stiffening is to be fitted to the lower or to a single cross-tie. Requirement `E' stiffening is to be fitted to the upper cross-ties where two cross-ties are arranged.

5. For shell stringers and vertical webs in fore peak:

   Requirement `A' stiffening is to extend the full length of the member.

10.7.2 The application of stiffening requirements to transverse structures where no cross-ties are fitted and within double hull structures are to be based on the results of direct calculation and will be specially considered.

10.8.1 The webs of continuous longitudinal deck and double bottom girders are to be stiffened longitudinally. Particular attention is to be given to the stiffening of docking girders, see also the buckling requirements in Pt 3, Ch 4, 7 Hull buckling strength.

10.8.2 The stiffeners on deck girders are to be spaced not more than 55t _w mm apart except in way of vertical webs and end brackets, where the spacing is not to exceed 45t _w mm. Alternatively, a combination of parallel stiffeners at 55t _w mm spacing and normal stiffeners at 45t _w mm spacing may be adopted. Particular attention is to be given to the stiffening of the docking girder.

10.8.3 The application of stiffening requirements to girders within double hull structures is to be based on the results of direct calculation, see also Pt 4, Ch 9, 10.10 Double bottom girders in way of docking supports 10.10.1.

10.8.4 The moment of inertia of the stiffeners is to comply with Pt 4, Ch 9, 10.6 Inertia and dimensions of stiffeners.

10.9.1 Vertical webs are to be fitted with stiffeners parallel to the face plate of the web and spaced not more than 60t _w mm apart. Stiffeners normal to the face plate are to be fitted when a vertical web supports horizontal stiffeners on transverse bulkheads. The length of stiffener is to be sufficient to distribute the load transmitted, and the connection between web stiffener and bulkhead stiffener is to comply with the relevant requirements of Pt 3, Ch 10, 5.2 Arrangements at intersections of continuous secondary and primary members.

10.9.2 The moment of inertia of the stiffeners is to comply with Pt 4, Ch 9, 10.6 Inertia and dimensions of stiffeners.

10.10.1 Additional vertical stiffeners may be required on the bottom panels of the girder to resist docking pressures.

10.11.1  Tripping brackets are generally to be fitted close to the toes of end brackets, in way of cross-ties and elsewhere, so that the spacing between brackets does not exceed the lesser of 4,5 m or 15 times the width of the face plate (20 times in the case of deck transverses). Arrangements in way of the intersections of primary members are to be such as to prevent tripping. A closer spacing of brackets may be required to be adopted with asymmetrical face plates.

10.11.2 To maintain continuity of strength, substantial horizontal and vertical brackets are to be fitted to transverses or stringers at ends of cross-ties. Horizontal brackets are to be aligned with the cross-tie face plates, and vertical end brackets are to be aligned with the cross-tie web.

10.11.3 Wide face plates may require additional support between brackets.

10.11.4 In the fore peak tank, if the angle between the normal to the shell plating and the vertical webs exceeds 20°, tripping brackets are to be fitted at the toes of end brackets and elsewhere, such that their spacing does not exceed 3 m.

10.12.1 Where openings are cut in the webs of primary supporting members, the greatest dimension of the opening is not to exceed 20 per cent of the web depth. The openings are to be kept equidistant from the corners of notches for frames and stiffeners. In the case of webs supporting single skin structures the openings are to be located so that the edges are not less than 40 per cent of web depth from the face plate. Openings are to be suitably framed where required.

10.12.2 In way of cross-ties and their end connections lightening holes are not to be cut in horizontal girders on the ship's side and longitudinal bulkheads, in symmetrical webs nor in vertical webs on longitudinal bulkheads and wing ballast tanks.

10.12.3 Holes cut in primary longitudinal members within 0,1D of the deck and bottom are, in general, to be reinforced as required by Pt 4, Ch 9, 4.10 Deck openings. Access holes may be cut in deep transverses and girders with suitable compensation to provide satisfactory resistance to deflection and shear buckling in the web.

10.12.4 All holes are to have smooth edges and are to be kept well clear of notches and the toes of brackets.

10.12.5  Small air and drain holes cut in primary members are to be kept clear of the toes of brackets and are to be well rounded with smooth edges. Where holes are cut in primary longitudinal members in areas of high stress, or where primary members are of higher tensile steel, they are to be elliptical, or equivalent, to minimise stress concentration.

10.12.6 Where holes are cut for heating coils, the lower edge of the hole is to be not less than 100 mm from the inner bottom. Where large notches are cut in the transverses for the passage of longitudinal framing, adjacent to openings for heating coils, the longitudinal notches are to be collared. Examination of the buckling strength of the web plate panel between notches for longitudinals may be required.

10.13.1 The arm length of brackets connecting primary supporting members should, in general, be not less than the depth of the member web, nor exceed 1,5 times the web depth. The two arms should be of approximately equal lengths.

10.13.2 In a ring system where the end bracket is integral with the webs of the members, and the face plate is carried continuously along the edges of the members and the bracket, the full area of the largest face plate is to be maintained to the mid-point of the bracket and gradually tapered to the smaller face plates. Butts in face plates are to be kept well clear of the toes of brackets. Where a wide face plate abuts on a narrower one, the taper is generally not to exceed 1 in 4. Where a thick face plate abuts against a thinner one, if the difference in thickness exceeds 3 mm, the taper on thickness is not to exceed 1 in 3.

10.13.3 The thickness of separate end brackets is generally to be not less than that of the thicker of the primary member webs being connected, but may be required to be increased locally at the toes. The bracket is to extend to adjacent tripping brackets, stiffeners or other support points. Bracket toes are to be well radiused. Where the bracket is attached to a corrugated bulkhead, suitable arrangements are to be made to dissipate the load at the bracket toe. Details of the welding to be used is way of toes separate brackets are to be submitted, see also Pt 3, Ch 10, 5.1 Continuity and alignment 5.1.7.

10.13.4 Brackets are to be fitted with suitable face plates and stiffeners. The maximum distance from the face plate to the first parallel stiffener is to be 30t _b. Subsequent stiffeners lying parallel to the face may be spaced not more than 45t _b apart. The maximum arm length for an unstiffened triangular panel is 100t _b, see Figure 9.10.2 Primary member end brackets. The depth of stiffeners is to be not less than 75 mm, and their moment of inertia is to comply with 10.6.

10.13.5 The area of discontinuous face plates is generally to be about 80 per cent of the area of the face plates of the adjacent members. However, where the stiffener adjacent to the face plate is of increased size, consideration will be given to the face area required. In addition, the following expression is to be satisfied:

10.13.6 The ends of discontinuous face plates are to be well tapered. The taper may be 1 in 3, but where the width of the face plate exceeds 500 mm, a taper not less than 1 in 4 is generally to be adopted. Stiffeners adjacent to the face plate should be tapered 1 in 2, and other stiffeners may be cut at 45°.

10.13.7 Face plates and web stiffeners are to be suitably supported against tripping, see Figure 9.10.2 Primary member end brackets.

10.13.8 In the case of very large brackets with heavy face plates, it is recommended that the effective span, l _b, be reduced by extending the primary member main supporting brackets to provide lateral stability to the face plate, see Figure 9.10.2 Primary member end brackets.

10.14.1 For details and connections of collars, see Pt 3, Ch 10, 5.2 Arrangements at intersections of continuous secondary and primary members.