Section 7 Construction details and minimum thickness

7.1.1 The symbols used in this Section are defined in Pt 4, Ch 9, 10.1 Symbols.

7.2.1 The requirements of Pt 4, Ch 9, 10.2 Compartment minimum thickness are also applicable to small conventional single hull tankers.

7.3.1 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.

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

nor is it to be less than:

7.3.3 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.

7.4.1 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.

7.4.2 The members are to have adequate end fixity, lateral support and web stiffening, and the structure is to be arranged to minimize 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.

7.5.1 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' and `D'. The application of these requirements is detailed in Pt 4, Ch 10, 7.5 Primary member web plate stiffening, and the corresponding locations indicated in Figure 10.7.1 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.

Figure 10.7.1 Key to application requirements

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

7.5.3 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.

7.5.4 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:

   

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.

7.5.5 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:

   

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.

7.5.6 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:

   

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.

7.5.7 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.

7.6.1 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 10.7.1 Maximum web thickness for stiffener inertia and p is to be obtained from Table 10.7.2 Coefficient for stiffener inertia.

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

   On transverses, webs and stringers

   

   On longitudinal deck and bottom girders

   

Table 10.7.1 Maximum web thickness for stiffener inertia

Requirement	Web thickness t w, in mm
`A'
`B' and `C'
`D'
Note Intermediate values by interpolation

7.6.2  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.

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

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

Table 10.7.2 Coefficient for stiffener inertia

Aspect ratio of plate panel,	1,0 or more	0,9	0,8	0,7	0,6	0,5	0,4	0,3 or less
p	1,5	2,1	2,9	4,2	6,1	9,2	14,6	30,0
Note 1. Intermediate values by interpolation. Note 2. The depth of panel, d, used in calculating aspect ratio may be measured from the face of the secondary member to which the primary member web stiffener is attached.

7.7.1 The requirements as detailed in Pt 4, Ch 10, 7.5 Primary member web plate stiffening and Pt 4, Ch 10, 7.6 Inertia and dimensions of stiffeners are to be applied in the following locations, see also Figure 10.7.1 Key to application requirements.

1. For bottom transverses:

   In the centre tank requirement `A' stiffening is to extend for 20 per cent of the breadth of the tank from the longitudinal bulkhead, but at least beyond the toe of the end bracket. In the wing tank, requirement `A' stiffening is to extend at least as far as the toes of the end brackets from the longitudinal bulkhead and the shell. Elsewhere, requirement `B' stiffening is to be fitted.

2. For transverses at side shell and 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.

3. For deck transverses:

   Requirement `C' stiffening is to be fitted.

4. For stringers and horizontal girders:

   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.

5. For cross-ties:

   Cross-ties are to be suitably stiffened to prevent buckling and twisting. Requirement `D' stiffening is to be fitted.

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

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

7.7.2 The application of stiffening requirements to transverse wing structures in wing tanks where no cross-ties are fitted is to be based on the results of direct calculation and will be specially considered.

7.8.1 The webs of continuous longitudinal deck and bottom girders are to be stiffened parallel to the girder face plate.

7.8.2 The stiffeners 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.

7.8.3 The moment of inertia of stiffeners is to comply with Pt 4, Ch 10, 7.6 Inertia and dimensions of stiffeners.

7.9.1 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.

7.9.2 The moment of inertia of the stiffeners is to comply with Pt 4, Ch 10, 7.6 Inertia and dimensions of stiffeners.

7.10.1 Stiffened docking brackets are to be fitted on both sides of the bottom centreline girder, midway between transverses, and are to be connected to a suitable bottom shell longitudinal. The bracket on one side is to be connected to the face plate of the girder but the other may be stopped at a suitable horizontal stiffener.

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

7.11.1 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.

7.11.2 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.

7.11.3 Tripping brackets are to be connected to the face plate of the bottom transverses. Elsewhere, other than for docking girders, the bracket is to be connected to the face plate whenever the unsupported width of the latter exceeds 150 mm. Where the width of symmetrically placed face plates exceeds 400 mm, a small bracket is to be fitted opposite, and in line with, the tripping bracket. Equivalent support arrangements are to be provided for cross-tie face plates. Particular attention is to be paid to the support of continuous face plates in way of the radius at toes of brackets.

7.11.4 Wide face plates may require additional support between brackets.

7.11.5 In the fore peak tank, if the angle to the normal of the shell plating and the vertical webs exceeds 20°, double tripping brackets are to be fitted to the web at about midspan, but in no case greater than 3,0 m apart.

7.12.1 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 opening is to be located so that the edges are not less than 40 per cent of the web depth from the face, and are equidistant from the corners of notches for frames or stiffeners. Openings are to be suitably framed where required.

7.12.2 Lightening holes are not to be cut in horizontal girders on the ship's side and longitudinal bulkheads, in symmetrical webs nor in side transverses and vertical webs in way of cross-ties and their end connections.

7.12.3 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.

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

7.12.5 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 of higher tensile steel, they are to be elliptical or equivalent to minimize stress concentration.

7.12.6 Where holes are cut for heating coils, the lower edge of the hole is to be not less than 100 mm from the inside of the shell plating. 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.

7.13.1 The requirements of Pt 4, Ch 9, 10.13 Brackets connecting primary members are also applicable to small conventional single hull tankers.

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