Section 4 Construction details for primary members

4.1.1 The requirements for section modulus and inertia (if applicable) of primary members are given in the appropriate Chapter. This Section includes the requirements for proportions, stiffening and construction details for primary members in dry spaces and in tanks of all ship types other than tankers.

4.1.2 The requirements for construction details for the primary structure of tankers are given in Pt 4, Ch 9, 10 Construction details and minimum thickness.

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

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

4.3.1 Primary members are to be so arranged as to ensure effective continuity of strength, and 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. Primary members in tanks are to form a continuous line of support and wherever possible, a complete ring system.

4.3.2 The members are to have adequate lateral stability and web stiffening and the structure is to be arranged to minimise hard spots and 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 panel.

4.3.3 Primary members are to be provided with adequate end fixity by end brackets or equivalent structure. 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 and effective distribution of the load from the member.

4.3.4 Where the primary member is supported by structure which provides only a low degree of restraint against rotation, the member is generally to be extended for at least two frame spaces, or equivalent, beyond the point of support before being tapered.

4.3.5 Where primary members are subject to concentrated loads, particularly if these are out of line with the member web, additional strengthening may be required.

4.4.1 The geometric properties of the members are to be calculated in association with an effective width of attached plating determined in accordance with Pt 3, Ch 3, 3.2 Geometric properties of section.

4.4.2 The minimum thickness or area of material in each component part of the primary member is given in Table 10.4.1 Minimum thickness of primary members.

4.4.3 Primary members constructed of higher tensile steel are to comply with Table 10.4.1 Minimum thickness of primary members.

4.5.1 Primary members are to be supported by tripping brackets. The tripping brackets supporting asymmetrical sections are to be spaced no more than two secondary frames apart. The tripping brackets supporting symmetrical sections are to be spaced no more than four secondary frames apart.

4.5.2 Tripping brackets are also to be fitted at the toes of end brackets and in way of heavy or concentrated loads such as the heels of pillars.

4.5.3 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.

4.5.4 Intermediate secondary members may be welded directly to the web or connected by lugs.

4.5.5 Where the depth of web of a longitudinal girder at the strength deck within 0,4L amidships exceeds:

1. 55t _w for mild steel members

2. for higher tensile steel members

additional longitudinal web stiffeners are to be fitted at a spacing not exceeding the value given in (a) or (b) as appropriate, with a maximum of for higher tensile steel members. In cases where this spacing is exceeded, the web thickness is, in general, to be suitably increased.

4.5.6 The arm length of unstiffened end brackets is not to exceed 100t _w. Stiffeners parallel to the bracket face plate are to be fitted where necessary to ensure that this limit is not exceeded.

4.5.7 Web stiffeners may be flat bars of thickness t _w and depth 0,1d _w, or 50 mm, whichever is the greater. Alternative sections of equivalent moment of inertia may be adopted.

4.6.1 Where openings are cut in the web, the depth of opening is not to exceed 25 per cent of the web depth, and the opening is to be so located that the edges are not less than 40 per cent of the web depth from the face plate. The length of opening is not to exceed the web depth or 60 per cent of the secondary member spacing, whichever is the greater, and the ends of the openings are to be equidistant from the corners of cut-outs for secondary members. Where larger openings are proposed, the arrangements and compensation required will be considered.

4.6.2 Openings are to have smooth edges and well rounded corners.

4.6.3 Cut-outs for the passage of secondary members are to be designed to minimise the creation of stress concentrations. The breadth of cut-out is to be kept as small as practicable and the top edge is to be rounded, or the corner radii made as large as practicable. The extent of direct connection of the web plating, or the scantlings of lugs or collars, is to be sufficient for the load to be transmitted from the secondary member.

4.7.1 End connections of primary members are generally to comply with the requirements of Pt 3, Ch 10, 3 Secondary member end connections, taking Z as the section modulus of the primary member.

4.7.2 The thickness of the bracket is to be not less than that of the primary member web. The free edge of the bracket is to be stiffened.

4.7.3 Where a deck girder or transverse is connected to a vertical member on the shell or bulkhead, the scantlings of the latter may be required to be increased to provide adequate stiffness to resist rotation of the joint.

4.7.4 Where a member is continued over a point of support, such as a pillar or pillar bulkhead stiffener, the design of the end connection is to be such as to ensure the effective distribution of the load into the support. Proposals to fit brackets of reduced scantlings, or alternative arrangements, will be considered.

4.7.5 Connections between primary members forming a 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.