Section 6 Bottom structure

6.1.1 This Section provides for longitudinal or transverse framing of the bottom structure of ships with single or double bottoms.

6.1.2 For ships intended to operate aground, see Pt 4, Ch 12, 7 Bottom strengthening for operating aground.

6.2.1 The scantlings of single bottom floors, extending for the full width of the ship, are to be in accordance with Pt 4, Ch 1, 7 Single bottom structure irrespective of the length of the ship. Floors below dredging pumps or similar items which could induce large concentrated loads or large dynamic forces, may be required to be of increased strength. Floors may be recessed locally in way of dredging pumps, etc. provided that suitable compensation is arranged.

6.2.2 The spacing of intercostals and longitudinal side girders is to be such as to ensure continuity of strength at bulkheads, ends of spoil spaces and wells and at ends of machinery seats so far as practicable, see also Pt 4, Ch 1, 7 Single bottom structure. An intercostal is to be fitted in the buoyancy space abreast hopper openings when the distance between the hopper opening and the ship's side exceeds 4,0 m.

6.2.3 Abreast of dredging wells and spoil spaces the minimum depth of floor at its inboard end is to be not less than:

The thickness of the web and area of the face plate are to be as required by Pt 4, Ch 1, 7.2 Girders and floors.

6.3.1 The spacing of transverses is to be in accordance with Pt 4, Ch 12, 5.3 Primary supporting structure at sides 5.3.1, and are to be supplemented by the following arrangements of brackets:

1. On the ship's centreline, or on each side of dredging wells where there is no structure on the centreline, the brackets are to be spaced not more than 1,25 m apart and are to extend outboard to the first longitudinal, port and starboard. The longitudinals supported by the brackets may be calculated using a nominal transverse spacing of 1,6 m.

2. On ships where the side are transversely framed, the brackets are to be fitted at every frame and are to extend inboard to the first longitudinal on the flat of bottom. This longitudinal is to be based on a span equal to the spacing of the transverses.

3. The thickness of these intermediate brackets is to be not less than:

   

6.3.2 In areas of high shear loading, the thickness and stiffening of the web plates on transverses, etc. may have to be increased. The depth of transverses is to be not less than 2,5 times the depth of the slot for the bottom longitudinals, and thickness of the web plates is to be not less than 8 mm.

6.3.3 Bottom transverses in spoil space side buoyancy tanks in way of cross-ties are to have a depth, d, of not less than:

Their arrangement, scantlings and end connections are to be such as to provide proper continuity of strength across the ship. The transverses are to be fitted with stiffeners in way of every shell longitudinal. The stiffeners should, in general, be equivalent to flat bars with a depth one-eighth of the transverse at that point and a thickness not less than the thickness of the transverse.

6.4.1 Self-propelled dredgers and reclamation ships of more than 500 tons gross and intended for International voyages are to be provided with a double bottom extending from the collision bulkhead to the after peak bulkhead, as far as this is practicable and compatible with the design and proper working of the ship.

6.4.2 A double bottom need not be fitted in way of watertight compartments used exclusively for the carriage of liquids provided the safety of the ship, in the event of bottom damage, is not thereby impaired.

6.4.3 The double bottom may, however, be interrupted locally, or fitted with wells in way of dredging pumps and other equipment. Where such openings are large, their scantlings and arrangements will be specially considered.

6.4.4 The scantlings are to be in accordance with Pt 4, Ch 1, 8 Double bottom structure except for the following:

1. The Rule thickness of centre girders may be reduced by 2,0 mm on ships classed A1 protected waters service.

2. The Rule thickness of side girders may be reduced by 1,0 mm on ships classed 100A1 extended protected waters service.

3. The scantlings of floors, longitudinals and plating supporting the bottom of spaces intended to carry spoil are to be determined in accordance with Pt 4, Ch 12, 8 Spoil space and well structure.

6.5.1 Plate floors may be fitted at every frame or may be spaced not more than 3,0 m apart with the shell and inner bottom plating between these floors supported by bracket floors. However, plate floors are to be fitted at every frame in the following areas:

1. As required for Pt 4, Ch 1, 8.5 Floors.

2. Below spaces from which dredged material will be discharged by grabs.

3. In main propulsion and dredging machinery rooms and in peak tanks.

4. For three frame spaces at ends of spoil spaces and dredging wells.

6.6.1 In locations other than below spaces intended for dredged spoil the section modulus of inner bottom longitudinals is to be not less than:

6.6.2 The section modulus of longitudinals below spaces intended for dredged spoil is to comply with the requirements of Pt 4, Ch 12, 8.3 Spoil space and well boundaries 8.3.7.

6.6.3 The spacing of transverses is generally to be as for dry cargo ships but is not to exceed 4,0 m. Below main dredging machinery the transverses are generally to be spaced not more than 1,0 m apart.

6.6.4 The ends of longitudinal girders under dredging machinery are to be tapered off or efficiently scarfed into other longitudinal structural items.