66.Double bottom tank construction

TRANSVERSELY FRAMED DOUBLE BOTTOM

If the double bottom is transversely framed, then transverse solid plate floors, and bracket floors with transverse frames,provide the principal support for the inner bottom and bottom shell plating (Figure 16.4). Solid plate floors are fitted at every frame space in the engine room and in the pounding region (see the end of this chapter). Also they are introduced in way of boiler seats, transverse bulkheads, toes of brackets supporting stiffeners on deep tank bulkheads, and in way of any change in depth of the double bottom. Where a ship is regularly discharged by grabs, solid plate floors are also fitted at each frame. 

Elsewhere the solid plate floors may be spaced up to 3.0 m apart, with bracket floors at frame spaces between the solid floors. The plate brackets of bracket floors are flanged and their breadth is at least 75 percent of the depth of the centre girder at the bracket floors. To reduce the span of the frames, which should not exceed 2.5 meters, at the bracket floor, vertical angle or channel bar struts may be fitted. Vertical stiffeners usually in the form of welded flats will be attached to the solid plate floors, which are further strengthened if they form a watertight or oiltight tank boundary. 

One intercostal side girder is provided port and starboard where the ship’s breadth exceeds 10 m but does not exceed 20 m and two are fitted port and starboard where the ship’s breadth is greater. In way of the bracket floors a vertical welded flat stiffener is attached to the side girder. Additional side girders are provided in the engine room, and also in the pounding region.

LONGITUDINALLY FRAMED DOUBLE BOTTOM 

In a longitudinally framed double bottom, solid plate floors are fitted at every frame space under the main engines, and at alternate frames outboard of the engine seating. They are also fitted under boiler seats, transverse bulkheads, and the toes of stiffener brackets on deep tank bulkheads. Elsewhere the spacing of solid plate floors does not exceed 3.8m, except in the pounding region where they are on alternate frame spaces. At intermediate frame spaces brackets are fitted at the tank side, and at the centre girder where they may be up to 1.25 m apart. Each bracket is flanged and will extend to the first longitudinal (Figure 16.5).

One intercostal side girder is fitted port and starboard if the ship’s breadth exceeds 14 m, and where the breadth exceeds 21 m two are fitted port and starboard. These side girders always extend as far forward and aft as possible. Additional side girders are provided in the engine room, and under the main machinery, and they should run the full length of the engine room, extending three frame spaces beyond this space. Forward the extension tapers into the longitudinal framing system. In the pounding region there will also be additional intercostal side girders. As the unsupported span of the bottom longitudinals should not exceed 2.5m, vertical angle or channel bar struts may be provided to support the longitudinals between widely spaced solid floors