Bottom Construction : Double-bottom Construction

The minimum depth is determined by rule requirements for the size of vessel but the actual depth issometimes increased in places to suit double-bottom tank capacities. The structure is made up of vertical floors which may be watertight, solid or of bracket construction. The floor structure is continuous from the center girder to the side shell and supports the inner bottom shell. Side girders are fitted in the longitudinal direction, their number depending on the width of the ship. These side girders are broken either side of the floors and are therefore termed intercostal girders.

Watertight or oiltight floors are fitted beneath the main bulkheads and are also used to subdivide the double-bottom space into tanks for various liquids. Solid plate floors of non-watertight construction, usually lightened by manholes, are positioned in other places as required to stiffen the structure. Between solid plate floors, bracket floors are fitted. Bracket floors consist of plate brackets attached to the center girder and the side shell with bulb plate stiffeners running between. The stiffeners are supported by angle bar struts at intervals and any side girders which are present in the structure. The arrangement of flooring will be determined by the type of framing system adopted, which may be either transverse or longitudinal.

Transversely framed double bottom

When transversely framed, the double-bottom structure consists of solid plate floors and bracket floors with transverse frames. The bracket floor is fitted between the widely spaced solid floors. It consists of transverse bulb angle sections stiffening the shell and inner bottom plating. Vertical support is provided by brackets at the side shell and center girder, any side girders and intermediate struts. The number of intercostal side girders fitted is determined by classification society rules.

 

 

Longitudinally framed double bottom

This is the system favored as a result of tests and it provides adequate resistance to distortion on ships of 120 m in length or greater. Offset bulb plates are used as longitudinal stiffeners on the shell and inner bottom plating, at intervals of about 1 m. Solid floors provide support at transverse bulkheads and at intervals not exceeding 3.8 m along the length of the ship. Brackets are fitted at the center girder and side shell at intermediate frame spaces between solid floors. These brackets are flanged at the free edge and extend to the first longitudinal. Channel bar or angle bar struts are provided to give support at intervals of not more than 2.5 m where solid floors are widely spaced. Intercostal side girders are again fitted, their number depending upon classification society rules.

 

 

 

When the longitudinals (stiffeners, frames) run longitudinally effectively continuous through transverse bulkheads, they contribute the section modulus of the hull girder and thus assist in resisting the longitudinal bending of the ship's hull. They also greatly increase the critical compressive buckling strength of the plating to which they are attached. ABS Rules provide for reduction of the required thickness of deck and bottom plating when longitudinally framed. For oil tankers, bulk carriers , general cargo vessels, containerships, and Great Lakes ore carriers, longitudinal framing is generally adopted, at least for the strength deck plating and bottom shell. An exception is made at the ends of the vessel, where the advantages of longitudinal framing disappear and where transverse framing is simpler to build.

In cargo ships and refrigerated cargo vessels, they interfere with cargo stowage and with the joiner work in passenger spaces in passenger vessels. Also, in passenger ships, the longitudinals interfere with running engineering service systems, such as wiring, ventilation, and piping. These supply systems generally run fore-and-aft over the passageways, and longitudinal framing interferes with the transverse branches to the various rooms and spaces being serviced. A practical solution for modern cargo ships is, to frame the bottom shell, inner bottom and strength deck longitudinally utilizing transverse floors every third frame.