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.
