Section
8 Double bottom structure
8.1 Symbols and definitions
8.1.1 The
symbols used in this Section are defined as follows:
|
d
DB
|
= |
Rule depth of centre girder, in mm |
|
d
DBA
|
= |
actual depth of centre girder, in mm |
|
h
DB
|
= |
head from top of inner bottom to top of over-flow pipe, in metres |
|
s
|
= |
spacing
of stiffeners, in mm |
|
H
DB
|
= |
height from tank top, at position under consideration, to deck
at side amidships, in metres |
|
Z
BF
|
= |
section modulus of bottom frame at bracket floor, in cm3.
|
8.2 General
8.2.1 Except as specified in Pt 4, Ch 1, 8.2 General 8.2.5, cargo ships other than tankers are to be fitted 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.
8.2.2 Where a double bottom is required to be fitted, its depth at the
centreline, d
DB, is to be in accordance with Pt 4, Ch 1, 8.3 Girders 8.3.1 and the inner bottom is to be continued out to the
ship's side in such a manner as to protect the bottom to the turn of the bilge.
8.2.4 Other well arrangements (e.g. for lubricating oil under main engines) can be considered
provided they give protection equivalent to that afforded by the double bottom. For a
cargo ship of 80 m in length and upwards or for a passenger ship, proof of equivalent
protection is to be shown by demonstrating that the ship is capable of withstanding
bottom damages as specified in paragraph 8 of SOLAS Chapter II-1 Regulation 9 - Double bottoms in passenger ships and cargo ships other than tankers. Alternatively, wells for
lubricating oil below main engines can protrude into the double bottom below the
boundary line defined by the distance B/20 provided that the vertical distance
between the well bottom and a plane coinciding with the keel line is not less than
B/40 or 500 mm, whichever is greater. For cargo ships of less than 80 m in
length, the arrangements shall provide a level of safety satisfactory to the Flag
Administration.
8.2.5 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. In addition, a double bottom need not be fitted
on the following ships:
-
Cargo ships of less than 500 tons gross tonnage.
-
Ships not propelled by mechanical means.
-
Trawlers and fishing vessels.
8.2.6 This Section provides for longitudinal or transverse framing in the double
bottom, but for ships exceeding 120 m in length and for ships strengthened for heavy
cargoes, longitudinal framing is, in general, to be adopted. For the additional
requirements for ships specially strengthened for heavy cargoes, see
Pt 4, Ch 7, 1.3 General class notations.
8.2.8 Girders and the side walls of duct keels are to be continuous, and the
structure in way is to be sufficient to withstand the forces imposed by dry-docking the
ship.
8.2.9 Adequate access is to be provided to all parts of the double bottom. The
edges of all holes are to be smooth. The size of opening should not, in general, exceed
50 per cent of the double bottom depth, unless edge reinforcement is provided. In way of
ends of floors and fore and aft girders at transverse bulkheads, the number and size of
holes are to be kept to a minimum, and the openings are to be circular or elliptical.
Edge stiffening may be required in these positions.
8.2.10 Provision is to be made for the free passage of air and water from all parts
of the tank to the air pipes and suctions, account being taken of the pumping rates
required. To ensure this, sufficient air holes and drain holes are to be provided in all
longitudinal and transverse non-watertight primary and secondary members. The drain
holes are to be located as close to the bottom as is practicable, and air holes are to
be located as close to the inner bottom as is practicable, see also
Pt 3, Ch 10, 5.3 Openings and Pt 4, Ch 9, 5.8 Openings in longitudinals.
8.3 Girders
8.3.1 The
minimum depth of the centre girder is to be taken as the greater of
the following:
-
-
d
DB = 50B mm, but need not be taken as greater than
2000 mm
-
d
DB = 760 mm.
8.3.3
In
transversely framed ships where the breadth, B,
does not exceed 10 m, no side girders are required, and one vertical
stiffener is to be fitted to the floors on each side, about midway
between the centreline and the margin plate. One side girder is to
be fitted where the breadth, B, exceeds 10 m but does
not exceed 20 m, and for greater breadths two girders are to be fitted
on each side of the centreline. The non-watertight side girders are
to extend as far forward and aft as practicable and are to have a
thickness not less than:
nor less than 6,0 mm.
8.3.4 Vertical
stiffeners are to be fitted at every bracket floor (see
Pt 4, Ch 1, 8.5 Floors 8.5.7), and are to have a depth
not less than the depth of the tank top frame or 150 mm, whichever
is the greater. For ships with a length, L, less than
90 m, stiffeners are to have a depth of not less than 1,65L mm
with a minimum of 50 mm. The thickness is to be as required for the
girder. Watertight side girders are to have a thickness 1 mm greater
than required by Pt 4, Ch 1, 8.3 Girders 8.3.3 for
non-watertight side girders. Where the double bottom tanks are interconnected
with side tanks or cofferdams, the thickness is to be as for deep
tanks (see
Pt 4, Ch 1, 9.2 Watertight and deep tank bulkheads 9.2.1)
with h, in metres, measured to the highest point at the
side tank or cofferdam.
8.3.5
In
longitudinally framed ships one side girder is to be fitted
where the breadth, B, exceeds 14 m, and two girders are
to be fitted on each side of the centreline where B exceeds
21 m. The girders are to extend as far forward and aft as practicable
and are to have a thickness not less than:
nor less than 6,0 mm.
In general, a vertical
stiffener, having a depth not less than 100 mm and a thickness equal
to the girder thickness, is to be arranged midway between floors.
8.3.6
Watertight
side girders are to have a plating thickness corresponding
to the greater of the following:
-
-
thickness t as
for deep tanks (see
Pt 4, Ch 1, 9.2 Watertight and deep tank bulkheads 9.2.1)
with h, in metres, measured to the highest point of the
side tank, or cofferdam if the double bottom is interconnected with
these tanks.
8.3.8
Duct
keels, where arranged, are to have a thickness of side plates
corresponding to the greater of the following:
-
-
thickness t,
as for deep tanks, (see
Pt 4, Ch 1, 9.2 Watertight and deep tank bulkheads 9.2.1) with h, in metres, measured to the highest point
of the side tank, or cofferdam if the double bottom tank is interconnected
with these tanks.
8.3.9 The
sides of duct keels are, in general, to be spaced not more than 2,0
m apart. Where the sides of the duct keels are arranged on either
side of a centreline or side girder, each side is, in general, to
be spaced not more than 2,0 m from the centreline or side girder.
The inner bottom and bottom shell within the duct keel are to be suitably
stiffened. The primary stiffening in the transverse direction is to
be suitably aligned with the floors in the adjacent double bottom
tanks. Where the duct keels are adjacent to double bottom tanks which
are interconnected with side tanks or cofferdams, the stiffening is
to be in accordance with the requirements for deep tanks, see
Pt 4, Ch 1, 9.2 Watertight and deep tank bulkheads 9.2.1. Access to the duct keel is
to be by watertight manholes or trunks.
8.4 Inner bottom plating and stiffening
8.4.1 The thickness of the inner bottom plating
in the holds is to be not less than:
nor less than 6,5 mm in holds and 7,5 mm under hatchways if no ceiling is
fitted.
8.4.3 A margin
plate, if fitted, is to have a thickness throughout 20 per cent greater
than that required for inner bottom plating.
8.4.4 Where
the double bottom tanks are common with side tanks or cofferdams,
the thickness of the inner bottom plating is to be not less than that
required for deep tanks (see
Pt 4, Ch 1, 9.2 Watertight and deep tank bulkheads 9.2.1), with h, in
metres, taken to the highest point of the side tank or cofferdam,
and K
1 is to be taken as ‘elsewhere’.
8.4.5 Inner
bottom longitudinals, or tank top frames at bracket floors within
the range of cargo holds, are to have a section modulus not less than
85 per cent of the Rule value for bottom longitudinals (see
Pt 4, Ch 1, 6.2 Longitudinal stiffening 6.2.1) or bottom frames in way of
bracket floors (see
Pt 4, Ch 1, 6.3 Transverse stiffening 6.3.1),
whichever is applicable. The unsupported span of tank top frames is
generally not to exceed 2,5 m. Where the double bottom tanks are interconnected
with side tanks, hopper and topside tanks or cofferdams, the scantlings
are to be not less than those required for deep tanks, see
Pt 4, Ch 1, 9.2 Watertight and deep tank bulkheads 9.2.1. For higher tensile steel
inner bottom longitudinals the requirements of Pt 4, Ch 1, 6.2 Longitudinal stiffening 6.2.2 are to be complied with where
applicable.
8.5 Floors
8.5.1
In
longitudinally framed ships, plate floors are to be fitted
under bulkheads and elsewhere at a spacing not exceeding 3,8 m. The
thickness of non-watertight plate floors is to be not less than:
nor less than 6,0 mm. The thickness need not be greater
than 15 mm, but the ratio between the depth of the double bottom and
the thickness of the floor is not to exceed  . This ratio may, however, be exceeded if suitable additional
stiffening is fitted. Vertical stiffeners are to be fitted at each
longitudinal, having a depth not less than 150 mm and a thickness
equal to the thickness of the floors. For ships of length, L,
less than 90 m, the depth is to be not less than 1,65L mm,
with a minimum of 50 mm.
8.5.2 The
thickness of watertight floors for longitudinally framed ships is
to be not less than:
-
-
-
whichever is the greater,
but need not exceed 15 mm on floors of normal depth. The thickness
is also to satisfy the requirements for deep tanks (see
Pt 4, Ch 1, 9.2 Watertight and deep tank bulkheads 9.2.1) with h, in metres,
taken to the highest point of the side tank, or cofferdam if the double
bottom tank is interconnected with these tanks. The scantlings of
stiffeners are to be in accordance with the requirements of Pt 4, Ch 1, 9.2 Watertight and deep tank bulkheads 9.2.1 for deep tanks, or as required
by Pt 4, Ch 1, 8.5 Floors 8.5.4 whichever is the
greater. The stiffeners are to be connected to the inner bottom and
shell longitudinals.
8.5.3 Between
plate floors, transverse brackets having a thickness not less than
0,009d
DB mm are to be fitted, extending from
the centre girder and margin plate to the adjacent longitudinal. The
brackets, which are to be suitably stiffened at the edge, are to be
fitted at every frame at the margin plate, and those at the centre
girder are to be spaced not more than 1,25 m.
8.5.4
In
transversely framed ships, plate floors are to be fitted under
bulkheads, in way of change in depth of double bottom and elsewhere
at a spacing not exceeding 3,0 m. The shell inner bottom plating between
these floors is to be supported by bracket floors. The thickness of
non-watertight plate floors is to be not less than:
but need not exceed 15 mm and is to be not less than
6 mm. Watertight or strengthened floors are to be fitted below, or
in the vicinity of, watertight bulkheads, and their thickness is to
be 2 mm greater than that derived above for non-watertight floors,
but need not exceed 15 mm on floors of normal depth. If the depth
of such floors exceeds 915 mm but does not exceed 2000 mm, the floors
are to be fitted with vertical stiffeners spaced not more than 915
mm apart and having a section modulus not less than:
|
Z
|
= |
5,41 d
DBA
2
h
DB
sk x
10-9 cm3
|
The ends of the stiffeners are to be sniped.
8.5.5 Where
the double bottom tanks are interconnected with side tanks or cofferdams,
or where the depth of floor exceeds 2000 mm, the scantlings of watertight
floors are to be not less than those required for deep tanks (see
Pt 4, Ch 1, 9.2 Watertight and deep tank bulkheads 9.2.1), and the ends of the stiffeners
are to be bracketed top and bottom.
8.5.6 Where
floors form the boundary of a sea inlet box, the thickness of the
plating is to be the same as the adjacent shell, but not less than
12,5 mm and need not exceed 25 mm. The scantlings of stiffeners, where
required are, in general, to comply with Pt 4, Ch 1, 9.2 Watertight and deep tank bulkheads 9.2.1 for deep tanks. Sniped ends for stiffeners on the boundaries
of these spaces are to be avoided wherever practicable. The stiffeners
should be bracketed or the free end suitably supported to provide
alignment with backing structure.
8.5.7
Where
bracket floors are fitted, the bottom frames are to be derived
from Pt 4, Ch 1, 6.3 Transverse stiffening 6.3.1. The unsupported
span of the frames is not to exceed 2,5 m. The breadth of the brackets
attaching the frames and the reverse frames to the centre girder and
margin plate is to be three-quarters of the depth of the centre girder.
The brackets are to be flanged on the unsupported edge and are to
have the same thickness as the plate floors.
8.5.8
Where
struts are fitted to reduce the unsupported span of the frames,
reverse frames and longitudinals, they are to have a cross-sectional
area of not less than:
-
-
where Z
BF is the modulus, in cm3,
of the frame or longitudinal based on the effective length between
floors as defined in Pt 3, Ch 3, 3 Structural idealisation.
|