Page 226 - Ship Construction.DJ Eyres 6Ed

Ch19-H8070.fm Page 215 Wednesday, October 18, 2006 6:58 AM
Decks, Hatches, and Superstructures 215
stiffening forward being designed to avoid buckling of the deck plating on
impact when shipping seas.
Transversely framed decks are fitted with deck beams at every frame, and
these have scantlings which are dependent on their span, spacing, and loca-
tion in the ship. Those fitted right forward on weather decks, like the longi-
tudinal framing forward, have heavier scantlings, and the frame spacing is
also decreased in this region so they will be closer together. Beams fitted in
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way of deep tanks, peak tanks, and oil bunkers may also have increased
scantlings as they are required to have the same rigidity as the stiffeners of
the tank boundary bulkheads. Deck beams are supported by longitudinal
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deck girders which have similar scantlings to deck transverses fitted with
any longitudinal framing. Within the forward 7.5 per cent of the ship’s
length these deck girders are more closely spaced on the forecastle and
weather decks. Elsewhere the spacing is arranged to suit the deck loads
carried and the pillar arrangements adopted. Each beam is connected to the
frame by a ‘beam knee’ and abreast the hatches ‘half beams’ are fitted with
a suitable supporting connection at the hatch side girder (see Figure 19.2).
Both longitudinals and deck beam scantlings are increased in way of
cargo decks where fork lift trucks, and other wheeled vehicles which cause
large point loads, are used.
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In way of the hatches fore and aft side girders are fitted to support the
inboard ends of the half beams, and transverses. At the ends of the hatches
heavy transverse beams are fitted and these may be connected at the inter-
section with the hatch side girder by horizontal gusset plates (Figure 19.4).
Where the deck plating extends inside the coamings of hatches amidships
the side coaming is extended in the form of tapered brackets.

Hatches

The basic regulations covering the construction and means of closing hatches
in weathertight decks are contained within the Conditions of Assignment of
Freeboard of the Load Line Convention (see Chapter 31). Lloyd’s Register
provides formulae for determining the minimum scantlings of steel covers,
which will be within the requirements of the Load Line Convention. Only the
maximum permitted stresses and deflections of covers under specified loadings
are given by the Load Line Convention. Under the Convention, ships fitted
with approved steel covers having direct securing arrangements may have
reduced B-100 or B-60 freeboards if they meet the subdivision requirements,
but in general they are assigned standard cargo ship Type B freeboards.
If steel pontoon type covers which are self-supporting and have no direct
securing arrangements are fitted, then the standard Type B freeboard only
is assigned. Where portable beams are fitted with wood or light steel covers
and tarpaulins, then the ship has an increased Type B freeboard, i.e. there