Page 223 - Ship Construction.DJ Eyres 6Ed

Ch19-H8070.fm Page 212 Wednesday, October 18, 2006 6:58 AM
212 Ship Construction
thickness may be increased where higher stresses occur owing to disconti-
nuities in the structure or concentrated loads.
Other thickness increases may occur where large deck loads are carried,
where fork lift trucks or other wheeled vehicles are to be used, and in way
of deep tanks. Where the strength deck plating exceeds 30 mm it is to be
Grade B steel and if it exceeds 40 mm Grade D over the midships region, at
the ends of the superstructure and in way of the cargo hold region in con-
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tainer ships. The stringer plate (i.e. the strake of deck plating adjacent to
the sheerstrake) of the strength deck, over the midship region and con-
tainer ship cargo hold area, of ships less than 260 metres in length, is to be of
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Grade B steel if 15 to 20mm thick, Grade D if 20 to 25 mm thick and Grade
E if more than 25 mm thick. Where the steel deck temperatures fall below
0 °C in refrigerated cargo ships the steel will be of Grades B, D and E
depending on thickness.
On decks other than the strength deck the variation in plate thickness is
similar, but lighter scantlings are in use.
Weather decks may be covered with wood sheathing or an approved
composition, which not only improves their appearance, but also provides
protection from heat in way of any accommodation. Since this provides
some additional strength, reductions in the deck plate thickness are permit-
ึ
ted; and on superstructure decks the plating thickness may be further
decreased within deckhouses, if sheathed. Before fitting any form of
sheathing the deck is treated to prevent corrosion between the deck plating
and sheathing (see Figure 19.3).
Any openings abreast the hatch openings in a deck are kept to a minimum
and clear of the hatch corners. If such openings are cut, compensation is
required to restore the sectional area of deck. All large openings in the
decks have well rounded corners, with insert plates fitted, unless the
corners are parabolic or elliptical with the major axis fore and aft, local
stress concentrations being reduced if the latter type of corner is cut (see
Figure 19.4).

DECK STIFFENING Decks may be framed transversely or longitudinally
but outside the line of openings it is preferred that longitudinal framing
should be adopted for the strength deck.
When the decks are longitudinally framed the scantlings of the longitud-
inals are dependent on their spacing, the length of ship, whether they are
inside or outside the line of hatch openings, their span and the deck load-
ing. Deck transverses support the longitudinals, and these are built from a
deep web plate with flange or welded face flat, and are bracketed to the side
frame (Figure 19.1). Within the forward 7.5 per cent of the ship’s length,
the forecastle and weather deck transverses are closely spaced and the
longitudinal scantlings increased, the additional transverse and longitudinal