Page 233 - Ship Construction.DJ Eyres 6Ed

Ch19-H8070.fm Page 222 Wednesday, October 18, 2006 6:58 AM
222 Ship Construction
BRIDGE STRUCTURES The side of bridge superstructures whose length
exceeds 15 per cent of the ship’s length will have a greater thickness than
the sides of other houses, the scantling being similar to that required for the
ship’s side shell. All bridge superstructures and midship deckhouses will have
a heavily plated bridge front, and the aft end plating will be lighter than the
front and sides. Likewise the stiffening members fitted at the forward end
will have greater scantlings than those at the sides and aft end. Additional
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stiffening in the form of web frames or partial bulkheads will be found
where there are large erections above the bridge deck. These are intended
to support the sides and ends of the houses above and are preferably
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arranged over the watertight bulkheads below. Under concentrated loads on
the superstructure decks, for example under lifeboat davits, web frames are
also provided.
The longer bridge superstructure which is transmitting the main hull
girder stresses requires considerable strengthening at the ends. At this
major discontinuity, the upper deck sheerstrake thickness is increased by 20
per cent, the upper deck stringer plate by 20 per cent, and the bridge side
plating by 25 per cent. The latter plating is tapered into the upper deck
sheerstrake with a generous radius, as shown in Figure 19.8(a), stiffened at
its upper edge, and supported by webs not more than 1.5 m apart. At the
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ends of short bridge superstructures less strengthening is required, but local
stresses may still be high and therefore the upper deck sheerstrake
thickness is still increased by 20 per cent and the upper deck stringer
by 20 per cent.

POOP STRUCTURE Where there is no midship deckhouse or bridge
superstructure the poop front will be heavily constructed, its scantlings being
similar to those required for a bridge front. In other ships it is relatively
exposed and therefore needs adequate strengthening in all cases. If the poop
front comes within 50 per cent of the ship’s length amidships, the discontinuity
formed in the main hull girder is to be considerably strengthened, as for a
long bridge exceeding 15 per cent of the ship’s length. Where deckhouses
are built above the poop deck these are supported by webs or short transverse
bulkheads in the same manner as those houses fitted amidships. The after
end of any poop house will have increased scantlings since it is more exposed
than other aft end house bulkheads.

PASSENGER SHIP SUPERSTRUCTURES It is shown in Chapter 8 that
with conventional beam theory the bending stress distribution is linear,
increasing from zero at the neutral axis to a maximum at the upper deck
and bottom. If a long superstructure is fitted the stress distribution remains
linear and the strength deck is above the upper deck in way of the super-
structure deck. If a short superstructure is fitted the stress distribution will
be broken at the upper deck, which is the strength deck, the stresses in the