Page 75 - Ship Construction.DJ Eyres 6Ed

Ch08-H8070.fm Page 64 Wednesday, October 18, 2006 7:29 AM
64 Ship Construction
is simply the ratio between the second moment of area and the distance of
the point considered from the neutral axis, i.e. I/y = Z.

The bending stress (s) is then given by s = M/Z.

THE SHIP AS A BEAM It was seen earlier that the ship bends like a
beam; and in fact the hull can be considered as a box-shaped girder for
--- ใช้เพื่อการศึกษาเท่านั้น---
which the position of the neutral axis and second moment of area may be cal-
culated. The deck and bottom shell form the flanges of the hull girder, and
are far more important to longitudinal strength than the sides which form
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
the web of the girder and carry the shear forces. The box shaped hull girder
and a conventional I girder may be compared as in Figure 8.3.
In a ship the neutral axis is generally nearer the bottom, since the bottom
shell will be heavier than the deck, having to resist water pressure as well as
the bending stresses. In calculating the second moment of an area of the
cross-section all longitudinal material is of greatest importance and the
further the material is from the neutral axis the greater will its second
moment of area about the neutral axis. However, at greater distances from
the neutral axis the sectional modulus will be reduced and correspondingly
higher stress may occur in extreme hull girder plates such as the deck
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stringer, sheerstrake, and bilge. These strakes of plating are generally heavier
than other plating.
Bending stresses are greater over the middle portion of the length and it
is owing to this variation that Lloyd’s give maximum scantlings over 40 per
cent of the length amidships. Other scantlings may taper towards the ends
of the ship, apart from locally highly stressed regions where other forms of
loading are encountered.

STRENGTH DECK The deck forming the uppermost flange of the main
hull girder is often referred to as the strength deck. This is to some extent
a misleading term since all continuous decks are in fact strength decks if
properly constructed. Along the length of the ship the top flange of the hull
girder, i.e. the strength deck, may step from deck to deck where large
superstructures are fitted or there is a natural break, for instance in way
of a raised quarter deck. Larger superstructures tend to deform with the
main hull and stresses of appreciable magnitude will occur in the structure.
Early vessels fitted with large superstructures of light construction demon-
strated this to their cost. Attempts to avoid fracture have been made by
fitting expansion joints which made the light structure discontinuous. These
were not entirely successful and the expansion joint may itself form a stress
concentration at the strength deck which one would wish to avoid. In modern
construction the superstructure is usually made continuous and of such
strength that its sectional modulus is equivalent to that which the strength
deck would have if no superstructure were fitted (see Chapter 19).