Page 235 - Ship Construction.DJ Eyres 6Ed

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Ch19-H8070.fm Page 224 Wednesday, October 18, 2006 6:58 AM
224 Ship Construction
superstructure deck being less than those in the upper deck. The long
superstructure is referred to as an ‘effective superstructure’ the erections
contributing to the overall strength of the hull girder, and therefore they
are substantially built.
In passenger ships with large superstructures the present practice is to
make the structure effective with adequate scantlings. Some older ships
have been fitted with expansion joints which are in effect transverse cuts
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introduced to relieve the hull bending stresses in the houses. It has been
shown that at the end of deck erections the stresses do not conform to beam
theory and the ends are ineffective in contributing to the longitudinal
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
strength. The expansion joints were therefore so arranged that this ‘end
effect’ extended from joint to joint, and lighter scantlings were then permit-
ted for the superstructure. Unfortunately the expansion joint often provided
an ideal ‘notch’ in the structure from which cracks initiated. Aluminium
alloy superstructures offer an alternative to the use of expansion joints,
since the low modulus of elasticity of the material results in lower stresses
in the houses than would be the case with a steel superstructure, all other
considerations being equal.

ึ
WEATHERTIGHT DOORS The integrity of houses on the freeboard
and other decks which protect the openings in these decks must be
maintained. Access openings must be provided to the houses and weath-
ertight doors are fitted to these openings. These must comply with the
requirements of the Load Line Convention (see Chapter 31) and are
steel doors which may be secured and made watertight from either side.
Weathertightness is maintained by a rubber gasket at the frame of the
door (see Figure 19.8(b)).