Page 157 - Introduction to Naval Architecture
P. 157
STRENGTH 143
A superstructure is joined to the main hull at its lower boundary. As
the ship sags or hogs this boundary becomes compressed and extended
respectively. Thus the superstructure tends to be arched in the opposite
sense to the main hull. If the two structures are not to separate, there
will be shear forces due to the stretch or compression and normal
forces trying to keep the two in contact,
The ability of the superstructure to accept these forces, and
contribute to the section modulus for longitudinal bending, is
regarded as an efficiency. It is expressed as:
where o" 0, o & and o are the upper deck stresses if no superstructure
were present, the stress calculated and that for a fully effective
superstructure.
The efficiency of superstructures can be increased by making them
long, extending them the full width of the hull, keeping their section
reasonably constant and paying careful attention to the securings to the
main hull. Using a low modulus material for the superstructure, for
15
instance GRP , can ease the interaction problems. With a Young's
modulus of the order of ^ of that of steel, the superstructure makes
little contribution to the longitudinal strength. In the past some
Figure 7.11 Superstructure mesh (courtesy RINA)
