Page 177 - Introduction to Naval Architecture

STRENGTH 163

14
Figure 7,18 Influence of lateral pressure (courtesy RINA )

The importance of the load-shortening curves is that they allow a
designer to establish how elements of the structure will behave both
before and after collapse and hence the behaviour of the ship section
as a whole. Even after collapse elements can still take some stress.
However, from Figure 7.16 for A equal to or greater than 0.6 the curves
show a drastic reduction in strength post collapse. For that reason it is
recommended that designs be based on A values of 0.4 or less and ft
values of 1.5 or less.
Using such approaches leads to a much more efficient structure than
would be the case if the designer did not allow the yield stress to be
exceeded.

TRANSVERSE STRENGTH

The loads on a transverse section of the ship in waves are those
calculated from the motions of the ship including the inertia and
gravity forces. Additionally there may be forces generated by the
movement of liquids within tanks, sloshing as it is termed. However, this
dynamic loading in a seaway is not the complete story. The scantlings of
the section must be able to withstand the loads at the waterline due to
berthing and the racking strains imposed during docking.
The most satisfying approach would be to analyse the three
dimensional section of the ship between main transverse bulkheads as
a whole, having ascertained the boundary conditions from a global
finite element analysis of the complete hull. This would be the
approach adopted by those with access to the necessary computers and
software. In many cases a simpler approach is needed.