Page 178 - Introduction to Naval Architecture

164 STRENGTH

For berthing loads it may be adequate to isolate a grillage in way of
the waterline and assess the stresses in it due to the loads on fenders in
coming alongside. In general, however, it is not reasonable to deal with
side frames, decks and double bottom separately because of the
difficulty of assessing the end fixities of the various members due to the
presence of the others, and the influence of longitudinal stiffening,
These are likely to be critical. For instance, a uniformly loaded beam,
simply supported at its ends, has a maximum bending moment at its
centre with zero moments at its ends. If the ends are fixed the
maximum bending moment reduces by a third and is at the ends.
The usual approximation is to take a slice through the ship
comprising deck beam, side frame and elements of plating and double
bottom structure. This section is then loaded and analysed as a
framework. The transverse strength of a superstructure is usually
analysed separately but by the same technique. The frameworks the
naval architect is concerned with are portals, in the superstructure, say,
shipshape rings in the main hull and circular rings in the case of
submarine hulls. Transverse bulkheads provide great strength against
racking of the framework. Some of this support will be transmitted to
frames remote from the bulkhead by longitudinal members although
these will themselves deflect under the loading as illustrated in Figure
7.19. Ignoring this support means results are likely to be conservative
and should really be used as a guide to distributing structure and for

Figure 7.19 Transverse strains