Page 338 - Practical Ship Design

Structural Design 295

The bottom can therefore resist the compressive loads from a hogging bending
moment better than the deck can resist the compressive loads from a sagging
moment. In addition, the hull form generally causes the sagging wave bending
moment to be greater than the hogging moment, whilst slamming can add another
component to the sagging bending moment.
The still water bending moment of a warship is usually relatively small when
compared with the wave bending moment, but if there is any choice in the matter it is
marginally advantageous to design so that there is a hogging still water bending
moment.
The compressive strength of the upper deck must therefore be evaluated as
accurately as possible if minimum structural weight is to be achieved. The most
probable form of failure of the deck is a column-like collapse of the longitudinals
and the thin plating to which it is attached between transverses. The well known
“Euler strut” equation is used for calculations of this sort in the elastic regime,
but in practice the material reaches yield point and there is an interaction between
buckling and yielding which depends on the standard of fabrication of the
structure with imperfections and “as built” stresses having a significant effect. The
behaviour is usually quantified in terms of a “column curve” relating failure load
and stiffener size, several versions of which are given in specialist books on the
subject.
Superstructures can play an important part in the strength of warships if required
to do so although the possibility of severe damage to the superstructure in action
together with the possibility of it having to be modified during service life causes
some prudent designers to ignore its contribution. Superstructures intended to
contribute to longitudinal strength should be made as long as possible, but there is
much to be said for the alternative philosophy of making the hull as large as
possible and reducing the superstructure to a minimum and not asking it to
contribute to longitudinal strength at all.
Superstructure contributions to longitudinal strength should be calculated on
the lines discussed for passenger liners.
Because of the light scantlings of warships, particular attention should be paid
to the suggestions made later on ways to minimise vibration and stress concent-
rations. Great attention should also be paid to the design of special strengthening at
the end of superstructures to marry these into the hull strength.
One difference between warship and merchant ship structural design is the need
to investigate and detail several structural sections in the former as opposed to the
“midship section” which has generally sufficed for the latter. This need is partly
due to the variation in the sections caused by the hull form of these fine lined ships
and partly to ensure that the strength is being maintained in way of large openings
and/or the ends of superstructures.