Page 336 - Practical Ship Design

Structural Design 293

Expansion joints, whilst relieving the superstructure of stress, caused stress
concentrations at their lower ends and this often led to fatigue cracking in this area.
The use of aluminium was discontinued some years ago mainly due to a
recognition of the increased danger of structural collapse in the event of fire, which
is a consequence of the material’s low melting point, but partly also due to a
realisation of its poor fatigue properties in a marine environment.
In modern cruise liners, designers in search of economic efficiency usually want
to fit extensive superstructures -both in length, which often extends from a short
distance abaft the bow to very near the stern, and in the number of tiers fitted. On a
number of these ships the superstructure is stepped in from the ship’s side at the
upper deck to accommodate lifeboats at this level, designers finding this
advantageous for a number of reasons:
- to improve the launching of the lifeboats as a contribution to passenger
safety;
- to reduce top weight enabling more accommodation to be fitted;
- to improve the amenity of the top decks for passengers.
In conjunction with these design decisions, designers then wanted to get the best
possible contribution from the superstructure to the longitudinal strength of the
ship and fortunately found a new design tool to hand in finite element calculations.
Finite element calculations can be used to solve the complex problems posed by
openings in the deckhouse sides, the stiffness of the deck on which the house sides
are supported and the three dimensional effects interrelating these.
Three-dimensional F.E.M’s cannot be used at the all-important initial design
stage, so naval architects involved in this type of work owe a considerable debt to
Professor Caldwell for his 1957 R.I.N.A. paper on the subject and to J.W.
Fransman for his 1988 R.I.N.A. paper “The influence of passenger ship super-
structures on the response of the hull girder”, in which analytical methods of
calculation are developed. Some appreciation of the approach adopted in these
papers may be given by Fig. 10.2 abstracted from the latter paper.

10.3.3 Warships, and more especially, frigates and corvettes
Although individual navies have their codes for structural design, none of these
rules for warships are as detailed or as freely available as merchant ship Classi-
fication Society rules. There are, however, a number of very good papers on the
subject and most of the following is abstracted from one or other of the papers
mentioned in the bibliography.
For an introduction it is hard to better the statement which J.D. Clarke of A.R.E.
(the British Admiralty Research Establishment) made in his 1986 paper “Wave
loading in warships” to the effect that “the most important loading exerted by the
sea on the usually slender hull of a warship (this mainly refers to frigates and