Page 171 - Introduction to Naval Architecture

STRENGTH 157

deal with them in any simple way although their effects will be included
In statistical data recorded at sea if the recorders are sited carefullv.

STRENGTH OF STRUCTURAL ELEMENTS

Up to this point it is the overall loading and strength of the hull that has
been considered. It was pointed out that in deciding which structure to
include in the section modulus care was necessary to ensure that the
elements chosen could in fact contribute and would not 'shirk' their
load. In this section the loading on, and strength of, individual
elements will be considered.
The basic structural element is a plate with some form of edge
support. Combining the plates and their supporting members leads to
grillages. Bulkheads, decks and shell are built up from grillages. Most of
the key elements are subject to varying loading so that at times they will
be in tension and at others in compression. Whilst a structure may be
more than adequate to take the direct stresses involved, premature
failure can occur through buckling in compression. This may be
aggravated by lateral pressure on the plating as occurs in the shell and
boundaries of tanks containing liquids.
Buckling
A structure subject to axial compression will be able to withstand
loading up to a critical load below which buckling will not occur. Above
this load a lateral deflection occurs and collapse will eventually follow,
Euler showed that for an ideally straight column the critical load is:

where:
I = column length.
I - second moment of area of the cross section.
This formula assumes the ends of the column are pin jointed. The
critical stress follows as:

where k is the radius of gyration.
If the ends of the strut were not pin jointed but prevented from
rotating, the critical load and stress are increased fourfold. The ratio l/k
is sometimes called the slenderness ratio. For a strip of plating between