Page 158 - Introduction to Naval Architecture

144 STRENGTH

designers have used expansion joints at points along the length of the
superstructure. The idea was to stop the superstructure taking load.
Unfortunately they also introduce a source of potential stress concen-
tration and are now avoided.
Nowadays a finite element analysis would be carried out to ensure the
stresses were acceptable where the ends joined the main hull. A typical
mesh is shown in Figure 7.11.

Example 7.2

The midship section of a steel ship has the following
particulars:
2
Cross-sectional area of longitudinal material = 2.3m
Distance from neutral axis to upper deck = 7,6 m
Second moment of area about the neutral axis = 58 m 4

A superstructure deck is to be added 2.6m above the upper deck.
This deck is 13m wide, 12mm thick and is constructed of
aluminium alloy. If the ship must withstand a sagging bending
moment of 450 MNm. Calculate the superstructure efficiency if,
with the superstructure deck fitted, the stress in the upper deck is
2
measured as 55 MN/m .

Solution
Since this is a composite structure, the second moment of an
equivalent steel section must be found first. The stress in the steel
sections can then be found and, after the use of the modular ratio,
the stress in the aluminium.
Taking the Young's modulus of aluminium as 0.322 that of steel,
the effective steel area of the new section is:

The movement upwards of the neutral axis due to adding the
deck:

The second moment of the new section about the old NA is: