Page 174 - Introduction to Naval Architecture

160 STRENGTH
2
Since the stress in the aluminium deck is 22.91 MN/m this deck
would fail by buckling. The transverse beam spacing would have to
be reduced to about 620 mm to prevent this.

These relationships indicate the key physical parameters involved
in buckling but do not go very far in providing solutions to ship
type problems.

Load-shortening curves
17
Theoretical and experimental studies show that the stiffness and
strength of rectangular plate elements of an orthogonally stiffened
shell are strongly influenced by imperfections including residual
stresses in the structure arising from the fabrication process and initial
deformations of plate and stiffener. These studies were the culmination
of a large research programme involving longitudinally loaded plates
with stringers b apart, between transverse frames a apart The plate
thickness was t, the radius of gyration of a stringer with a width b of
plating was rand the stringer area was AS. The stress was o and strain e
with subscript o denoting yield. Stringers used were tee bars and flat
plate. The following parameters were used:

The outcome of the research was a series of load-shortening curves as
shown in Figure 7.16. These are for a range of stringer and plate
slenderness with average imperfections. Average imperfections were
defined as a residual stress 15 per cent of yield and a maximum initial
plate deflection of 0.1 fP.
The results are sensitive to stiffener area ratio, particularly for low A
and high /?, Figure 7.17, in which a' u is the ratio of the average
compressive stress at failure over the plate and stiffener cross section to
the yield stress. Peak stresses in Figure 7.16 correspond to the strengths
indicated in Figure 7.17(b). Figure 7.18 shows the influence of lateral
pressure on compressive strength for the conditions of Figure 7.16. The
s tne
effect is most marked for high A and increases with /?. Q *
corresponding head of seawater.