Page 334 - Practical Ship Design

Structural Design 29 1

O,l(-l.l).C, .C, .L2 B(C, +0.7)
M, -
(T=- - kN/m2
2 f, k, .C, .L2 B(C, +0.7)x
.
which reduces to:
-0.11 .c,
o= W/m2
f, . k, x

For unrestricted service, the amidships stress and mild steel structure, all the above
constants are unity and the formula corresponds to a wave bending moment stress
ow = 1 10 N/m2
It should be noted that this stress is based on the sagging wave bending moment
with the corresponding calculation for the hogging moment being a little more
complicated.
The permissible combined stress for still water plus wave bending moments is
given by: o = 175/kL N/mm2.
Built into a modulus derived using the wave bending moment only therefore is
an allowance for the still water bending moment being

175- 100 or 59% of the wave bending moment.
100

If the actual still water bending moment is in excess of these figures, the
modulus must be adjusted accordingly.

10.2.2 Structural decisions based on longitudinal strength

One of the first decisions which must be taken in structural design is whether to use
longitudinal or transverse framing.
For large ships (over about 200 m) longitudinal framing will generally be a
classification requirement, but even if this is not the case its use will usually be
desirable on economic grounds because it results in a lighter steelweight.
For small ships (under about 65 m) longitudinal strength is of secondary
importance and longitudinal framing brings no advantage in steelweight, whilst
the greater complexity of this system of construction increases fabrication costs.
For medium-sized ships - between these limits - the choice lies with the
designer, who can decide whether it is more advantageous to minimise steel
material weight or steel work man-hours.
It is worth noting that it need not be a straight choice between longitudinal and
transverse framing however as a combination of these methods can have advantages
in some ship typedsizes. Such a combination will generally use longitudinal
framing for the bottom framing and for the strength deck, i.e., for the two flanges of