Page 77 - Ship Construction.DJ Eyres 6Ed

Ch08-H8070.fm Page 66 Wednesday, October 18, 2006 7:29 AM
66 Ship Construction
Transverse Stresses

When a ship experiences transverse forces these tend to change the shape
of the vessel’s cross sections and thereby introduce transverse stresses.
These forces may be produced by hydrostatic loads and impact of seas or
cargo and structural weights both directly and as the result of reactions due
to change of ship motion.
--- ใช้เพื่อการศึกษาเท่านั้น---
RACKING When a ship is rolling, the deck tends to move laterally relative
to the bottom structure, and the shell on one side to move vertically
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
relative to the other side. This type of deformation is referred to as ‘rack-
ing’. Transverse bulkheads primarily resist such transverse deformation,
the side frames contribution being insignificant provided the transverse
bulkheads are at their usual regular spacings. Where transverse bulkheads
are widely spaced deep web frames and beams may be introduced to
compensate.

TORSION When any body is subject to a twisting moment which is
commonly referred to as torque, that body is said to be in ‘torsion’. A ship
heading obliquely (45°) to a wave will be subjected to righting moments of
ึ
opposite direction at its ends twisting the hull and putting it in ‘torsion’. In
most ships these torsional moments and stresses are negligible but in ships
with extremely wide and long deck openings they are significant. A particular
example is the larger container ship where at the topsides a heavy torsion box
girder structure including the upper deck is provided to accommodate the
torsional stresses (see Figures 8.4 and 17.9).

Local Stresses
PANTING Panting refers to a tendency for the shell plating to work ‘in’
and ‘out’ in a bellows like fashion, and is caused by the fluctuating pressures
on the hull at the ends when the ship is amongst waves. These forces are
most severe when the vessel is running into waves and is pitching heavily, the
large pressures occurring over a short time cycle. Strengthening to resist
panting both forward and aft is covered in Chapter 17.

POUNDING Severe local stresses occur in way of the bottom shell and
framing forward when a vessel is driven into head seas. These pounding
stresses, as they are known; are likely to be most severe in a lightly ballasted
condition, and occur over an area of the bottom shell aft of the collision
bulkhead. Additional stiffening is required in this region, and this is dealt
with in Chapter 16.