Construction of a Torsion Box - Why Torsion Box is Required?
When a body is subjected to a twisting moment which is commonly referred to as torque, that body is said to be in torsion.
A ship heading obliquely to a wave while being subjected to righting moments of opposite direction at its ends cause the ship to put into a torsion.
Most of the ships are designed with appropriate deck openings to mitigate the effect of this torsion, or to provide torsional strength to the ship.
Oil tankers have many transverse bulkheads which act as main stiffening members against racking and twisting. The uppermost continuous deck does not have many opening hatches compared to the dry cargo vessel.
In bulk carrier, the hatch openings are relatively smaller and provided with sufficient deck spacing and deck stiffening members which are sufficient to counteract twisting moments.
Racking effect
When a ship is rolling, the deck tends to move laterally relative to the bottom structure, and the shell on one side move vertically relative to the other side. This type of deformation is referred to as 'Racking'.
It is reduced or prevented by the beam knee and tank side bracket connections together with the transverse bulkheads, the latter having the greatest effect.
Transverse bulkheads primarily resist such transverse deformation, the side frames contribution being insignificant provided the transverse bulkheads are at their usual regular spacings.
Torsion Box
In Container ships, Problems may arise due to the lack of torsional strength caused by the large hatches. This problem is overcome to some extent by fitting torsion boxes on each side of the ship.
These boxes are formed by the upper deck, the top part of the longitudinal bulkhead, sheer strake and upper platform, all of which are of thick material. The boxes are supported inside by transverses and wash bulkheads in addition to the longitudinal framing. These boxes are only effective if they are efficiently tied at their ends. At the aft end, they extend into the engine room and are tied to deep transverse webs. Similarly, at the fore-end, they are carried as far forward as the form of the ship will allow and are welded to transverse webs.
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