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Hull Girder Shear Forces and Bending Moments - Bulk Carrier Limitations
The bending moment is the amount of bending caused
to the ship's hull by external forces. For example, the
bending moment is the highest in the midship section
when the ship's ends are supported by crests of a
wave known as `sagging' or `positive bending'. When
the ship is riding the crest of a wave at its midships,
the bending moment is known as `hogging' or `negative
bending'. Bending moments are measured in tonne-
When two external parallel forces act in opposite
directions on any part of a structure to break it apar
or shear it, the forces are known as shearing forces
and are measured in tonnes. Shearing stress is,
therefore, the stress that may break or shear the
All classification society member bulk carriers are assigned with permissible still
water shear forces (SWSF) and still water bending moment (SWBM) limits. There are
normally two sets of permissible SWSF and SWBM limits assigned to each ship,
i) Seagoing (at sea) SWSF and SWBM limits.
ii) Harbour (in port) SWSF and SWBM limits.
The seagoing SWSF and SWBM limits are not to be exceeded when the ship puts to sea
or during any part of a seagoing voyage. In harbour, where the ship is in sheltered
water and is subjected to reduced dynamic loads, the hull girder is permitted to carry a
higher level of stress imposed by the static loads. The harbour SWSF and SWBM limits
are not to be exceeded during any stage of harbour cargo operations.
When a ship is floating in still water, the ship's lightweight (the weight of the ship's structure and its
machinery) and deadweight (all other weights, such as the weight of the bunkers, ballast, provisions
and cargo) are supported by the global buoyancy upthrust acting on the exterior of the hull. Along
the ship's length there will be local differences in the vertical forces of buoyancy and the ship's
weight. These unbalanced net vertical forces acting along the length of the ship will cause the hull
girder to shear and to bend ,inducing a vertical still water shear force (SWSF)
and still water bending moment (SWBM) at each section of the hull.
At sea, the ship is subjected to cyclical shearing and bending actions induced by continuously
changing wave pressures acting on the hull. These cyclical shearing and bending actions give rise to
an additional component of dynamic, wave induced, shear force and bending moment in the hull
girder. At any one time, the hull girder is subjected to a combination of still water and wave
induced shear forces and bending moments.
The stresses in the hull section caused by these shearing forces and bending moments are carried by
continuous longitudinal structural members. These structural members are the strength deck, side
shell and bottom shell plating and longitudinals, inner bottom plating and longitudinals, double
bottom girders and topside and hopper tank sloping plating and longitudinals, which are generally
defined as the hull girder.
Risk of Heavy cargoes & Monitoring the Ship's Loading limits
How to avoid risk of vessel being overloaded
Local loading criteria defining maximum allowable cargo weight in each cargo hold
Structural standards & strengthening of bulk carriers
Our detail pages illustrated many safety aspects of Bulk carrier
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- Suitability of Shore Terminals for handling bulk cargo
- Preparation for ships carrying bulk cargo & standard loading condition
Operation of sea going bulk carriers involved numerous hazards . Careful planning and exercising due caution for all critical shipboard matters are important . This site is a quick reference to international shipping community with guidance and information on the loading and discharging of modern bulk carriers so as to remain within the limitations as specified by the classification society.
It is vital to reduce the likelihood of over-stressing the ship's structure and also complying with all essential safety measures for a safe passage at sea. Our detail pages contain various bulk carrier related topics that might be useful for people working on board and those who working ashore in the terminal. For any remarks please
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