Various stresses in ships structure
Stress is defined as a force (or forces) acting on a solid structure that causes a deformation, known as strain. Stresses are defined by how forces act on any material:
Caused by forces that tend to pull the material in parallel but opposite directions away from the centre, increasing the tension and the length of the material before breaking it in two.
Caused by forces that tend to push the material in parallel but opposite directions towards the centre, causing the material to buckle or crush and decreasing its length until the force exerted becomes equal to the strength of the material.
Caused by non-parallel forces that tend to pull the material in opposite directions away from the centre, causing the material to shear under the influence of the exerted force.
Caused by forces acting in opposite directions, causing a twisting moment in the material. In other words, if a shear stress occurs in a transverse direction, it is referred to as torsional stress.
The following terminology is important in understanding stresses on ships:
This is the effect of a force applied around any pivot point on a structure, causing it to turn around that point. For example, if a force `F' is applied at a distance `d' from a pivot point, the moment `M' can be given by the formula M = F x d.
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- metres.
When two external parallel forces act in opposite directions on any part of a structure to break it apart 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 structure apart.
For ships, this force is considered by taking the difference between the buoyancy and the weight force acting on it. According to Archimedes' principle, for a ship to float the weight force must be balanced by the buoyancy force. Wherever one of these forces exceeds the other, shearing stresses are likely to occur.
The most likely areas for shearing stresses are at about 25% of the ship's length from stem or stern, particularly at the transverse watertight divisions at this length. For bulk carriers, it has been found that the highest shearing stresses occur at the boundary of the accommodation and the last cargo hold.
Shearing Stress is calculated by the formula :
Shear Stress = F / A , where
F = Shear Force, and
A = Area of cross section.
The Chief Officer must plan the stowage to ensure that loads are distributed evenly within the ship at all stages. Structural members that increase a vessel's resistance to shearing forces are the centre girder, intercostal side girders, deck and side plating and longitudinal frames where fitted. They should be given particular attention during inspections to detect any signs of damage to the structure.
Since shear forces are greatest at the bulkheads, this is compensated for by placing angle brackets at each side of the bulkheads.
The stresses on ships can be categorised by the forces causing them
These result from differences in the weights loaded/ discharged or moved within a ship. They tend to act constantly and continuously as long as the source of stress is not moved, eg by a change in load distribution on the ship. These stresses can be caused by any load, from the weight of the ship itself, its structure, equipment, machinery, cargo, bunkers, stores or the constant pressure of seawater on the hull.
These result from the movement of the ship at sea, the varying effects of sea and swell, and the motion of the ship through the water. The ship may also be subjected to internal dynamic stresses caused by the movement of liquids within holds or tanks.
These include all stresses not classified as static or dynamic stresses. They tend to change with the operation, eg using a crane causes vibration and point load stresses.
Categories of ship stresses :Pounding, hogging, sagging, panting, drydocking, racking and more
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