Structural strength In Chapter 1 it was stated that one of the requirements in th design of a ship was that the streture should be sufficiently strong tu withstand without failure the forces imposed upon it when the ship is at sea. In this chapter the problem of structural strength will be studied in more detail. The problem consists first of all in assessing the foreces acting on the ship and secondly in determining the response of the structureand also the deformating of the structure. The structural strength problem is really a dynamic one. It has been seen that the ship is rarely in calm water and in consequence the motion of the sea generates motions in the ship itself. The motions generated because of the six degrees of freedom of the ship, i.e.heaving, swaying and surging, whick are linear motions, and rolling, pitching and yawing, which are ratations, all involve accelerations which generate forces on the structure. It is also important to recognise that even in still water the ship is subjected to forces which distort the structure, the forces being due to hydrostatic pressure and the weight of the ship itself and all that is carries. A complete study of structural strength should take into account all these forces and in the present day development of the subject this is in fact what is done. It is fitting, however, to examine the problem the static point of view first of all. Static forces on a ship in still water It has been seen that the hydrostatic forces on a floating body or ship in still water provide a vertical force B, say, which is exactly equal to the gravitated force acting on the mass M of the ship, i.e.Mg.Hence B=Mg. If the distribution of these forces along the length of the ship is examined it will be found that the gravitational force per unit length is not equal to the buoyancy per unit length at every point. If the mass per unit length at every point is m and the immersed cross—sectional area at that point is a then Buoyancy per unit length=and Weight per unit length= So that the net unit length is
The ship under these circumastances carries a load of this magnitude whick varies along the length and is therefore loaded like a beam. It follwa that if this load is integrated along the length there will be a force tending to shear the structure so that Shearing force= On integrating a second time the bending moment causing the ship to bend in a longitudinal vertical plane can be determined. Hence Bendinh moment It will be seen that what is called longtitudinal bending of the structure can be distinguished and this generates shear and bending stresses in the material. Longitudinal bending is then a most important aspect of the strength of the structure of a ship and an accurate assessment of the longitudinal shearing force and bending moment is necessary in order to ensure safety of the structure. The accurate determination of the still water force and bending moment is a relatively easy task and whil it does not give a complete picture of the longitudinal bending of the structure at sea it is most useful to calculate these quantities. High values of shear force and bending moment in still water will usually indicate high values at sea, so that from still water calculations it is possible to obtain some idea of loading distributions which are likely to be undesirable. The calculations of shear and bending stresses in the material of the structure will be dealt with later. The other result arising from these forces and moments is that there is overall deflection of the structure ,i.e.the ends of the ship move vertically relative to the certre. When the ends move upwards relative to the centre the ship is said to ‘sag’ and the deck is in compression while the bottom is in tension. If the reverse is the case then the ship ‘hogs’, with the deck in tension and the bottom in compression. The longitudinal bending of the ship due to t
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