الفهرس 
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Abstract
Recently, the higher strength steel is widely used in ship structure. Especially, in the parts subjected to high longitudinal bending stresses, such as the deck and the bottom of ship.
Therefore, these structural members are recommended to built using the higher strength steel.
Using such steel in ship structure lead to lighter scantlings, then the dead weight capacity of ship will be increased. However, the reduction of thickness will lead to a lower buckling strength. So, the buckling failure is most significant mode with using such steel. Therefore, the possible modes of buckling of stiffened plate are studied. Based on known behavior of structural members it is show, that the buckling of plate panels between stiffeners (local buckling) is less dangerous among other modes of buckling, since the local buckling may not directly lead to a collapse.
In this work, a new design philosophy allowing the local buckling is established for perfectly flat plate panel. The maximum allowable stress, the ultimate strength and the allowed deflection of the plate panel are the main criteria to ensure that the structural strength is equivalent to that of conventional ship constructed of the ordinary strength steel.
The analytical studies are performed to know the effect of stiffener space, plate thickness and material yielding strength on the carrying capacity per unit breadth of the plate panel. Moreover, the deflection of the replaced plates subjected to an uniaxial compression have to be less than or equal to the allowable deflection.
Finally, the strength analysis and the economical analysis are applied to two types of large ships, a bulk carrier and a large crude carrier, to determine the saving weight and the total cost of the composite ship.
Hence, the deck and bottom constructed with higher strength steel while the other parts are built with ordinary strength steel.
This composite structure gives the optimum design of ship hull. That means, less weight with less cost.