Abstract
Recently, in a certain ship (three islander of length 128 metre), some cracks occurred at the corners of the entrance of bridge aft bulkhead, and it seems to be impossible to explain the cause of these damages by the old theories. The author analysed this cause that, the deck girder were pushed up through a pillar by the action of bottom water pressure and as a result of this displacement the plate around the entrance were subjected to the shearing strain and the large stresses occurred at the corners of the entrance. The cause of damages of this kind seems to be cleared by this analysis, because the positions, where the cracks occurred in actual ship, can thereby be understood easily, although this analysis contains some assumptions about the load condition, boundary conditions of the girders, effective breadth of the deck plate, stress concentration factor at the corners of the entrance etc..
The counter-measures of damages of this kind considered as a result of the above analysis are
1) to enlarge the radius of corners of the entrance as large as possible.
2) to change the connections of bridge aft bulkhead from weld to rivet and if possible make some expansion joints in order to prevent the transmission of shearing strain to bulkhead plate.
3) to reinforce the corner parts of the entrance, where the stress concentrates locally.
The author thinks that 1) and 2) are good, especially 2) is available and advisable, because, by the analysis of this paper, the cause of occurrence of cracks is related to strain and the displacement itself is about 1mm, although the stresses of the corners are very large.
