高張力鋼を使用した船体パネルの設計応力

抄録

Mild steels have mainly been used as ship structural members in the shipyards of Japan except for particular longitudinal members in spite that qualified high tensile steels have been prevailed. In these years, however, there has been a growing tendency using high tensile steels from the economical requirement. Report of the 200 th Research Committee of Shipbuilding Research Association of Japan stated that the amount of high tensile steels used in ship structures would much increase in the near future. Investigation from the standpoint of design aspects as well as from that of economical aspects will be greatly appreciated in these circumstances.
In this paper, the problems of allowable stresses and the decrease of compressive strength of high tensile steel plates subjected to compression is studied. Firstly the ultimate compressive strength of deck and bottom panels when compressive loadings are loaded to ship structures due to the longitudinal bending moment is analysed. Secondly the probability of failure of panels was calculated based on the reliability analysis after the examination of statistical data on longitudinal bending moments.
The analytical method which was developed by the authors using the concepts of limit analysis in the finite deformation was applied to obtain the compressive strength of panels. In order to get probability of failure, a great many statistical data are required. In this paper, the yield stress of materials and initial deflections of panels are considered to be probabilistic variables as factors of strength R, and still water bending stresses and wave induced bending stresses are also assumed to be probabilistic ones as factors of applied stress S.
Reliability analysis method which was originally introduced by Freudenthal et al. and developed by Yasukawa et al. were applied here.
The following results were obtained from the calculations carried out in this paper.
1) When a high tensile steel plate is used as a deck panel instead of a mild steel plate, it is quite natural that the thickness of a high tensile steel plate may be thinner than that of a mild steel one. However, it can be concluded that the allowable stresses of a high tensile steel plate depends on not only yield stresses of materials but also breadth-thickness ratio (b/t) of the plate.
According to the result obtained by the reliability analysis, the ratio of allowable stress of plate used MS and that of plate used HY 32 is given in equation (9).
2) When plates of the same value b/t are used for the bottom and deck plating, the probability of failure of the former is a little smaller than that of the latter. Especially for the bottom plating, it can be said that the longitudinal systems have the advantage of the transverse systems from the aspect of longitudinal compressive strength.