Local loads such as water pressure and cargo loads reduce the ultimate longitudinal bending strength of ship hull girder in hogging condition. In the present paper, the authors have developed a simplified method to estimate the stress distribution in the double bottom structure under local loads, based on mechanical consideration. The developed method needs only dimensions of cross section of primary members, and does not require numerical calculations such as FEA.
Recently, the structural requirements of the ultimate longitudinal bending strength based on Smith's method (Smith, C.S. 1977; Dow, R.S., 1981) have been introduced into various structural standards such as Common Structural Rules (CSR, 2006). However, it has been revealed in recent investigations (Fujikubo, 2015; Amlashi H.K.K. and Torgeir Moan, 2008, 2009) that local loads such as water pressure and cargo loads reduce the ultimate longitudinal bending strength of ship hull girder in hogging condition, because double bottom structures are pushed upward in way of hold spaces by water pressure, and biaxial stresses are induced in the bottom panels significantly as shown in Fig. 1. Such stress can be regarded as "initial stresses" when estimating the ultimate hull girder strength by Smith's method. In other words, the collapse may occur below the estimated bending moment without considering local load effects.
In the present paper, the authors have developed a simplified method and formula to estimate the stress distribution in the double bottom structure under local loads based on mechanical consideration. Regarding the formularization, a double bottom structure is regarded as an anisotropic plate for simplification, and a deformation of the double bottom under local loads is derived by using energy method, whereas the other structural members which affect the stress distribution on the double bottom are also taken into consideration by each mechanical modeling.
