Effective arrangement of transverse strucrtural members of car carriers in preventing excessive racking deformation

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Wide openings have been arranged in the transverse bulkheads of roll on/roll off ships in order to keep good behicles access on decks, which is further pursued in recent design by eliminating the transverse bulkheads in the upper part of the freeboard deck of a hull girder. Since its transverse sections are very weakly stiffened, they often exhibit significant shearing deformation so-called racking due to asymmetric loads with respect to the center line of the hull girder.We first consider the effects of transverse partial bulkheads and strong web frames, whose relative rigidities to the main hull girder are defined in a non-dimensional form. Using these relative rigidities, we propose a simple formula to predict the racking deformation at the sections where the bulkheads or strong web frames are installed. The predicted values are found to be in good agreement with the numerical calculations.Considerations are then made for the effects of the flexural rigidities of the transverse frames and the thickness of the shell platings, respectively. Then the combined effects of the bulkheads/strong web frames and transverse frames/shell platings are examined by numerical calculations. It is found that in some cases the combined effects may reduce the racking deformation 10-20 percents.Finally case studies are made for two actual car carriers, which have essentially different structural configurations; the one has partial bulkheads and the other has strong web frames. The racking deformation estimated by the proposed simple method is in conformity with the results ob<ained by the complex three-dimensional hull girder analysis.