Objectives  In order to efficiently reduce the bending stress of a grillages in a rectangular cabin under internal pressure, mathematical models for the optimization of the vertical positions of platforms and size and layout of pillars are proposed respectively.

  Methods  The vertical positions of the two internal platforms are taken as the design variables, the maximum bending stress of the transverse and longitudinal bulkhead structure is minimized, and the optimal positions of the internal platforms are obtained via a genetic algorithm. The optimization results show that the best positioning of platforms is close to the vertical uniform distribution. A stepwise optimal pillar design method is proposed. First, the maximum bending stress of the top deck structure is minimized by taking the positions of pillars with the same stiffness as the design variables. Through the repeated use of the model, optimal layout schemes under different numbers of pillars can be obtained in succession. The number and layout of the pillars are then selected according to the stress constraints. To further reduce the maximum bending stress of the top deck structure under a given number and layout of pillars, a mathematical model for the optimal variable stiffness of pillars is proposed. In this study, pillar cross-section size is treated as a design variable, the weight of the pillars in the previous round of optimization design is treated as the constraint, and the maximum bending stress of the top deck structure is minimized.

  Results  The optimization results show that the pillars in the central zone are large than those in other regions. By using the proposed optimal design models, the maximum bending stress of the transverse and longitudinal bulkheads and top deck is reduced by 28.3%, 25.7% and 13.9% respectively.

  Conclusions  The proposed method can provide reference points for comparable structural design.