Objective This study analyzes the impact of initial imperfections and the combined loading of bending and torsion on the ultimate strength, residual bearing capacity, and collapse characteristics of box girders under single/cyclic bending and torsion combined loading.

Method ABAQUS software is used to conduct nonlinear finite element numerical calculations of the ultimate strength of box girders under single bending, combined loading of single bending moment and torsion, cyclic bending, and combined loading of cyclic bending and torsion. The effects of initial imperfections, materials, and loadings are considered.

Results The box girder model incorporating five sets of cyclic amplitudes with welding residual stress shows a maximum variation of 0.51% between the box girder models with the highest and lowest ultimate strengths. Similarly, the box girder model without welding residual stress exhibits a maximum variation of 1.94% between the models with the highest and lowest ultimate strengths. Under combined loading of cyclic bending moment and torsion, the ultimate strength of the box girder is lower than that under cyclic bending moment. The combination of cyclic bending moment and torsion loading more effectively relieves welding residual stresses, and the box girder model with four sets of residual stresses in five cyclic amplitudes exhibits higher ultimate strength than that without welding residual stresses. Once the ultimate strength is reached, the box girder model with both torsion and welding residual stresses has slightly higher residual bearing capacity.

Conclusion The impact of torsion on the ultimate strength of box girders is limited. The findings of this study can provide valuable references for further research on the ultimate strength of ship hull girders under combined loading of cyclic bending moment and torsion.