To fully consider intact initial imperfections, including welding initial deformation and residual stress, it is necessary to conduct a study on the plastic deformation distribution and ultimate strength of a box girder under different cyclic bending moments.

First, an isotropic material model and the Chaboche material model are chosen. Using the Python language, programs are developed to directly apply the initial deformation in finite element software. Subsequently, ABAQUS software is employed to conduct nonlinear finite element numerical simulations of the ultimate strength of a box girder under different cyclic bending moment patterns. The analysis considers the influence of welding initial deformation, residual stress, material hardening and the Bauschinger effect.

The results show that under unidirectional cyclic bending moments, the bending stiffness and ultimate strength of the box girder decrease continuously as the number of cycles increases. The plastic deformation extends from the upper deck of the box girder to the side plate region. The plastic distribution region of the box girder with welding initial deformation and residual stress is wider in the ultimate state. Compared to a single loading and considering only the initial deformation, after three bidirectional cycles, the ultimate strength of the box girder decreases by 10.44% to 15.15%. Considering intact initial imperfections, the ultimate strength decreases by 8.41% to 14.50%.

The ultimate strength of a box girder considering intact initial imperfections under different cyclic bending moments decreases more moderately. The obtained results can serve as a reference for the study of the ultimate strength of box girders under different cyclic bending moments.