The determination of thermodeformed state of large-size structures is a crucial step in optimizing assembly welding and assessing structural reliability. Conducting extensive laboratory tests for this purpose can be challenging, which high-lights the need for modern modeling methods to predict stress and strain kinetics and its impact on the structural integrity under different external loads. However, the resource-intensive nature of computer simulations, particularly in finite-element analysis, imposes limitations on numerical analyses. To overcome these limitations and reduce computational time and resource requirements, a multi-scale modeling methodology has been developed. This methodology involves two levels of analysis: the level of welding stresses and strains, and the level of macroscopic deformation of the entire structure. The connection between these levels is achieved through the averaging of properties and numerical parameters of specific finite elements. The developed methodology has been successfully applied to a typical case study involving the assembly welding of a large aluminum pressure vessel. The study also demonstrates the limitations of the proposed approach, highlighting its applicability.