In recent years, the construction industry has witnessed a growing preference for welded box-sections due to their ease of fabrication and improved stability characteristics. Nonetheless, welded box-sections are prone to various instabilities, which can be classified into three categories: global buckling, local buckling, and interactive buckling. amongst these, interactive buckling is of primary concern as research in this area is still limited. Currently, there is a lack of viable options to effectively address the nonlinear interaction of global and local buckling. The equivalent imperfections provided by the Eurocode were developed based on an elastic design approach and are not suitable for geometrically and materially nonlinear analysis (GMNIA). Therefore, the applicable equivalent geometric imperfections should be revised and improved to make them suitable for plastic analysis. The authors previously investigated the interactive buckling resistance of welded square box-sections and developed suitable imperfections that can be used for GMNI analysis. However, the study was focused only on square box-sections. In the current research, the interactive buckling capacities of square and rectangular welded box-sections are investigated. A general rule for imperfection combination is proposed, which can be applied to all welded box-section columns. This makes the application of numerical model-based design more accurate for box-section columns, as well as significantly improves the general applicability of GMNI analysis-based resistance calculations.