This paper presents a comprehensive numerical study into the in-plane member stability and design of normal and high strength steel (NSS and HSS) non-slender welded I-section beam–columns subjected to compression plus uniaxial bending. Finite element (FE) models were firstly developed to replicate the structural performance of welded I-section beam–columns made of different steel grades, as observed in existing tests collected from the literature. The validated FE models were then employed in a comprehensive parametric study to generate extensive numerical data covering a wide range of cross-section geometries, steel grades, member slendernesses and loading combinations. The numerically obtained data were utilised to evaluate the accuracy of the current design methods set out in the European Standards EN 1993-1-1:2005 and EN 1993-1-12:2007 as well as the American Specification AISC 360-16 for both NSS and HSS non-slender welded I-section beam–columns. The comparisons revealed that the codified design methods yield varying levels of accuracy in predicting the ultimate capacities of welded I-section beam–columns depending on the steel grade. To address this issue, new design proposals that are compatible with the European beam–column design framework have been developed, featuring more accurate yield strength-dependent interaction curves anchored to improved resistance predictions for members subjected to isolated loading scenarios (i.e. the compression and bending end points of the interaction curves). The new design proposals have been found to provide more accurate and consistent resistance predictions than the existing codified design provisions for NSS and HSS non-slender welded I-section members under compression plus uniaxial bending. The reliability of the new proposals has been confirmed through statistical analyses according to EN 1990: 2002.