Stability failure due to plate buckling presents a central challenge in steel construction, in particular in the design of thin-walled, plate-shaped load-bearing structures. According to EN 1993-1-5 and the new prEN 1993-1-14, load-bearing capacity can be verified by means of analytical formulae or the finite element method (FE method). A geometrically and materially non-linear analysis with imperfections (GMNIA) requires the consideration of suitable types and forms of imperfection. The variety of possible imperfection approaches and their combination complicates their conservative application significantly. This study considers the ultimate loads for stiffened plates using GMNIA and investigates simplified imperfection approaches. The results show that a global imperfection with e0=min{a/400; b/400} in the negative direction leads to the minimum ultimate load in the majority of cases. A modified global imperfection with e0=max{a/600; b/350} in the negative direction results in the minimum ultimate load in an even larger number of cases. This enables conservative FE modelling practical for the construction industry with the use of only one imperfection.