Due to inevitability of initial geometric imperfections in the structure and necessity of considering them in direct analysis approach, determining the amount of geometric imperfections of the members is essential on the behavior and performance of the structure. These imperfections include out-of-plumb and out-of-straightness and significantly reduce the buckling resistance and stiffness of the members.Therefore, it is necessary to consider these imperfection in the analysis of stability and design of structures as two important patterns of member imperfections. At the beginning of this research, the eva‎luation of capabilities of the Abaqus software for accuracy in obtaining results and simplicity in modeling has been specified. As a result, the verification of steel frames with variable sections using the spectral finite element method in Mathematica and SAP2000 software has been achieved. This capability leads to medeling of geometric imperfections of members using this software, which is one of the most accurate finite element software. In this research, the modeling of geometric imperfections of the members has been carried out by adopting a linear combination of buckling modes according to the scientific refrence of Shayan et al. in 2014. In this regard, firstly, an introduction about Abaqus software and its modeling method has been presented. To verify the accuracy of the mentioned method and ensure the correctness of the results, several steel frames with constant sections have been verified with the results of obtained from the mentioned refrence. Then, modeling of several steel frames with variable sections using Static Riks nonlinear analysis has been discussed. Finally, different types of patterns for applying geometric imperfections are implemented using a linear combination of buckling modes on variable section frames. The results of this method have been compared with the results of applying geometric imperfections in valid codes, including the American and Iranian codes. By comparing the mentioned methodes, it can be concluded that the modeling of initial geometric imperfections using the linear combinations of buckling modes is a more accurate eva‎luation. It includes the final capacity of steel frames with variable sections compared to the method introduced in the codes (NHF method). In other words, the methodes approved in international codes in some cases designs the structure contray to reliability and are considered non-conservative.

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سعيد صرامي , عبدالرضا عطائي