عنوان مقاله [English]
In this paper, some material and geometrical nonlinear effects, and methods of second order nonlinear advanced analysis are investigated. Numerical methods are used to investigate the effect of replacing concentric braces on steel frame performance. Three, six, ten and twenty story 2D frames with eight different arrays of braces and four loadings are analyzed. ANSYS finite element software was utilized to perform second order nonlinear analysis and Lateral Load-Displacement curves were drawn. Curves linear regression was calculated to define new lateral stiffness showing the frames performance. The method used in this investigation is plastic region second-order nonlinear analysis and is applied to structural system analysis, based on the modified finite element method. The plastic-elastic hinge model refers to the simplest analysis. By comparison, the plastic-elastic region model illustrates the most reformations. The plastic region analysis method models plastic development in the whole structure. This method is performed in two ways. The first one is lattice, using finite element, and the second is based on the Beam-Column theory. The second order nonlinear analysis used in this paper enumerated plastic region advanced analysis, and includes nonlinearity in geometry and material, like second order effects (P-$\pounds$ and P-$\Delta$), redistribution of internal loads, because of plastic region formation, lateral stiffness degradation, based on steel yielding, and shear deformations. To apply all the mentioned factors in the analysis, many methods and codes were suggested by other authors. For example, the AISC-LRFD code applies an effective length factor, enhancement factor and interaction design term of the mentioned factors in analysis and design. The reason is because of the low quality of computers when codes were originally established. Second order nonlinear advanced analysis could apply all the mentioned terms in the design procedure directly. This method assimilates analysis and design, so, designers do not need different codes. It is hoped that these methods will be used as a common structural analysis/design method by developing computers. Each analysis that could determine the strength and stability of structural systems and isolated members in such a way that does not need to control isolated members capacity and the definition of effective length coefficient is enumerated as advanced analysis. This method gives much information to designers about the behavior of structures exposed to external loads and environmental conditions. In this investigation, a BEAM element was utilized to model structures. Results show that applying the method used in ANSYS could determine the frame ultimate loading coefficient of Vogel Frame by 3 percent tolerance, compared to the plastic region analysis method and plastic hinge analysis. Performing about eighty analyses denoted that changing bracing arrays could increase or decrease ultimate strength, ultimate displacement and lateral stiffness multiple times. To increase the ultimate strength coefficient, the best option is using an X-brace frame. Furthermore, applying diagonal bracing in off side spans could decline the lateral displacement of the structure. Frames without a bracing system and with Chevron and eccentric bracing systems demonstrate the most nonlinearity.