This paper presents the importance of the second-order effects in behaviour of steel frame structures. EN 1993-1-1 were distinguished three main methods: taking / without taking into account second order effects depending on the sensitivity of horizontal design effects and tolerance. Using these methods, the susceptibility of steel frame structures to second-order impacts is considered in accordance with EN 1993-1-1. The study was performed taking into account also the rotational stiffness of the joints of the frame structure. In this way, trying to determine the effect of the joint on the rotational rigidity makes the frame structure more sensitive to second order impacts. It is also considered a semi-rigid joints effects the general frame-tailed P-Δ and the second row stress. This establishes the link between the effects of the second-order and the semi-rigid joints in the overall (general) analysis. Frame-building sensitivity makes it possible to compare and measure the importance of such dependencies.Keywords: steel frame, structural analysis, second order analysis, semi-rigid joints, finite element method, sensitivity of the second order effects.

The evaluation of second-order effects of steel framed structures can provide different analysis results than using linear analysis methods. In various structural engineering literature were distinguished different methods of analysis: taking or without taking into account second-order effects. It depends on the sensitivity to the horizontal actions. The slenderer the structure, the more sensitive it is to horizontal actions. Using nonlinear methods, the sensitivity of steel frame to second-order impact is considered. This paper shows the importance of evaluations of the second-order effects in behaviour of steel frame structures. Performed investigations reveal the influence of the rotational stiffness of the joints to the behaviour of whole framed structure. Calculation results show that decreased flexibility of the semi-rigid joints increase sensitivity of the framed structure to the second-order effects and vice versa. The identified interdependence between the sensitivity to the second-order effects and the flexibility of the semi-rigid joints highlights the importance of evaluation of such dependencies.

This paper describes the use of stainless‐steel as a structural material, discusses current structural design considerations, reviews recent research work, and highlights important findings and developments. Over the past 25 years, structural design codes for stainless‐steel have been introduced or substantially altered or are still being modified, while at the same time there is a growing interest in the use of stainless‐steel in construction. Realistic modeling of structural elements of stainless‐steel plays an important role in the analysis and design of steel framed structures. Structural element modeling and optimization using stainless‐steel, in case a built‐up column, can contribute to existing research on stainless‐steel. The object under consideration is a stainless‐steel built‐up column, which is classified as atypical structural elements. The purpose of this optimization problem is to perform the minimization of the weight of the selected built‐up column, when the height of the column and the load acting on it are known. The built‐up column is considered with two different types of cross‐sectional chords: at first with circular (CHS) cross‐sectional chords and then with elliptical (EHS) cross‐sectional chords. A genetic algorithm based optimum design is presented for such a built‐up column. The design algorithm obtains a built‐up column with the least weight by selecting appropriate sections from standard set of duplex stainless‐steel hollow elliptical cross‐sections. The optimization model accounts for the strength and stability constraints. The built‐up column is under action of compressive axial force, bending moment and combination compressive axial force and bending moment. The parameters under consideration include the height and width of the outer cross‐section of the column's chords, the ratio of the outer cross‐sectional dimensions to the thickness and the initial eccentricity of the load. The computation results showed that the chosen optimization model and genetic algorithm allow efficiently obtaining the column of minimum weight, where the variables are column chord cross‐section and diagonals cross‐section. The design algorithm used to solve the problem is described in detail in this work. Until today, there is known that the lack of information prevented the recommended methodology from being properly established. Because of this, the obtained results confirmed that these types of structural elements of the steel in question are needed to develop a more efficient design of stainless‐steel structures in the design method and thus contribute to its wider use in construction.