Document Type : Article
Authors
1
Research institute of new technology in Civil Engineering Semnan University
2
Dept. of Civil Engineering Islamic Azad University, Bandar-Abbas
Abstract
Various factors affect the behavior of steel structures, including: second-order effects, redistribution of internal forces due to formation of plastic hinges, residual stresses, degradation of element stiffness due to the plastification of steel, and initial geometric imperfections. The effect of geometrical imperfections on the behavior of frames may be high or low. In bracing structures, the out-of- straightness imperfection of the member, and in unbracing structures, the out-of-plumbness imperfection of the member, are further effects. According to the AISC-LRFD code, maximum abrication and erection tolerance for out-of- plumbness is limited to 1/500 times the column length. Obviously, analytical ethods that can further involve the nonlinear elements of their calculations, directly and accurately, are more advanced. Various methods of analysis, in order to be advanced, are included: first-order elastic analysis, second-order elastic analysis, first-order inelastic advanced analysis and second-order inelastic advanced analysis. Advanced analysis mentions that any analysis/design that directly affects key factors can influence the behavior of the structure in the process of analysis. The types of method for implementing advanced analysis are: elastic-plastic hinge method, refined plastic-hinge method and plastic-zone method. The elastic-plastic hinge method is the simplest approximation of the inelastic behavior of material by assuming that all its inelastic effects are concentrated at plastic hinge locations. In this idealization, it assumes that the element remains elastic, except at the ends, where zero-length plastic hinges can form. This method accounts for inelasticity but not the spread of yielding or plasticity at sections, or the influence of residual stresses. The refined plastic-hinge methods are based on some simple modifications of the elastic-plastic hinge method. The notional-load concept is first introduced to the conventional elastic-plastic hinge method by applying additional fictitious equivalent lateral loads to account for the influence of residual stresses, member imperfections, and distributed plasticity that are not included in conventional procedures. Second-order elastic analysis methods are used by most researchers to assess the initial out-of- plumbness imperfection of columns, and are inaccurate for highly indeterminate structures. No researchers have studied the behavior of structures with initial out-of- plumbness imperfection of columns using advanced analysis. The purpose of this paper is to present a new method for considering the effects of the initial out-of- plumbness imperfection of columns, with much accuracy. For this purpose, the proposed method is based on the plastic-zone advanced analysis theory. This method is accurate and all factors mentioned above can be involved in the calculations directly, being based on the refined finite element method. The plastic zone method models the spread of the plastification volume of the structure directly and explicitly. In general, this style is done in two ways. One is networking with finite elements, and the other is based on beam-column theory. In the beam- column method, every member is divided into several beam-column components and its section is divided into several fibre components. Some Software has the ability to undertake the analysis, accurately. Of these, ANSYS and ABAQUS are named. The plastic zone analysis used in this study, with ANSYS software, is a beam-column plastic zone. In this study, three bending steel frames of three types; without initial out-of-plumbness imperfection, and an initial out-of-plumbness of 1/500 and 1/200, are analyzed by the proposed advanced plastic-zone analysis method, and the load-displacement curves of each case are plotted. For popular analytical results, in each frame, three types of loading have been considered, and for contribution to the effects of gravity loads, it also increases, associated with side loads and to the same factor. The effect of out-of- plumbness imperfection begins in the structural linear behavior range, and reduces the resistance and stiffness of the structure. In the large gravity loads range, this effect is larger. In a single story, Vogels frame and a twenty story frame, 1/500 out-of- plumbness imperfection would be reduced to 1, 3 and 7 percent of the resistance of the tructure, respectively. For Vogels six-story frame, the ultimate load from the proposed method by ANSYS software compares well with Vogels plastic zone and Kims plastic hinge method, with max 3\% difference.
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