Considering Member Imperfections in RFEM 6
Imperfections in construction engineering are associated with production-related deviation of structural components from their ideal shape. They are often used in a calculation to determine the equilibrium of forces for structural components on a deformed system.
In RFEM 6, imperfections are related to imperfection cases that can be defined using the Data tab of the Navigator as shown in Image 1. The imperfection type can be set in the Imperfection Case window. The available types are: local imperfections, notional loads from a load case, initial sway via table, static deformation, and group of imperfection cases (Image 2). To represent the member's deviation from its ideal shape, it is important to choose local imperfections as an imperfection type.
In the Assignment tab of the same window, the imperfection case can be assigned to existing loads and/or load combinations (Image 3). Alternatively, the imperfection case in terms of load cases and/or load combinations can be considered using the Load Cases and Combinations window (Image 4).
In RFEM 6, the deviation of the structural components from their ideal shape can be represented by Member / Member Set imperfections as a local imperfection type (Image 5). These member imperfections can be defined as an initial sway or an initial bow (Image 6). The initial sway simulates a component as inclined over its entire length, whereas the initial bow simulates a straight structural element as a bow (Image 7). For the latter, activity criteria can be also set (e.g., always, according to standard, or with respect to manually defined member slenderness).
Both the initial sway and the initial bow can be defined as absolute or relative values. In addition, they can be defined according to the standards available for selection under "Definition Type". Finally, the coordinate system (local or principal) and the direction with respect to which the imperfection is simulated should be selected.
In RFEM 6, the deviation of a structural component from an ideal shape can be simulated by member imperfections. First, it is necessary to create imperfection case with "Local Imperfections" as the imperfection type and relate them to existing loads and/or load combinations. Later, the member imperfections can be defined in terms of separate imperfection cases. These imperfections can be represented by initial sway or initial bow with respect to the member's local or principal coordinate system. For the latter, activity criteria can also be set.
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The stability checks for the equivalent member design according to EN 1993-1-1, AISC 360, CSA S16, and other international standards require consideration of the design length (i.e., the effective length of the members). In RFEM 6, it is possible to determine the effective length manually by assigning nodal supports and effective length factors or, on the other hand, by importing it from the stability analysis. Both options will be demonstrated in this article by determining the effective length of the framed column in Image 1.
For the joint components, it is possible to check whether the stability failure is relevant (requires the Structure Stability add-on for RFEM 6 / RSTAB 9).
In this case, the critical load factor for all analyzed load combinations and the selected number of mode shapes is calculated for the connection model. The smallest critical load factor is compared with the limit value 15 from the standard EN 1993‑1‑1, Clause 5. Furthermore, a user-defined adjustment of the limit value is possible. Moreover, the corresponding mode shapes are displayed graphically as the result of the stability analysis.
For the stability analysis, an adapted surface model is used to specifically recognize the local buckling shapes. The model of the stability analysis, including the results, can also be saved and used as a separate model file.
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RFEM 6 structural analysis software is the basis of a modular software system. The main RFEM 6 program is used to define structures, materials, and loads of planar and spatial structural systems consisting of plates, walls, shells, and members. The program also allows you to create combined structures as well as to model solid and contact elements.