The structural analysis software RFEM 6 is the basis of a modular software system. The main program RFEM 6 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.
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Since the boundary surfaces of contact solids can only be of the Null Surface type, it is necessary to model additional surfaces with a section. The following is an example of this.
There is a reinforcement of a plate with an additional plate. Both plates are connected by means of a contact solid:
In order to simulate a weld, borders are required on both plates:
You can simply create the bottom border by inserting a new rectangular surface. In this case, the program asks whether the surface should be integrated into the base surface, which must be confirmed by clicking Yes. There are then two openings in the bottom surface; one in the outermost surface in which the surface for the border lies, and one in the surface for the border in which the bottom surface of the contact solid lies:
For the upper border, it is also necessary to create a new surface, and the opening to the upper surface of the contact solid must be inserted manually:
After creating the borders, you can insert the actual surfaces for the weld:
If the reinforcing plate is subjected to tension now, the modeled weld transfers the tension stresses:
Based only on the surface results or surface stresses, it is not possible to make a statement about the buckling behavior of the box girder. The stability behavior can be analyzed using the RF‑STABILITY add-on module. It determines the buckling shapes and critical load factors that allow a statement about the buckling behavior.
However, the buckling design has not yet been provided. For this purpose, the buckling shape would have to be transferred to the model, so it can be calculated according to the second-order analysis on the imperfect structural system. A stress analysis with the RF‑STEEL add-on module could then be used to carry out a buckling design.
The RF‑IMP add-on module facilitates the transfer of the buckling shape. Using this module, you can generate the equivalent geometry based on the stability mode so that you can perform a buckling design with a second-order stress analysis on the predeformed structure using RF‑STEEL.
The procedure in RFEM could look like this:
As an alternative, you can use the PLATE-BUCKLING module to analyze the buckling behavior.
There are some very interesting technical articles on this topic on our website.
It is possible to display or calculate the stresses in RFEM as well as in the RF‑STEEL Surfaces add-on module by using the following smoothing options:
To compare the results correctly, it is necessary to select the same display type and calculation type in RFEM 5 and RF‑STEEL Surfaces.
In RFEM 5, it is possible to do this in Project Navigator Display → Results → Surfaces → Distribution of Internal Forces/Stresses (Image 02). In the RF‑STEEL Surfaces module, this can be displayed or changed in Details → "Options" tab (Image 03).
The stability analysis for plate structures can be converted into a pure stress analysis if the calculation is performed according to the second-order analysis and the imperfection required by the standard has been applied to the structure.
Using the RF‑STABILITY and RF‑IMP add-on modules, you can create imperfections (or a predeformed FE mesh). The type of imperfection depends strongly on the structural component and the corresponding standard. For members that have been modeled as a plate structure, you can use the values from DIN EN 1993‑1‑1:2005 5.3. For planar surfaces, it is possible to use the values from DIN EN 1993‑1‑5:2006, Annex C, for example. For shells, the problem is much more complex and there are different approaches. I would advise against the generation of imperfections and perform the plate buckling design using the MNA/LBA concept according to DIN EN 1993‑1‑6, which does not require the application of an imperfection.
For example, if you want to design a surface model of a steel beam, you can proceed as follows:
1. Select a load with relatively high axial forces (compared to other internal forces in the load case), in most cases you can use the self-weight load case or a load combination with the corresponding self-weight. It may be necessary for each load combination to have an individual imperfection.
2. Calculate the load combination according to the linear static analysis and use it as a basis for RF‑STABILITY.
3. Find the first mode shape of a global failure using RF‑STABILITY.
4. Use the calculated mode shape as a basis for an imperfection using RF‑IMP. It is possible to apply 1/300 of the beam length as an amplitude, for example.
5. Create a load case combination that uses the created imperfection as a basis and is calculated according to the second-order analysis.
6. Perform a stress analysis using this load combination, which is also the stability analysis of the structure.
Yes, it is possible. The "Generate Loads → From Surface Load to Openings" function allows you to do just that (see Image 01). For the load, you can select in which direction the load should act, the area load distribution, the area load direction, the load distribution type, and of course, the surface load (see Image 02).
Thus, it is possible to set the loads (for example) for non-load bearing elements such as windows.