Parameters from Cross-Sections
Tips & Tricks
In RFEM and RSTAB, parameterization provides you many options, especially for recurring structural elements. Within the parameterization tool, you can access the internal values of a model, for example the values of a selected cross‑section. The following example shows you how this can work.
The aim is to have a surface load of p = 50 kN/m² acting only on a girder flange. This cross‑section is a square tube with the flange width of 200 mm; you should take this width from the cross‑section properties in order to convert the surface load into a corresponding line load. There are also several ways to do that. The figure shows one of them.
First of all, insert a new “Line Load - Graphically”. In the Load Parameters section, click the little black triangle to the right of the value to open the “Edit Formulas” window. Here you can also enter the formula directly. However, this is not the purpose of our example. In order to control the input values by using a parameter list, you need to create these parameters first. Click the [Edit parameters] button to open the window of the same name.
Define three parameters:
- p for surface load (Force per Area) in kN/m²
- b for length in mm
- l for line load (Line Force) in kN/m
As you can see in the list of parameters, the formulas for b and l are recorded and therefore the “Value” cells are shaded in gray. The formula of b for the flange width is especially important here. This involves two special formulas.
- The formula Cross Section() is embedded in the formula GetCsPar(;). CrossSection(n) is taken as a string from the n‑th row of the cross‑section table.
- GetCsPar ("Cross-section", "ID") requires the cross‑section as a string and the value ID that is sought. You can obtain the ID from the window “Edit parameters” using the Info button, as shown in the figure.
There are a lot of parameters that can help you also find the eccentric loads, for example. After you create the parameters, you can apply the l parameter in the “Edit Formulas” window of the line load. Then, close the window and also the line load window and you will now see the correct line load.
You can do any changes in the surface load by means of “Edit” → “Edit Parameters” and the line load is automatically adjusted with a click on [OK], just like it is after cross-section modification in row 1 of the cross‑section table.
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The structural engineering software for design of frame, beam and truss structures, performing linear and nonlinear calculations of internal forces, deformations, and support reactions