# Frequently Asked Questions (FAQ)

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• ### How will I get the actual member length of e.g. cables after the calculation by RF-FORM-FINDING?

#### Answer

This information is available in the Results navigator for members (see Figure 01). There, the member lengths are displayed in relation to the loaded system and the unloaded system. The "Loaded length" is obtained from the form-finding under consideration of the prestresses

The recalculation to the "unloaded length" can be done manually using Hooke's law:

${\mathrm l}_{\mathrm{unloaded}}\;=\;{\mathrm l}_{\mathrm{loaded}}\;-\;\mathrm{Δl}\\\\\mathrm\sigma\;=\;\mathrm E\;\cdot\;\mathrm\varepsilon\\\frac{\mathrm F}{\mathrm A}\;=\;\mathrm E\;\cdot\;\frac{\mathrm{Δl}}{\mathrm l}\\\mathrm{Δl}\;=\;\frac{\mathrm F\;\cdot\;\mathrm l}{\mathrm E\;\cdot\;\mathrm A}\\\\{\mathrm l}_{\mathrm{unloaded}}\;=\;{\mathrm l}_{\mathrm{loaded}}\;-\;\frac{\mathrm F\;\cdot\;{\mathrm l}_{\mathrm{loaded}}}{\mathrm E\;\cdot\;\mathrm A}\;=\;{\mathrm l}_{\mathrm{loaded}}\;\cdot\;\left(1-\;\frac{\mathrm F\;}{\mathrm E\;\cdot\;\mathrm A}\right)$

• ### Is it also possible to specify a cable length in RF‑FORM‑FINDING?

#### Answer

In addition to a prestressing force or the target cable sag, it is also possible to specify the cable length, as shown in Figure 01.

The program then tries to fit the cable subjected to the acting force (a load case of the "form-finding" type with a dead load, for example) in the way that the length corresponds to the specified length.

• ### How is it possible to create the cutting pattern geometry for a decorative sticker on a biaxially curved membrane roof?

#### Answer

The geometry of a sticker on a biaxially curved membrane roof depends on its position on the roof and the projection direction of it. To ensure that the sticker elements can be applied to the curved surface without any creases, flatten the projection of the graphic to be displayed on the curved surface. You can perform the geometric transversion in RFEM as follows:

1. Import the graphic to be displayed as a background layer into the model by using the DXF interface.

2. Depending on the imported DXF graphic, define cutting lines on the curved basic shape in the desired projection direction.

3. Create nodes on the basic shape at the intersection points of the intersection lines and the respective intermediate areas.

4. Copy the newly created node construct on the basic shape into a new model file.

5. Create spline lines between the intersection and intermediate nodes in the new model for each section.

6. Define a membrane surface element, depending on the material properties of the sticker in the surface surrounding the spline lines.

7. Create a cutting pattern for each surface unit and calculate the flattened sticker geometries.
• ### Why does the form-finding not work for the "Cable on Pulleys" member type?

#### Answer

The "Cable on Pulleys" member type can only describe the forces N and displacements u‑x in the direction of the cable. Therefore, it cannot be combined with regard to the form‑finding, for example, at the edge of membrane surfaces.

For this, see also Chapter 4.17 in the online manual for RFEM 5.

• ### How are the properties of the elasticity and shear modulus of a membrane fabric with the usual force/length syntax transformed into the general force/surface syntax to be entered in RFEM?

#### Answer

The thickness of membranes is usually very thin compared to the planar extension. Due to these extreme geometric conditions, the stiffness of membrane fabrics is usually related directly to a strip width, that is the line (compare with a line spring), without considering the thickness.

In contrast, the general FEA software RFEM processes the material definitions (E, G, ν, and so on) and surface properties (shell, membrane, and so on) independently of each other. Thus, the pure definition of the material still does not clarify whether there is a rigid plate structure or a flexible membrane structure subjected to a tensile load. The final element specification is not clear until the surface properties are considered additionally for the simulation. Therefore, RFEM always requires the description of stiffness in the general unit syntax of force/surface, regardless of the geometric conditions of the structural component to be simulated.

Thus, the line-related membrane stiffness in the force/length syntax can be transferred to the force/surface syntax in RFEM by considering the reference thickness d:

$\frac{\mathrm F}{\mathrm A}=\frac{\left({\displaystyle\frac{\mathrm F}{\mathrm L}}\right)}{\mathrm d}$

where
F is the force,
L is the length,
d is the reference thickness,
A is the surface.

The stiffness transformed into the force/surface format in this way is thus related to the reference thickness and can convert the initially specified membrane stiffness in the force/length format in RFEM by specifying the reference thickness d as the membrane surface thickness.

• ### When generating the FE mesh of an ETFE cushion, some problems occur. I cannot find any mistake. What can I do?

#### Answer

In most cases, the problem is that the finite elements of the cover surfaces are very close to each other in the edge area of the cushion when using Bézier surfaces. The FE mesh nodes of the bottom and top surface might be combined then. In principle, there are two approaches:

1. Numerical solution of the problem:

You can adjust the tolerance for combining FE mesh nodes by using the menu "Tools" → "Regenerate model."

2. Geometrical solution of the problem:

A more workable solution is to increase the distance between surfaces by using the boundary surfaces as shown in Figure 01.
• ### Are the models and presentations from Info Day 2015 freely available, and can you send them to me?

#### Answer

Yes, the data is freely available. You can download the presentations and finished models by the speakers under Downloads below.
• ### I cannot define the prestress separately for warp and weft in RF‑FORM‑FINDING. How can I activate it?

#### Answer

The warp and weft directions are linked to the axes of the surface. The default setting allows you to only apply isotropic prestress. If the axes are aligned, the orthotropic or radial prestress will be available as well.

The video shows the procedure.

• ### How can I influence the sag of the found shape in RF‑FORM‑FINDING?

#### Answer

The sag can be influenced by the prestress of the membrane. Please note that the uniform increment in both directions does not cause any changes in the shape. Thus, a modification must be made in the desired direction only. The video shows the procedure.
• ### It seems that the members stay not deformed after my RF-FORM-FINDING calculation. What went wrong?

#### Answer

Deformation of members are displayed as single colored lines per default. Please let them displayed as colored cross-sections. Therefore, it is always a good idea to decrease the Display factor by the deformation to 1.0, otherwise, the results might be misinterpreted.

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If not, contact us via our free e-mail, chat, or forum support, or send us your question via the online form.

#### First Steps

We provide hints and tips to help you get started with the main programs RFEM and RSTAB.

#### Wind Simulation & Wind Load Generation

With the stand-alone program RWIND Simulation, wind flows around simple or complex structures can be simulated by means of a digital wind tunnel.

The generated wind loads acting on these objects can be imported to RFEM or RSTAB.

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