#### Further Information

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• ### How do you transform the elasticity and shear modulus properties of a membrane fabric with the usual syntax force/length into the general syntax force/surface for the input in RFEM?

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 directly related to a strip width, ie line (cf. line spring), without considering the thickness.

In contrast, the general FEM program RFEM processes material definitions (E, G, ν, etc.) and surface properties (shell, membrane, etc.) independently of each other. Thus, with the pure definition of the material, it is still unclear whether B. is a rigid plate structure or a tension-loaded flexible membrane structure. The final element specification only results with the additional consideration of the surface properties for the simulation. Therefore, RFEM always requires the description of stiffness in the general unit syntax 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 a reference thickness d :

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

where
F ... Force
L ... Length
d ... Reference thickness
A ... Surface

The stiffness thus transformed in the force/surface format is thus related to the reference thickness used 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.

• ### Are the models and presentations from Info Day 2015 freely available, and can you send them to me?

Yes, the data is freely available. Use the following download option to load the presentations and finished models of the consultants.
• ### How does RFEM effectively determine the shape of a curved plate structure (compression shell) subjected to compression-only?

The effectiveness of a plate structure subjected to compression depends on its shape. For each load distribution, there is an associated optimal shape that transfers the loads to the foundations using as little material as possible.

Figure 01 - Form-Finding of Compression-Loaded Plate Structure

Since the use of a structural system usually specifies several load states, it is recommended to align the optimal shape with the main load (permanent load) of the model for purely strategic reasons.

The RFEM program helps you to find these model shapes in conjunction with the RF-FORM-FINDING add-on module. The underlying form-finding process shifts the position of the FE mesh nodes in space based on a specified load so that the new position of the elements transfers the load mainly via the membrane's load-bearing capacity. The explicit use of the normal force resistance of members and the membrane force resistance of the surface elements results in constant stress states in the cross-sections. This stress-free state makes optimal use of the cross-sections and allows for a very effective structure with minimum material input.

This process can be applied both for the usual cable and membrane structures consisting of flexible elements under tension as well as for pressure-loaded rigid beams and shell structures.

To develop an optimal pressure plate shape in RFEM, proceed as follows:

1. Activation of the RF-FORM-FINDING module in the 'General Data' of the 'Options' tab
Figure 02 - Activation of Form-Finding Options

2. Specification of form-finding settings for the structural elements
Figure 03 - Form-Finding Setting for Beam
Figure 04 - Form-Finding Setting for Surface Element

4. Form-finding analysis
The finally developed shape can then be used in RFEM for the structural analysis with regard to the further variable loads.

• ### In RF-FORM-FINDING, I cannot define the prestress separately for warp and weft. How can I activate it?

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

The procedure is demonstrated in the video.

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

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

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.
• ### Why is the membrane constricted so much during the form-finding? I obtain very large deformation values.

Such an effect could be caused by free edges, which can be deformed almost infinitely during the form-finding process. In this case, it is difficult to achieve the state of equilibrium.

As a basis for the form-finding, there should be no free edges on the membrane surface. There must be cables arranged on the free edges of the membrane whose stiffness can be based, for example, on the folded or strengthened membrane edge strips.
• ### How do I recognize that the flattening process in RF‑CUTTING‑PATTERN has found the convergence?

The geometrically nonlinear flattening process transfers the real mesh geometry of the planar, buckled, curved or double-curved surface components from the selected set of cutting patterns, and flattens these planar components by minimizing the distortion energy, assuming the defined material behavior.

The iterative calculation used for this is controlled by the parameters in the "Calculation Parameters" menu, "Cutting Patterns" tab.

The "Maximum number of iterations" parameter limits the scope of the calculation, and stops the process when the set maximum iteration is reached. If the convergence criterion does not depend on the "Tolerance for convergence criteria" parameter in the convergence range when the maximum iteration has been reached, the program displays Error 10154.

If there is no error message displayed by the program, it is reasonable to assume the proper convergence.

You can usually resolve the error by adjusting the flattening geometry or increasing the maximum number of iterations.

• ### Is it possible to perform the form-finding of membranes regardless of the semi-rigid supporting structure?

Generally, the form-finding process in RFEM always considers all model data. In form-finding, the elastic supporting elements on the membrane react with the deformations in the direction of the membrane tensile stress acting. The integral process gives the prestressed model to the subsequent calculation if the forces of all deformed elements are in equilibrium with the membrane geometry subjected to the specified prestress.

Figure 01 - Form-Finding with Deformed Supporting Structure

However, the flexibility of the supporting structure can be suppressed for the pure form-finding by defining the support with the "Form-finding stage only" nonlinearity (also called form-finding support) at the membrane edges.

In this case, the form-finding process results in a shape whose prestress is in equilibrium with the specified form-finding support forces and the remaining boundary conditions.

For the structural analysis of all other load cases and combinations, the form-finding supports are deactivated, and the form-finding support forces are applied as external loads to keep the global equilibrium on the entire model.

Due to the removal of the supports, the membrane reacts with the semi-rigid supporting structure in the subsequent analyses (LC and CO). Without additional loading, this reaction is similar to the relaxation with the associated reduction of prestress.

• ### Why does form-finding warp membrane surfaces into each other?

The form-finding warps the surfaces into each other because the defined prestress on the boundary line between two membranes does not coincide at the same angle, and thus the affected FE nodes are not in equilibrium with the boundary conditions. In this case, the FE nodes of the membrane surfaces move in the direction of the remaining resultants until the form-finding process finds a solution within the tolerance limit. Since it is usually possible to only achieve this solution with large displacement of FE nodes in space, the surfaces assigned to the FE nodes seem to "swim" into each other.

This behavior results in an unfavorable FE mesh arrangement in conjunction with a singular membrane force distribution. In this case, it is necessary to adjust the prestress in order to reach the equilibrium, or to set an element at the limit situation in order to transfer the forces due to the disequilibrium.

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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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