Frequently Asked Questions (FAQ)

Further Information

In addition to our technical support (e.g. via chat), you’ll find resources on our website that may help you with your design using Dlubal Software.

Receive information including news, useful tips, scheduled events, special offers, and vouchers on a regular basis.

• 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?

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.

• 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.

• How can I smooth continuous boundary lines of membrane cuttings?

The perimeter of membrane cutting patterns is described by boundary lines. These boundary lines can be as follows:

1. Globally defined boundary lines of the assigned membrane surfaces, or
2. Cutting lines applied subsequently for the distribution of cutting patterns on the membrane surfaces
In this case, the cutting patterns can only consist of a global line definition or the cutting lines applied subsequently, or the mixtures of both types.

Figure 01 - Boundary Lines of Cutting Patterns

The global boundary lines are untouchable due to their fixed geometry description (arc, circle, spline, and so on), and are also implemented in the design of planar cutting patterns in this way.

On the other hand, the subsequently applied cutting lines are based on the FE mesh of the surfaces assigned in the cutting line specification and have no influence on the mesh itself.

The cutting units encircled by the boundary lines and cutting lines take over the FE mesh of the assigned surfaces for flattening. Since the cutting lines run through the FE elements themselves, with no regard to the global mesh in the edge area, the edges of the original FE elements cannot be used to describe the cutting pattern limitation. In this case, the affected FE elements within the cutting line area are divided by the cutting lines.

Depending on the orientation of the cutting line, the FE elements are cut in the middle or almost on the edge. Since the FE elements on the edge can cause some problems with the geometry, a certain tolerance limit has been entered for the decision. This limit controls the critical length ratio between the FE edge length specified by the cutting line and the original FE edge length. If the ratio is smaller than the given limiting value, the cutting line refers to the original FE node.

Figure 03 - Smoothing

This fact may lead to an "Irritation" of the cutting line if the cutting lines are close to the FE element edges. This situation can be optimized by reducing the given tolerance limit.

• Why is the compensated cutting pattern length of a membrane surface incompatible with the defined compensation?

The integral flattening process in RF-CUTTING-PATTERN does not flatten each cutting pattern individually, but rather the complete model geometry in one step. In this case, the line type of the respective cutting pattern units also has an effect on the adjacent cutting patterns.

If there is a welding line between two cutting patterns, the program ensures that the lines in the connecting area are equal. The geometry of the cutting patterns is determined in the way that the relevant edge lengths of the cutting patterns are identical.

The line type of "boundary line" allows for an independent examination of the adjacent cutting pattern units. The length of the boundary lines in the adjacent area may be different.

Basically, the cutting pattern units meet the following conditions:

1. Surface compensation,
2. Boundary line compensation, and
3. Boundary line type.
These requirements are numerically precisely represented in the integral calculation. Especially in the case of cutting patterns in the boundary area, the process is usually not able to determine a universally applicable shape of the cutting pattern as there are too many boundary conditions.

Thus, the program is close to the optimum solution. The affected cutting patterns are particularly noticeable due to their curvature in relation to the uncompensated solution. Due to the greater degree of modification, these cutting patterns also differ slightly from the defined compensation specifications, but are in equilibrium with all other cutting patterns because of the overall calculation.

• By which length is the boundary line compensation distributed in the surface?

The program does not define any fixed length for the distribution of the boundary line compensation. In fact, the compensation is defined at the boundary line nodes and reacts in connection with the integral flattening depending on the geometry and stiffness.

In the calculation, the integral flattening considers all compensation strains on the surface and on the edges with the strains of flattening itself.

• How can I flatten the shape found using RF‑FORM‑FINDING?

This requires the RF‑CUTTING‑PATTERN add-on module. This calculates and organizes cutting patterns for membrane structures. The boundary conditions of cutting patterns on the curved geometry are determined by means of boundary lines or by independent planar or geodesic cutting lines. For detailed information, see the links below and the videos.
• How does the flattening process consider the compensation in the determination of cutting patterns?

The flattening process in the RF‑CUTTING‑PATTERN add-on module is an iterative process that flattens the respective areas of a cutting pattern by minimizing the distortion energy assuming the assigned material behavior.

In simplified terms, the method compresses the initial geometry in a press assuming a frictionless contact until the stresses due to flattening are in equilibrium with each other.

→ See the video

Since this process covers the complete mechanics of the curved structural component, you it is possible to additionally consider the compensation directly as an applied strain load.

Figure 01 - Compensation

Since the strains from the compensation specification interact with the strains from the flattening in the algorithm of RF‑CUTTING‑PATTERN, this kind of compensation consideration cannot be compared with the usual, flat scaling of the non-compensated cutting patterns. The consideration of the complete "cutting pattern mechanics" with all strain terms provides very high-quality geometry of the cutting pattern.

Before the flattening itself, the integral flattening process determines a coordinate system (warp and weft direction) from the mean orientation of FE elements, and uses the initially defined coordinate system with no regard to the respective position of the FE elements in the coordinate system orientation in order to describe the compensation strain and stiffness. Thus, the comparison shown in the video is only valid for an isotropic linear elastic membrane model.

• Why does the direction of a cutting pattern edge line in the support area change abruptly?

The direction change results from the 0% line compensation in the "Different Compensation by Line" tab for the boundary line in the support area.

This setting requires that the boundary line in the support area keeps the line length, regardless of the compensation set in plane. Since the weld lines adjoining the adjacent cutting patterns are allowed to change their lengths due to the compensation set in plane, the algorithm seeks the geometrically poor but energetically balanced solution.

You can avoid the abrupt change if the relevant boundary lines are allowed to relax freely in the flattening process.

Did you find your question?
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.

Your support is by far the best

“Thank you for the valuable information.

I would like to pay a compliment to your support team. I am always impressed how quickly and professionally the questions are answered. I have used a lot of software with a support contract in the field of structural analysis, but your support is by far the best. ”