#### 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.

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• ### When creating a material, there are no nonlinear material models available for surfaces or solids. Why?

The nonlinear material models are only available in the 3D environment. Please make sure that the model type is set to "3D" (see Figure 02).
• ### What is the formula used to calculate the ASCE 7 wind velocity profile?

The wind velocity profile in RWIND Simulation according to the ASCE 7-16 standard [1] is calculated based on Eq. 26.10-1. The coefficients and basic wind speed in this equation below are incorporated in the wind pressure equation.

Velocity wind pressure (imperial): qz = 0.00256 Kz Kzt Kd Ke V2

We must reference this equation to calculate the inlet velocity relative to elevation for the RWIND Simulation CFD wind tunnel. To consider only velocity rather than pressure from this equation, the basic wind speed is multiplied by the square-root of each coefficient. Notice the velocity variable in Eq. 26.10-1 is squared which requires the square root of the coefficients to be considered.

$Inlet\;velocity\;=\;V\sqrt{K_e\;\cdot\;K_{d\;}\cdot\;K_z\;\cdot\;K_{zt}}$

Because the ASCE 7-16 standard does not address wind CFD analysis and magnitude of the required inlet velocity, it is difficult to draw comparisons. Therefore, this is the closest estimate for calculating the RWIND Simulation inlet wind velocity per the code.

• ### How can I deduce the axial stress and hoop stress of a circular container?

Since there are only the directions x- and y- in the plane for surfaces, it is first necessary to define which should be the hoop stress and which the axial stress. In the following example, sigma_x should be the axial stress and sigma_y the hoop stress.

The example consists of an inclined circular container (Figure 01). After the modeling, the program tries to align the local axis systems on the global axis system (Figure 02). However, the x‑axis should run along the container for all surfaces in this case. This orientation can be achieved as follows.

First, the z‑axis of all surfaces must point inwards or outwards. In the example, the outside direction has been selected. If this is not the case for the surface, you can right-click the surface and use the "Reverse Local Axis System" feature to move the z‑axis to the other surface side. Then, select all surfaces and open the Axes tab in the Edit Surface dialog box. Figure 03 shows the dialog box. In this case, one of the boundary lines oriented axially has been selected. Figure 04 shows the aligned local axis systems now. All x-axes are axial and all y-axes run in the circumferential (hoop) direction.

Figure 05 shows the results of the membrane stresses axial (sigma‑x,m) and along the circumference (sigma‑y,m).

• ### Are the Dlubal Software programs applicable for the calculations in mechanical engineering?

Both RFEM and RSTAB are ideally suited for the application in mechanical engineering and are used by numerous users in daily practice. Due to the modular system, you can only use the add-on modules required for your purposes.

###### Main Programs RFEM or RSTAB
The main programs are used to define structures, materials, and loads.

RSTAB provides optimal tools for beam, truss or frame structures.
In addition, RFEM provides you with additional options to calculate the structures with surface or solid elements.

Dynamic Analyses
In case a seismic analysis or vibration designs are required, the RF‑/DYNAM Pro add-on modules provide special tools for determining natural frequencies and mode shapes, for an analysis of forced vibrations, a generation of equivalent loads, or for a nonlinear time history analysis.

If you have any question about the Dlubal Software programs, please do not hesitate to contact the sales department.

• ### How can I generate moving loads on a surface for a circular ring?

In RF-MOVE Surfaces, it is also possible to select a circle as a set of lines. A simple example is shown in the video.
• ### Is it possible to display or export certain results over a period of time from the time history calculation in RF‑DYNAM Pro - Forced Vibrations?

With a time course monitor, you can view all results over a period of time. In this case, it is also possible to select several parts of the structure and then export the results directly to Excel.
• ### My aim is to mesh a circular hole plate in a mapped way. Is it possible to generate such a mapped mesh in RFEM?

By using the FE mesh refinement, the program allows you to create a mapped FE mesh. The automatic FE mesh generator can thus be controlled to a certain extent. However, a specified mesh geometry cannot be set.
• ### The calculation of my model results in unrealistically high stresses at many locations. What is the reason?

The most common causes are listed below:
###### 1. Singularities
Singularities appear in a limited range due to the concentration of the result values that depend on the stress. They are conditioned by the FEM methodology: In theory, the stiffness and/or the stress in an infinite size concentrate on an infinitesimal small area. Therefore, the singularities occur especially at point supports, load application locations, reentrant corners, or in the area of stiffness peaks.

If the result value of the stress peak is greater and the area of this stress peak is smaller in the case of a finer FE mesh, the singularity is very likely to occur.

The recommendations for dealing with the singularity locations are included in the following technical articles of our Knowledge Base, for example:
###### 2. Unrealistic Support Definition
Rigid supports (infinitely stiff supports) are rather unrealistic in many cases. Therefore, it is recommended to display the supports as elastic supports. In this case, the stiffness of the adjacent structural components should be estimated realistically.

For checking purposes, the deformation diagram is suitable, possibly with a stronger precamber, as well as the result display of the support reactions or contact stresses. For a better overview, the simplest possible loads should be used for the check.
###### 3. Incorrectly Defined Directions / Nonlinearities
A mistake in the direction definition, for example, of loads, member hinges, or line and surface releases, is often the cause of the unrealistic behavior. When using the local or rotated coordinate systems as a reference systems, you must pay attention to the correct definition. For example, nonlinearities defined in the opposite direction are typical for the supports that fail due to tension or compression.
Incorrectly defined loads can be easily identified by displaying the loading. The loads applied for the calculation can be easily displayed in the Results navigator by using the "Load Distribution" option.

Furthermore, modeling inaccuracies can also lead to the incorrect definition of directions. By importing a DXF file, you can introduce inaccuracies into the model, for example, nodes that are not on top of each other or the lines skewed in the wrong direction.
The "Regenerate Model" feature is very helpful for treating the minor inaccuracies.

Incorrectly defined releases and hinges can usually be identified by means of the deformation image and the diagrams of internal forces. Again, it is recommended to work with simple loads for checking purposes.
###### 4. Model does not correspond to reality
It may often happen that not all external or internal influences from a structure to be modeled have been considered sufficiently and accurately in the model. Supports or supporting structural components may not have been modeled or they are in a  wrong place. The realistic estimation of the stiffness of the adjacent structural components is also important. Provided that it has been over- or underestimated, the load transfer in the model is sometimes changed significantly.

However, it is possible to simple check the deformation, possibly by using a stronger precamber.
The following questions may help you to find the solution if the real structure is known: Is the magnitude of the deformations close to reality? Is the deformation diagram qualitatively consistent with my expectations?

A good example is presented in the following Knowledge Base article:

• ### Is it possible to use RFEM for modeling a channel that is distorted from a rectangular cross-section to a pipe cross-section?

Yes, it is possible. The FAQ video shows step by step how to create a model of such a component with quadrangle surfaces.
• ### What is the effect of the "Without membrane tension" stiffness type in surface properties?

The "Without membrane tension" stiffness type describes the plate and plane stiffness of surfaces.

The plate stiffness including the shear stiffness perpendicular to the surface plane is defined linearly elastic and the plate stiffness in the surface plane is defined nonlinearly elastic with the material model according to "Drucker-Prager" depending on the defined thickness and the assigned material.

In order to ensure that the surface behavior is "Without tension" in the pane direction, the nonlinear material model reacts with yielding tensile stress fy,t going to almost zero in the elements subjected to tension in combination with a relatively small strain hardening modulus Ep. On the other hand, in the case of compressive forces, the elements remain virtually unaffected linearly elastic due to the relatively high yielded compressive stress fy,c and react with an unrestricted compression transmission.

Since the "Without membrane tension" stiffness type basically modifies the pane-related degrees of freedom of the surfaces, it is only applied to the surfaces of the relevant model types 3D, 2D - XZ (uX/uZY) and 2D - XY (uX/uYZ).

To better describe the partial nonlinearity of this surface stiffness type, it was renamed from "Without tension" to "Without membrane tension" as of the Version RFEM 5.06.1103 on 4.02.2016.

The Knowledge Base article "Modeling Approaches for Shear/Hole Bearing Connections by Means of FEa" shows possible application.

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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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