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Frequently Asked Questions (FAQ)
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An overpressed joint between two members can be controlled in RFEM by means of the member stress results. For members, this stress result represents the effective stress as a color gradient across the member surface depending on the assigned cross-section.
Figure 01 - Stresses on Members
Based on the local member axis, the member stress result gives the following stress components and reference stresses with an associated color palette:
- Elastic stress component
- Elastic equivalent stresses
Using active displaying for the members connected to the joint, and displaying the σx stresses, it is possible to visualize the state of stress on and thus also between the members. If only negative stresses occur in the area between the members, the joint is overpressed.
It is possible to display or calculate the stresses in RFEM as well as in the add-on module by means of the following smoothing options:
- Constant on elements
- Not continuous
- Continuous within surfaces
- Continuous total
- Continuously by groups or Continuous by groups
To compare the results, the same display type and calculation type must be selected in RFEM and RF-STEEL Surfaces.
In RFEM, it is possible to do this in the Project Navigator Show → Results → Surfaces → Distribution of Internal Forces/Stresses (Figure 02). In RF-STEEL Surfaces, this can be displayed or changed in the Details → 'Options' tab (Figure 03).
In the Result Diagrams dialog box, it is possible to create average regions to prepare the results for engineering. This function is available with the 'Edit Average Regions' button. The dialog box in Figure 1 opens.
Define the average regions in the table columns on the left; entries for Start, End, and Length are interdependent. Each region can be activated separately. The 'Use for Results' section controls for which deformations, internal forces, stresses, or strains a smoothing is to be performed. The smoothing can be constant or linear for all average regions.
Furthermore, an average line can be displayed over the entire result diagram via the button of the same name (Figure 2).
The integral of the average region is displayed if the 'With Result Interpretations' checkbox is selected in the settings of the result diagrams (Figure 3).
Since surfaces only have the directions x- and y- in the plane, it is necessary to define which should be the hoop stress and 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 modeling, the program tries to align the local axis systems on the global axis system (Figure 02). In the present case, however, the x-axis should run along the container for all surfaces. This orientation can be achieved as follows.
First, the z-axis of all surfaces must point inwards or outwards. In the example, the outward direction has been selected. If this is not the case for a surface, you can right-click the surface and use the function "Reverse Local Axis System" to move the z-axis to the other surface side. Then, select all surfaces and select the Axes tab in the surface dialog box. Figure 03 shows the dialog box. In this case, one of the axially extending boundary lines has been selected for the orientation. Figure 04 shows the now aligned local axis systems. All x-axes are axial and all y-axes are circumferential.
Figure 05 shows the results of the membrane stresses axial (sigma-x, m) and over the circumference (sigma-y, m).
The definition of surface supports should be as realistic as possible. Experience shows that the equation solver works most effectively with this. To simplify matters, degrees of freedom are often defined as 'fixed'. However, this can have a big impact on the overall stiffness matrix and cause numerical problems (see Figure 01).
It is better to work with springs in order to avoid the numerical problems. It is often sufficient to define very stiff springs (see Figure 02). The same applies to the foundation perpendicular to the surface. You can find more information in  and in the links below this FAQ.
It is very likely caused by missing uniform staff orientation, see Figure 1.
As soon as the member orientation is uniform (right-click on the member → 'Reverse Member Orientation'), the signs of the shear force diagram also match, see Figure 2.
There is no load distribution displayed between the external facade elements at the example shown in Figure 01. Unstressed cells are not displayed according to the color scale during load distribution, but remain empty. Thus, the value on these elements is 0. This has the advantage that it is recognized immediately that the FE elements are not stressed.The cause of the problem can be visualized directly in RWIND Simulation. By default, calculations are based on a simplified model. Depending on the setting, the shell of the model can be refined or coarsened. An FE mesh is placed over the structure, so to speak, and depending on the level of detail, this FE mesh clings to the model. Figure 02 shows the extent of the level of detail that is too small. The surfaces standing on the façade are not displayed well enough and no wind flows between the cantilevered surfaces in the simulation, which is why these internal surfaces do not experience any wind pressure.The level of detail can be adjusted in RWIND Simulation by 'Edit Model' or directly in RFEM in the settings for the wind load simulation (see Figure 03). Optionally, the simplified model can also be completely deactivated in RWIND Simulation.In the case of a greater level of detail (corresponds to a finer FE mesh), the cantilevered surfaces are displayed cleanly and the FE elements are stressed accordingly (see Figure 04 and Figure 05).
For a resultant, a concrete combination of loads is required, which result combinations cannot provide.
The problem is apparent in the following example. A single-span beam is loaded with three different load cases. For the support at node 1, the result envelope of the 6 possible load combinations gives a maximum P-Z of 11.25 kN based on the result of CO2 (see Figure 01). The support at node 2 has a maximum P-Z of 12 kN based on the result of CO1. The resultant of 23.25 kN, however, does not exist in any of the involved load combinations and is therefore too large (maximum LK 1 and LK 2 with 22.5 kN).
The situation is similar to the pure result combination of the load cases which have the same maximum values P-Z of the nodal supports 1 and 2. However, it is not apparent here that a resultant would give incorrect results.
For this reason, a resultant is not used for result combinations, since the results can be incorrect.
AnswerThe default definition of surface elements assumes an isotropic material behavior. The load attempts to get to the supports as quickly as possible. The stiffness of the elements also plays a role here.
For plates, the structural behavior or the load transfer is best represented and understood with the trajectories of the principal moments αb. For wall elements, it is necessary to consider the trajectories of the principal axial forces αm.
In this example, the load is not applied parallel to the free plate´s edges but almost perpendicular to the supports, because this is the shortest path of the load transfer.
At the blunt corners of the system, the load absorption area is larger than in the support centers, corresponds to a singularity point and has -as a consequence of that- large peak values.
In order to force the system to remove the load parallel to free plate edges, the following procedure is the fastest:
Definition of an orthotropic plate. It is recommended to use the orthotropy type 'Effective Thicknesses'. The effective plate thickness has to be specified in the support direction and a very small thickness (e.g. 1mm) in the secondary support effect.
The second graphic shows the difference between both models.
AnswerIn general, an imperfection describes the Incompleteness of a structure or the deviation from an ideal shape caused by the production. There are different ways to simulate the imperfection. In RSTAB and RFEM, imperfections are represented as equivalent loads. The definition of equivalent loads is shown in Figure 01 and is taken from  . The same is described in EC3  . Since these are equivalent loads that are dependent on the axial force, they are also taken into account for a calculation according to the 1st Order Theory. It is recommended to manage loads and imperfections in separate load cases. They can be combined in an appropriate way with each other in load combinations. Load cases with pure imperfections have to be categorized as action type 'Imperfection' in the general data for load cases (see Figure 02).
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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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