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Frequently Asked Questions (FAQ)
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Since surfaces only have the directions x- and y- in the plane, it is necessary to define which should be the circumferential and the axial stress. In the following example, sigma_x should be the axial stress and sigma_y the circumferential stress.
The example consists of an inclined circular container (Figure 01). After modeling, the program tries to align the local axis systems with 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).
In RFEM, the interface application 'Simulate and Generate Wind Loads' makes it possible to exchange member, surface, and solid elements, and in RSTAB to exchange the member elements.
To avoid generating a too fine mesh along with a corresponding long calculation time, the program simulates all members with a rectangular cross-section as standard. The size of the rectangular cross-section is selected in such a way that the real cross-section geometry is barely included.
By deactivating the option 'Export Optimized Member Topology', you can avoid this additional optimization of the model and allow consideration of the real cross-section geometry within existing cross-section settings.
If the exact representation of the cross-section geometry requires more than 1000000 elements, the interface automatically changes to the simplified rectangular section display of the cross-sections.
AnswerWith the FE mesh refinement, it is also possible to create an aligned FE mesh in the program. Thus, the automatic FE mesh generator can be controlled to a certain extent. However, it is not possible to use it for setting of a specified mesh geometry.
Yes, that is possible. How to create a model of such a component with quadrangle surfaces is shown in the video of the FAQ step by step.
AnswerTo analyze a gas-tight object, it must be modeled in RFEM first. Then, you can create a new solid from the boundary surfaces. In this case, it is necessary to select the "Gas" solid type and it must have the corresponding material (see Figure 01). In the "Gas" tab (see Figure 02), you can define additional gas parameters. As a result, the resulting gas pressure, the overpressure and the initial pressure can be graphically displayed for the gas solid. Furthermore, the Results navigator provides the deformed volume of the gas as well as its temperature.
Please check the orientations of all bearings, rods and surfaces and the associated orientation of the loads. The display of the respective coordinate systems via the pointing navigator (Fig. 01).
If this does not solve the problem, first use a simple load case where the deformation is easy to understand. Refer to internal forces and deformations to detect possible twists or missing connections (Fig. 02). To detect distorted loads on surfaces, you can also use the Load Balancing option in the results (Fig. 03)
Not infrequently, a supposed connection between a surface and a rod does not exist. Here the FE network can also show if a connection has been established (Fig. 04).
If this also leaves the imbalance, use a copy of the model and erase successive elements to find the missing symmetry.
If all this fails, then please send us the model to our hotline with a note of which tests you have already performed.
Rotated surfaces are often used in RFEM to easily model and calculate geometrical conditions. However, if you use a large number of rotated surfaces in the model, you may experience performance drop. This is due to the definition of the surfaces, because they have to be generated in the background again and again using the boundary conditions if a change occurs in the structural system.
Therefore, it is recommended to replace the rotated surfaces by the surfaces of the 'Quadrangle' type, which significantly reduce the amount of data. Furthermore, the quadrangle surfaces also allow you to apply loads to parts of the surface only.
The Stability Analysis for plate structures can be converted into a pure stress analysis if the calculation is in quadratic theory and the imperfection required by the standard has been applied to the structure.
With the help of the RF-STABILITY and RF-IMP modules, you can create imperfections (or a pre-formed FE mesh). The type of imperfection depends heavily on the structural component and the standard used. For members that have been modeled as surface-tensioned structures, the values from DIN EN 1993-1-1: 2005 5.3 can be used. For plane surfaces, it is possible, for example, to use the values from DIN EN 1993-1-5: 2006 Annex C. For shells, the problem is much more complex and there are different approaches. I would advise against the generation of imperfections and perform the buckle integrity design by means of the MNA / LBA concept according to DIN EN 1993-1-6, which does not require the application of an imperfection.
For example, if you want to design the surface model of a steel beam, you can do the following:
1. Select a load with relatively high axial forces (compared to other internal forces in the load case), in most cases you can use the self-weight load case or a load combination with the corresponding self-weight. It may be necessary for each load combination to have an individual imperfection.
2. Calculate load combination according to the theory of one-order and use it as basis for RF-STABILITY
3. Find the first eigenform of a global failure with RF-STABILITY
4. Use the calculated mode shape as the basis for an imperfection with RF-IMP. It is possible to apply 1/300 of the beam length as amplitude, for example.
5. Generate a load combination that uses the generated imperfection as a basis and is calculated according to the linear deformation analysis.
6. Perform a stress analysis on the basis of this load combination which is also a stability analysis of the structure.
AnswerYou can use the surface releases, but you need to separate the overlapping part with an own surface. Overlapping surfaces should be avoided in general and can be checked by the "Model Check" (Tools → Model Check → Overlapping Surfaces). Please watch the video file (see link below) to see the approach. Alternatively you can add a "Contact Solid" between the two surfaces.
For this, you can use the load type of 'Free Variable Load'. It allows you to define a load that acts variably along the perimeter, when specifying the rotation axis. Also, it is possible to separate the perimeter in several segments.
Moreover, with this load, it is possible to generate a load that is variable by height. In the provided model file, a load with both variables is applied by way of example.
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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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