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Frequently Asked Questions (FAQ)
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The term "shear panel" indicates that the translational spring, which is created along the beam length by means of the shear panel type including the corresponding parameters, is smoothed, see Figure 01.
This is also the case of the "Bracing" shear panel type, so that the mode shape always seems to be arbitrary at this location, see Figure 02.
In order to obtain the exact results, it is recommended to manually define the lateral support by using a nodal support according to the general method (Figure 03) or to define the effective lengths according to the equivalent member method, including intermediate lateral restraints, if necessary. Thus, a mode shape with the visible lateral restraint is created in the mid-span (Figure 04).
AnswerThere can be many reasons for the small effective modal mass factors. This can often be observed in the case of large structures. In most cases, the reason for this problem is the fact that only local mode shapes occur. The following text describes how you can handle this:
If you have paid attention to all of these notes, the global mode shapes of the structure should only be activated, which also activate a high mass.
- You should recognize from the result graphic whether the local mode shapes are really there. If the individual members or surfaces have a very low natural frequency, these occur first.
- In the case of including these local eigenvectors in the calculation anyhow, you should increase the number of mode shapes to be calculated.
- If the local mode shapes occur on surfaces, the masses of the affected surfaces can be neglected. This feature is described in this technical article.
- In the case of the local mode shapes on members, it is recommended to deactivate the FE mesh division on members.
In a dynamic analysis, you can only calculate as many mode shapes as the structural system has degrees of freedom. The degrees of freedom mean the number of mass points multiplied by the number of the directions in which the masses act.
A cantilever that is not split by the FE mesh has a mass point at the end. The action direction of the masses in RF‑/DYNAM Pro is limited to the X and Y direction. Thus, the system has 2 degrees of freedom, thus 2 mode shapes can only be calculated.
Please note: In this case, the selection of the method for solving eigenvalue problem is very important. In contrast to the "Root of the characteristic polynomial" method, the Lanczos method cannot calculate all eigenvalues of the system, but only n -1, which means only 1 eigenvalue in this example.
To display the mode shape, the calculation must be successfully performed. If this is not possible, there are either invalid boundary conditions or no stability problem was found (convergence not reached). No critical buckling shape can be considered then.
The 'Mode Shape - Overview' window allows you to check the plausibility of the eigenvalue analysis. You can open this window by clicking the button marked in the figure.
The graphical control of the mode shape graphic is available if the eigenvalue solver was used in the calculation, for example, in the case of the analysis according to General Method in compliance with EN 1993‑1‑1.
The symbols in the RF-/DYNAM Pro graphic should indicate that these members have a local torsional rotation.
Since the members are related to the centroidal axis in the graphic, rotations cannot be detected otherwise. For this reason, we have selected the display with the short vertical line with the small end circles.
In the animation, you can see that these symbols rotate and thus represent the rotations.
In RSBUCK and RF‑STABILITY, the lowest critical load is calculated first. This is obtained, for example, for a hinged column (Euler buckling mode 1, IPE cross-section) for the buckling about the z-axis. With this buckling load, the effective length Lcr,y is determined retrospectively.
In order to obtain the correct effective lengths for Lcr,y, it is necessary to also consider the second buckling mode (mode shape). For this, specify at least two or more buckling modes for the calculation in the calculation parameters. In the second buckling mode, you obtain a higher buckling load (sway about the y-axis), from which you obtain the correct buckling load Lcr,y.
As shown in the example, RSBUCK or RF‑STABILITY requires the calculation of several buckling modes (mode shapes). Thus, you can obtain results for the individual directions (see Figure).
According to EN 1998-1, so many modal forms should be used, that the sum of the effective modal masses is at least 90% of the effective total mass (usually this corresponds to the total mass of the structure). This regulation can be different in other seismic standards.
In the input window of RF-/DYNAM Pro, you can find a tab 'Mode Shapes' in the Dynamic Load Cases where frequencies and effective modal mass factors are summarized in a table. So you can check before the calculation whether you need to determine the required number of eigenvalues or if you need to increase the number.
After the calculation, the effective modal masses and the factors are displayed in the table 5.7 Effective modal mass factors.
This FAQ lists causes for small effective modal mass factors and possible solutions.
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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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