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Frequently Asked Questions (FAQ)
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AnswerFor seismic design, the add-on modules RF-/DYNAM Pro - Natural Vibrations and RF-/DYNAM Pro - Equivalent Loads are available. They allow you to perform the multimodal response spectrum analysis. After the analysis in the add-on modules, the calculated seismic loads are exported to load cases, which can be evaluated as usual.Furthermore, you can evaluate the story drift and the horizontal shear of the building. These and other features are described in detail in the webinar "Response Spectrum Analysis in RFEM."
AnswerNo, this option does not necessarily have to be activated to consider the dead load. If the masses are imported from a load case that already contains the dead load, it is not necessary to activate this option. Otherwise, the dead load of the structure will be doubled.
The RF-/DYNAM Pro - Equivalent Loads add-on module only contains a linear analysis of structures. If you now apply a nonlinear model for the calculation, RF‑/DYNAM Pro - Equivalent Loads will modify it internally and treat it as a linear model. The nonlinearity in your model is the masonry, which cannot absorb any tensile forces.
The problem is as follows: RF-/DYNAM Pro - Equivalent Loads calculates the equivalent loads linearly and exports the load cases from them. However, the load cases are subsequently calculated nonlinearly on the basis of the material model, which is not entirely correct. Furthermore, the results are superimposed according to the SRSS or CQC method, which results in tensile and compressive forces being present in the model.
In this case, you could change the masonry to isotropic linear and work with linear properties of the material model, for example. Additionally, it is possible to insert line hinges at this location, which could be used to avoid the moment restraint, for example.
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.
The prestress of cables has a governing influence on the behavior of the structure. Therefore, it must also be considered in the dynamic analysis.In the natural vibration cases of the add-on module, it is possible to consider stiffness modifications on the basis of a load case. This option allows you to rewrite the geometric stiffness matrix for the natural vibration analysis. In order to avoid the falsification of the results or to consider any unwanted effects, it is important to only include the prestress and no further loading in the load case to be imported.If you want to perform the time history analysis, make sure that the implicit Newmark analysis and the explicit analysis do not use the natural vibration cases, but that the calculation parameters are defined directly in the dynamic load case. Therefore, it is also necessary to consider the prestress. For this, the "Stationary State" feature is available. This option allows you to consider the stiffness modifications resulting from the prestress.There is also an interesting technical article about this topic.
For members (RSTAB) or members, surfaces, and solids (RFEM), you can display the material weight W. For this, simply select the entire structure or a part of it and right-click to open the shortcut menu. Here, you can find the "Center of Gravity and Info ..." option.However, the nodal masses are not apparent from this.To generate the nodal masses, you can use the RF‑/DYNAM Pro - Natural Vibrations add-on module. In this case, it is possible to create a natural vibration case where the mass only actsin the Z direction, for example. After the natural mode case has been calculated, the "nodal masses" are available in the result tables of RF‑/DYNAM Pro.In RF‑/DYNAM Pro, you can optionally convert the nodal loads into masses by applying the force components of the respective load case to the natural vibration case calculation.
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.
With the COM interface, you can access most operating elements as well as results of the following programs or add-on modules:
- RF-/STEEL EC3
- RF-/TIMBER Pro
- RF-/DYNAM Pro
AnswerBefore you start the calculation in the add-on module, it is recommended to check all tabs. This message appears if you did not open the calculation parameters of the natural mode case at least once. In spite of having defined a mass case, it has not yet been assigned to the natural vibration case.
AnswerIn this case, you have the option to print the image in the timeline diagram directly into the printout report. Here you proceed as described in the picture.
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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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