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Frequently Asked Questions (FAQ)
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AnswerNo, this option does not necessarily have to be activated to consider the self-weight. If the masses are imported from a load case that already contains the dead load, this option must not be activated. Otherwise, the structure's own weight is doubled.
The cause of small effective modal mass factors can be very diverse. Often, this can be observed for larger structures. The reason for this problem consists in most cases in the fact that only local eigenvectors occur. The following describes how you can handle this point:
If all of these notes have been observed, only the global eigenmodes of the structure should be activated, which also activate a high mass.
- If they really refer to local eigenvectors, or not, should be analyzed in the results graphic. If single members or surfaces have a very low angular frequency, they occur first.
- If you want to include these local eigenvectors in the calculation, you should increase the number of eigenvectors to be calculated.
- If local eigenvectors occur on surfaces, the masses of the affected surfaces can be neglected. This feature is described in this article .
- In case of local eigenmodes on members, it is recommended to deactivate the FE mesh splitting on members.
The prestressing of cables has a governing influence on the behavior of a structure. Therefore, it must also be considered in the dynamics.In the natural vibration cases of an add-on module, it is possible to consider stiffness modifications on the basis of a load case. With this option, the geometric stiffness matrix is rewritten for the natural vibration analysis. In order to avoid falsifying the results or to consider any unwanted effects, it is important that the load case to be imported contains only the prestress and no further loading.If you want to perform a 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, the prestress must also be taken into account. The "Stationary State" function is available for this. With this option, the stiffness modifications resulting from the prestress are taken into account.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. Simply select the entire structure or a part thereof and right-click to open the shortcut menu. Here you will find the entry "Center of Gravity and Info ...".However, the nodal masses are not apparent from this.To generate nodal masses, you can use the RF-/DYNAM Pro Natural Vibrations add-on module. In this case, you it is possible to create a natural vibration case where the mass acts e.g. only in the Z direction. After having calculated the natural mode case, the "nodal masses" are available in the results 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 case calculation.
In a dynamic analysis, you can only calculate the eigen modes as many degrees of freedom as the system has. With degrees of freedom, the number of mass points multiplied by the number of directions into which the masses act is meant.
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, so only 2 eigen-modes can be calculated.
Caution: In this case, the choice of the solution method is very important. In contrast to the "Root of Characteristic Polynomial" method, the Lanczos method can not calculate all eigenvalues of the system, but only n -1, which means in this example only 1 eigenvalue.
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
- RX-TIMBER Glued-Laminated Beam
- RF-/TIMBER Pro
- RF-/DYNAM Pro
Before starting the calculation in the add-on module, it is recommended to check each tab. This message appears if you have not opened 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.
In this case, you have the option to print the image in the time course diagram directly into the printout report. Proceed as described in the picture.
Each multi-mass system can usually be represented by a single-mass system. When you do this transformation, you need the modal mass of the system. This mass is needed to generate the frequency of the equivalent single-frequency oscillator.Beiteilungsfaktor
This factor can also be negative because it is composed of the substitute mass at a node and the associated displacement due to the eigenform. If the deflection is in the negative direction, the participation factor becomes negative. The replacement mass factor is then still positive, since the participation factor is squared. (see formula)equivalent mass
The equivalent mass of a system is a part of the total mass which is excited due to the vibration of the multi-mass oscillator. The equivalent mass of a system can be between zero and the total mass. The replacement mass factor is only the quotient of the total mass to the substitute mass. As a rule, this makes it possible to check more quickly what proportion of the excited mass of the respective eigenform is. Should it happen that the substitute mass factor is greater than 1, one should check the discretization of the system and, if necessary, refine the division of the nodes or the FE mesh.For an earth analysis, the substitute mass factor and the substitute mass are usually decisive, since these values are used to calculate the dynamic equivalent loads on the building.
AnswerThis setting defines how the defined loads in RFEM / RSTAB are handled in the dynamics module, since these must first be transformed in bulk.Z-load components: When this option is selected, only those masses that act in the Z direction are considered. This includes the positive as well as the negative direction. This option is the default because it is desired in most applications. Loads, such as earth pressure, defined in a horizontal direction are not taken into account.Z load shares (in the direction of gravity): Here the loads are considered only in positive Z-axis. If you want to use this option, you should check in which direction the Z-axis was defined. With this option, it does not matter in which direction you set the dead weight in a load case.Full load as mass: This option converts all loads into masses, both horizontal and vertical.
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