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Frequently Asked Questions (FAQ)
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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.
AnswerYes the possibility exists. In many standards we have implemented this possibility. Here you can select the option "Other" as subsoil class and thus adapt some parameters according to your wishes.The following annexes to EN 1998-1 have this option: CEN, NBN, CSN, DIN, UNI, NP, STN, CYS, BS, NS and NF.
Each multi-mass system can usually be represented by a single-mass system. When performing this transformation, the modal mass of the system is necessary. This mass is required to generate the frequency of the equivalent oscillators with a single degree of freedom.Participation factor
This factor can also be negative because it consists of the equivalent mass on a node and the corresponding displacement due to the mode shape. 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 equivalent mass factor is only the quotient from the total mass to the equivalent mass. Thus, it is usually possible to check more quickly what the ratio of the excited mode mass of the respective mode shape is. Should it happen that the equivalent mass factor is greater than 1, you should check the discretization of the system and, if necessary, refine the division of the nodes or the FE mesh.For an earthquake analysis, the equivalent mass factor and the equivalent mass are generally decisive because these values of the dynamic equivalent loads are calculated for the building.
AnswerWhen calculating by using the Equivalent Load Method, an equivalent load is determined from the individual mode shapes as the name implies. After a successful determination, you can export these equivalent loads in RFEM. RFEM automatically creates load cases in this way.These exported load cases are now displayed in a result combination is superimposed. One always uses a quadratic superposition for earthquake storm loads. Two types of superposition are possible here. First, the frequently used SRSS Rule and the CQC Rule. All load cases from the earthquake will always be considered as Stöndig and superimposed according to the respective regulations. The calculation rules are governed by the common standards.
In RF‑DYNAM Pro ‑ Natural Vibrations, it is possible to import the geometrical stiffness matrix. You can use it to consider the second‑order analysis. The equivalent lateral loads for earthquakes generated with the RF-DYNAM Pro - Equivalent Loads add-on module take this stiffness modification into account.
In the attached file, it is described when the second-order analysis according to EC 8 has to be considered and which options are offered by the RF-DYNAM Pro modules.
The module 'RF- / DYNAM Pro - Equivalent Loads' is used to perform the multi-modal response spectrum analysis. The equivalent loads are determined explicitly and exported to the main program RFEM / RSTAB in load cases. Load cases are generated separately for each selected mode shape and excitation direction. Such a load case is illustrated in Figure 1 as an example, but you can display the loads in these exported load cases as usual. You can find a list of generated loads in the results tables of the Dynamic Load Cases, see Figure 2. Equivalent loads are created on each finite element node (or each inner node). If the number of generated loads is too large, the graphical display is deactivated. You can adjust this setting in the 'Details' of the RF- / DYNAM Pro add-on module as shown in Figure 3.
AnswerTo study a system with tension members in RF-/DYNAM Pro it is recommended to define several Natural Vibration Cases (NVCs) and to use the stiffness modification options. The natural vibration analysis is a linear analysis of frequencies and mode shapes, tension members are considered as trusses in RF-/DYNAM Pro and can cope with tension and compression to the same degree. To estimate how the frequencies change when the tension members fail you can import this initial state into RF-/DYNAM Pro via a Load Case where you deactivate the members manually or via a Load Case where the tension members failed due to the load state. The defined NVCs can be assigned to Dynamic Load Cases (DLCs) to perform, for example, a response spectrum anaylsis with the export of equivalent loads (add-on module RF-/DYNAM Pro Equivalent Loads).
The equivalent loads that are exported into Load Cases in the main program RFEM / RSTAB are evaluated with consideration of all nonlinearities, in some cases this can lead to conflicts. An example is illustrated in the picture. In the first case no stiffness modifications are used in RF-/DYNAM Pro and therefore all members are considered in the dynamic analysis, but in RFEM / RSTAB the tension member fail due to equivalent loads. When you want to perform a completely linear analysis you must deactivate the nonlinearities in RFEM / RSTAB. In the second and third illustrated case an initial condition is imported where the tension members are not available. The equivalent loads are therefore based on the same structure as the structure for the calculation of internal forces and deformations.
In the response spectrum analysis or the time history analysis with the add-on module RF-/DYNAM Pro Forced Vibrations the modified stiffness of the Natural Vibration Cases (NVCs) is also used for the calculation of further results. The overall linear analysis with consideration of initial conditions is performed within the module RF-/DYNAM Pro.
The RF-/DYNAM Pro - Equivalent Loads add-on module analyzes structural equivalent loads and export them into load cases. Further analysis is performed in RFEM or RSTAB then. Result combinations are created.
The RF-/DYNAM Pro - Forced Vibrations add-on module provides the multi-point response spectrum analysis. It means that buildings on different supports can excite different situations. Only the final result combination is created here, so you receive no equivalent load and the traceability is more difficult.
AnswerNo, the calculation is always linear. All nonlinarities (for example failure at tension, nonlinear material, etc.) are ignored. Cables or tension members can be replaced by the "Truss" member type, for example.
Yes, they are considered in the eigenvalue analysis (Natural Vibrations module) and in the time history analysis (Forced Vibrations module) as well as in the response spectrum analysis (Forced Vibrations and Equivalent Loads modules).
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