Modal Analysis in RFEM 6 Using a Practical Example

Technical Article on the Topic Structural Analysis Using Dlubal Software

  • Knowledge Base

Technical Article

Modal analysis is the starting point for the dynamic analysis of structural systems. You can use it to determine natural vibration values such as natural frequencies, mode shapes, modal masses, and effective modal mass factors. This outcome can already be used for vibration design, and it can be used for further dynamic analyses (for example, loading by a response spectrum).

In RFEM 6 and RSTAB 9, you can perform modal analysis by using the Modal Analysis add-on as one of the add-ons available for dynamic analysis. The features of this add-on were discussed in a previous Knowledge Base article titled "Features of the Modal Analysis Add-on for RFEM 6". This article will show you a practical example of how to use this add-on for determining the natural vibration values of a reinforced concrete multi-story structure.

A Practical Example

You can activate the Modal Analysis add-on in the model's Base Data. In the Standards II tab of the Base Data window, you can select the standard used for the dynamic analysis, so that the input dialog boxes of the dynamic analysis are automatically adjusted to the selected standard (Image 1).

In contrast to RFEM 5, where you are asked to provide the input data for the modal analysis in the associated add-on module, the Modal Analysis add-on in RFEM 6 is fully integrated into the program itself. Thus, the user interface is extended by new entries in the navigator, tables, and dialog boxes after activating the add-on and selecting the design standard.

This way, you can initialize the input of data for the modal analysis directly in the Load Cases & Combinations window. The first step is to create a load case with modal analysis as an analysis type (Image 2) and import masses directly from load cases or load combinations of interest.

As discussed in the previous Knowledge Base article titled “Features of the Modal Analysis Add-on for RFEM 6", you can create a design situation based on a selected design code and use it for the modal analysis, as in the load combination selected in Image 2. In this example, the combinations are created according to the EN 1990 standard and the German national annex (DIN | 2012-08) shown in Image 3. Hence, you can create a design situation with a seismic mass combination type based on which the program will automatically generate a load combination with the preset combination coefficients for the selected standard. This load combination, in fact, contains the masses to be used for the modal analysis (Image 4). When importing the masses, load components in the global Z-direction are considered by default (Image 5).

The modal analysis settings can be defined further in the Modal Analysis Settings window, where you can select the method for determining the number of mode shapes (Image 6). In this example, the number of the smallest mode shapes to be calculated is manually set as 12. Another option is to set the maximum natural frequency so that the mode shapes are automatically determined until the set natural frequency has been reached.

The method for solving the eigenvalue problem is also to be selected from the three available methods in RFEM 6: Lanczos, roots of characteristic polynomial, and subspace iteration. In RSTAB 9, on the other hand, two methods are available: subspace iteration and shifted inverse iteration. Although all of them are suitable for determining the exact eigenvalues, the choice is conditioned by the size of the support system to be considered. In this example, the Lanczos method is employed to determine the n-lowest eigenmodes and the corresponding eigenvalues of the structure. Next, the masses are defined as acting in the global X and Y directions. You can also consider masses rotating around the global axes X, Y, and Z, but considering the structure in this example, this is not necessary.

Once the modal analysis settings have been defined, you can start the calculation and obtain the results in both graphical and tabular form. Hence, in addition to the display of masses (discussed in more detail in the Knowledge Base article titled “Features of the Modal Analysis Add-on for RFEM 6"), the Results Navigator allows you to see all the mode shapes of the structures, as shown in Image 7.

The natural frequencies of the corresponding eigenmodes can be found in the Navigator, but also in the Results table as shown in Image 7. As a matter of fact, the Natural Frequency tab of the Modal Analysis results table provides you with an overview of the eigenvalues, angular frequencies, natural frequencies, and natural periods of your undamped system. The values are obtained by calculating the equation of motion of a multiple degree of freedom system, without damping with the set eigenvalue solver. Based on the eigenvalues λ [1/s2], the angular frequencies ω [rad/s] are derived, given that they are related though the relation λi = ωi2. Next, the natural frequency f [Hz] is obtained by considering that f = ω/2π. Finally, the natural period T [s] can be calculated as the reciprocal of the frequency (that is, T = 1/f).

In the Results tables for the modal analysis, you can also display the effective modal masses (which describe how much mass is activated in each direction by each eigenmode of the system), the corresponding modal mass factors, and the participation factors. For instance, if you are interested in performing a response spectra analysis afterwards, you can check whether the effective modal mass factors of a specific shape have to be considered for the response spectrum calculation according to the requirements of the certain standard. This is shown in Image 8.

Final Remarks

You can use the Modal Analysis add-on to determine the natural vibration values of structures, such as natural frequencies, mode shapes, modal masses, and effective modal mass factors in RFEM 6. The features of the add-on are discussed in more detail in a previous Knowledge Base article titled "Features of the Modal Analysis Add-on for RFEM 6".

This article, on the other hand, shows you a summary of how to perform a modal analysis in RFEM 6. Thus, all you must do is to create a load case of a modal analysis type, import masses directly from load cases or/and load combinations of interest, and define the analysis settings. Once you do the calculation, the results in terms of natural frequency values, effective modal masses, participation factors, and masses in mesh points are available to you. This outcome can be used for design purposes and for further dynamic analysis in the program (for example, loading by a response spectrum).

Author

Irena Kirova, M.Sc.

Irena Kirova, M.Sc.

Marketing & Customer Support

Ms. Kirova is responsible for creating technical articles and provides technical support to the Dlubal customers.

Keywords

Modal analysis Natural vibration values

Links

Write Comment...

Write Comment...

  • Views 93x
  • Updated 6 May 2022

Contact Us

Contact Dlubal

Do you have any further questions or need advice? Contact us via phone, email, chat or forum or find suggested solutions and useful tips on our FAQ page, available 24/7.

+49 9673 9203-0

[email protected]

Online Training | English

Eurocode 2 | Concrete Structures According to DIN EN 1992-1-1

Online Training 25 May 2022 8:30 AM - 12:30 PM CEST

Online Training | English

RFEM 6 | Students | Introduction to Timber Design

Online Training 25 May 2022 4:00 PM - 5:00 PM CEST

ADM 2020 Aluminum Design in \n RFEM 6

ADM 2020 Aluminum Design in RFEM 6

Webinar 25 May 2022 2:00 PM - 3:00 PM EST

Online Training | English

RFEM 6 | Dynamic Analysis and Seismic Design According to EC 8

Online Training 9 June 2022 8:30 AM - 12:30 PM CEST

Online Training | English

Eurocode 5 | Timber Structures According to DIN EN 1995-1-1

Online Training 15 June 2022 8:30 AM - 12:30 PM CEST

ASCE 7-16 Response Spectrum Analysis in RFEM 6

ASCE 7-16 Response Spectrum Analysis in RFEM 6

Webinar 5 May 2022 2:00 PM - 3:00 PM EST

Webservice & API in RFEM 6

Webservice & API in RFEM 6

Webinar 20 April 2022 2:00 PM - 3:00 PM CEST

Geotechnical Analysis in RFEM 6

Geotechnical Analysis in RFEM 6

Webinar 7 April 2022 2:00 PM - 3:00 PM CEST

Masonry Design Using Finite Element Method in RFEM 6

Masonry Design Using Finite Element Method in RFEM 6

Webinar 31 March 2022 2:00 PM - 3:00 PM CEST

Tensile Membrane Structure Design in RFEM 6

Tensile Membrane Structure Design in RFEM 6

Webinar 17 March 2022 2:00 PM - 3:00 PM EST

Stability and Torsional Warping Analysis in RFEM 6

Stability Analysis and Torsional Warping (7 DOF) in RFEM 6

Webinar 17 March 2022 2:00 PM - 3:00 PM CET

Data Exchange Between Rhino/Grasshopper and RFEM 6

Data Exchange Between Rhino/Grasshopper and RFEM 6

Webinar 10 March 2022 2:00 PM - 3:00 PM CET

Wind Load Generation Based on CFD in RWIND 2

Wind Load Generation Based on CFD in RWIND 2

Webinar 4 March 2022 10:00 AM - 11:00 AM CET

NDS 2018 Timber Member Design \n in RFEM 6

NDS 2018 Timber Member Design in RFEM 6

Webinar 24 February 2022 2:00 PM - 3:00 PM EST

Analysis of Steel Joints Using FE Model in RFEM 6

Analysis of Steel Joints Using FE Model in RFEM 6

Webinar 24 February 2022 2:00 PM - 3:00 PM CET

Form-Finding and Calculation of Membrane Structures in RFEM 6

Form-Finding and Calculation of Membrane Structures in RFEM 6

Webinar 3 February 2022 2:00 PM - 3:00 PM CET

Design Aluminum Structures \n in RFEM 6 and RSTAB 9

Model and Design Aluminum Structures in RFEM 6 and RSTAB 9

Webinar 27 January 2022 2:00 PM - 3:00 PM CET

ACI 318-19 Concrete Design in RFEM 6

ACI 318-19 Concrete Design in RFEM 6

Webinar 20 January 2022 2:00 PM - 3:00 PM EST

RFEM 6
Hall with Arched Roof

Main Program

The structural analysis program RFEM 6 is the basis of a modular software system. The main program RFEM 6 is used to define structures, materials, and loads of planar and spatial structural systems consisting of plates, walls, shells, and members. The program can also design combined structures as well as solid and contact elements.

Price of First License
3,990.00 USD
RSTAB 9
Structural Frame and Truss Analysis Software

Main Program

The structural frame & truss analysis and design program RSTAB 9 contains a similar range of functions as the FEA software RFEM, paying special attention to frames and trusses. Therefore, it is very easy to use and for many years, it has been the best choice for the structural analysis of beam structures consisting of steel, concrete, timber, aluminum, and other materials.

Price of First License
2,550.00 USD
RFEM 6
Modal Analysis

Dynamic Analysis

Analysis of Natural Vibrations

Price of First License
1,030.00 USD
RSTAB 9
Modal Analysis

Dynamic Analysis

Analysis of Natural Vibrations

Price of First License
1,030.00 USD