In the Modal Analysis add-on, you have the option to automatically increase the sought eigenvalues until reaching a defined effective modal mass factor. All translational directions activated as masses for the modal analysis are taken into account.
Thus, it is possible to easily calculate the required 90% of the effective modal mass for the response spectrum method.
For each load case, the deformations can be displayed at the end time.
These results are also documented for you in the printout report of RFEM and RSTAB. You can select the report contents and extent specifically for the individual design checks.
You have several options available to define masses for a modal analysis. While the masses due to self-weight are considered automatically, you can consider the loads and masses directly in a load case of the modal analysis type. Do you need more options? Select whether to consider full loads as masses, load components in the global Z-direction, or only the load components in the direction of gravity.
The program offers you an additional or alternative option for importing masses: A manual definition of load combinations as of which are the masses considered in the modal analysis. Have you selected a design standard? You can then create a design situation with the Seismic Mass combination type. Thus, the program automatically calculates a mass situation for the modal analysis according to the preferred design standard. In other words: The program creates a load combination on the basis of the preset combination coefficients for the selected standard. This contains the masses used for the modal analysis.
For a response spectrum analysis of building models, you can display the sensitivity coefficients for the horizontal directions by story.
These key figures allow you to interpret the sensitivity to stability effects.
In RFEM, you can use these three powerful eigenvalue solvers:
- Root of Characteristic Polynomial
- Method by Lanczos
- Subspace Iteration
RSTAB, on the other hand, provides you with these two eigenvalue solvers:
- Subspace Iteration
- Shifted inverse power method
The selection of the eigenvalue solver depends primarily on your model size.
- Automatic consideration of masses from self-weight
- Direct import of masses from load cases or load combinations
- Optional definition of additional masses (nodal, linear, or surface masses, as well as inertia masses) directly in the load cases
- Optional neglect of masses (for example, mass of foundations)
- Combination of masses in different load cases and load combinations
- Preset combination coefficients for various standards (EC 8, SIA 261, ASCE 7,...)
- Optional import of initial states (for example, to consider prestress and imperfection)
- Structure Modification
- Consideration of failed supports or members/surfaces/solids
- Definition of several modal analyses (for example, to analyze different masses or stiffness modifications)
- Selection of mass matrix type (diagonal matrix, consistent matrix, unit matrix), including user-defined specification of translational and rotational degrees of freedom
- Methods for determining the number of mode shapes (user-defined, automatic - to reach effective modal mass factors, automatic - to reach the maximum natural frequency - only available in RSTAB)
- Determination of mode shapes and masses in nodes or FE mesh points
- Results of eigenvalue, angular frequency, natural frequency, and period
- Output of modal masses, effective modal masses, modal mass factors, and participation factors
- Masses in mesh points displayed in tables and graphics
- Visualization and animation of mode shapes
- Various scaling options for mode shapes
- Documentation of numerical and graphical results in printout report
In the modal analysis settings, you have to enter all data that are necessary for the determination of the natural frequencies. These are, for example, mass shapes and eigenvalue solvers.
The Modal Analysis add-on determines the lowest eigenvalues of the structure. Either you adjust the number of eigenvalues or let them determined automatically. Thus, you should reach either effective modal mass factors or maximum natural frequencies. Masses are imported directly from load cases and load combinations. In this case, you have the option to consider the total mass, load components in the global Z-direction, or only the load component in the direction of gravity.
You can manually define additional masses at nodes, lines, members, or surfaces. Furthermore, you can influence the stiffness matrix by importing axial forces or stiffness modifications of a load case or load combination.
Do you have great respect for the ravages of time? After all, it eventually gnaws at your construction projects. Use the Time-Dependent Analysis (TDA) add-on to consider the time-dependent material behavior of members. Long-term effects, such as creep, shrinkage, and aging, can influence the distribution of internal forces, depending on the structure. Prepare for this optimally with this add-on.
Do you want to consider other loads as masses in addition to the static loads? The program allows that for nodal, member, line and surface loads. For this, you need to select the Mass load type when defining the load of interest. Define a mass or mass components in the X, Y, and Z directions for such loads. For nodal masses, you have an additional option to also specify moments of inertia X, Y, and Z in order to model more complex mass points.
The Dlubal structural analysis software does a lot of work for you. The input parameters, which are relevant for the selected standards, are suggested by the program in accordance with the rules. Furthermore, you can enter response spectra manually.
Load cases of the type Response Spectrum Analysis define the direction in which response spectra act and which eigenvalues of the structure are relevant for the analysis. In the spectral analysis settings, you can define details for the combination rules, damping (if applicable), and zero-period acceleration (ZPA).
Have you activated the Time-Dependent Analysis (TDA) add-on? Very well, now you can add time data to load cases. After you have defined the start and end of the load, the influence of creep at the end of the load is taken into account. The program allows you to model creep effects for frame and truss structures made of reinforced concrete.
In this case, the calculation is performed nonlinearly according to the rheological model (Kelvin and Maxwell model).
Was the calculation successful? You can now display the determined internal forces in tables and graphics, and consider them in the design.
As soon as the program has completed the calculation, the eigenvalues, natural frequencies and periods are listed. These result windows are integrated in the main program RFEM/RSTAB. You can find all mode shapes of the structure in tables and also have an option to display them graphically and to animate them.
All result tables and graphics are part of the RFEM/RSTAB printout report. In this way, you can ensure clearly arranged documentation. You can also export the tables to MS Excel.
It is often necessary to neglect masses. This is particularly the case when you want to use the output of the modal analysis for the seismic analysis. For this, 90% of the effective modal mass in each direction is required for the calculation. So you can neglect the mass in all fixed nodal and line supports. The program automatically deactivates the associated masses for you.
You can also manually select the objects whose masses are to be neglected for the modal analysis. We have shown the latter in the image for a better view. A user-defined selection is made the and the objects with their associated mass components are selected to neglect the masses.
You can already see it in the image: Imperfections can also be taken into account when defining a modal analysis load case. The imperfection types that you can use in the modal analysis are notional loads from load case, initial sway via table, static deformation, buckling mode, dynamic mode shape, and group of imperfection cases.
When defining the input data for the modal analysis load case, you can consider a load case whose stiffnesses represent the initial position for the modal analysis. How do you do that? As shown in the image, select the "Consider initial state from" option. Now, open the "Initial State Settings" dialog box and define the type Stiffness as the initial state. In this load case, as of which is the initial state taken into account, you can consider the stiffness of the structural system when the tension members fail. The purpose of all of this: The stiffness from this load case is considered in the modal analysis. Thus, you obtain a clearly flexible system.
Compared to the RF‑/DYNAM Pro - Natural Vibrations add-on module (RFEM 5 / RSTAB 8), the following new features have been added to the Modal Analysis add-on for RFEM 6 / RSTAB 9:
- Preset combination coefficients for various standards (EC 8, ASCE, and so on)
- Optional neglect of masses (for example, mass of foundations)
- Methods for determining the number of mode shapes (user-defined, automatic - to reach effective modal mass factors, automatic - to reach the maximum natural frequency)
- Output of modal masses, effective modal masses, modal mass factors, and participation factors
- Masses in mesh points displayed in tables and graphics
- Various scaling options for mode shapes in the Result navigator
The stress and strain results by surface can be output in the surface result table according to the thickness layer.
The form-finding process gives you a structural model with active forces in the "prestress load case" This load case shows the displacement from the initial input position to the form-found geometry in the deformation results. In the force or stress-based results (member and surface internal forces, solid stresses, gas pressures, and so on), it clarifies the state for maintaining the found form. For the analysis of the shape geometry, the program offers you a two-dimensional contour line plot with the output of the absolute height and an inclination plot for the visualization of the slope situation.
Now, a further calculation and structural analysis of the entire model is performed. For this purpose, the program transfers the form-found geometry including the element-wise strains into a universally applicable initial state. You can now use it in the load cases and load combinations.
Did you know? You can easily define structural modifications in load cases of the Modal Analysis type. This allows you, for example, to individually adjust the stiffnesses of materials, cross-sections, members, surfaces, hinges, and supports. You can also modify stiffnesses for some design add-ons. Once you select the objects, their stiffness properties are adapted to the object type. In this way, you can define them in separate tabs.
Do you want to analyze the failure of an object (for example, a column) in the modal analysis? This is also possible without any problems. Simply switch to the Structure Modification window and deactivate the relevant objects.
The Ponding load type allows you to simulate rain actions on multi-curved surfaces, taking into account the displacements according to the large deformation analysis.
This numerical rainfall process examines the assigned surface geometry and determines which rainfall portions drain away and which rainfall portions accumulate in puddles (water pockets) on the surface. The puddle size then results in a corresponding vertical load for the structural analysis.
For example, you can use this feature in the analysis of approximately horizontal membrane roof geometries subjected to rain loading.
Go to Explanatory VideoOnce you activate the Form-Finding add-on in the Base Data, a form-finding effect is assigned to the load cases with the load case category "Prestress" in conjunction with the form-finding loads from the member, surface, and solid load catalog. This is a prestress load case. It thus mutates into a form-finding analysis for the entire model with all member, surface, and solid elements defined in it. You reach the form-finding of the relevant member and membrane elements amid the overall model by using special form-finding loads and regular load definitions. These form-finding loads describe the expected state of deformation or force after the form-finding in the elements. The regular loads describe the external loading of the entire system.
The load cases of the type Response Spectrum Analysis contain the generated equivalent loads. First, the modal contributions have to be superimposed with the SRSS or CQC rule. In this case, you can use the signed results based on the dominant mode shape.
Afterwards, the directional components of earthquake actions are combined with the SRSS or the 100% / 30% rule.
Compared to the RF-/DYNAM Pro - Equivalent Loads add-on module (RFEM 5 / RSTAB 8), the following new features have been added to the Response Spectrum Analysis add-on for RFEM 6 / RSTAB 9:
- Response spectra of numerous standards (EN 1998, DIN 4149, IBC 2018, and so on)
- User-defined response spectra or those generated from accelerograms
- Direction-relative response spectrum approach
- Results are stored centrally in a load case with underlying levels to ensure clarity
- Accidental torsional actions can be taken into account automatically
- Automatic combinations of seismic loads with the other load cases for use in an accidental design situation
Compared to the RF-FORM-FINDING add-on module (RFEM 5), the following new features have been added to the Form-Finding add-on for RFEM 6:
- Specification of all form-finding load boundary conditions in one load case
- Storage of form-finding results as initial state for further model analysis
- Automatic assignment of the form-finding initial state via combination wizards to all load situations of a design situation
- Additional form-finding geometry boundary conditions for members (unstressed length, maximum vertical sag, low-point vertical sag)
- Additional form-finding load boundary conditions for members (maximum force in member, minimum force in member, horizontal tension component, tension at i-end, tension at j-end, minimum tension at i-end, minimum tension at j-end)
- Material types "Fabric" and "Foil" in material library
- Parallel form-findings in one model
- Simulation of sequentially building form-finding states in connection with the Construction Stages Analysis (CSA) add-on
Did you know that Equivalent static loads are generated separately for each relevant eigenvalue and excitation direction. These loads are saved in a load case of the Response Spectrum Analysis type and RFEM/RSTAB performs a linear static analysis.
Have you already discovered the tabular and graphical output of masses in mesh points? That's right, this is also part of the modal analysis results in RFEM 6. This way, you can check the imported masses that depend on various settings of the modal analysis. They can be displayed in the Masses in Mesh Points tab of the Results table. The table provides you with an overview of the following results: Mass - Translational Direction (mX, mY, mZ), Mass - Rotational Direction (mφX, mφY, mφZ), and the Sum of Masses. Would it be best for you to have a graphical evaluation as quickly as possible? Then you can also graphically display the masses in mesh points.
Do you know exactly how the form-finding is performed? First, the form-finding process of the load cases with the load case category "Prestress" shifts the initial mesh geometry to an optimally balanced position by means of iterative calculation loops. For this task, the program uses the Updated Reference Strategy (URS) method by Prof. Bletzinger and Prof. Ramm. This technology is characterized by equilibrium shapes that, after the calculation, comply almost exactly with the initially specified form-finding boundary conditions (sag, force, and prestress).
In addition to the pure description of the expected forces or sags on the elements to be formed, the integral approach of the URS also enables a consideration of regular forces. In the overall process, this allows, for example, for a description of the self-weight or a pneumatic pressure by means of corresponding element loads.
All these options give the calculation kernel the potential to calculate anticlastic and synclastic forms that are in an equilibrium of forces for planar or rotationally symmetric geometries. In order to be able to realistically implement both types individually or together in one environment, the calculation provide you with two ways to describe the form-finding force vectors:
- Tension method - description of the form-finding force vectors in space for planar geometries
- Projection method - description of the form-finding force vectors on a projection plane with fixation of the horizontal position for conical geometries
- You can activate or deactivate the use of torsional warping in the Add-ons tab of the model's Base Data.
- After activating the add-on, the user interface in RFEM is extended by some new entries in the navigator, tables, and dialog boxes.
A library for cross-laminated timber panels is implemented in RFEM, from which you can import the manufacturer's layer structures (for example, Binderholz, KLH, Piveteaubois, Södra, Züblin Timber, Schilliger, Stora Enso). In addition to the layer thicknesses and materials, there is also the information about stiffness reductions and the narrow side bonding.
Go to Explanatory VideoThe Concrete Design add-on allows you to perform the seismic design of reinforced concrete members according to EC 8. This includes, among other things, the following functionalities:
- Seismic design configurations
- Differentiation of the ductility classes DCL, DCM, DCH
- Option to transfer the behavior factor from a dynamic analysis
- Check of the limit value for the behavior factor
- Capacity design checks of "Strong column - weak beam"
- Detailing and particular rules for curvature ductility factor
- Detailing and particular rules for local ductility