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  • Answer

    The shear stiffness was probably not considered in the manual calculation or it was assumed as infinite. In RFEM/RSTAB, this is taken into account according to reality by default, so that the deformation is more unfavorable than in the manual calculation, which only includes the deformation component from bending.

    The shear stiffness can be deactivated under the global calculation parameters (menu "Calculation" → "Calculation Parameters"), see the figure.

    Image 01 - Deactivating Shear Stiffness

  • Answer

    It is highly probable that the software is blocked by an antivirus program.

    We recommend to add the Dlubal Software programs, including the working directory, to the exceptions of all existing antivirus programs.

    You can find the path of the working directory in the menu "Options" → "Program Options," tab "Data Files" → category "Working Directory," see the figure.

    Figure 01 - Path of Working Directory

  • Answer

    The general FE mesh settings can be modified using the IFeMeshSettings interface. This interface is located under IModel > IModelData > ICalculation. Figure 01 shows which elements can be modified / displayed.

    Here is a code example where the target length of the FE elements is set to 100 mm. Furthermore, the division of the members with the same element size is activated and the minimum division is set to 3 elements:

    Sub mesh_params()

    Dim iApp As RFEM5.Application

    '   get interface for model data
    Dim iModel As RFEM5.model
    Set iModel = GetObject(, "RFEM5.Model")

    On Error GoTo e

    If Not iModel Is Nothing Then
        
        '   get interface for application and lock licence
        Set iApp = iModel.GetApplication()
        iApp.LockLicense
        
        '   get interface for model dat
        Dim iModdata As RFEM5.IModelData2
        Set iModdata = iModel.GetModelData
        
        '   get interface for calculation
        Dim iCalc As RFEM5.ICalculation2
        Set iCalc = iModel.GetCalculation()
        
        '   get interface for mesh settings
        Dim iMeshSet As RFEM5.IFeMeshSettings
        Set iMeshSet = iCalc.GetFeMeshSettings
        
        '   get general mesh settings
        Dim meshGen As RFEM5.FeMeshGeneralSettings
        meshGen = iMeshSet.GetGeneral
        
        meshGen.ElementLength = 0.1
        
        '   set new general mesh settings
        iModdata.PrepareModification
        iMeshSet.SetGeneral meshGen
        iModdata.FinishModification
        
        '   get mesh member settings
        Dim meshMem As RFEM5.FeMeshMembersSettings
        meshMem = iMeshSet.GetMembers
        
        meshMem.DivideStraightMembers = True
        meshMem.ElementLength = 0.1
        meshMem.MinStraightMemberDivisions = 3
        
        '   set new mesh member settings
        iModdata.PrepareModification
        iMeshSet.SetMembers meshMem
        iModdata.FinishModification
        
        
        iApp.UnlockLicense
    End If

    e:  If Err.Number <> 0 Then
        MsgBox Err.description, , Err.Source
        End If
        iApp.UnlockLicense
        Set iApp = Nothing
        Set iModel = Nothing

    End Sub


    The subroutine is also completed by an error interceptio routine (On Error GoTo e) and the Prepare-/FinishModification block is required as in the case of modifying other elements. Here, the block is created via the IFeMeshSettings interface.

  • Answer

    The number of load cases and combinations calculated at one time is limited to 9,999. This limitation can be avoided by using the "To Calculate" option, which allows you to select the cases to be calculated. Thus, load cases and load combinations can be divided into several groups with a smaller number of cases.
  • Answer

    Basically, there are six methods available for solving the nonlinear algebraic system of equations. If your model does not include any nonlinearities (that is, you have created a purely linear system), the access of this selection field is blocked.

    Figure 01 - System Without Nonlinearities

    As soon as you define a nonlinearity in the model, the selection field for the solving methods is activated, and you can select the desired solving method.

    Figure 02 - System with Nonlinearity

  • Answer

    In principle, the calculation with RF‑FORM‑FINDING has the same requirements as the calculation without this add-on module, so the criteria for general instabilities should be checked (see Links).

    However, problems often occur with RF‑FORM‑FINDING because the form-finding process has been activated for too many elements. Therefore, you should check whether the correct (necessary) elements have been activated (see Figure 01).

    In the following example in Figure 02, the form-finding has been activated for all horizontal members. This results in the error "The stiffness matrix is singular!" (see Figure 03). If you deactivate the beam members, the calculation is successful (see Figure 04).

  • Answer

    In the calculation parameters of RFEM or RSTAB, there are the "Number of load increments for load cases/load combinations" text boxes under the "Global Calculation Parameters" tab. These two entries control the numerical incremental application of the defined load boundary conditions in the respective load cases and load combinations. The reciprocal value of the entry describes a fraction of the load. The solving process then applies the defined load fractions successively to the model in so-called load increments until the complete load is reached. In the respective load increments, the equation solver tries to find an equilibrium within the maximum allowed iterations, and thus to specify suitable start values for the next load increment.

    Figure 02 - Calculation Parameters

    It is possible to imagine that the solving process collects the complete load of a load case or a load combination in a "watering can" and pours it onto the load-collecting model in portions. In this case, the number of load increments correlates with the speed of the load application. The speed is not to be understood as a real time parameter, but purely numerically.

    Figure 01 - Deformation Development Dependent on Load Increment

    The incremental load application has only an effect in the case of nonlinear structural systems. It usually provides a correspondingly higher result quality with increasing number of load increments. The basic aim of this method is to find a micro convergence in the respective load increments to specify new high-quality start values for the next load increment, and thus finally to achieve a macro convergence for the entire load case.

    Figure 03 - Calculation Diagrams

  • Answer

    If a member is connected eccentrically to a surface or to another member, you can also imagine that each node (RSTAB) and each FE node (RFEM) of each element is coupled to the member (see Figure 01 on top). The result is identical to that of the defined eccentricities (see Figure 01 on bottom).

    Figure 01 - Eccentricity with Rigid Members (top) and Defined Eccentricities (bottom)

    The structural system shown in the figure is nothing more than a truss with an upper chord and a lower chord, which is connected to each other by means of verticals. As you known, the chords are increasingly stressed by axial forces and less by bending moments due to the geometry.

    Figure 02 - Distribution of Bending Moment (top) and Axial Force (bottom)

  • Answer

    There are two ways to do this:

    1. You can define the corresponding member as a null member. Thus, it is not considered in the calculation of all load cases and load combinations.
    2. You can deactivate the corresponding member in all or only for certain load cases and/or load combinations. To do this, it is necessary to activate the "Modify stiffnesses" option in the calculation parameters of the load case or load combination. Then, you can deactivate this member in the additional tab window.

    However, you should pay attention to the following points:

    • When using the null member, a warning message appears if the member loads have been defined.
    • In the case of generated loads, the loads are redistributed automatically when using the null member.
    • If deactivating the member in the calculation parameters, the member loads and the determined generated loads are not considered. No error message appears in this case. It is necessary to redistribute the loads manually.
  • Answer

    In addition to a prestressing force or the target cable sag, it is also possible to specify the cable length, as shown in Figure 01.

    Figure 01 - Entering Cable Length

    The program then tries to fit the cable subjected to the acting force (a load case of the "form-finding" type with a dead load, for example) in the way that the length corresponds to the specified length.

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First Steps

First steps

We provide hints and tips to help you get started with the main programs RFEM and RSTAB.

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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