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  • 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.

  • Answer

    The solids are connected by using surface release with the "fixed if negative pz" nonlinearity. The force pz is derived on the basis of the orientation of the contact surfaces. If the orientation of both surfaces is the same and you want the compressive load to be transferred between the surfaces, it is necessary to use the "fixed if positive pz" nonlinearity.
  • Answer

    The working directory is a local path where the data of the currently opened structure is temporarily handled and saved. It consists of the first letters of the file. Only ASCII characters may be used.

    The non-ASCII characters are, for example, "ä," "ö," and "ß."

    To avoid this problem, replace the special characters in the file name by the ASCII characters, such as "ae" and "ss" instead of "ä" and "ß." When you open the file again, the message will no longer appear.
  • Answer

    In order to only calculate specific load cases, load combinations, or result combinations in the same way as the "To Calculate..." command (see Figure 01), you can use the CalculateBatch method of the ICalculation interface. For the transfer, the method expects a field with the load type of Loading. This Loading includes the number of the load, and the type (for example, a load combination):

    Sub batch_test()
        
    '   get interface from the opened model and lock the licence/program
        Dim iModel As RFEM5.IModel3
        Set iModel = GetObject(, "RFEM5.Model")
        iModel.GetApplication.LockLicense
        

    On Error GoTo e
        
        '   get interface for calculation
        Dim iCalc As ICalculation2
        Set iCalc = iModel.GetCalculation
        
        '   create array with loading types
        Dim loadings(3) As Loading
        loadings(0).no = 1
        loadings(0).Type = LoadCaseType
        
        loadings(1).no = 4
        loadings(1).Type = LoadCaseType
        
        loadings(2).no = 4
        loadings(2).Type = LoadCombinationType
        
        '   calculate all loadings from the array at once
        iCalc.CalculateBatch loadings

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

    End Sub
  • Answer

    The shear correction factor is considered in the RF‑LAMINATE add-on module by using the following equation.


    $k_{z}=\frac{{\displaystyle\sum_i}G_{xz,i}A_i}{\left(\int_{-h/2}^{h/2}E_x(z)z^2\operatorname dz\right)^2}\int_{-h/2}^{h/2}\frac{\left(\int_z^{h/2}E_x(z)zd\overline z\right)^2}{G_{xz}(z)}\operatorname dz$

    with $\int_{-h/2}^{h/2}E_x(z)z^2\operatorname dz=EI_{,net}$

    The calculation of shear stiffness can be found in the English version of the RF-LAMINATE manual, page 15 ff.

    For a plate with the thickness of 10 cm in Figure 01, the calculation of the shear correction factor is shown. The equations used here are only valid for simplified symmetrical plate structures!

    Layerz_minz_maxE_x(z)(N/mm²)G_xz(z)(N/mm²)
    1-50-3011,000690
    2-30-1030050
    3-101011,000690
    4103030050
    5305011,000690

    $\sum_iG_{xz,i}A_i=3\times0.02\times690+2\times0.02\times50=43.4N$

    $EI_{,net}=\sum_{i=1}^nE_{i;x}\frac{\mbox{$z$}_{i,max}^3-\mbox{$z$}_{i,min}^3}3$

    $=11,000\left(\frac{-30^3}3+\frac{50^3}3\right)+300\left(\frac{-10^3}3+\frac{30^3}3\right)$

    $+11,000\left(\frac{10^3}3+\frac{10^3}3\right)+300\left(\frac{30^3}3-\frac{10^3}3\right)+11,000\left(\frac{50^3}3-\frac{30^3}3\right)$

    $=731.2\times10^6 Nmm$

    $\int_{-h/2}^{h/2}\frac{\left(\int_z^{h/2}E_x(z)zd\overline z\right)^2}{G_{xz}(z)}\operatorname dz=\sum_{i=1}^n\frac1{G_{i;xz}}\left(χ_i^2(z_{i,max}-z_{i,min})\;χ_iE_{i,x}\frac{z_{i,max}^3-z_{i,min}^3}3+E_{i,x}^2\frac{z_{i,max}^5-z_{i,min}^5}{20}\right)$

    $χ_i=E_{i;x}\frac{z_{i,max}^2}2+\sum_{k=i+1}^nE_{k;x}\frac{z_{k,max}^2-z_{k,min}^2}2$


    χ113.75 106
    χ2
    8.935 106
    χ3
    9.47 106
    χ4
    8.935 106
    χ5
    13.75 106


    $\sum_{i=1}^n\frac1{G_{i;yz}}\left(χ_i^2(z_{i,max}-z_{i,min})-χ_iE_{i,y}\frac{z_{i,max}^3-z_{i,min}^3}3+{E^2}_{i,y}\frac{z_{i,max}^5-z_{i,min}^5}{20}\right)=$


    8.4642 1011
    3.147 1013
    2.5 1012
    3.147 1013
    8.4642 1011

    Total 6.7133 x 1013

    $k_z=\frac{43.4}{{(731.2e^6)}^2}6.713284\;e^{13}=5.449\;e^{-3}$

    $D_{44}=\frac{{\displaystyle\sum_i}G_{xz,i}A_i}{k_z}=\frac{43.4}{5.449\;e^{-3}}=7,964.7 N/mm$

    This corresponds to the resulting value in RF‑LAMINATE (Figure 02).
  • Answer

    In RF‑GLASS, there are two different types of calculations. On the one hand, there is the "2D" calculation. In this case, a glass structure is displayed as a surface element. When considering the shear coupling, the program determines an equivalent cross-section by using the laminate theory On the other hand, there is the "3D" calculation. In this case, the composition is modeled as a solid element in the calculation, and thus the effectiveness of stiffnesses between the foil and glass is determined exactly when considering the coupling.

    Further information about the calculation methods can be found in the RF‑GLASS manual, Chapter 2.

  • Answer

    Only the default setting of 1 load increment can be set when a complex nonlinear material model is defined. The reason for this is because the program cannot determine the correct material stiffness for each incremental loading amount. The exact maximum load needs to be applied to the structure in order to determine the state of the material's stress/strain diagram. 


    Figure 01 - Material Model - Nonlinear material defined

    This setting can be found and changed under "Calculation Parameters" as well as under the "Calculation Parameters" in the load cases and combinations dialog box. 


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