# Frequently Asked Questions (FAQ)

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• ### In which case do I use which superposition expression for the modal response spectrum method?

FAQ 000244 EN Calculation RF-DYNAM Pro - Forced Vibrations

The complete quadratic combination (Complete Quadratic Combination, CQC) is used if in the analysis of spacial models with mixed torsional / translational eigenvectors there are adjacent mode shapes, whose periods differ by less than 10 %.

If this is not the case, the quadratic combination (SRSS) is used.

• ### How do I choose the correct time step for the integration?

FAQ 000243 EN Calculation

The correct time step depends on the natural frequency of the system or the frequency of the exciting forces.

To achieve a sufficient precision, the governing period T = 1/f is divided in about 10 steps; that is, the time step Delta t is to be such that Delta t < t/10="1/(10f)" =="" 2="" pi="" (10="" omega).="">

Furthermore, you can specify in this definition point whether the results of each time step are to be shown or, for example, only for each fifth time step.

• ### How many mode shapes do I need to consider for an earthquake analysis?

In accordance to the EN 1998.-1 you need to calculate so many mode shapes that the sum of the effective modal masses is at least 90 % of the effective total mass (this is usually the overall structural mass). This can be regulated differently in other earthquake standards.

In the module RF-DYNAM Pro you find those values together with the frequencies in the Dynamic Load Cases (DLCs) in a tab called 'Mode Shapes', please see attached picture. You can check whether or not you calculated a sufficient number of eigenvalues before starting the calculation, it might be that you have ti increase the number of eigenvalues.

After calculation you find the effective modal masses and the factors in Table 5.7 5.7 Effective Modal Mass Factors.
• ### I have problems to understand how to model elastic surface foundations. Where do I enter my foundation modulus? And what is the meaning of the other surface elastic foundation parameters?

FAQ 000241 EN Modeling | Structure RFEM

An elastic surface foundation in RFEM is described by five governing parameters:

On the one hand, there are three subgrade moduli in the respective directions related to the local surface axes (Cu,x, Cu,y and Cu,z).

On the other hand, the parameters for considering foundation shear capacity (Cv,xz and Cv,yz) are relevant.

You can assign the nonlinearity to the define elastic surface foundation by defining the failure criterion, for example 'under tension'.

The parameter Cu,z is practically equal to the Winkler foundation coefficient and can be found in a soil expertise.

The parameters Cu,x and Cu,y are friction factors indicating the foundation resistance against horizontal displacement in the respective directions. Thus, you can enter the foundation modulus, for example for the foundation support, in the Cu,z text box as the support is to be acting perpendicularly to the surface.
• ### Which bending theory should I use for the calculation of plates and shells (Kirchhoff / Mindlin)?

FAQ 000239 EN Calculation RFEM

You can select between two bending theories for plate and shell elements in the "Calculation Parameters" dialog box.

In the calculation according to the Mindlin theory, shear force deformations are included, but are not considered according to the Kirchhoff theory.

If thicker plates and shell elements are calculated, the Mindlin theory results in more precise results. When calculating thin or very thin elements, the Kirchhoff theory is appropriate.
• ### I defined several materials; SHAPE-THIN determines strange cross-section properties.

FAQ 000238 EN Calculation SHAPE-THIN

For cross-section with several material, the ideal cross-section properties and cross-section diagrams are determined.

For this, the reference material defined in the General Data under [Settings] is used.

Thus, the surface of the individual materials is recalculated to the surface of the reference material by using:

n = E_i / E_ref.

• ### What is the capacity of the RFEM analysis core?

FAQ 000237 EN Calculation RFEM

The following values represent the upper limits in the RFEM 5 data structure:

• 99,999 objects of each category (nodes, lines, surfaces, cross-sections, etc.)
• 99,999 objects of each type of load per load case
• 9,999 Load cases
• 9,999 Load combinations
Notice that the sum of the nonlinearly analyzed load cases and load combinations may not be higher than 9,999.
• 9,999 Result combinations

Notice that the limit for efficiently working in RFEM may be lower in dependence of the used hardware and the complexity of the structure.

• ### Is it possible to export data from RFEM to RSTAB?

FAQ 000236 EN Import / Export & BIM RFEM RSTAB

The export is not possible in this direction.

In RFEM, surface elements may be available in addition to member elements. RSTAB is not able to import these surface elments because this would lead to a falsification of the structural system and so to wrong results.

• ### How can I rotate a cross-section (IPE 270) set in 2D mode by 90° so that it can be loaded about the minor axis? When I try to rotate the section by 90°, an error message appears.

FAQ 000235 EN Modeling | Structure RSTAB

When entering models in the two-dimensional space, only section rotation angles of either 0 or a multiple of 180 are allowed. This is because the direction of the local axis system in the member (x-y-z) is clearly defined and specified. Moreover, only the member internal forces My and Vz are calculated for 2D structures.

The major axis of the cross-section (y-axis) in the 2D space is always oriented in the direction of the global Y-axis. The minor axis (z-axis) are not considered (as if it was completely undisplaceable).

In case of the cross-section rotations allowed, biaxial bending could occur since loads in the 2D space can only be defined in the X-Z plane. However, the resulting Vy, T and Mz portion would remain unconsidered. This is the reason why only those cross-section rotations are allowed for which the section local z-axis is directed to the X-Z plane.
• ### The top and bottom reinforcement seem to be reversed in the graphic.

FAQ 000233 EN Display Settings RF-CONCRETE

Background: Each surface has a local coordinate system in RFEM (x,y,z).

The coordinates x and y are in-plane, z runs perpendicular to the surface.

The top and bottom side of a surface is determined by the direction of the z-axis.

The direction of the local z-axis determines the bottom side of a surface. Usually, this local z-axis is orientated downwards. Depending on the defined direction of the boundary lines and the sequence in which the lines were clicked., it is also possible that the local z-axis points upwards.

It is easy to correct this. First, display the local surface coordinate system.

To do this, proceed as follows:

Go to the "Display" navigator on the left. Here, you can set all display properties in a tree-like structure.

Open the entries "Model" -> "Surfaces" and select the "Surface Axis Systems x,y,z" check box.

The bottom reinforcement is on the side of the slab, where the z-direction is positive.

To reverse the direction of the local z-axis, click the corresponding surface, and then select the "Reverse axis system" option in the context menu.

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