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• ### I use member hinges with slippage in the longitudinal direction of the member. However, the calculation is always aborted because of an instability message. How can I optimize the system accordingly?

New FAQ 003585 EN-US

Especially the definition of slippage is a challenge for the solver due to the nonlinear calculation. In the following, hints are given how instabilities can be avoided.

###### Sliding definition
Slippage (eg in one connection) is defined by means of the "Partial Effect" nonlinearity (see Figure 03). It can be used to define the hinge displacement from which the forces should be transferred. As can be seen in the diagram, the stop, that is, the stiffness that acts according to the corresponding hinge displacement, is considered as rigid (vertical branch, see the red arrows). However, under certain circumstances, this may lead to numerical problems in the calculation. To avoid this, the stiffness that acts after the hinge displacement should be reduced slightly. This is achieved by defining a very stiff spring (see Figure 04).

In addition to the very stiff stop, numerical problems may occur within the slippage. In this case, a small stiffness has to be considered for the effect of the slippage in order to increase the horizontal branch a little bit. The stiffness should be selected so small that it has no decisive influence (see Figure 05). This situation is possible by using the "Diagram" nonlinearity.

###### Arrangement of Member Hinges
When arranging the hinges, care should be taken to ensure that they are not defined in the same direction on both member ends. Thus, there is a state in which the member is not sufficiently supported and the system fails already in the first iterations. In such a case, the slippage on only one side of the member should be defined and the size of the slippage adjusted accordingly (see Figure 06).

• ### An apparently insufficiently supported member does not lead to instability of the entire system. What is the reason for this?

New FAQ 003583 EN-US

The cause may be the definition of member eccentricities. For a better overview, the origin lines are automatically hidden for eccentric members. In some cases, it may appear that a member end is not sufficiently supported (see Figure 01, left). However, the meshed line acts in the background. This can be clearly shown by hiding the members in the Display navigator (see Figure 01, right).

In Figure 01, the line at the common node has not been geometrically separated. For this reason, no graphical connection line that represents the eccentricity results in this case. However, because of the setting shown in Figure 02, the node is still networked with the vertical member.

To visualize the graphical connection line, it is recommended to divide the member or line at this node (see Figure 03)
• ### SHAPE-THIN calculates a very small shear area. What is the reason for this?

New FAQ 003547 EN-US

The shear area is calculated as follows:

${\mathrm A}_{\mathrm y}\;=\;\frac{{\mathrm I}_{\mathrm z}^2}{\int_{\mathrm A^\ast}\left({\displaystyle\frac{{\mathrm S}_{\mathrm z}}{\mathrm t^\ast}}\right)^2\operatorname d\mathrm A^\ast}$

${\mathrm A}_{\mathrm z}\;=\;\frac{{\mathrm I}_{\mathrm y}^2}{\int_{\mathrm A^\ast}\left({\displaystyle\frac{{\mathrm S}_{\mathrm y}}{\mathrm t^\ast}}\right)^2\operatorname d\mathrm A^\ast}$

These include:

 I z or I y : 2nd-order moment of area in relation to the axis z or y S z or S y : 1st degree moment of area in relation to the axis z or y t *: effective element thickness for shear transfer A *: Surface Area Based on Effective Shear Thickness t *

The effective element thickness for shear transfer t * has a significant influence on the shear area. Therefore, the defined effective element thickness for shear transfer t * (Figure 1) of the elements should be checked.

• ### In a comparison between the partial calculation of a single floor and the total calculation of the building, the local deformations are very different. What is the cause?

New FAQ 003540 EN-US

First, please note that the local deformations of surfaces are always related to the undeformed system. Therefore, for a multi-storey building, the deformations of the top floor also include the deformations of the lower floors, as shown in Figure 01 on the left.

Figure 01 on the right shows the corresponding bending moment my. As with this simple model, it is identical for the floors. In such a case, the partial calculation of the individual floors is no problem, because the relative deformation seems to be identical for each floor.

However, it becomes problematic if the supporting elements are loaded differently or if the stiffness of the supporting elements within a floor is different. Figure 02 shows the bending moment my of such a system. It can be seen that the distribution, especially between the bottom ceiling and the top floor, shows the greatest differences. In this particular case, internal columns with a less stiff cross-section were added in addition to the corner columns. For this reason, the relative deformation increases more with each additional floor in the middle than at the corner columns.

In reality, this structure will not be available in this way because the floors are manufactured one after the other and thus the deformations (for example due to self-weight) are compensated for the structure from ceiling to floor. Thus, it is a typical structural state problem. Thus, the question arises whether the effects can be neglected or if, for example, For example, you have to analyze the results with the add-on module RF-STAGES.

• ### I have inserted a result beam above a door opening. Why do the shear forces and moments differ so strongly from an ideal single-span beam?

New FAQ 003531 EN-US

If the result beam is defined correctly, there may be a coarse FE mesh in the lintel area, which leads to rough inaccuracies in the results (in this case, shear forces, see Figure 1).

It is recommended to create about ten finite elements above the height of the door opening. For example, if the height above the door opening is 0.5 m, a target FE mesh width of 0.05 m is adequate in this area so that the desired results are achieved (see Figure 2: Shear forces).

The settings are set globally (Menu → Calculation → FE Mesh Settings) or by using a local FE mesh refinement.

• ### After the calculation, a circularly modeled opening appears as a quadrilateral.How can I fix this problem?

New FAQ 003528 EN-US

The consideration of curved elements depends on the fineness of the FE mesh.
An FE element always has straight outer edges.
If the FE mesh is selected too roughly, the result is a very inaccurate mapping of the curved elements with straight lines.
If the global setting of the target length of the finite elements is specified with sufficient accuracy under "Calculation" → "FE Mesh Settings", you can also perform a local FE mesh refinement on the nodes, lines, or surfaces.
• ### Unfortunately, the module results are not visible in the RFEM Viewer.

New FAQ 003522 EN-US

In the Viewer mode, all results, including the module results in the printout report, are available to you.

However, there are the following restrictions:
No calculation, editing and saving, as well as import and export is possible. In addition, you can not start any modules.
• ### The design ratio of the cross-section check is different for the RF-/STEEL and RF-/STEEL EC3 add-on module. What is the reason?

New FAQ 003489 EN-US

Cross-sections assigned to class 1 or 2 are designed plastically by RF-/STEEL EC3 by default. In order to be able to compare the results with RF-/STEEL, please activate the elastic design of class 1 and 2 cross-sections (Figure 2) in the Details of RF-/STEEL EC3.

Please also check whether the partial safety factors γ for the resistances of the cross-sections are defined identically in both add-on modules (Figure 3 and 4).

• ### How many divisions must a member have for the calculation in RFEM so that I get accurate results?

New FAQ 003485 EN-US

It is not possible to globally answer this question because it depends on the system. There are several divisions to be considered in RFEM.

###### 1. Member divisions for results tables
You can create member divisions for result values by using the menu 'Insert > Model Data > 'Member Divisions'. This division ensures that - e.g. in the RFEM results tables - the internal forces of members can also be displayed at intermediate points. The graphical output remains unaffected.

###### 2. Member divisions
The divisions for the graphical result diagram and the determination of the extreme value can be viewed and influenced in the FE mesh settings (see Figure 1).

For cable, foundation, and tapered members or members with plastic properties, you can specify the number of internal divisions. They lead to a real division of the member by intermediate nodes. However, if a member is arranged on the boundary line of a surface or if the definition line has an FE mesh refinement, the specification has no effect.

Select the 'Activate Internal Member Divisions for Large Deformation Analysis' option to divide also beams by intermediate nodes for the calculation according to large deformation analysis so that these members are calculated with higher accuracy. The number of member divisions is taken over into the input field above.

If using the division even for straight members, which are not integrated into surfaces, FE nodes are generated on all free members and considered for calculations according to the linear static and second-order analysis. The length of the finite elements is either determined by the global target length l FE set in the General dialog section or entered manually.

With the option 'Use Division for Members with Nodes Lying on Them', RFEM generates FE nodes on those locations of the member where end nodes of other members are lying, without having any connection existing between these members.
• ### Result diagrams are not displayed in the entire area of a section through the surface. What is the reason?

New FAQ 003473 EN-US

If the section was defined on an unused line, activate the 'Integrate Unutilized Objects into Surfaces' checkbox in the FE-Mesh settings. There is a section defined in lines 7 and 12 in Figure 1. Line 7 has no other function for surface 1. It is neither the boundary line of surface 1, nor does it have a support or a load. In the FE-Mesh settings, the 'Integrate Unutilized Objects into Surfaces' checkbox is deactivated so that no result diagrams are displayed on line 7. Conversely, line 12 is stressed so that result diagrams are displayed on this line. If the 'Integrate Unutilized Objects into Surfaces' checkbox is selected, result diagrams are also displayed on line 7.

If the section runs through several surfaces, the respective surfaces have to be specified in the 'On Surface No.' section. Figure 2 shows a section through Surfaces 3 and 4. In the section 'On Surfaces No.' only the surface 3 is specified so that only on this surface result diagrams are displayed. Surface 4 would have to be specified in the section 'On Surfaces No.' so that result diagrams are also displayed at this place.

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