Frequently Asked Questions (FAQ) - Search term: Nodes of ...

In the standards, no torsion is taken into account in the stability analysis for the equivalent member method or the general method. The warning message is a reminder to the user of this fact.

First of all, the magnitude of the torsional stress or moments should be analyzed. Very small torsional moments in the members to be designed often prevent certain design formats. In order to neglect them and suppress the warning, you can change the limit values in the Ultimate Configuration. There is no normative basis for this limit value and therefore, no general specification. Find more information here:
Neglecting Torsion

In the case of open cross-sections, it can generally be assumed that the acting torsional moments are not transferred by primary torsion, but by secondary (warping) torsion. In RFEM 6 and RSTAB 9, the internal forces are initially only determined with six degrees of freedom without considering warping. For small acting torsional moments, it seems obvious to neglect the torsional stresses in the design. For large torsional moments, however, a calculation should be carried out with seven degrees of freedom in order to also consider the influence of the warping moment and the secondary torsional shear stresses. For this, you can use the Torsional Warping (7 DOF) for RFEM 6 / RSTAB 9 add-on, for example.

In order to specify a uniaxial load transfer for a plate, you can assign orthotropic properties to the surface. This way, you can control the stiffnesses for specific directions so that the loads are mainly transferred in one direction. You have two options.

Orthotropic Thickness for Isotropic Material

• Double-click the surface to open the dialog box for editing the slab.
• In the Thickness with Material section, click the button to the right, to create a new thickness.
• In the "New Thickness" dialog box, select the Shape Orthotropy thickness type.

• Switch to the "Shape Orthotropy" tab. The Effective Thicknesses orthotropy type is set by default.
• For the uniaxial load transfer, enter the effective thicknesses as orthotropic properties in such a way that the stiffness is very high in the desired main support direction and very low in the other direction.

• To consider the self-weight, you can apply a user-defined fictitious thickness that corresponds to the full surface thickness.
• Click OK to save the thickness.
• Click OK again in the "Edit Surface" dialog box to assign the thickness to the plate.

Orthotropic Material Model for Constant Thickness

• Double-click the surface to open the dialog box for editing the slab.
• In the Thickness with Material section, click the button to the right, to create a new material.
• In the "New Material" dialog box, select the material from the library as usual. However, change the material model to Orthotropic | Linear Elastic (Surfaces) (1). Also, activate the User-defined material option (2).

• Switch to the "Orthotropic | Linear Elastic (Surfaces)" tab.
• For a uniaxial load transfer, change the modulus of elasticity of the secondary load direction to a very small value. You can also specify very small values for the shear moduli in the secondary support direction. Furthermore, set the Poisson's ratios to zero.

• Click OK to save the material.
• Click OK again in the 'Edit Surface' dialog box to assign the material of the plate.

Requirements

• RFEM 6 / RSTAB 9 is installed
• You have a valid license of WebServices.
• The Web Services are activated in the settings of RFEM 6 / RSTAB 9.
• IDEA StatiCa is installed.
• The BIM Link to RFEM 6 / RSTAB 9 by IDEA StatiCa is installed (IDEA StatiCa → "BIM" tab → "Activate Your BIM Link" → RFEM 6 / RSTAB 9).

Procedure

1. Create and calculate your RFEM 6 / RSTAB 9 model.
2. Open the shortcut "IDEA StatiCa 2X.X for RFEM 6" or" IDEA StatiCa 2X.X for RSTAB 9".
3. Select joint nodes and connecting member(s) in RFEM 6 / RSTAB 9.
4. Import the geometry into the checkbot via "Import" → "Connections".
5. Transfer the connection or member from the checkbot to IDEA StatiCa.

The following programs and add-ons are perfectly suitable for your reinforced concrete projects.

Main Program

To model your structure and determine the internal forces, you need one of the main programs. Depending on the application area, you can use RFEM 6 or RSTAB 9.

• RSTAB: Analysis of planar or spatial frame & truss structures
• RFEM: Finite element analysis of plate, wall, shell, and member structures

If your model has surface components, such as plates and walls, in addition to membe elements, you need RFEM.

Concrete Design Add-on

Use the Concrete Design add-on to design the reinforced concrete components of your model. The design checks according to specific standard include not only members (bending beams, columns) and surfaces (plates, walls), but also surfaces susceptible to punching shear. For columns, the stability analysis according to the nominal curvature method is integrated in the add-on.

You can specify user-defined reinforcement or determine it by using the program. With these basic conditions, you can perform the ultimate and serviceability limit state design checks according to one of the following standards:

• EN 1992, including National Annexes
• ACI 318
• CSA A23.3
• SP 63.13330

The add-on also allows for deformation analyses for cracked reinforced concrete surfaces (state II) using the approximation methods described in the standards. Tension stiffening as well as creep and shrinkage can be taken into account.

Additional Add-ons

If you have special requirements for the reinforced concrete structure, we recommend further add-ons and additional programs to help you solve the task.

To use a cross-section that is not included in the extensive cross-section library of RFEM 6 or RSTAB 9, the RSECTION program helps you when modeling the cross-section including reinforcement. You can then design this cross-section as usual in the Concrete Design add-on.

If you need to perform seismic or vibration analysis, the dynamic analysis add-ons provide you with powerful tools for your structural analysis and design. The Modal Analysis add-on enables the calculation of natural frequencies, and in combination with the Response Spectrum Analysis add-on, you can also perform seismic analysis according to numerous standards.

If you are designing a building with a complicated shape, so that the determination of wind loads according to the relevant standards reaches its limits, the RWIND RWIND program will help you: Using a CFD analysis, you can determine the wind loads even for highly complex building shapes.

If the construction stages affect the final internal forces in a structure, or if the governing internal forces for the structural design result from certain construction stages, you should consider the modeling using the Construction Stages Analysis add-on. The Building Model add-on allows you to easily enter large multi-story concrete structures.

The "Beam Panel" element will allow you to model entire buildings or only certain areas with using surfaces for timber shear wall panels.
By introducing the necessary line hinges in the rigid couplings, the "Beam Panel" element was also released, which will now be further optimized step-by-step in order to achieve the goal mentioned above.
Currently, the application is limited to very simple structures. However, we are very focused on improving the application as quickly as possible. The current state of development is summarized in this FAQ:

Current State and Planned Development:

• Modeling of single, independent surfaces
• Boundary lines orthogonal to each other and not interrupted (four boundary lines, except across surface cells)
• Openings possible via surface cells
• Automatic connection stiffness calculation according to EN 1995‑1‑1 (linear) for nails and fasteners
• More convenient input and extension for all standards (in development)
• Rotation orthotropy direction of sheathing (in development)
• Semi-rigid coupling of adjacent elements (in development)
• Different cross-sections for boundary members in the positive and negative directions (in development)
• Optional no definition of boundary members (in development)
• Connections of panel joints (corners, T-joint, wall-slab) (in development)
• Integration of modeling tools (drag and drop, copy...) (in development)

There can be several reasons for this error.

Missing Surface Reinforcement

In the surface with the punching nodes is no reinforcement available.

You can find out how to create surface reinforcement here: Surface Reinforcement

Missing Dimensions

In RFEM 6, the geometry of punching nodes is usually determined using the cross-section of the connected structural component (wall, column). If a slab or a plate is designed separately within a 2D positional analysis and thus no walls or columns in the form of surfaces or members exist in the model, RFEM cannot automatically determine the structural component dimensions. In this case, it is necessary to manually enter the dimensions:

Not Supported Nodes

In the case of punching nodes that are not supported by a nodal nor linear support, the dimensions of the connected components can be defined via the ultimate configurations.

More information can be found here: Ultimate Configurations (Punching)

Supported Nodes

In the case of punching nodes that are supported by a nodal nor linear support, which represent a connected component, you can specify the dimensions in the support settings.

More information can be found here:
Fictitious Columns (Nodal Supports) | Fictitious Wall (Line Supports)

Not Valid Nodes

In the punching nodes are located in the middle of a line load or a line support, no punching shear occurs. Therefore, these nodes are indicated as not valid.

Both features known from RFEM 5, "Area Load on Members via Planes" and "Area Load on Members via Cells", can be found in RFEM 6 in the load wizard "Member Loads from Area Load". You can either define planes via nodes (marked in green in the image) or select the cells (marked in blue in the image).

The Structure Stability add-on is not dependent on the settings made in the Steel Design add-on. It is a global stability analysis of the entire structure according to how the model is created. The setting of boundary conditions is only used for the design in the Steel Design add-on.
If you want the top flange to be supported, for example, we recommend using objects from Types for Members → Member Supports → Shear Panel. These objects are reflected in the results of the add-ons for dynamic and stability analysis.
In order to take into account the shear panel, it is necessary to activate this option of structural modification for the selected load cases or load combinations.

RFEM allows you to perform structural analysis and design of laminate and sandwich structures. The same applies to the cross-laminated timber. Stress and deflection analysis of laminate and sandwich surfaces is performed according to the laminate theory, taking into account the shear coupling.

Programs and Add-ons

RFEM is the main program that you can use to define the model and actions. You can model planar and spatial structures, consisting of plates, walls, shells, and members.

For the stress and deflection analysis, you need the Multilayer Surfaces add-on. It allows you to define and analyze layer structures.

Use the Timber Design add-on to also design the member supporting elements of the structure according to Eurocode 5 or ANSI/AWC NDS, for example.

Dynamic Analysis

If you need to perform a seismic or vibration analysis, the corresponding Dynamic Analysis add-ons are the perfect tools for determining natural frequencies and mode shapes, or for the analysis of external excitations.

In case of any questions about the Dlubal timber design solutions, our sales team will be happy to assist you.

Both RFEM and RSTAB provide the solution. There is a design add-on available for the European and international standards that can facilitate your daily work in structural concrete design.

Main Programs RFEM and RSTAB

The main programs RFEM and RSTAB are used to define the model with its properties and actions. For concrete structures, RFEM is clearly the first choice: In addition to spatial frame and truss structures, you can also model plate, wall, and shell structures here.

Available Standards

• EN 1992 (Eurocode 2)
• ACI 318 (US standard)
• CSA A23.3 (Canadian standard)
• SP 63.13330 (Russian standard)

Add-on for Concrete Structures

The Concrete Design add-on allows you to perform the ultimate and serviceability limit state design checks according to the standards listed above. The stability analysis according to the equivalent member method (nominal curvature method) can be carried out for rectangular or circular cross-sections. The design add-on also covers the punching shear design checks for the surfaces with nodal, line, and surface supports.

The scope of functions of the add-on includes the design check for limiting the deflection of reinforced concrete components. The analytical deformation analysis takes into account the different cross-section states "uncracked" and "cracked" (state I / state II).

In case of any questions about the Dlubal concrete design solutions, our sales team will be happy to assist you.

The unit symbol for axial and shear strain shown in Image 01, o/oo, represents "per mille" which means part per thousand.

This is not to be confused with %, which represents parts per hundred.
Example: 1 o/oo would equal 1/1000 or 0.001.

In order to evaluate user-defined points in addition to the result values on grid points and FE mesh nodes, there is the option to specify "Additional Surface Result Points" in RFEM 6. You can find them under "Types for Surfaces", as shown in Image 01.

The coordinates for these points are entered directly in the dialog box or selected in the model; see Image 02. Furthermore, it is necessary to assign the surface whose result values should be displayed.

In the Results navigator (the upper half), you can activate and deactivate the display of the manually specified result values. In the lower half of the Results navigator, you can set the display for grid points and/or surface result points; see Image 03.

When displaying the values in FE mesh points, the additional surface result points are deactivated.

To avoid punching reinforcement, it may help to apply additional reinforcement in some areas. If the existing reinforcement in the punching shear area does not increase despite the reinforcement, the following problem is the cause:

• The additional reinforcement requires the minimum dimensions or a required reinforcement length to be applied for the calculation.

It is calculated according to Section 6.4.4 / Equation 6.47 of EN 1992‑1‑1. In the example, this is shown for a rectangular column.
The basic reinforcement is 3.35 cm²/m, the additional reinforcement is also 3.35 cm²/m. If both reinforcements are effective, the result is a reinforcement of 6.7 cm²/m.

• The program does not consider the required anchorage length of the additional reinforcement. It is necessary to adjust it manually.

• In order to illustrate the problem, we calculate with a very fine mesh in the example. The required dimensions when entering additional reinforcements may increase if a very coarse mesh is set. Therefore, the required minimum length can then jump to the next mesh element here.

This can be quickly done by activating the nodes with support (1) in the Navigator – Views, and displaying the associated supports (2).

Then, you can activate the numbering (menu "View" → "Display Numbering").

Corrugated web beams allow for further optimization of steel structures. They are often used to reduce the self-weight of main load-bearing elements and to prevent buckling failure of webs due to the corrugation.

This example will be based on a WTA – 1250 – 300x20 Beam from the Zeman catalogue.

To create a model of this beam, it's best to start by modeling a sine wave that will be used to edit the web. This consists of the steps shown below and in the attached images.

• Create nodes representing sine function
• Copy nodes to desired length of beam
• In RFEM's Lines table, manually create spline from 1st to last node

After these steps, it's recommended to create a parametric section with the dimensions of the corrugated beam. Members using this section can later be transformed into surfaces.

• Create a parametric section with the geometry based on desired beam
• Generate surfaces from members

With this set of surfaces, it is recommended to copy the sine spline into the position of the top and bottom edge of the surface representing the web.
Later, it is required to change the geometry type of the web surface to Quadrangle and redefine the boundary lines of this element.

• Copy sine into top and bottom of web
• Change web geometry type to Quadrangle
• Exclude straight lines from web surface and add sine lines

After preparing the geometry, it's required to focus on finishing the model for the calculation. Select the start and end lines of the member and create a member with the type Rigid. After that, it's possible to define nodal supports on the beam and loads in the model. In order to check the resulting internal forces of a member, it is recommended to define a result beam on the main center line. Finishing all of these steps allows us to start the analysis of the beam.

• Define rigid members on external lines of surfaces
• Add nodal supports
• Add loads – use line loads, not member loads for continuous loading
• Define a result beam on the center line
• Perform the analysis

Remember to define proper mesh settings with an adequate target length of the finite elements and to use a line mesh refinement on the sine lines.

Generally, the elements in RFEM 6 are rigidly connected by default. But by using special RFEM objects, we may change the way the model responds to the actions. For this purpose, we may use line hinges and line releases. The difference between them is explained below.

Line Hinge

Line hinges change the characteristics of connections between surfaces.
By using line hinges, we can allow for rotation between two surfaces on a selected edge or we may define a full translational release between elements.

Aside from these options, we can also define a spring stiffness in every direction or allow for nonlinearities in this hinge, a new feature compared to the previous RFEM program.

Moreover, line hinges allow for special options such as a slab-wall connection which is described here.

Line Release

Line releases work similarly to line hinges, by changing the behavior of two connected elements, but unlike a hinge it also creates a new line and nodes in its place.

By using line releases, we can define/select more objects to be released rather than only connected surfaces. Line releases may be used to also include members and solids in their definition.

Similarly to line hinges, we can allow for rotation and translational releases between elements with options for a spring stiffness and nonlinearities.

The line release is a very powerful tool in managing more complicated geometries and connections between specific elements.

Requirements

In order to transfer support forces from other models, two conditions need to be met:

1. The load wizard "Import Support Reactions" is available as of the program version X.02.0071 (or later).
2. Both models must be in the same project (the same project ID). It is necessary to reconnect the existing projects in the Dlubal Center, if necessary.
3. Both models must have different model IDs.
4. In the model where the loads are to be applied (hereinafter referred to as a "child" model), load cases need to be created.

To ensure that the models have different IDs, it is necessary to create each model via "File → New...". As an alternative, you can also save the existing model as a copy via "File → Save As..."; thus, it obtains a different ID. You can view the IDs under the model parameters in the Base data of a model.

Procedure

If the requirements are met, you can select the "Import Support Reaction" load wizard in the "child" model and assign the "parent" model there in the Model tab (Image 01).

In the Objects tab, the assigned lines and nodes of the "parent" model are assigned to the lines and nodes to be loaded in the "child" model (Image 02).

Assigning Nodes and Lines

It is important to note that the nodes and lines are assigned using their numbering. Pay attention to the order of the entry and the orientation of the lines.

Finally, you can select the assignment of the load cases and the load directions to be imported in the Load tab. Image 03 shows a pure transfer of the vertical loads in Load Cases 1-3.

As soon as you confirm the load wizard, the applied loads appear with the value "??" first. The correct value is only displayed once the respective load case has been calculated (Image 04). This calculation also triggers the calculation in the "parent" model.

Changes in Related Model

If there are changes made in the "parent" model, the load in the "child" model is reverted to "??", provided that the "parent" model has been saved. A recalculation of the load case in the "child" model transfers the new values.

Bolt holes in thin-walled cross-sections calculated with the “Thin-Walled Analysis” design method can be modeled using null elements.

In general, you can use null elements to bridge failure locations or holes in the cross-section, for example, if the cross-section only has a hole at this analyzed member location and before / after the complete cross-section provides the connection in the member. Using the null element, it is possible to transfer the shear in the failure location, and also to apply Steiner's components to the section properties.

In the case of a null element, the thickness t is equal to zero. For the calculation of shear stresses, the thickness of the adjacent elements is used as usual, as these elements exist in front of and behind the hole (viewed in the longitudinal direction of the member). For the shear thickness t *, we recommend specifying the thickness of the thinnest element adjacent to the null element.

Elements are connected if they share a common point.

Image 01 shows two elements. Element 1 has Point 1 and Point 2, and Element 2 has Point 2 and Point 7. Elements 1 and 2 are connected, because they share the same Point 2.

Image 02 shows two elements that cross each other. Element 1 has Point 1 and Point 2, and Element 2 has Point 7 and Point 8. Elements 1 and 2 are not connected, because they do not share the same point.

You can connect the crossing elements using the "Connect Lines and Elements" function. Simply drag a window over the area where you want to connect the elements. It is not necessary to fully capture the objects (Image 03).

After applying the function, Element 1 is divided into Element 1 and Element 3 (Image 04). Elements 1, 2, and 3 are now connected with each other via the common Point 7.

Image 05 shows two elements. Element 1 has Point 1 and Point 2, and Element 2 has Point 7 and Point 8. Elements 1 and 2 are not connected, because they do not share the same point.

It is possible to create a connection between Point 2 and Point 7 using a null element (Element 3 in Image 06). A null element has a thickness t = 0 mm and a shear thickness t* > 0 mm. Element 1 is connected to Element 3 via Point 2, and Element 3 is connected to Element 2 via Point 7, so the entire cross-section is connected.

Image 07 shows two cross-sections. Cross-Section 1 is an IPE 200, containing Elements 1 through 5. Cross-Section 2 is a UPN 200 that contains Elements 6 through 8. The elements of Cross-Section 1 are not connected to the elements of Cross-Section 2, as they do not share any common points.

It is first necessary to divide the cross-sections into their individual objects (points, lines, elements, parts, openings) in order to edit them. You can do this by using the "Convert to Normal Geometry" function in the shortcut menu of the cross-sections (Image 08).

The connection of Element 7 of Cross-Section 1 with Element 1 and Element 2 of Cross-Section 2 can be created using a null element. Element 7 is to be divided at the connection point. The division is carried out automatically when setting the element, if the "Auto Connect Lines and Elements" option is activated. In Image 09, Element 7 was divided into Elements 7 and 10. Element 9 connects Element 10 and Element 7 via Point 39, and Element 1 and Element 2 via Point 2.

The Building Model and the feature of modeling stories with "Rigid Diaphragm" is not designed for all building types.

The function was primarily developed for 3D buildings with 5-10 stories (or more) with a regular or the same floor plan. This means that you should only assign the "Rigid Diaphragm" function to the slabs where the walls and columns are positioned identically in the stories above and below. If this is not the case, instability may occur.

If the model has been entered correctly according to this convention, three options for the result display will be available to you:

• Total
• Floors only
• Combination

The "Total" result display shows the results on the entire vertical components (that is, walls, shear walls, columns, and so on). See Image 02. If you select "Floors only", the results for the separate calculation of slabs are displayed as a 2D model. The "Combination" option corresponds to the same results from both result types mentioned above.

For smaller 3D models and buildings with different floor plans, it is still better to work with the usual modeling as a 3D model. If you are working with models that sometimes have regular floor plans, you can alternatively assign the "Rigid Diaphragm" option to the individual floor slabs. The floor plan geometry of the stories above and below this slab should then be the same again.

The fundamental extraction of a 2D floor from any 3D model is not possible with the technology implemented for this add-on function.

If you want to generate an arc in RSTAB 9, you can use the parameterized circular arc in the block manager. To do this, simply open the block manager as shown in Image 01 and select the corresponding block.

In the following dialog box, you can see a preview image of the inserted block. Using the block nodes shown, you can select an insertion point and set it to a node in the model or to manually defined coordinates; see Image 02.

The parameters and member properties can be edited in the Structure tab. This is shown in Image 03.

Image 04 then shows the created round arch.

By default, the Shear plane in thread option is activated and the lower strength according to the selected design standard is considered for the bolt shear check.

In AISC, the bolt nominal shear strengths are listed in Table J3.2. As an example, Group A (for example, A325) bolt has a nominal shear strength of 54 ksi (372 MPa) when the threads are not excluded from the shear planes. To use the higher strength of 68 ksi (469 MPa), the option can be unchecked to exclude threads from the shear planes.

In the example program, a cantilever is initially created from IPE 200. This is subjected to a member load of 3.5 kN and the calculation is carried out.

This table is accessed on Line 34:

The ResultTables.NodesDeformations() method requires three arguments. First, determine which type of results are to be read out. These can be the results of

• Load cases
• Load combinations
• Result combinations
• Design situations
• Construction stages

sein.

Second, specify the number of the load case, load combination, and so on. Finally, transfer the node number to the method.

The return value d of the method is a list included in the dictionary. On Line 37, d is displayed completely. Line 40 shows how to access a specific value. [0] is the list index and ['displacement_z'] is the key of the dictionary.

A calculation break‑off due to an unstable system can have different reasons. On one hand, it can indicate a "real" instability due to overloading the structural system; on the other hand, modeling inaccuracies may also be responsible for this error message. In the following text, you can find a possible procedure for finding the cause of the instability.

1. Modeling Check

First, you should check if the modeling of the structural system is correct. You can use the model check tools available in RFEM or RSTAB (Tools → Model Check) for this purpose. This way, you can find, for example, the identical nodes or overlapping members, and fix them, if necessary.

Furthermore, you can calculate the structure subjected to pure dead load in a load case according to the linear static analysis, for example. If results are displayed, the structure regarding the modeling is stable. If this is not the case, the most common causes are listed below:

• Incorrect Definition of Supports / Lack of Supports

This can lead to instabilities, as the structure is not supported in all directions. Therefore, the support conditions must be in equilibrium with the structural system as well as with the external boundary conditions. Statically underdetermined systems can also lead to calculation aborts due to a lack of boundary conditions.

• Torsion of Members About Their Own Axes

If members rotate about their own axes, that is, a member is not supported about its own axis, this can lead to instabilities. This is often caused by the settings of member hinges. Thus, it may happen that the torsional releases are entered at both the start node and the end node.

• Missing Connection of Members

In the case of large and complex models in particular, it may quickly happen that some members are not connected to each other, thus they "float in the air." Also, if you forget about crossing members that should intersect with each other, this can lead to instabilities as well. A solution provides the model check of "Crossing Unconnected Members", which searches for members that cross each other, but do not have a common node at the intersection point.

• No Common Node

The nodes seem to be at the same location, but on closer inspection, they deviate slightly from each other. This is often caused by CAD imports, and you can correct it using the model check.

• Formation of Hinge Chain

Too many member end hinges on a node can cause a hinge chain that leads to a calculation abort. For each node, only n‑1 hinges with the same degree of freedom relative to the global coordinate system may be defined, where "n" is the number of connected members. The same applies to line releases.

2. Check of Stiffening

If the stiffening is missing, it may also lead to calculation aborts due to instabilities. Therefore, you should always check whether the structure is stiffened sufficiently in all directions.

3. Numerical Problems

An example of this is shown as follows: A hinged frame is stiffened by tension members. Because of the column contractions due to vertical loads, the tension members receive small compressive forces in the first calculation step. They are removed from the structure (since only tension can be absorbed). In the second calculation step, the model is unstable without these tension members.

There are several ways to solve this problem. You can apply a prestress (member load) to the tension members in order to "eliminate" the small compressive forces, to assign small stiffness to the members, or also to remove the members one by one in the calculation. The calculation setting is automated in RSTAB 9, and it can be optionally activated in RFEM 6.

4. Detecting Causes of Instability

• Automatic Model Check with Graphical Result Display

To get a graphical representation of the instability cause, the Structure Stability add-on can help you. Select the "Calculate eigenvector for unstable model…", and it is possible to calculate the unstable structure. An eigenvalue analysis is performed using the structural data so that the instability of the affected structural component is displayed graphically as a result.

• Critical Load Problem

If load cases or load combinations are calculated according to the geometrically linear analysis, and the calculation is only aborted as of the second-order analysis, there is a stability problem (critical load factor less than 1.00). The critical load factor indicates the factor by which the loading must be multiplied in order for the model to become unstable (for example, buckling) under the corresponding load. Consequently: A critical load factor smaller than 1.00 means that the structure is unstable. Only a positive critical load factor greater than 1.00 means that the load due to the predefined axial forces multiplied by this factor leads to the buckling failure of the stable structure. In order to be able to find the "weak point", the following procedure is recommended, which requires the Structure Stability add-on.

First, it is necessary to reduce the load of the affected load combination until the load combination becomes stable. The load factor in the calculation parameters of the load combination can help. This also corresponds to a manual determination of the critical load factor if the Structure Stability add-on is not available. For purely linear structural elements, it may already be sufficient to calculate the load combination according to the linear static analysis, and determine the critical load using the add-on. Using the graphical buckling mode of this load combination, you can possibly find the problematic location in the structure and fix the problem.

In order to avoid inconsistencies in a model caused by data imports from CAD programs or by modeling, RFEM 6 provides the option to define model tolerances for nodes, lines, surfaces, and directions in the input data. In this way, you can specify as of which tolerance are the nodes, lines, surfaces and distances to be considered as identical and combined together.

These model tolerances are taken into account in the program, on the one hand, when individual model parameters are modified using the "Move/Copy" function and, on the other hand, when the "Drag & Drop" function is used. However, if the position of the model parameters is defined using the table, the specified tolerances can be fallen short of at will. (Note: In the video, the model tolerance has been set a little coarser so that the effects of this setting become clear. Usually, a model tolerance of 0.0005 m is preset by the program.)

In RFEM 5, such inconsistencies in the model could be automatically eliminated using the "Regenerate Model" function.

1) In the material library, set the region to “All” and the material type to “Fabric” in the filter. Select any one of the fabric materials from the list.

2) Activate the “User-defined material” option and specify the user-defined name.

3) Under the Material Values tab, revise the fictitious thickness, density, and so on. The strengths and basis weight (m_s) do not affect the calculation and can be disregarded.

4) To specify the modulus of elasticity and shear modulus in terms of force/area, select the Orthotropic Linear Elastic (Surfaces) tab and enter the values there. Note: Changing the thickness in Step 3 affects the values entered in this tab.

To access the user-defined materials and sections for future models, a template can be created. This is shown in FAQ 005109 .

The activation of the "Auto connect lines/members" function is shown in Image 01.

In RFEM 6 and RSTAB 9, the program creates a node of the "on member/on line" type on the existing line or member by default.

However, if you would like to divide the existing line or member automatically, you can change this in the program options. To do this, open the "Options" → "Program Options..." menu. A dialog box appears where you can deactivate the connection "by internal nodes" option, see Image 02.

There are two options when it comes to defining effective lengths in RFEM 6. First, edit the member and navigate to the "Design Types" Tab. Second, create a new effective lengths definition.

Third, set whether you would like to calculate the critical moment according to the Eigenvalue method or Chapter F from the AISC. Fourth, navigate to the "Nodal Supports and Effective Lengths" tab. Within this tab there are two different methods that can be used.

Method 1: Nodal Supports

Referencing Member No. 1 in the attached model, for this column you can see a how the effective lengths for the Start, End, and Intermediate nodes are defined. First, click on Select Member or Member Set and then select the member. This will activate the intermediate nodes along the member length in the table. Next, check whether the node can move in the y/z axis (weak/strong axis), rotate about its local x axis (torsion), and about its local z axis.

Top and bottom flanges can also be restrained separately by fixing the y axis and by releasing (unchecking) the rotation about the local x axis restraint (torsion).

Method 2: "K" Factors and Absolute Values

Referencing Member No. 5 in the attached model, the effective length factors can be used to define the unbraced length directly and/or apply the appropriate member end conditions. To adjust the unbraced lengths directly instead of utilizing the nodes on the member (Method 1), the "K" factors can be entered manually in the table below. Or the "Absolute Values" can be entered by selecting the option "Absolute Values". Then the unbraced length itself can be entered manually instead. This method is best used when there are no intermediate nodes currently present on the member.

There are two different options for doing this.

Option #1:
Right-click the line and use the "Divide Line" tool. In the following dialog box, select the "Create 'On Line' nodes without dividing line" option.

Option #2:
Manually draw a node and snap to the intersection point.

Double-click this node to edit it and change it to an "On Line" node type.