Effective Lengths

For stability proofs of members using the design column method or nominal curvature method, the definition of buckling lengths is necessary. When you assign a buckling length to a member, the corresponding boundary conditions and buckling lengths for the stability analysis of the member are taken into account. The cross-section design then follows with the internal forces of this stability analysis.

Dialog Box "New Effective Lengths"

Basic

In the Basic tab, fundamental settings must be made. You can define the buckling length factors and support conditions in the tab Supports and Buckling Lengths.

Type of Analysis

Use the checkboxes to determine which forms of stability failure should be checked for the member (or member set). Under compression loading, flexural buckling around the strong or weak axis may be decisive.

If you disable a failure mode, the corresponding option in the 'Structural Type' section and in the 'Supports and Buckling Lengths' tab is disabled. This failure mode will not be investigated in the proof.

Structural Type

With the checkboxes in this section, you can determine whether the member system is 'Translational' or 'Non-Translational'. This setting can be made separately for both buckling directions. The structural type affects the determination of the equivalent moment M_0e according to EN 1992-1-1, 5.8.8.2(2) Eq. (5.32).

Options

With the checkbox 'Import from Stability Analysis', you have the option to apply buckling length factors based on buckling modes. You can make the corresponding entries in the additional tab Import from Stability Analysis.

Supports and Buckling Lengths

In the Supports and Buckling Lengths tab, you define the boundary conditions for the stability proof and set the buckling length factors.

Defining Nodal Supports and Effective Lengths

Supports

The support of the member (or member set) defines the boundary conditions for the stability proof. Additionally, the node supports are used for subdividing the member into segments.

To define the supports, you can select typical variants from a list in the 'Support Type' column. Alternatively, you can check the boxes in the 'Fixed' columns individually (fixed support) or uncheck them (no support).

Selecting Support Type

You can define the node supports for the beginning and the end of the member. If the member is supported by a node on the member itself, check the 'Intermediate Node' box (see image Specify Buckling Length Factors Segment-Wise). Intermediate nodes are standard nodes between members of a member set and nodes of the type 'On Member' (see chapter Nodes in the RFEM manual).

The intermediate nodes are not defined by node numbers but by the order on the member: '.1' refers to the first intermediate node from the member start, '.2' to the second intermediate node, etc. If a member, to which this buckling length is assigned, has more (or fewer) intermediate nodes, the consideration starts from the member start; excess inputs or nodes are ignored.

The button adds a new intermediate node above the selected row. To delete an intermediate node, select the row and then click the button . The table context menu also offers possibilities for editing rows.

With the button , you can select a member or member set in the model to transfer its number of intermediate nodes into the table. If the buckling length is already assigned to a member, you can select a node with the button . The table will then select the row of the corresponding intermediate support (if available).

Buckling Length Factors

The 'Buckling Length Factors' table is aligned with the number of node supports. If no intermediate nodes are defined, only one 'Segment' exists. You can adjust the buckling length factors of this segment to the boundary conditions by extending or shortening the buckling length for the different failure modes using factors.

Supports at intermediate nodes (see image Select Support Type) divide the member or member set into multiple segments for the different failure cases:

• Without: Buckling length factor from segment No. 1 applies to all subsequent segments
• Fixed in z: Individual buckling length factors k_y for buckling around the y-axis
• Fixed in y: Individual buckling length factors k_z for buckling around the z-axis
• Fixed in all: Individual buckling length factors for both buckling directions and all segments

An arrow symbolizes a cross-segment buckling length factor if no corresponding intermediate support is present in the 'Node Supports' table. You can specify the buckling length factors of the individual segments in the table rows and thereby adjust the buckling lengths of the segments.

Specifying Effective Length Factors by Segment

The buckling length used for the proof of failure at a point in this segment results from multiplication of the segment length with the corresponding buckling length factor.

Enter the buckling length factors for each segment that are decisive for buckling around the y- or z-axis. The list also includes typical buckling length factors.

Selecting Effective Length Factor

You can also specify 'Absolute Values' for the buckling lengths. It should be noted that these values are used for all assigned objects. Unlike buckling length factors, there is no relative adjustment to the actual segment length! Therefore, definition via buckling length factors instead of absolute values is preferred.

The automated determination of buckling lengths for concrete construction using the k_A-k_B method is not yet available in the current development stage.

Import from Stability Analysis

The Import from Stability Analysis tab is available when you have enabled the corresponding checkbox in the Basic tab. Here you can select the buckling modes and members whose buckling length factors k_y or k_z should be applied.

Import of Effective Lengths from Stability Analysis

Around Axis

Eigenmodes are properties of a load case or a load combination. First, select in the 'Load Case/Combination' list which load situation is decisive for the buckling mode. The list contains only load cases and combinations for which a stability analysis has been specified. You can specify the eigenmode of a specific load case for each main axis.

In the next step, set the decisive 'Mode No.'. The list of eigenmodes is available for all calculated load cases and combinations.

Selecting Mode Shape

With the button , you can display the eigenmodes in the workspace.

Finally, select the 'Member No.' in the list. With the button , you can also graphically determine the member in the workspace.

Buckling Length Factors

The table shows the buckling length factors imported from the stability analysis for the two main axes. If you want to adjust a value, activate the 'Custom' checkbox in the 'Around Axis' section. This makes the input field accessible.

The buckling length factors displayed here are transferred to the tab Supports and Buckling Lengths and can no longer be edited there. With the 'Absolute Values' option, you can also adopt the buckling lengths L_cr,y and L_cr,z of the members from the stability analysis results. This option can be used, for example, if the buckling length for a member set is to be applied from a member contained within it.