Effective Lengths

For stability analysis according to the equivalent member method, it is necessary to define effective lengths in order to determine a critical load for the stability failure.

After assigning an effective length to an object to be designed, the corresponding settings and the effective lengths for this object are considered in the stability analysis. If there is no effective length defined, but the stability analysis is activated, a warning appears (see Result Table Errors & Warnings).

Main

In the "Main" tab, you can make basic specifications; in the Nodal Supports & Effective Lengths tab, you can enter the effective length factors and define the nodal supports to be considered. The input options are adjusted according to the selected design standard, so not all options are always available. The design standard is selected in the Base Data of the model; see also Standards I.

Determination Type

Use the check boxes to specify which forms of stability failure are to be checked for the member or member set. Under compression, flexural buckling about the major or minor axis, as well as torsional buckling, can become governing. For unsymmetrical cross-sections with compressive loading, flexural-torsional buckling under pure compressive loading is also analyzed as a combination of flexural and torsional buckling.

The "Lateral-torsional buckling" option activates the design for lateral-torsional buckling under bending. Depending on the selected design standard, there are various options for calculating the critical lateral-torsional buckling moment M_cr.

Buckling Axes

It is usually necessary to analyze the buckling behavior about the "principal section axes y/u and z/v". However, in the case of unsymmetrical cross-sections, it may be necessary to consider the flexural buckling about the "section axes y and z" in addition to the axes u and v. This special case is relevant, for example, for the design of angle sections in lattice towers.

Member Type

Some design standards differentiate between different structure types in a stability analysis. Depending on the selected standard, you can classify the member in this section, for example, as a cantilever or a beam supported on both sides.

Type of Effective Length Factors

American design standards differentiate between the theoretical and the recommended values of the effective length factors. The effective length factors that can be defined as a template for members restrained on one side, for example, are adjusted according to the selection.

Options

The "Import from Stability Analysis" check box allows you to apply the effective length coefficients based on buckling modes. The corresponding entries can be specified in the additional tab "Import from Stability Analysis" (see image Importing Effective Lengths). This option is available if the Structural Stability Analysis add-on has been activated.

Nodal Supports & Effective Lengths

Nodal Supports

By entering nodal supports, you define the boundary conditions for the lateral-torsional buckling analysis. Furthermore, the nodal supports are used to subdivide the member or the member set into segments.

To define the supports, you can select common types from the list in the left column, activate the check boxes in the cells individually (fixed supports), or deactivate them (no supports).

In addition to fixed or rigid supports, you can also define spring parameters for some directions; for this, use the cell shortcut menu. You can enter a spring stiffness in the Nodal Supports section.

You can define the support conditions at the start, at the end, and at intermediate nodes. Standard nodes between members or a member set and "nodes on members" are considered as intermediate nodes (see Chapter Nodes of the RFEM manual).

The definition of intermediate nodes is not based on node numbers, but on the order on the member: .1 designates the first intermediate node from the member start, .2 the second one, and so on. If a member with this effective length has more or less intermediate nodes, the consideration based on the member start applies and the excess entries or nodes are ignored.

Using the button, you can insert a new intermediate node above the selected line. To delete an intermediate node, select the line and click the button. The table shortcut menu also provides the options for editing rows.
Use the button on the left to select a member or a member set in the model, and to automatically transfer the number of intermediate nodes on this member set to the table. If the effective length is already assigned to a member or a member set, you can select a node by clicking the button on the right. In the table with nodal supports, the row of the associated intermediate support is selected automatically (if available).

Effective Length Factors

The "Effective Length Factors" table is adjusted to the number of the nodal supports. If no intermediate nodes are defined, there is only one "segment". You can adjust the effective length factor of this segment to fit the boundary conditions by using the factors to increase or reduce the effective length for various failure modes.

Supports at the intermediate nodes subdivide the member or member set into several segments for various failure cases:

• Supports "in z/v" divide the length for buckling around the major axis by the k_y/u factor.
• Supports "in y/u" divide the length for buckling around the major axis by the k_z/v factor.
• A restraint "about x" divides the length for torsional buckling by the k_T factor.

An arrow symbolizes an effective length factor across segments, if there is no corresponding intermediate support in the "Nodal Support" table. You can define the effective length factors of the individual segments in the table rows, and thus adapt the effective lengths of the sections. For common cases, you can use the predefined values from the cell shortcut menu.

The effective length used for the design of a failure mode at a location in this segment results from the multiplication of the segment length by the corresponding effective length factor.

You can also specify absolute values for the effective lengths. Please note that these values are used for all assigned objects. In contrast to the use of the effective length factors, there is no relative adjustment to the actual segment length. Therefore, you should prefer the definition via the effective length factors to the entry of absolute values for the effective length.

For the lateral-torsional buckling design with the eigenvalue solver, the total length of the object with the corresponding supports is always taken into account. The program determines the critical lateral-torsional buckling moment M_cr on an internal equivalent member model with 4 degrees of freedom (φ_x, φ_z, u_y, ω) and the defined nodal supports. If you have selected the user-defined entry of M_cr, you can enter the value of M_cr for each segment. This is used for all design locations within the segment.

Nodal Supports - Additional Data

This section is displayed if a spring has been defined as a support in the selected row, or if there is a lateral support in y/u without a rigid restraint about x. You can define the parameters in detail here.

Enter the characteristic values of springs that are available for the lateral support or the rotation about the supported axes. You can also specify stiffnesses for warping springs.

The Eccentricity refers to the lateral support in y/u and, depending on the position of the compression flange, it can have a stabilizing or destabilizing effect for the lateral-torsional buckling. The list offers a support on an upper or a lower flange, as well as the option for defining it manually.

The spring values and eccentricities are taken into account in the eigenvalue solver when determining the elastic critical moment for lateral-torsional buckling.

Import from Stability Analysis

The Import from Stability Analysis tab is displayed if activating the corresponding check box in the "Main" tab. Here, you can select the buckling modes and members whose effective length factors k_y/u or k_z/v should be applied.

About Axis

Specify the load cases of the stability analysis from which the effective lengths are to be imported. You can define a mode shape of a specific load case for each principal axis.

The mode shapes are properties of a load case or a load combination. First, select in the "Load Case / Load Combination" list, which load situation is governing for the buckling mode. The list only contains the load cases and load combinations with a specified stability analysis.

In the next step, define the governing "Mode No." The list of mode shapes is available for all calculated load cases and load combinations.

Click the button to display the mode shapes in the graphic window of the main program.

Finally, select the "Member No." in the list. You can also use the button to define the member graphically in the work window.

Effective Length Factors

The table shows the effective length factors that were imported from the stability analysis for both principal axes. If you want to adjust the value manually, activate the "User-Defined" check box in the "About Axis" section. Thus, the text box becomes accessible.

The effective length factors displayed here are transferred to the Nodal Supports & Effective Lengths tab and can no longer be edited there. The "Absolute Values" option allows you to transfer the effective lengths L_cr,y/v and L_cr,z/v of the members from the stability analysis results. You can use this option, for example, if you want to apply the effective length to a member set from a member included therein.