Effective Lengths

For stability checks according to the equivalent member method, e.g. in accordance with EN 1995-1-1 [1] 6.3, it is necessary to define buckling lengths (or lateral-torsional buckling lengths) so that the program can determine the critical load for stability failure.

If a buckling length is assigned to a member or member set, the corresponding settings and buckling lengths are taken into account in the stability design of the object. If no buckling length is defined, but the stability design is activated, a warning is issued as a result in the table Errors & Warnings.

Basic

In the Basic tab, fundamental settings are to be defined. The buckling length factors and nodal supports can then be defined in the Nodal Supports and Buckling Lengths tab. The input options are aligned with the Standard stored in the model's basic data.

Method of Determination

Consider buckling lengths

Use the check boxes to define which types of stability failure are to be checked for the member or member set. Under compressive loading, flexural buckling about the strong or weak axis may be decisive. The 'Lateral-torsional buckling' option activates the check for lateral-torsional buckling under bending loading.

Method of determining the ideal bifurcation moment

Depending on the standard, various options are available for determining the ideal bifurcation moment M_cr or the critical bending stress. The analytical method for rectangular sections is preset. If you want to define the value manually, activate the 'User-defined' option. You can then define M_cr in the Nodal Supports and Buckling Lengths tab under the buckling length factors.

Buckling Length Factor Type

American design standards distinguish between theoretical and recommended values for buckling length factors. The buckling length factors that can be defined as templates, e.g. for members with one end fixed, are adjusted according to the selection.

Member Type

For stability design according to standards such as CSA O86 [2], the structural type plays a role. In this section, you can classify the member as a simply supported beam or a cantilever.

Options

The Import from stability analysis check box provides the option of applying buckling length factors on the basis of buckling modes. You can make the corresponding inputs in the additional Import from Stability Analysis tab.

If, for the fire design, different equivalent member lengths are relevant than for the cold design, select the Different buckling lengths for fire design check box. You can then define the boundary conditions in the Nodal Supports and Buckling Lengths - Fire Design tab.

Factors

For standards such as CSA O86 [2] or NBR 7190 [3], modification factors can be defined in an additional section, which can be used, for example, to take into account the moment distribution when determining the buckling lengths.

Nodal Supports and Buckling Lengths

Nodal Supports

By entering the nodal supports, you define the boundary conditions for the lateral-torsional buckling check (only for the 'Eigenvalue method' method of determination). In addition, the nodal supports are used for dividing the member or member set into segments.

To define the supports, you can select typical variants from a list in the left column or activate the buttons in the cells individually (fixed support) or deactivate them (no support).

In addition to a fixed or loose support, spring properties are also possible for the relevant directions. To do so, use the cell context menu. You can enter the spring stiffness in the section Nodal Supports - Additional Data.

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

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

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

With the left button, a member or member set can be selected in the model and the number of intermediate nodes on this member set is automatically transferred to the table. If the buckling length has already been assigned to a member or member set, a node can be selected with the right button. In the nodal support table, the row of the corresponding intermediate support is automatically selected (if available).

Buckling Length Factors

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

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

• Support 'in z/v' divides the length for buckling about the strong principal axis with the factor k_y/u
• Support 'in y/u' divides the length for buckling about the weak principal axis with the factor k_z/v and the lateral-torsional buckling length with the factor k_LT
• Fixation 'about x' divides the lateral-torsional buckling length with the factor k_LT

The designation of the factors may vary depending on the selected Design Standard.

An arrow symbolizes a segment-spanning buckling length factor if no corresponding intermediate support is present in the 'Nodal Supports' table. You can define the buckling length factors of the individual segments in the table rows and thus adjust the buckling lengths of the sections. For typical cases, you can use the predefined values from the cell context menu.

The buckling length used for the check of a failure mode at a point in this segment results from multiplying the segment length by the corresponding buckling length factor.

You can also specify absolute values for the buckling lengths. Note that these values are used for all assigned objects. In contrast to using buckling length factors, no relative adjustment to the actual segment length is made. Therefore, you should prefer defining buckling length factors over entering absolute values for the buckling length.

For the lateral-torsional buckling check with the eigenvalue solver, the total length of the object with the corresponding restraints is always considered. The program determines the critical lateral-torsional buckling moment M_cr on an internal equivalent member model with four degrees of freedom (φ_x, φ_z, u_y, ω) and the defined nodal supports. If you have selected the user-defined input of M_cr, you can enter a value for M_cr for each segment. This is used for all design points 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 a lateral support in y/u without rigid fixation about x is present. Here you can define the parameters in detail.

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

The eccentricity refers to the lateral support in y/u and can have a stabilizing or destabilizing effect depending on the position of the compression flange in lateral-torsional buckling. The list offers support at the top or bottom flange as well as the option for manual definition.

Import from Stability Analysis

The Import from Stability Analysis tab is displayed when the corresponding check box is activated in the 'Basic' tab. Here you can select the buckling modes and members whose buckling length factors k_y/u or k_z/v are to be applied.

About Axis

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

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

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

With the button, you can display the eigenmodes in the graphic window of the main program.

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

Buckling Length Factors

The table lists the buckling length factors imported from the stability analysis for the two principal axes. If you want to adjust a value manually, activate the 'User-defined' check box in the 'About Axis' section. This makes the input field accessible.

The buckling length factors displayed here are transferred to the Nodal Supports and Buckling Lengths tab and can no longer be edited there. With the 'Absolute Values' option, you can also transfer the buckling lengths L_cr,y/v and L_cr,z/v 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 derived from a member contained within it.

Nodal Supports and Buckling Lengths - Fire Design

This tab is displayed if you have selected the Different buckling lengths for fire design option in the 'Basic' tab.

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