For stability analyses according to the general method of EN 1993‑1‑1 (described in Clause 6.3.4 of the standard), the definition of boundary conditions is required in order to determine the critical load for the stability failure. Therefore, this type of steel design is only available if the design standard EN 1993 is selected in the Base Data (see also Standards I).
After assigning a boundary condition to an object to be designed, the settings for this object are taken into account in the stability design. If an effective length is assigned at the same time (a required entry for the equivalent member design), a warning appears as there must be a clear entry for the stability analysis (see the results table Errors & Warnings). You also get a warning if neither effective lengths nor boundary conditions are assigned while the stability analysis is activated.
According to the standard specifications, the general method only analyzes the stability of structures for failure cases out of the main load-bearing plane (lateral-torsional buckling, flexural buckling in the direction of the minor axis). If required, it is necessary to consider the flexural buckling in the main load-bearing plane when determining internal forces (imperfections and a second-order analysis). The procedure for considering possible stability failure in the main load-bearing plane is described in the following technical article on our website: General Method for Stability Analyses According to EN 1993-1-1 and Buckling in Main Load-Bearing Plane.
Main
The "Main" tab shows the definition type and the coordinate system for entering nodal supports. The nodal supports to be considered are defined in the Nodal Supports tab.
The program determines the critical load factor αcrit,op on an internal equivalent model with four degrees of freedom (φx, φz, uy, ω), the defined nodal supports, and member hinges.
Nodal Supports
By entering nodal supports, you define the boundary conditions for the internal eigenvalue solver.
To define the supports, you can select common types from the list in the left column, or 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 each direction; for this, use the cell shortcut menu. You can enter the spring stiffness in the Additional Parameters 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, and so on. Thus, the is considered based on the member start and 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 a boundary condition 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).
Once the boundary condition have been assigned to a member or member set, you can check the supports using the
button in the dialog box graphic.
Additional Parameters
This section is displayed if selecting a row with an activated support in the upper table. You can define further 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 can be defined in the direction of the x-axis and z-axis. For the eccentricity in the direction of the z-axis, you can select a support on the upper or lower flange as well as the manual definition option.
The rotation angle β allows you to rotate the support in the xz-plane of the member set.
Member Hinges
In the Member Hinges tab, you can define the releases of the displacement y, the rotations about x and about z, and the warping between the members. At each segment start and end, you can select the corresponding release using the check boxes in the table. The directions refer to the coordinate system of the segment. When defining a spring, there is the input window for Additional Parameters available, as in the case of the nodal support input.
