The Members | Stability tab is available if the “Perform stability design” check box is selected in the Members | Main tab. Here, you can specify additional settings for the stability design checks.
The “Design parameters” are divided into several categories, which differ depending on the design standard.
EN 1993
Calculation Method
For design according to the Equivalent member method in compliance with Sections 6.3.1 to 6.3.3, the definition of effective lengths is required. The "Structure Type acc. to Table B.3" is relevant for structural components with buckling in the form of lateral yielding. The two check boxes allow you to control whether the equivalent moment factors are assumed to be Cmy = 0.9 and Cmz = 0.9.
On the other hand, the General method according to Section 6.3.4 requires the definition of boundary conditions. This design method is subject to certain application limits in some National Annexes. For example, you can use the “Enable also for non-I-sections” check box to remove the restriction according to the German Annex. The options for the “Extension methods” also allow you to partially remove the application limits for the general method. Thus, you can perform the interpolation of the reduction factor χop according to Eq. (6.66) or use the European bending-torsional buckling curve according to Naumes [1]. The “Extension method” option allows you to apply the general method according to Section 6.3.4 for additional transverse bending and torsion (see [2]). In this way, you can design asymmetric cross-sections as well as tapered members and member sets with biaxial bending.
Including Second-Order Effects
Second-order analysis according to 5.2.2(4) can be taken into account manually by increasing the bending moment about the major axis y or the minor axis z. For example, in the case of a frame whose governing buckling mode is lateral deflection, the internal forces can be determined according to linear static analysis and increased by corresponding factors. To do this, select the check box and specify the increasing factor α. The increase in the bending moment has no effect on the flexural buckling design according to Section 6.3.1. This design is performed with the axial forces.
Position of Positive Transverse Load Application
If lateral loads (in the direction of the major axis) are present, it is important to define where these forces act on the cross-section. You can significantly influence the ideal elastic critical moment Mcr: a load acting on the upper chord of a bending beam in the direction of the shear center has a destabilizing effect. If the load acts on the lower chord, it has a stabilizing effect. Use the selection boxes to define the “Vertical location.”
Parameters for Lateral-Torsional Buckling
The reduction factor χLT for determining the lateral-torsional buckling lines can be determined generally according to Eq. (6.56) or specifically for rolled and similar welded cross-sections according to Eq. (6.57).
- With the default setting “If possible, according to Eq. 6.57, otherwise according to Eq. 6.56”, the program automatically uses the more favorable equation. As an alternative, you can have χLT determined “Always according to Eq. 6.56 General case (conservative)”.
The “Use factor f for modification of χLT acc. to 6.3.2.3(2)” check box controls whether the coefficient specified in the National Annex for considering the moment distribution between the lateral supports is applied. This factor leads to an increase in χLT.
6.3.3(4) Parameters kyy, kyz, kzy, kzz
The standard provides two methods for determining the interaction factors for 6.3.3(4). They apply to uniform members subjected to bending and compression. The interaction factors depend on the selected method and are specified in Appendix A (Method 1) or Appendix B (Method 2).
Lateral-Torsional Buckling of Hollow Sections
For hollow sections, the standard does not explicitly regulate the design checks for lateral-torsional buckling: there is only a low risk of this type of stability failure. You can use the check box to control whether the design should also be performed for double-symmetrical hollow sections, with the exception of circular hollow sections.
Stability Analysis of Cold-Formed Sections
The stability analysis of cold-formed cross-sections is performed according to EN 1993-1-3, Section 6.2. For the combination of bending and axial force, Eq. (6.36) is used, if possible. Otherwise, the design is performed according to EN 1993-1-1, Section 6.3.3 or 6.3.4.
AISC 360
Position of Positive Transverse Load Application
If lateral loads (in the direction of the major axis) are present, it is important to define where these forces act on the cross-section. You can significantly influence the ideal elastic critical moment Mcr: a load acting on the upper chord of a bending beam in the direction of the shear center has a destabilizing effect. If the load acts on the lower chord, it has a stabilizing effect. Use the selection boxes to define the “Vertical location.”