3.1.2 Stability

Stability

The Perform stability analysis check box controls whether to run a stability analysis in addition to the cross-section designs. If you clear the check box, Windows 1.4 to 1.8 are not displayed.

When the check box is selected, you can define the axes relevant for the analysis of Flexural buckling according to 6.3 of [1].

Furthermore, it is possible to consider second-order effects acc. to 5.2.2(4) by a factor for bending moments that can be defined manually. In this way, for example, when designing a frame with its governing buckling mode represented by lateral displacement, you can determine the internal forces according to linear static analysis and increase them by appropriate factors. Increasing the bending moments does not affect the flexural-buckling analysis according to [1] clause 6.3.1, which is performed with the axial forces.

By default, RF-/STEEL EC3 determines the elastic critical moment Automatically by Eigenvalue Method. For the calculation, the program uses a finite member model to determine M_cr taking into account the following items:

• Dimensions of gross cross-section
• Load type and position of load application point
• Effective distribution of moments
• Lateral restraints (by support conditions)
• Effective boundary conditions

You can specify the degrees of freedom by the factors k_z and k_w (see Chapter 2.5).

When determining the elastic critical moment Automatically by comparison of moment distribution, factor C₁ is determined by means of the moment distribution. Click the [Info] button to open a dialog box showing the load and moment distributions.

The Tolerance for moment distribution in this dialog box allows you to control the degree up to which deviations are acceptable for the moment distributions.

The coefficients C₂ and C₃ will be determined automatically by the Eigenvalue Method, if required.

With the option Manual definition in Window 1.5, the title of column J in Window 1.5 changes to M_cr so that you can enter the elastic critical moment for LTB directly.

If transverse loads are available, it is important to define the location where these forces are acting on the cross-section: Depending on the load application, transverse loads can be stabilizing or destabilizing, and thus have a major impact on the elastic critical moment.

The signs of the eccentricities are related to the cross-section's shear center M. An article in our Knowledge Base provides more information about the sign convention for transverse loads.

According to [1] Annex B, Table B.3, the equivalent moment factor for structural components with buckling in the form of lateral deflection should be assumed as C_my = 0.9 or C_mz = 0.9. Both check boxes are cleared by default. If they are selected, the program determines the factors C_my and C_mz according to the criteria defined in Table B.3.

Small compression forces do not allow for a check of pure bending according to [1] clause 6.3.2. A user-defined limit ratio of N_c,Ed / N_pl allows you to hide small compression forces for this design.

To design asymmetric cross-sections with the intended axial compression according to [1] clause 6.3.1, you can analogously neglect small moments about the major and the minor axis with the settings defined in this dialog section.

According to [1] clause 6.3.4, the general method is allowed for unsymmetric cross-sections or tapered members only if they are subjected to compression and/or uniaxial bending in the principal plane. In order to neglect a minor moment loading about the minor axis, you can define a limit for the moments ratio M_z,Ed / M_pl,z,Rd.

Scheduled Torsion is not clearly specified in [1]. If there is a torsional stress that does not exceed the predefined ratio of existing shear stress to Limit shear stress due to torsion of 5%, it is neglected for the stability analysis. In this case, results for flexural buckling and lateral-torsional buckling are displayed.

If one of the limit values in this dialog section is exceeded, a note appears in the results window and the program won't perform any stability analysis. However, the cross-section designs are performed independently. These limit settings are not part of the Standard [1] or any National Annex. Modifying the limits is the user's responsibility.

In our Knowledge Base, you can find an example explaining the application of limit values:
https://www.dlubal.com/en-US/support-and-learning/support/knowledge-base/001498

According to 6.3.1 ... 6.3.3 (Equivalent Member Method), it is possible to handle sets of members as one large single member. For this, define the factors k_z and k_w in the 1.6 Effective Lengths - Sets of Members window. They are used to determine the support conditions β, u_y, φ_x, φ_z and ω. In this case, Windows 1.7 and 1.8 are not displayed. Please note that the factors k_z and k_w are identical for each section or partial member of the set of members. Therefore, the equivalent member method should be used only for straight sets of members.

With the default setting of 6.3.4 (General Method), the program performs a general analysis according to [1] clause 6.3.4 which is based on the coefficient α_cr. In Windows 1.7 Nodal Supports and 1.8 Member Hinges, the boundary conditions must be defined with regard to the stability failure (buckling and lateral-torsional buckling) separately for each set of members. The factors k_z and k_w from Window 1.5 are not used.

Find more information about the general method in an article of the Knowledge Base.

The options are locked if the stability analysis with warping torsion is set (see Chapter 3.1.5).

For structural components with bending and compression, the method according to 6.3.3 is only applicable for double-symmetric cross-sections. However, components with single-symmetrical cross-sections or a tapered member diagram can be designed with the General Method according to 6.3.4. In Germany, only I-sections are allowed, which is why a warning appears during calculation. In this case, it would be possible to deviate from the National Annex in the National Annex Settings dialog box and also allow the General Method for non-I-sections (see Figure 2.10).

If the check box is selected, RF-/STEEL automatically selects the appropriate design method. The buckling lengths defined in Window 1.5 and 1.6 for buckling about the principal axis are not considered in the design according to Section 6.3.4.