RF-/STEEL EC3 Version 5/8

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RF-/STEEL EC3 Version 5/8

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3.1.2 Stability


Figure 3.2 Dialog box Details, tab Stability
Stability Analysis

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 doesn't affect the flexural-buckling analysis according to [1] clause 6.3.1 which is performed with axial forces.

Determination of Elastic Critical Moment for LTB

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 Mcr 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 kz and kw (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.

Figure 3.3 Dialog box Moment Coefficients C1 for Determination of Lateral-Torsional Buckling Moments

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 Mcr so that you can enter the elastic critical moment for LTB directly.

Mcr user-defined
Mcr user-defined

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.

Model Type According to Table B.3

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 Cmy = 0.9 or Cmz = 0.9. Both check boxes are cleared by default. If they are selected, the program determines the factors Cmy and Cmz according to the criteria defined in Table B.3.

Limit Values for Special Cases

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

Analogously, it is possible for the pure check of bending according to [1] clause 6.3.2 to neglect small compression forces by defining a limit ratio of Nc,Ed / Npl.

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 Mz,Ed / Mpl,z,Rd.

The intended Torsion is not clearly specified in [1]. If there is a torsional stress not exceeding the shear stress ratio of 5% preset by default, it is neglected for the stability design; only results for flexural and lateral-torsional buckling are displayed.

If one of the limits 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.

Stability Analysis Method of Sets of Members

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 kz and kw in the 1.6 Effective Lengths - Sets of Members window. They are used to determine the support conditions β, uy, φx, φz and ω. In this case, Windows 1.7 and 1.8 are not displayed. Please note that the factors kz and kw 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 αcr. 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 kz and kw 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).

[1] Eurocode 3: Design of steel structures - Part 1-1: General rules and rules for buildings; EN 1993-1-1:2010-12