The ultimate configuration is divided into several tabs, which clearly structure the specifications for the individual design checks.
Members | Main
In the Members | Main tab of the ‘‘Ultimate Configuration’’ dialog box, you can define the basic settings for the design of members and member sets.
General
The “Perform stability design” check box controls whether stability analyses, such as against flexural buckling or lateral-torsional buckling, are performed in addition to cross-section checks. You should deactivate the check box if you do not want to perform stability design or if you have already considered the stability effects in the internal force calculation (for example, by performing a second-order analysis with imperfections).
If the stability analysis is activated, it is necessary to define and assign effective lengths to the respective members or member sets. If this is not the case, no stability analysis is possible. After the calculation, a corresponding message is displayed in the Errors & Warnings result table. Cable members and tension members are excluded from this, as they can only absorb tensile forces and do not require stability analysis.
Design Ratio Limits for Ignoring Internal Forces and Stresses
The design standards, which are mostly based on manual calculations, contain design check formulas or interaction conditions that are often only designed for certain combinations of internal forces. However, when calculating the structural design on a 3D model, the values for internal forces are usually very small. Although thez are insignificant from an engineering point of view in terms of load-bearing capacity, strict adherence to the design standard may prevent certain design checks or force the use of less favorable interaction formulas. The “Design ratio limits for ignoring internal forces and stresses” therefore provide a simple and transparent way of neglecting certain internal forces during the design and avoiding the above-mentioned problems.
The limit value η describes the ratio of the effective shear force to the cross-section resistance, which is determined in a simplified manner using the cross-section properties. These are not standard-compliant design capacities: Standard-specific regulations (for example, for the cross-section classification) are therefore not taken into account; this is not the purpose of setting limit values. You should therefore only use high limit values for testing purposes.
If a shear force is below the limit value condition, the design is performed without a warning that the shear force is neglected. However, in the design check details, you can use the “Negligible” note for the design internal forces to check which internal forces were neglected as a result of the limit values and were not taken into account in the design. If all internal forces are negligible at a point, only the check of the negligible internal forces is specified as design.
If the Warping Torsion (7 DOF) add-on is activated, the limit value for the second moment of area and the shear stresses from secondary torsion are also available.
Analysis of Thin-Walled Structures
Determining the effective cross-section requires an iterative approach. In the “Maximum number of iterations” text box, you can specify the maximum number of calculation runs. As soon as the “Maximum difference between iterations” between the results of two iterations is exceeded, the calculation ends.
Options
Cross-sections of Classes 1 and 2 may be designed plastically. Select the “Elastic design (also for class 1 and 2 sections)” check box if these sections are to be designed without using the plastic reserves.
The “Design according to 6.2.1(5)” check box allows you to control whether the general design should be performed according to the elasticity theory. The design according to 6.2.1(5) can be conservative, as it does not take into account the limitation on plastic stress redistribution that is allowed in an elastic design. You can define the parameter C in the Standard Parameters tab.
The “Shear buckling design” check box is activated by default. It checks whether the slenderness λ of the web requires a shear buckling design according to EN 1999-1-1, Section 6.7.4. If the limit value λlim is complied with, the design is considered to be fulfilled. However, if the slenderness is above the limit value, according to Section 6.7.4.2(2), it is necessary to add member transverse stiffeners at the supports in order to perform the shear buckling design.
Alternative Values
This section lists factors that can be determined differently according to EN 1999.
If the check box is not selected, the default value for the design checks is used.
The relevant sections of the standard are specified in each case.
Members | Stability
The Members | Stability tab is available if the you have selected the ‘'Perform stability design’' check box in the ‘'Members | Main’' tab. You can specify additional settings for the stability analyses here.
Position of Positive Transverse Load Application
If there are transverse loads (in the direction of the minor axis), it is important to define where these forces act on the cross-section. They can significantly influence the ideal elastic critical moment Mcr: A load acting on the upper flange of a bending beam in the direction of the shear center has a destabilizing effect.
If the load acts on the bottom chord, it has a stabilizing effect. Use the selection boxes to define the “Vertical position”.
Flexural-Torsional and Lateral-Torsional Buckling
EN 1999-1-1, Section 6.3.1.4 (1) lists section types for which the analysis of flexural-torsional and lateral-torsional buckling can be omitted: hollow sections, double-symmetrical I-sections, and sections composed of radially protruding sub-sections (angles, T-sections) and in Class 1, the following: lateral-torsional buckling: hollow sections, double-symmetrical I-sections, and cross-sections composed of radially projecting partial surfaces (angles, T-sections) that are classified in Class 1 or 2. The check box allows stability analyses to be performed for such cross-sections.
Surfaces
In the “Surfaces” tab of the “Ultimate Configuration” dialog box, you can configure basic settings for the design of surfaces.
Design Ratio Limits for Ignoring Internal Forces and Stresses
This category manages a limit value that applies to very small stresses. If the stress is below the defined ratio η, it is not taken into account in the design. The background for this option is described in closer detail in the chapter Members | Main.
Standard Parameters
The Standard Parameters tab lists all relevant coefficients according to the selected National Annex.
To set a user-defined value, you can adjust it in the corresponding input box. Use the
button to reset the values to the default values of the selected National Annex.