Lateraltorsional Buckling  EC 3
The Lateraltorsional buckling dialog box allows the definition of the lateraltorsional buckling lengths for each flange of the selected steel element. The color code designated for each flange (blue for the upper flange and orange for the lower flange) highlights the corresponding tab and is also displayed in the explanatory picture and in the field corresponding to the calculated value.
How to access the command
Select a steel linear element:
Lateraltorsional Buckling  EC 3
Description of the Lateraltorsional Buckling dialog box
When EC 3 standard is the steelwork standard selected for the current project, the Lateraltorsional buckling dialog box allows the definition of the following parameters:

Viewer: On the topright part of the window, a special viewer is available. It displays the preview of the real geometry of the selected linear elements, with the current location of defined restraints. The same type of symbols as on a 3D model are used. Using the mouse, it is possible to rotate, zoom or pan the element on the preview. Also, the button located on the topright corner of the viewer resets the view to the starting position.
 Restraints
 Graphical definition: temporarily hides the dialog and allows for graphical selection, on the 3D model, of the points where the restraints are located.
 Auto detect: finds and adds to the table below it all the points where other linear elements intersect the element under consideration.
 Copy to other flange: copies the table with the restraints definition to the other (top or bottom) flange tab.
 Add, Equal spacing, Remove, Remove all: buttons used for editing the content of the table above them.
 Definition type  options for a quick definition of the restraints’ location, by using three methods:
 Number: number of restraints;
 Abscissas: list of abscissas along the element; a space or a semicolon could be used as a separator of next values;
 Ratio: list of locations using relative length of the element (e.g. 0.25 0.5); a space or a semicolon could be used as a separator of next values.
 Restraint type: pinned (allowed rotation about the local y axis) or fixed (no rotation about the y local axis).
 Define: add new restraints to the table.
 Lateraltorsional buckling length: allows the definition of the lateraltorsional buckling lengths calculation method (as for the buckling lengths).
 Auto calculation: the lateraltorsional buckling is calculated with the method specified in the Steel Design Settings  EC 3 dialog box.
 Imposed value: calculates the buckling lengths according to a user defined value (entered in the corresponding field).
 Super element ratio: calculates the buckling lengths by a custom value multiplied by the superelement length.
 Element ratio: calculates the buckling lengths by a custom value multiplied by the element length.
 Mesh size ratio: the lateraltorsional buckling length is equal to a user defined value multiplied by the length of one meshing size.
 Effective length factors: the buckling length coefficients. kz is automatically calculated and kw can be specified.
 Critical moment Mcr: to allow verification of profiles in cases of solutions for which the user already knows the value of the critical moment, this selection is available in order to impose this value manually. By default, the value of critical moment is calculated automatically, according to a set of parameters defined in the Lateraltorsional Buckling dialog. When the value of the critical moment is imposed by the user, all the other parameters in the dialog are ignored and disabled with regards to editing.
 Critical moment Mcr0: the Mcr0 critical moment value can be entered by the user, determined automatically or assumed to be equal to the Mcr moment. Thanks to this, you can control the nondimensional slenderness for lateraltorsional buckling due to uniform bending moment (λ0).
 Select curve: define the buckling curve according to Table 6.3, EN 199311.
 Load type: defines the load type according to the selected element. Select the corresponding option from the dropdown list. Click the button near the dropdown list to display the Bending moment diagram for each case.
Load type
 General case: according to EN 199311, art. 6.3.2.2, refers to the lateral torsional buckling curves given in table 6.4.
 Rolled/welded profiles: according to EN 199311, art. 6.3.2.3, refers to the lateral torsional buckling curves given in table 6.5.
 The least/most favorable: the lateral torsional buckling calculation is made according to EN 199311, 6.3.2.2 to get a value  χLT(1) and another calculation according to EN 199311, art. 6.3.2.3 to get a value  χLT(2). If the least favorable option is selected, the smaller χLT value is taken into account; if the most favorable option is selected, the bigger χLT value is taken into account.
 Reduction factor f: the reduction factor as defined in EN 199311, art. 6.3.2.3 (2). It can reduce the lateral torsional buckling coefficient χLT and lead to smaller work ratios, as defined in formula (6.58).
 Auto: the reduction factor f is calculated based on kc values given in table 6.6.
 None: the reduction factor f is not taken into account.
 kc value: enter a custom value for kc.
 f value: enter a custom value for f.
 Load application point: defines the point of load application on the element. The specified numerical value is relative to the neutral axis.
 Loading and end supporting conditions: defines the coefficients depending on the end supporting conditions and on loading.
For the lateral buckling verification calculations according to EN 199311, the procedures for calculating the moment factors Cmy/Cmz/CmLT according to Table B.3 of Annex B include how the bending moment diagram is interpreted and the consideration of sway buckling mode in the element. For the last point the info if the element is braced or unbraced on a given direction is retrieved directly from the steel element properties, from the Buckling group.
Braced or unbraced options
These factors determine the interaction factors using Method 2 (used by some National Annexes, including Poland and the Czech Republic).
You can select the steelwork standard using the Localization settings dialog box. For detailed information, refer to the following section: How to configure the project standards.
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