In the Cross-sections table of the 'Steel Design' category, all cross-sections that you have created in the model are listed.
Valid Cross-sections
Steel design is possible for many cross-section types. The font color of the cross-sections provides information about the use and validity of the cross-section. The colors mean:
- black: cross-section is suitable for design and is used for the objects to be designed
- red: cross-section is not suitable for steel design
- blue: cross-section is not used in the model
- gray: cross-section is not used for the objects to be designed
Furthermore, thin-walled cross-sections from the cross-section properties program RSECTION can be designed (see section RSECTION Cross-sections). The categories of the individual cross-sections are specified in the 'Cross-section type' table column.
Objects with an invalid cross-section are not taken into account in the design – even if the 'To be designed' option is activated for this cross-section. They are automatically displayed as 'Not valid / deactivated' in the Objects to Be Designed table.
Removing Cross-section from Design
For a valid cross-section, you can exclude all objects to which the cross-section is assigned from the design by deselecting the 'To be designed' check box. These objects are then classified as 'Not valid / deactivated' in the Objects to Be Designed table and are not analyzed in the design.
Cross-section Classification (for EN 1993, NTC, and SIA 263)
For some standards, the cross-section classification covers the limitation of the resistance and rotation capacity due to local buckling of cross-section parts. If single panels are defined for a cross-section, the automatic cross-section classification is activated by default. The program checks the c/t-limits of the compressed cross-section parts and classifies the cross-section into cross-section classes 1 to 4.
In the list of the table column, you can also set the cross-section class manually.
- Class 1/2: The design is performed with the plastic cross-section resistances without further checking of the c/t parts.
- Class 3: An elastic design is performed without further checking for local buckling.
- Class 4 possible: In the design, it is checked whether effective cross-section properties have to be taken into account. Depending on the result, the cross-section is classified as Class 3 or Class 4 and designed accordingly.
Buckling Curves (for EN 1993, NTC, and SIA 263)
According to some standards, buckling curves describe the reduction factor χ as a function of the relative slenderness λ̄. This reduces the ultimate limit state of a member with compressive force. Buckling curves take into account the imperfections of real members (such as geometric precambers and internal stresses from rolling or welding) by means of the imperfection factor α. In [1] Table 6.1, for example, the buckling curves a0, a, b, c, and d are given with α = 0.13 / 0.21 / 0.34 / 0.49 / 0.76. Table 6.2 defines which buckling curve applies, depending on the cross-section shape, manufacturing type (rolled or welded), h/b ratio, flange thickness, buckling direction (y-y or z-z), and steel grade.
For the design, the buckling curves defined in the cross-section properties are used automatically. However, you can also define the effective length manually.
Design of Cold-formed Sections (for EN 1993 and AISC)
Cold-formed sections usually require special designs with regard to local buckling and cross-section stability. For a cross-section of the 'Cold-formed' manufacturing type and the EN 1993 design standard, the objects with this cross-section are automatically designed according to EN 1993-1-3 [1] (with the exception of cold-formed hollow sections). For the AISC 360 design standard, cold-formed sections are designed according to AISI S100 [2].
For other design standards, no specific design of cold-formed sections is implemented.
Adjusting Cross-section
If necessary, you can adjust the properties of a cross-section in the editing dialog box. To do this, double-click the row of the cross-section or click the button
in the table toolbar.
In the 'Options' table column, icons are displayed for settings or modified values (see image 'Cross-sections' Table). They provide a quick overview of the cross-sections used. For example, the icon
is assigned to all cross-sections that are calculated according to thin-walled theory.
Notes on Stress Determination
The stress-based designs of a cross-section are based on the selected method of analysis in the cross-section. If the 'Thin-walled model' is activated for the cross-section, both the cross-section properties and the stresses used for the design are determined according to thin-walled theory ("TWA"). If this option is not activated, the cross-section properties and stresses are determined on the basis of an FEM solution.
The unit stresses for the respective stress determination provide a good basis for understanding the stress values used in a design. You can check these in the editing dialog box of the cross-section.
RSECTION Cross-sections
The options for designing cross-sections from the program RSECTION depend on the modeling of the cross-section: If the cross-section has elements, single panels (cross-section parts) are automatically created that enable the calculation of the effective section. If the cross-section is defined by parts without elements, however, no single panels can be created. Automatic cross-section classification is then not possible.
Using Another Cross-section for Design
You can use a different cross-section for the design than for the RFEM/RSTAB calculation: To do this, click the corresponding row of the 'Use another cross-section for design' table column. Using the button
, you can then import a cross-section from the library.
If another cross-section is defined in the column, further options are available in the shortcut menu. These allow you to transfer the new cross-section to RFEM or RSTAB so that it is also used in the structural analysis. Likewise, it is possible to import the original cross-section back into the table.
Optimizing Cross-section
The Steel Design add-on offers the option of optimizing standard cross-sections. In this simplified optimization, the program searches for the smallest possible cross-section within the same cross-section series that fulfills all designs with a design criterion smaller than the maximum permissible design ratio defined in the Global Settings dialog box.
To optimize a cross-section, click the corresponding cell of the 'Use another cross-section for design' table column. Then select the Optimize option in the list.
For a parametric cross-section, the editing dialog box of the cross-section appears. In the 'Optimization | Steel Design' tab, you can define the parameters for the optimization.
Check the parameter(s) to be changed in the 'Geometry' column. In the 'Min' and 'Max' columns, you can then enter the lower and upper bounds of the parameter. The 'Step' controls the interval in which the dimensions of the parameter vary during the optimization process. If the 'aspect ratios' are to be retained, check the corresponding check box and specify the parameters for the optimization of the outer dimensions.
The same conditions apply for cross-section optimization as for using another cross-section (see above). The designs are performed for all cross-section variants with the internal forces of the structural analysis. The optimal variant is then output in the table column. The effects of the changed cross-section on the stiffnesses and internal forces in the RFEM/RSTAB model are not taken into account. Therefore, use the options of the shortcut menu to transfer the optimized cross-sections into the model (see image Shortcut Menu with Options for Exporting or Importing).
A comprehensive structural optimization is possible with the Model Optimization add-on.
