In the table Cross-Sections of the 'Steel Design' category, all cross-sections that you have created in the model are listed.
Permitted Cross-Sections
Steel design is possible for many cross-section types. The font color of the cross-sections indicates the use and admissibility 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 table column 'Cross-Section Type'.
Objects with an invalid cross-section are not considered in the design – even if the 'To Be Designed' option is enabled for this cross-section. They are automatically classified as 'Not valid / deactivated' in the table Objects to Be Designed.
Remove Cross-Section from Design
For a valid cross-section, you can exclude all objects assigned to the cross-section from the design by deactivating the 'To Be Designed' check box. These objects are then classified as 'Not valid / deactivated' in the table Objects to Be Designed and are not checked in the design.
Cross-Section Classification (for EN 1993, NTC and SIA 263)
Cross-section classification is used in some standards to account for the limitation of resistance and rotation capacity due to local buckling of cross-section parts. If individual panels are stored for a cross-section, automatic cross-section classification is activated by default. The program checks the c/t ratios of the compression parts of the cross-section and assigns the cross-section to classes 1 to 4.
You can also define the cross-section class manually in the list of the table column.
- Class 1/2: The design is carried out with the plastic cross-section resistances without further check of the c/t parts.
- Class 3: Elastic design is performed without further check for local buckling.
- Class 4 possible: During the design, it is checked whether effective cross-section properties are to be considered. Depending on the result, the cross-section is classified as Class 3 or Class 4 and designed accordingly.
Design of Cold-Formed Cross-Sections (for EN 1993 and AISC)
Cold-formed cross-sections usually require special verifications with regard to local buckling and cross-section stability. For a cross-section of the manufacturing type 'Cold-Formed' and the design standard EN 1993, the objects with this cross-section are automatically verified according to EN 1993-1-3 [1] (with the exception of cold-formed hollow sections). For the design standard AISC 360, cold-formed cross-sections are verified according to AISI S100 [2].
No specific design of cold-formed cross-sections is implemented for other design standards.
Modify Cross-Section
If required, you can adjust the properties of a cross-section in the editing dialog. To do so, double-click the row of the cross-section or click the
button in the table toolbar.
Icons for settings or modified values are displayed in the table column 'Options' (see image Table 'Cross-Sections'). They provide a quick overview of the cross-sections used. The icon
, for example, is assigned to all cross-sections that are calculated according to thin-walled theory.
Notes on Stress Determination
The stress-based verifications of a cross-section are based on the selected calculation theory in the cross-section. If 'Thin-Walled Model' is activated for the cross-section, both the cross-section properties and the stresses used for 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 a FEM solution.
The unit stresses for the respective stress determination provide a good basis for tracing the stress values used in a verification. You can check them in the editing dialog of the cross-section.
RSECTION Cross-Sections
The possibilities for designing cross-sections from the RSECTION program depend on the modeling of the cross-section: If the cross-section has elements, individual panels (cross-section parts) are automatically generated, which enable a calculation of the effective cross-section. However, if the cross-section is defined by parts without elements, no individual panels can be generated. Automatic cross-section classification is then not possible.
Use Different Cross-Section for Design
You can use a different cross-section for design than for the RFEM/RSTAB calculation: To do so, click the corresponding row in the table column 'Use Different Cross-Section for Design'. Then use the
button to import a cross-section from the library.
If another cross-section is stored in the column, further options are available in the context menu. These allow you to transfer the new cross-section to RFEM or RSTAB so that it is also used in the structural analysis. It is also possible to import the original cross-section back into the table.
Optimize Cross-Section
The Steel Design Add-On offers the possibility to optimize 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 verifications with a design criterion smaller than the maximum allowable utilization defined in the Global Settings dialog.
To optimize a cross-section, click the corresponding cell in the table column 'Use Different Cross-Section for Design'. Then select the Optimize option from the list.
For a parametric section, the editing dialog of the cross-section appears. In the 'Optimization | Steel Design' tab, you can define the parameters for the optimization.
In the 'Geometry' column, check the parameter(s) to be changed. In the 'Min' and 'Max' columns, you can then enter the lower and upper limits of the parameter. The 'Step' controls the interval at which the dimensions of the parameter vary during the optimization process. If the 'Aspect Ratios' are to be maintained, check the corresponding check box and enter the parameters for optimizing the outer dimensions.
The same conditions apply to cross-section optimization as when using a different cross-section (see above). The verifications are carried out 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 modified cross-section on the stiffnesses and internal forces in the RFEM/RSTAB model are not taken into account. Therefore, use the context menu options to transfer the optimized cross-sections to the model (see image Context menu with options for exporting or importing).
Comprehensive structural optimization is possible with the Model Optimization Add-On.
