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7.2 Cross-Section Optimization
The design module offers you the possibility to optimize overloaded or little utilized cross-sections: Define the relevant sections in the 1.3 Cross-Sections window by opening the drop-down list in column E or F where you decide if the cross-sections are to be determined From current row or from user-defined Favorites (see Figure 2.17). You can also start the optimization in the result windows by using the shortcut menu.
During the optimization process, RF-/STEEL EC3 determines the cross-section that fulfills the ultimate limit state (!) design in the most "optimal" way, that is, comes as close as possible to the maximum allowable design ratio specified in the Details dialog box (see Figure 3.11). The required cross-section properties are determined with the internal forces as available from RFEM or RSTAB. If another cross-section proves to be more favorable, it is used for the design. Then, the graphic in Window 1.3 shows two cross-sections – the original cross-section from RFEM or RSTAB and the optimized cross-section (see Figure 7.7).
If you select the Optimize option for a parametric cross-section, the following dialog box appears.
You determine the parameter(s) that you want to modify by selecting the Optimize check box(es). This enables the Minimum and Maximum columns where you can define the upper and lower limits of the parameter. The Increment column controls the interval in which the size of the parameter varies during the optimization process.
If you want to Keep current side proportions, activate the corresponding check box. In addition, you have to select at least two parameters for optimization.
Cross-sections composed of rolled cross-sections cannot be optimized.
Please note that during the optimization the internal forces won't be automatically recalculated with the modified cross-sections: It is up to you to decide which cross-sections should be transferred to RFEM or RSTAB for recalculation. As a result of optimized cross-sections, the internal forces may differ significantly because of the modified stiffnesses in the structural system. Therefore, it is recommended to recalculate the internal forces with the modified cross-sections after the first optimization, and then to optimize the cross-sections once again.
You can export the modified cross-sections to RFEM or RSTAB: Go to the 1.3 Cross-Sections window and select on the menu
- Edit → Export All Cross-Sections to RFEM/RSTAB.
You can also use the shortcut menu in Window 1.3 to export optimized cross-sections to RFEM or RSTAB.
Before the modified cross-sections are transferred, a query appears asking if the results of RFEM or RSTAB should be deleted.
After starting the [Calculation] in RF-/STEEL EC3, the internal forces and design ratios are determined in one calculation run.
If the modified cross-sections have not yet been exported to RFEM or RSTAB, you can reimport the original cross-sections to the design module by using the options shown in Figure 7.7. Please note that this possibility is only available in the 1.3 Cross-sections window.
If you optimize a tapered member, the program modifies the member start and end and linearly interpolates the second moments of area for the intermediate locations. Since these moments are considered with the fourth power, the designs may be inaccurate if the depths of the start and end cross-section differ considerably. In such a case, it is recommended to divide the taper into several members, thus modeling the taper layout manually.