A design variable is a numerical input that is allowed to change during the design optimization. When creating the model and specifying the input, the following should be done for a design variable:
- Enter a reasonable value for the design variable. The value entered will be used as the Current Value in the design optimization dialog. Naturally, the entered value will be used if a regular analysis is performed on the model.
- While entering the value for beam elements or nonlinear shell elements, right-click in the field and choose Set as Design Variable. The
symbol will appear next to the field. (To remove the variable, right-click again and choose Remove as Design Variable.) For beam cross sections chosen from a section library, right-click the selection in the list box and choose Set as Design Variable. For linear plate elements, a check box appears in the Design Variable column to indicate that the plate thickness is a design variable. Note: Although the Element Definition for multiple parts can be entered simultaneously, the Set as Design Variable will only be set for the first selected part. Visit each Element Definition screen to set the design variable for the other parts. (Likewise, the Element Definition can be copied from one part and pasted to another, but the flag to set the input as a design variable is not copied.)
Use Analysis
Analysis Optimization in the FEA Editor or Results environment to access the Design Optimization dialog. The Design Variable tab will list each of the design variables that are defined. Should you decide that a predefined variable is not required, select the row or multiple rows and use the Delete Rows button.
The input for the Design Variable tab is as follows:
Variable:
The part number and variable defined as design variables are listed. New variables can only be added through the FEA Editor as described above.
Current Value:
Design optimization uses numerical techniques to find the optimum solution to a set of design variables. The routine begins the search with each design variable set to the value entered in the Current Value column. By default, the value in this column is the number entered when the Set as Design Variable command was used. Any of the values can be changed in the Design Variable tab; however, the corresponding value in the model is not updated (nor does it need to be).
If the Discrete column is activated, placing the cursor in the Current Value cell will show two extra controls to the right of the cell. Select one of the previously defined discrete values. The button 
will access the Discrete Design Variable dialog. When working with dimensions, use the Single Value or Increment fields to enter values; then click the Add button. The values will be added to the list of discrete values. When working with a beam cross section from a section library, choose the library, section type, and available section; then click the Add button to populate the list of selected sections.
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- Continuous values are used for the design variables during the iteration process to find the optimum result.
- The discrete design variable nearest to the optimum result is chosen.
- A final iteration using the discrete design variable is performed. Thus the optimum model is based on the discrete values.
When using discrete design variables, the optimum design is calculated as follows:
Lower Limit:
Specify the minimum value for the design variable.
Upper Limit:
Specify the maximum value for the design variable.
Discrete:
If activated, individual values of the user-specified design variable are used during the optimization analysis. If not activated, then the design variable can be any value between the lower and upper limits.
Some variables, such as selecting a beam cross section from the AISC library, will force the design variable to be a discrete value.