To add trimmer bars to an existing opening

Have a slab placed in your model.

Switch to elevation view, and draw an opening shape on the slab.
If desired, switch back to isometric view.
Select (Add Trimmers to Openings).
The Opening Trimmers dialog opens.
On your model, click to select the slab.
Adjust the settings on the Opening Trimmers dialog.
Select Select Concrete .
On the model, select the slab.
On the model, click on the Smartline to select the opening.
Click OK .