You can curve the edge of a floor opening using the Curve tool. Once the tool is active, you can click and drag the opening edge to curve it, or select a point to curve to.
To curve an opening edge by clicking and dragging:
In 2D plan view, click on the opening edge you want to curve.
Right-click and select Curve, or select Edit > Modify Elements > Curve.
Click and drag the opening edge to the desired curve.
Release your mouse button.
To curve an opening edge to a selected point:
In 2D plan view, click on the opening edge you want to curve.
Right-click in and select Curve, or select Edit > Modify Elements > Curve.
Select the point you want to curve to. The opening edge automatically curves to the point.
Click to finish.