AutoCAD Plant 3D: How to create circular floor plate

First of all, you need to know that it is not possible to create perfectly circular floor plate in Plant 3D. 

It is possible to create an almost circular plate with the following steps:

1. Use the POLYGON command to create a polygon with enough sides so it appears to be a segmented circle.
2. Draw the plate using the Existing polyline option to select the polygon: