Creating a Mushroom kind of geometry

Hi,

I am trying to create a mushroom from of geometry, with a circular platform above and a frame structure below to support the platform. 

I started creating this form by creating a cone and use subsurface component to create a ring kind of shape. I am now at the stage where I want to separate this form into two part, where half of the ring will be a platform and  the other have to be a frame structure (using polyline and pipe). 

Please have a look at the picture below.

*The component covered by the [list item description] is Replace, because I want to create the framework like the picture on the up left corner.