This example shows how a custom column can be created in the family editor. The example is a simple concrete column with a flared top. The height of the flare at the top can be controled as well as the radius of the column and the radius of the flare at the top.
Start the column family by using the Structural Column.rft family template. Using this template makes sure your family comes into the project under the correct category and it will behave like other structural columns in your project.
First create an extrusion to form the shaft of the column. In the sketch of the extrusion create a parameter to control the raduis of the column.
In an elevation view draw a reference plane below the "Upper Ref Level". Align and lock the top edge of the extrusion previously drawn to the new reference plane. Create a dimension and assign a parameter label to it to be able to control the height of the flared top. Flex the family to make sure the top of the extrusion is moving when parameter controling the position of the reference plane is changed.
Now in a plan view create a blend. This will become the flared top of the column. In the sketch of the bottom of the blend create a raduis dimension and use the parameter used for the column radius. Then sketch the top of the blend and create a new parameter to control the raduis of the top of the colum flare.
In the elevation view align an lock the top of the blend to the "upper ref level" and align and lock the lower edge of the blend to the referecne plane you created just below the "upper ref level".
Now flex the family again trying different radius values for the column and the top of the flare. Also test different flare heights.
As an optional step select the blend and the column extrusion apply a material parameter so the material of the column can be controled once it is loaded into the project.
Example family file (Flared_Column.rfa) is attached to the article (at the bottom of the page).
You can consider making multiple type in the family with the standard dimensions typically used. This makes the flexing process go faster and makes the column family easier to use once loaded into the project.
This example article was inspired by the following discussion forum post on the subject.
For more information on creating families in Revit see the Revit Families Guide.