Autodesk CIVIL3D - CIVILDESIGNCOMP 2007 SLM CD Tutorials Manual page 737

from the Rim work plane. With these relationships, the geometry of the
entire vault can be controlled with a few parameters.
17 Right-click Model Parameter and then click Edit. Change the Equation
for LenA5 to Wth. Change the value for Wth to 4. Wall thickness (Wth)
is one of the size parameters that is built in to this part type.
18 Click View tab
and then click Add Extrusion. You are prompted to select a profile.
19 Click the outer rectangle. Enter 48 for Distance and check the box next
to Flip. Click OK.
An extrusion is created that extends downward 48 inches.
Exercise 3: Defining the Vault Box Geometry | 725
Views panel SW Isometric. Right-click Modifiers