The Display menu > Show Interaction Surface command is only available for concrete sections that have reinforcing specified. Clicking the Display menu > Show Interaction Surface command accesses the Interaction Surface form.
In Section Designer the interaction surface is defined by a series of PM curves that are equally spaced around a 360 degree circle. For example, if 24 curves are specified (the default), there is one curve every 360°/24 curves = 15°.
Use the following links to access information about the various areas of the Interaction Surface form, including the table, charts, Options area, 3D View area and Edit menu.
Interaction Surface Form Table
Interaction Surface Form Charts
Interaction Surface Form Options Area
Interaction Surface Form 3D View Area
Interaction Surface Form Edit menu