The procedure to define a non-prismatic is very straight forward. You might find it useful to watch the video "Nonprismatic Sections" available at http://www.csiberkeley.com/Support_WL_SAP.html for an example. Also, find below a brief outline of how to define a nonprismatic section:
1) Define the starting and end sections of the tapered section and click the “Add New Property” from the “Frame Properties” window, choose “Other” frame section property type from the drop-down menu and click the “Nonprismatic” button to define the tapered section.
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2) Choose the starting and end sections from the drop-down menu and define the length of the variation.
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Figure 1 - Caption