I have idealized a taper by using a start and end cross-section (calculated with SHAPE-THIN). Then, I have calculated the frame. When I carry out the stress calculation in STEEL, the error message "Tapered member contains incompatible cross-sections" appears. What can be the cause of the error?
The stresses in STEEL are determined with the second moments of area and the distances to the stress points. Now, if the taper's start and end cross-section have different numbers and locations of stress points, the program cannot interpolate the intermediate values.
Furthermore, in this case the rendering mode often does not show a taper but only the two cross-sections at the start and end. You can see a "break" in the middle of the member.
You can look at the stress points of a cross-section in its details. With a click on the button [Details of Stress Points] you can open a table showing the exact values for the stress points (number, location ...).
So, to solve the problem, you have to create the same number of stress points.
You can do it, for example by modeling a very small strengthening for the cross-section also at the end of the taper. Maybe you can create the tapered cross-sections with the sections for example from RSTAB instead of SHAPE-THIN.
For this purpose, the cross-section library of RSTAB offers the "Strengthened I-Section with Lower T-Section" among the welded cross-sections (Parametric - Thin-Walled).
Cross-Section Properties Software
Section properties, stress analysis, and plastic design of open and closed thin-walled cross-sections
Stress analysis of steel surfaces and members