Answer:
This message appears if both cross-sections of the taper have a different number of stress points.
The stresses in STEEL are determined with the second moments of area and the distances to the stress points. Now, if the start and end cross-sections of the taper have different numbers and locations of stress points, the program cannot interpolate the intermediate values. Often, in the rendering mode, no taper is displayed; 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. By clicking the [Details of Stress Points] button, you can open a table showing the exact values for the stress points (number, location, and so on).
To solve the problem, you have to create the same number of stress points. You can do this, for example, by creating a cross-section at the end of the taper as a copy of the start cross-section and adjusting the height. However, the original form must be preserved. Therefore, it may also be necessary to model a very small strengthening.
You can also create tapered cross-sections with sections of the RSTAB library: For this purpose, "Strengthened I-Section with Lower T-Section" are among the welded cross-sections.
