In some cases when you try to create a lofted plate or a lofted slab by using construction lines, arcs, or circles, the geometry of the part would become self-intersecting. In these situations the part is not created, and Tekla Structures displays the following status bar message: The resulting surface would be self intersecting.
With construction lines or arcs you can try to resolve the situation by changing the modeling direction of the construction lines or arcs. Select the construction line or arc, and on the contextual toolbar clickSwap ends.
With construction circles you can try to resolve the situation by moving either of the circles.