I have defined only one element. If I swap the long and short side (that is, the thickness becomes the length), the St. Venant torsion constant is, in my opinion, calculated incorrectly. If true, which consequence does this have for the design in RSTAB?
SHAPE-THIN is a program for designing thin-walled cross-sections, the cross-section properties are determined according to the theory for thin-walled sections. Strictly speaking, this theory applies only if the width-to-length ratio is below approx. 1/10.
SHAPE-THIN determines the torsional constant according to the expression included in chapter 7.2.1 of the manual. The element thickness t is included with the third power, explaining the differences for swapped thicknesses.
These It,St.Ven. results obtained with a very great element thickness are therefore useless. When using them in RSTAB, you should evaluate this accordingly.
Thus, you should confirm the query appearing before SHAPE-THIN shows a incorrectly modeled element (see figure).