How is the change in the torsional moment Iy taken into account for variable cross-sections (tapered members)?
RSTAB and RFEM interpolate the variable stiffnesses over the member by higher-order polynomials.
The internal determination of tapered cross-sections is done by using the taper shape.
If different cross-sections are defined for the member start and end, you can select in this column or text box the "Options" tab either the linear or quadratic taper shape (figure 1). In this way, you can consider the taper geometry for the determination of the interpolated cross-section properties.
In most cases, the layout of the taper will be linear; that is, the member tapers and extends regularly only over the depth of the start to the end section. If, however, the width and depth of the section changes, the quadratic shape is recommended for the interpolation of the cross-section properties.
For more precise results, it might make sense to divide the tapered member and add additional cross-sections. The division of the tapers and members with elastic has also an influence on the calculation of tapers (figure 2).