In the Mechanica help it shows the following: Beam Section Property Calculations Depending on the type of beam cross section, Mechanica uses the following formulations for beam section property calculations: Solid Sections â€” The shear center is not calculated for solid sections. It is assumed to be at the centroid of the section, coincident with the neutral axis. You can modify the location of the shear center. Torsional stiffness, the second polar moment of area J, is approximated as: J = 4 Iy Iz / (Iy + Iz) Note: This equation gives the exact value only for circular sections and can have an error as high as 20% for rectangular sections. For other shapes, the error can be even higher. Exercise caution when using the calculated value of J. You can find exact values of J for torsional stiffness in R.J. Roark and W.C. Young, Formulas for Stress and Strain, 6th edition, Table 20, pages 348â€“359. I'll try and find out why it doesn't use the correct calculation for J