# For cross-section design of a flat steel, I obtain abnormally high shear stresses due to the torsion in the STEEL EC3 add-on module, which can be disproved by a simple manual calculation. What is the error?

Most likely, the error is in the selection of the cross-section:

For steel design, a thin-walled flat bar cross-section should be selected instead of a rectangular solid cross-section, see Figure 01.

The reason for the high shear stress of a solid cross-section is caused by the existing stress points of the cross-section or by the corresponding thickness of this stress point.

In the case of a thin-walled flat bar cross-section, there are four stress points at the corner points of the cross-section with the corresponding thickness t = 10 mm, see Figure 02.

For a solid cross-section, however, there is another stress point in the center, where the maximum of height h or width b is assumed as the thickness t for this cross-section type. In this case, the width b is 200 mm, see Figure 03.

This results in a small torsional section modulus Wt and the correspondingly high shear stress.

The solution is, as described above, to select a flat bar in the main program.