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

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

For a steel design, a thin-walled flat steel cross-section should be selected instead of a rectangular solid cross-section, see Figure 1.

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 steel cross-section, there are four stress points at the corner points of the cross-section with the corresponding thickness t = 10 mm, see Figure 2.

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 3.

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

Therefore, the solution is, as described above, to select flat steel within the main program.