Cause of Error in Cross-Section Design of Flat Steel in STEEL EC3

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 Image 01.

1 Flat Bar

The high shear stress of a solid cross-section is due to the existing stress points of the cross-section, or 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 Image 02.

2 Stress Points of Flat Bar

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 Image 03.

3 Stress Points of Solid Cross-Section

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

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