Field note / structural geometry
The flange pulls the centroid upward.
With the flange at the top, more area sits near the top of the section. The centroid therefore lies above mid-depth, and the distances to the top and bottom fibres are different.
Section modulus depends on the fibre.
The same Ix produces different elastic section moduli at the two extreme fibres: Ztop = Ix/ȳ and Zbottom = Ix/(D - ȳ). Use the modulus for the fibre where the bending stress is being evaluated.
What the calculator reports
- Area A combines the flange and web areas.
- Centroid ȳ is measured from the top face of the flange.
- Ix and Iy are centroidal strong- and weak-axis properties.
- Ztop and Zbottom distinguish the two extreme fibres, while rx and ry are the radii of gyration.
What still needs a design check
This is a gross geometry screen. Reinforcement, cracking, composite action, shear, local stability, load combinations, material resistance, and code-specific serviceability remain outside it.