Field note / structural geometry
One diameter, the same value about every centroidal axis.
For a perfect solid circle, there is no strong-axis or weak-axis distinction through the centre. Rotating the section about its centroid leaves the area and second moment unchanged.
The four reported properties
- Area A is the gross cross-sectional area used in an idealised axial-stress or weight calculation.
- Second moment I describes the geometric distribution of area about a centroidal diameter and is used with stiffness and elastic bending relationships.
- Section modulus Z is I divided by the outer-fibre distance, so for a circle Z = πD3/32.
- Radius of gyration r is √(I/A), which simplifies to D/4 for a solid circle.
What the quick check leaves out
The calculator does not model a hollow tube, holes, reinforcing steel, cracking, composite action, local instability, buckling length, stress limits, deflection limits, or a code-specific resistance. Those choices belong to the design problem around the geometry.