Start with the extreme-fibre stress.
The calculator divides the applied moment by the elastic section modulus about the selected bending axis. It then compares the resulting stress with an allowable value entered by the user and reports the equivalent allowable moment and required section modulus.
The allowable is not universal.
An allowable bending stress should be traceable to the material, design standard, load combination, serviceability basis, and project brief. CivilKit does not choose those inputs or infer a resistance from a material name.
What still needs judgement
- Lateral-torsional buckling, local buckling, section classification, imperfections, and second-order effects.
- Shear interaction, axial force, torsion, fatigue, deflection, vibration, fire, durability, and dynamic effects.
- Connections, supports, load paths, effective or composite properties, resistance factors, national annexes, and professional review.
The basic relationship is commonly written as f = M/S; compare the FHWA bridge design example for that flexural notation and context.