A structural steel member typically comprises elements named webs and flanges. In a roofing profile the sides of the rib act as a web, and the pan and crest act as flanges.
Steel is strong in tension but can be weak in compression if the profile is not optimised.
Profiled metal cladding acts as a beam, which derives its strength from the ability of its flanges, separated by the web to resist tensile and compressive forces. This strength can only be maintained while those parts in compression are restrained from buckling.
The geometry of the profile shape, together with the material thickness and strength, determines the load capacity of the profile. Manufacturing variations of tested profiles will produce different results in pierced-fastened profiles, under load and may produce vastly different results in clip-fixed profiles.
Profile strength can be determined by analysis of sectional properties, but this will not accurately predict their performance under load, as the profile shape, and therefore the sectional properties, change during deflection. Only physical testing can prove the actual capabilities of the profile.