The design method is referred to generally as "load and resistance factor" design.
The strength reduction (phi) factor is a resistance factor, and not exactly a factor of safety. Phi factors vary by member and what is being resisted (flexure, shear, torsion, bearing).
The resistance factor reflects material properties. It is multiplied with the computed resistance (strength) provided by the element to determine design (nominal) strength. (ACI 318 9.3)
The load factors are multiplied with code-required loads to determine minimum required strength for each load combination. (ACI 318 9.2)
For example, a typical live load with a factor of 1.6, on a tension-controlled flexural section using phi=0.9, this translates to a live load FOS of 1.78 for flexure; for shear, the phi=0.75, giving a FOS of 2.13.
As explained, ACI does n ot use individual material factors like Eurocode or british codes. It uses an overall capacity reduction factor mto reflect the overall variation in all material properties. Because of this, different capacity reduction factors are used in different areas of design.
In The Material Factor logic, more variable materials (eg concrete) have much higher factors than less variable materials (eg steel). In EC2, concrete would ghave a factor of 1.5 and steel 1.15. So where steel dominates the capacity the factor is really mainly affected by the steel factor so is close to 1.15 overall (eg flexural tension). Where concrete dominates the capacity, the concrete factor has most effect (eg concrete in pure axial compression in a column interaction diagram).
For a Capacity Reduction Factor code like ACI, for pure bending in the interaction diagram, the factor is .9. For pure axial compression, it is .65 or .75.
The Material Factor code is much more seamless and logical but, as long as the Capacity Reduction Factor approach covers all options logically, they come out to similar results.
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