The paper discusses the components of reduction factor to be accounted for design purposes. A parametric study of moment resisting steel frames (MRSF) designed according Eurocode 3 (EC3) and Euro-code 8 (EC8) is performed. The structural supply of the designed structures exceeds largely the seismic demand in spite of the fact that Ultimate Limit State (ULS) verification of EC8 for MRSF is not conservative. The structural performance is poorly correlated with ULS verification of EC8 but with others code restrains and with parameters like base shear ratio or fundamental period. The practice that deduces the values of reduction factor on the observation of structural performance under past earthquake is not rationally based. This practice tends to make useless the ULS verification for structural systems like MRSF, in which structural performance is poorly correlated with the ULS verification. This practice is responsible of the unsafe values of reduction factors assigned in the seismic codes.
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