Per ASCE7 section 12.3.4.1 there are conditions where ρ factor can be 1.0, one of them is for drift calculation and P-delta effects. However per section 12.12.1.1 for seismic force-resisting systems solely comprising moment frames in structures assigned to seismic design category D,E or F the design story drift (Δ) shall not exceed Δa/ρ for any story. ρ shall be determined per section 12.3.4.2, assuming this section supersedes the section 12.3.4.1 will entail ρ shall not be taken as 1.0 for allowable deflection calculation of moment frame structures as described above, whatever ρ factor determined per section 12.3.4.2 shall be used for this purpose.
Per section 12.3.4.2 if the requirements in table 12.3-3 are met the ρ can be 1.0. Per table 12.3-3 ρ=1.0 for three cantilever columns while ρ=1.3 for two moment frames. Even if there are moment frames with fixity at column bases, still ρ=1.3 for two moment frames in one line of resistance, for this example there are eight connection nodes that resist the lateral load, compare it to three base fixity connections for three cantilever columns with ρ=1.0 which are inherently less redundant lateral systems due to their single point of fixity at base.
I have nothing against considering ρ factor in deflection calculation of moment frames, but I am wondering why would the ASCE7 not make it a requirement for cantilever columns as well? Part of the reason might be there are very few major structures with cantilever column lateral system and even those are only in one story, compared to multistory moment frame structures on large buildings. Moment frames are more redundant than cantilever columns in general, and imposing these provisions on moment frames and not cantilever columns for the case of seismic retrofit of soft story buildings puts moment frames at a disadvantage and encourages the engineers to use more cantilever column in their design than the moment frames.
Hoping to have constructive discussion about this matter and finally make it to modify ASCE7 to be more moment-frame -friendly.
Edmond Hagopyan, P.E.
