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Re: Consideration of Column Cracking

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> My client insists that the effect of cracking in RC columns must be
> considered.
> According to some researchs, the gross inertias of columns are reduced to
> 80%.
> In my opinion it's overestimated.

Reducing gross inertia of columns by 80% seems reasonable. You can
also check the table 4.1 on page 163 of the following book for
more accurate values of effective member moment of inertia:
  Seismic Design of Reinforced Concrete and Masonry Buildings
    By: T. Paulay and M.J.N. Priestley
The Clause 10.14.1 of the Canadian standard CSA A23.3-94 
and also the explanatory notes N21.2.2.1 and N21.2.2.2 of 
this standard suggests that flexural rigidity of the beams
may be taken as 0.35 of their gross section value. Also there, it
is suggested that the flexural rigidity of the columns   
may be taken as 0.70 of their gross section value.

Abdi (A.S. Moghadam)