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Dear waqar saleem, Here are my 2 cents: 1. Section 21.5.3.2 of ACI 318-08 deals with flexural members i.e., beams and NOT columns. In case of columns of SMRF, ACI section 21.6.4.3 is applicable. 2. For calculating the tie spacing, the applicable member parameter is 'minimum member dimension' of the column, which is 12 inches in case of 12"x15" column. So, the controlling tie spacing for seismic zone 3 will be 12"/4 = 3" (and, NOT 2.625", as calculated in your post.) 3. A column tie spacing of 3" has generally no problem in concreting, even if commonly used nominal maximum aggregate size of 3/4 inch is used. I frequently use this spacing in concrete columns, for buildings located in seismic zones 3 & 4 areas, without getting a single complaint due to closeness of ties. 4. As clarified in the commentary section R21.6.4.3, this maximum tie spacing is for confinement of concrete, rather than for shear. As such, use of a larger size tie bar would not help increase required spacing of ties in case of an SMRF. IMHO, The only parameter that may be helpful in increasing the required tie spacing, is the 'smaller dimension of the column'. However, in a double storey building, I do not see a need to increase smaller dimension of the column beyond 12", solely for providing ties at a spacing larger than 3". Regards.