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thank you sir

well this is not always true. Last year I did a similar comparative study in Saudi for one of our projects. UBC-97 gave higher base shear due to the fact that the actual calculated base shear exceeds the maximum allowed value that is independent of time period. Buildings were in moderate seismic zones and the height of building where the actual base shear was less than the max base shear was 16m for concrete and 18m for steel structures. These buildings were low rise having 3 stories max. In IBC 2006/ASCE 05 maximum base shear formula has T and actual base shear is constant for all heights. That is the reason max base shear governed upto 14m height of concrete and 12m height for steel structures. In nutshell IBC 2006 gave us upto 64% reduction in base shear magnitude in concrete and upto 56% reduction in steel buildings as compared to that of UBC-97.

Centre of rigidity is a measure of 'torsional susceptibility' of buildings, and is computed by calculating stiffness of vertical members, at a particular floor level, to lateral loads. Hence, in elastic range, it is independent of loading. But loading can change the state of member: can push a member into inelastic region. In in-elastic region, center of rigidity will change continuously because stiffness of some members will diminish, depending on the state of members and their mechanical properties, which will depend on loading condition. In design offices, and as far as building codes is concerned, you, primarily, deal with elastic response, and hence it is independent of loading.