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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.

i hv little query about the centre of rigidity... is centre of rigidity is independant of loading..?? if yes then how material non linearity or second degree analysis affects the centre of rigidity....

Material non-linearity would effect value of "EI" and would result accordingly when calculating the center of rigidity. Similarly, Second degree analysis would effect to the extent of change in EI value as calculated for the of Center of Rigidity. My answer is just the mathematical explanation of equations solved for center of rigidity.

by default etabs gives 10% of the beam depth as its top and bottom cover. Most of the times you can rely on that in order to save time by not getting lost too much into tiny details. I for example use 65mm for beams depth less than 750mm (40mm clear cover+10mm stirrups dia+half of 25mm dia bar considering it to be 1 layer) and 90mm for beams depth more than 750mm (considering two layers) you really need to worry about this issue when you have shallow beams, for deeper beams, etabs assumption of 10% of depth is fine. and 65mm for columns (they always have one layer, assume bar dia of 25mm or 1inch #8)