Dear Vijaya, seismic drifts are evaluated with non-reduced seismic loads (without R) for the Colombian case. You can use load combinations (including seismic load case), but vertical loads, by nature, will reduce lateral displacement. A conservative way to evaluate drifts is only considering the non-reduced response spectrum.
Regards,
AG
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Andres Guzman D.Eng., MEng, Ing., M.ASCE
Associate Professor
UNIVERSIDAD DEL NORTE
Barranquilla
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