Dear Sir,
Whenever try to find story drift for spectrum load case it gives warning as" NOTE: STORY DRIFT WILL NOT BE CALCULATED CORRESPONDING TO RESPONSE SPECTRUM LOAD CASES.
RESPONSE SPECTRUM COMPUTES JOINT DISPLACEMENTS THAT REPRESENTS MAXIMUM MAGNITUDE
OF THE RESPONSE QUANTITY THAT IS LIKELY TO OCCUR DURING SEISMIC LOADING. ANY
RESPONSE QUANTITY LIKE STORY DRIFT, TORSIONAL IRREGULARITY,ETC SHOULD BE
CALCULATED FROM ACTUAL DISPLACEMENTS OF EACH MODE CONSIDERED DURING ANALYSIS.
THE RESPONSE QUANTITY FROM EACH MODE SHOULD BE COMBINED USING MODAL COMBINATION
TO GET THE MAXIMUM MAGNITUDE OF THAT RESPONSE QUANTITY."