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First of all my apologies, if my question seems stupid because of my unfamiliarity with the topic. When defining response spectrum load case in ETABS, we have three directions; u1, u2 and u3. Lets assume the excitation angle = 0 and building principle direction is u1 So I will apply response spectrum function (per IBC 2006) in u1 direction by a scale factor. My question is why we don't apply u2 and u3 in the SAME LOAD CASE. My understanding is, according to WILSON in his book on STATIC & DYNAMIC ANALYSIS, CHAPTER 15. The response is calculated for each direction separately. Then this response is combined by different methods (CQC, SRSS etc) which is called MODEL COMBINATION. In CQC method response in 2 directions is assumed to be a portion of 1 direction means S2=a.S1 Where a = 0 to 1.0, recommended value is 0.50 to 0.85 I know in EQ building has only one principle direction. Although EQ can hit from any direction but there is always one major direction. or you can say major acceleration in 1 direction. Lets say u1. As in CQC method the response in other direction will be some portion of response in major direction. I also know that If we have equal spectra CQC is reduced to SRSS method which is independent from the excitation angle theta. My confusion is we will do MODAL COMBINATION (cqc or srss) only when we want to combine u1, u2 and u3 at the same time. But if we are applying only u1 in one response spectrum load case, then we DONT have to do modal combination. It is the same thing as in CQC S2=a. S1. Now in this case a=0. so S2=0. All we have is only response in u1 direction. Then in my building models what i do? include u1 and u2 both in one load case or in separate load cases?