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Response Spectrum Load Case


WR1
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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?

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ok thanks...i understand what u said...my building is not regular thats y im concerned with orthogonal effect..anyways...i have this confusion:-

I understand that u cannot take both directions in one spectrum load case because it will be 100 and 100 which is not realistic..

but my point is why then CSI team has put this option in ETABS..they could have omitted u2 text box from there....there should be some reason why they put it?

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  • 11 months later...
  • 4 years later...
  • Moderator
On 11/21/2012 at 6:21 AM, UmarMakhzumi said:

You should define separate load cases.

It would help you understand mode shapes, assign scale factors so that modal mass to be 90% or whatever code criteria is, and make your combos compatible too.

Umar sb i am not clear bout this 90% contribution how do we check this and what is meant by this ? whatever the mode is there is contribution of mass either in direction X or Y.

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Yar, it is a requirement that your model should include enough modes that the sum of mass participation of all the modes should be greater than 85 or 90 (whatever is in the code). You can download a tutorial that I posted some time ago that shows how to check this participation and how to scale the values. Check the link below.

Thanks.

 

 

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  • 3 years later...

Aoa,  i was working on G+35 tower , I've preformed response spectrum analysis ,scaled dynamic base shear for more than 85% respect to static base shear, all the shear walls fails pu/po>.35 , initial sections sections were 12'' up to the top, i increased sections and fc'. but my senior told me not to increase size of walls instead replace  static Eq ( all six) load case with spectrum load case , he gave me reference of model which is done by other structural design firm. seniors here kindly elaborate what would be the reason not to include Static Eq cases in load combinations.

note. diaphragm is semi rigid

check attached excel sheets of load combos.other firm.xlsx

 

my model.xlsx other firm.xlsx

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  • 2 weeks later...

I am a bit confused by your question. My understanding is that you designed the structure for Response Spectrum loads and as well as for equivalent static loads and then your senior told you use response spectrum only and you want to know why is that ? 

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  • 1 month later...

Depends on Building Code. For some structures with irregularities response spectrum analysis is mandated by codes since equivalent static is more applicable for regular shaped buildings with continuous load path and might not give  accurate seismic force distribution. 

 

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thank you for response . yes i agree, lda analysis should be preformed  for irregular structures, my confusion is about load combination, which load combinations should i use , RS only or both Static and Rs . 

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