Response Spectra Analysis

[alphaxy]

I have defined the response spectra in both direction

SPECX
U1 = UBC97FUNC = 9.81m/s^2
U2 = -------- = ---------

SPECY
U1 = -------- = ---------
U2 = UBC97FUNC = 9.81m/s^2

Then, Analyzed and view the story shears

I have noticed that static and dynamic values is the same for both direction

Story Load Loc P VX VY
STORY1 EQX Top 0 -291.16 0
STORY1 EQX Bottom 0 -291.16 0
STORY1 EQY Top 0 0 -291.16
STORY1 EQY Bottom 0 0 -291.16
STORY1 SPECX Top 0 1058.4 0
STORY1 SPECX Bottom 0 1058.4 0
STORY1 SPECY Top 0 0 1058.4
STORY1 SPECY Bottom 0 0 1058.4

Why is that?
Do I have to get the first direction first then move on to other direction to get the correct scaling factor?

I defined SPECCOMB in RS definition as a combination of both direction. Is this correct?

Pls. Advise

Thanks

 

[zeemas]

Alphaxy

Check the following thread, hope it will help you.

http://www.eng-tips.com/viewthread.cfm?qid=233037

http://www.eng-tips.com/viewthread.cfm?qid=236265

 

[alphaxy]

Zeemas,

Thank you for your quick response.

I have check that link you've given. As, expected We have the same values for scaling factors except for the Y-direction. You got 102.82, but in my solution I get 103.03. Please take a look at it so I can correct my self if I miss something.

I have the same problem with aqibusman. I am getting different results in dynamic base shear when getting it straight in story shears rather than in response spectrum base reaction which I think this two "should" be the same. Is there something wrong with these values?

I have attached an excel file for you to look at. Please correct me if you think I didnt make it right.

After scaling these factors, should I expect the value of Vy in EQSX to be 30% of Vx for orthogonal effect? Or it may happen that the value is significantly different?

Hoping for your response
AlphaXY

 

[alphaxy]

Zeemas,

Forget the first question. I already got the same value of 102.9. I downloaded the original model provided by aqibusman and edited the model again and finally got the same value.

Newei, I hope you can help me with the other question.

Regards,
AlphaXY

 

[zeemas]

You are getting different value because you are adding them by SRSS or CQC. I will say that dont include both acceleration in one case because if you do that then you are adding these two values by SRSS or CQC.
Define 2 different cases one with Vx and other one is Vy and add them mathematically in load combination.

Best
Z

 

[alphaxy]

z,

Got it. Thanks for the info. I will check it in my model as soon as I can and get back to you at once. However, just to make it clear, this is what you suggest:

Define SPECX and SPECY in response spectrum and combine it in load combination mathematically when you obtain scale factors for each direction (Say, 1.2DL + 1.6LL + 1.1SPECX + 1.1SPECY) "RATHER THAN" having SPECXY in response spectrum definition with scale factors of both direction to be used in load combination (1.2DL + 1.6LL + 1.1SPECXY).

Thanks
AlphaXY