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# Civil Engineering/Dynamic Analysis vs Static Analysis on ETABS

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QUESTION: Dear Mr. Khan,

My model consists of a 5 story building with irregular shaped floors to be done on ETABS. I ran the model using both dynamic and static analysis. As we know, when running the model under dynamic analysis, one should check the “Dynamic Analysis” option and set the “Dynamic Parameters” in the “Analysis Options”. One should also define a response spectrum function and response spectrum cases. My question concerns the results I obtained in the Summary Report. In fact I got different results concerning the base shear of the earthquake static load case (EX and EY) in both the dynamic analysis and the static analysis.

Base Shear EX (dynamic analysis) = 4475T      VS     Base Shear EX (static analysis) = 3995T
Base Shear EY (dynamic analysis) = 5098T      VS     Base Shear EY (dynamic analysis) = 4538T

Is this normal? If no how can I solve this problem?
From my basic knowledge, the base shear of the earthquake static load case (NOT SPECTRUM CASE!) whether obtained from dynamic analysis or static analysis should be equal.

Thank you for your help,
Sincerely,
Farid

ANSWER: Hi Farid,

What you need to do is to modify that factors in the RSA definitions so that the dynamic base shear falls within 90-100% of the static. So, in your case modify the X direction factor by multiplying it by 3995/4475, and the Y-direction by 4538/5098. Then re-run the analysis and compare the base shears.

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Static vs Dynamic Anal
QUESTION: Dear Mr. Khan,

Thank you for your help I appreciate your level of expertise and the negligible time you took to answer.

I tried what you said; unfortunately nothing’s changed.

I guess what you meant with dynamic base shear is the base shear obtained from the response spectrum analysis that I call “SPECX” and “SPECY” (highlighted in yellow in the attached image) whereas I was mentioning the base shear from the static load case of type “QUAKE” and auto lateral load “UBC 97” (highlighted in red) that I call “EX” and “EY”.

The attached image shows the results tables (total reactive forces - recovered loads - at origin taken from the summary report) obtained from the DYNAMIC ANALYSIS (RSA factors = 1) versus the results obtained from the STATIC ANALYSIS. The values highlighted in red in both tables are the ones obtained from the static analysis and correspond, as mentioned earlier, to the base shear obtained from the static load case of type “QUAKE” and auto lateral load “UBC 97”. My question is: shouldn’t the values highlighted in red be equal? I got them equal in previous projects but the last couple of projects I did not get them equal and I am wondering why.

Thank you again for your time,
Sincerely,
Farid

Answer
Hi Farid,

Yes, I checked some of my models and can confirm that in the summary report these values should be equal. I can only suggest that you check your model again to see that you are getting at least 90% mass participation, and also if you have ramps make sure the masses are lumped at floor levels.

Maybe that will resolve the discrepancy.

If all else fails, export the model to an e2k text file, then import it back and re-run it.
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#### Arshad Khan

##### Expertise

I can answer any questions to do with civil and structural engineering consultancy and construction industry in East Africa and the Middle East, and specifically with the analysis and design of reinforced concrete structures. My particular expertise is in the aseismic design and optimisation of tall buildings.

##### Experience

Employment history: 40 years in Construction and consultancy in the UK, Kenya, Tanzania, Uganda, Ethiopia, Eritrea, South Africa, Somalia, Zambia, Austria, Bahrain, Kuwait, Doha and the U.A.E.

Organizations
.Fellow of Institution of Structural Engineers (UK) .Fellow of Institution of Civil Engineers(UK) .Member of the Institution of Engineers, Kenya .Registered Engineer, ERB, Kenya .Member of the Architectural Association of Kenya (Engineers Chapter) .Chartered Engineer (UK)

Publications
•1984: International Conference on the Art and Practice of Structural Design, London •1994: 3rd Int. Kerensky Conference in Structural Engineering, Singapore •2008: International Conference on High-Rise Towers, Abu Dhabi •2013: IEK International Conference, Kisumu, Kenya

Education/Credentials
BSc, 1st Class Hons, in Building Engineering, University of Bath, UK MSc in Concrete Structures and Technology, University of London. Diploma of Imperial College, UK.

Awards and Honors
•Science Congress Special Award (for 2-seater Hovercraft - 1968) •Institution of Civil Engineers Award for outstanding performance at Bath University (1975) •Concrete Society Postgraduate study Bursary Award (1976) •Consular Representative for British High Commission, Nairobi. (1995 to 1998) •Examiner for Institution of Civil Engineers Professional Interviews, Nairobi. (1997 to 1998) •Branch Representative in Vienna for PI assessment for Inst. of Struct. Engrs. (1999 to 2010)

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