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Civil Engineering/Vertical Earthquake Analysis on ETABS


QUESTION: Dear Mr. Khan,

Hope this message finds you well.
I would kindly like to know how we do the vertical earthquake analysis on ETABS using static analysis on the one hand and dynamic analysis on the other hand.

Thank you for your time,

ANSWER: Hi Farid,

For static and dynamic analysis you can make allowances for the vertical effects by adding a factor in the load combos, as per UBC-97 (+/- 0.5 x Ca x I).

Alternatively you can include acceleration in the Z-direction when selecting the analysis parameters.

[an error occurred while processing this directive]---------- FOLLOW-UP ----------

QUESTION: Dear Mr. Khan,

Thank you for your quick reply as my case is very urgent.

What about unchecking the "Include Lateral Mass Only" in "Define Mass Source"? In fact I have read about it after I asked you my question and the Help section on ETABS says that we should uncheck it in order to consider vertical dynamics.

Moreover, I did not quite understand when you said "Alternatively you can include acceleration in the Z-direction when selecting the analysis parameters". Does this apply for the dynamic analysis?

Finally, I have read in section 1631.2 (point #5) UBC 97 that in dynamic analysis, "the vertical component of ground motion may be defined by scaling corresponding horizontal accelerations by a factor of two-thirds". Personally I understand from this that a scale factor of 0.666 (=2/3) in the direction of "UZ" should be introduced along with the initial scale factors of 1 in the directions of "U1" and "U2". Obviously all these factors are to be scaled accordingly later on after the analysis.

Thank you for your time,

Hi Farid,

Yes, what you say is correct.

Yes, when you go to analyis, look at dynamic parameters and include ACCEL Z in the Ritz Load Vectors.

Again, what you have quoted is correct, but that procedure is an alternative to the others I mentioned earlier (i.e. by adding/subtracting vertical load in the combos).

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Arshad Khan


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.


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.

.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)

•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

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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