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Civil Engineering/Checking seismic drift in ETABS


Dear sir, According to ASCE 7-05

"12.8.6 Story Drift Determination. The design story drift (A)
shall be computed as the difference of the deflections at the centers of mass at the top and bottom of the story under consideration."

My question, we dont need to calculate story drift at edges of floor diaphragm?

Similary according to FEMA 451 part 2,

""Total displacement at center of mass = δavg = 3.640 in. (see Table 6-9a)
Rotation at center of mass = 0.000189 radians
Maximum displacement at corner of floor plate = dmax = 3.640 + 0.000189(102.5)(12)/2 = 3.756 in.
Ratio δmax/δavg = 3.756/3.640 = 1.03 < 1.20, so no torsional irregularity exists."

How to know the rotation of center of mass in ETABS and the displacement? Because there is no point at center of mass in ETABS.

Hi Waseem,

If your building is fairly symmetrical and centre of mass coincides with centre of stiffness then the drift will be the same for both locations. And remember that story drift affects not just curtain walling but internal partitions too.

Rotation of diaphragm in radians will be constant whether you consider it at centre of mass or elsewhere, so you can compare that with maximum displacement as noted above.

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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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