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# Civil Engineering/Checking seismic drift in ETABS

Question
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.

Civil Engineering

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•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

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•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)