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Civil Engineering/story shear in seismic design of buliding


story shear
story shear  
QUESTION: hello sir,
i am designing a 31 floors building using etabs 2013 software but i have a problem with the story shear which is given in the image below. Can u tell me what should be the reason?
the period given by the first mode is large equal to 6,2 sec

ANSWER: Hi Batoul,

The story shear diagram looks OK- one normally expects a sharp reduction of story shear at GFL diaphragm level and below as the basement walls act as a stiff box. You need to design carefully the diaphragm at GFL.

As for your time period, I regret to say it is too high- for a 31 story building the first and second mode time periods should be between 2.5 and 3.5 seconds maximum- better to be around 3.0 seconds. Check your mass source and if that is OK, then you need to stiffen up the frames and or shear walls.

---------- FOLLOW-UP ----------

QUESTION: thank u very much !
What do you mean the walls act as a stiff box! and what can i do to resolve this problem.
About the period, the mass source is Selfweight + SDL + 25%LIVE. Isn't ok?
How can i stiffen up the shear walls?

Hi Batoul,

If your GF slab is connected to the basement walls, then they act as very stiff shear walls in X and Y directions- the whole basement is a stiff box.

Your mass source looks OK. You can either widen or thicken your shear walls or make the frames stiffer, or reduce the self weight by optimising the floor slabs, or increase the grade/E-value of the concrete. All these will improve the time periods.

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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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