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Civil Engineering/modal analysis period


hello dear expert
i would like to ask about time period of modal analysis for high rise building.
i design a structure consists of 26 stories, height about 83.25 meter, i define a response spectrum function, and define mass of source from loads only , when i run the model
the time period resulted from response spectrum was 7.21 sec in x direction and 5.32 sec in y direction,
the time period using Rayleigh formula was 3.28 sec.
why is this mistake in time period of response spectrum or modal analysis?????
what should i do to reduce the period to an acceptable range ?????

Hi Farah,

For your building the 1st mode should not be much more than 2.5 to 3 seconds. The reasons why it is much higher could be due to one or a combination of:

1. Modelling errors (loss of connectivity, density of materials, young's modulus, etc).
2. The structure is too heavy
3. The structure is too flexible  

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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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