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Civil Engineering/ETABS Period/Rigid Diaphragm

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Expert: - 11/30/2007


Question
Hello,

I built a model of a 5 story building with slabs at each level, 2 elevator shafts and a continuous wall around the lowest level. The area of each floor is about 50,000 sf and the floor to floor height is 13 ft. I have a 12 thick slab and a rigid diaphragm defined at each floor but my period is around 20 seconds when it should be around a second, it almost seems like the program has a different horizontal and vertical scale factor because the building is responding as though it is 700 ft tall instead of 700 inches tall. The wall around the bottom story is orders of magnitude stiffer than the columns but the columns still picking up about 10% of the base shear. Any advice you could give me would be greatly appreciated.

thanks,
Peter

Answer
Hi Peter,

You are right, in that the period should be 0.5-1.0 second. I acn only suggest 2 possible problems:

1. That your units are not consistent- recheck these.
2. That you have a member that is not properly connected to the main structure and that is what is giving you a first mode high period. Check the first mode deflection, increase the scale factor and then animate it- you may well spot the problem member!

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Arshad Khan

Expertise

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.

Experience

Employment history: 36 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.

Organizations
.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)

Publications
•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

Education/Credentials
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 date)

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