Civil Engineering/Diaphragm

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Expert: - 10/27/2010


Question
Hi,

I am analysing an 80 storey RC building in ETABS, with 2 outrigger and belt walls on 1/3 and 2/3 of building height.

I am getting very high horizontal shear stress in the columns and walls at a number of levels above and below the outrigger level due to torsional moment.  Why is the horizontal shear stress so high?  The horizontal shear stress seems to disappear after a few floors.  Where has it gone?  What is the load path for the torsional moment?

At the outrigger floor, shall I assume the floor slab as rigid or flexible diaphragm?  If I assume rigid diaphragm, the horizontal shear stress within the floor slab is acceptable, close to concrete shear capacity.  However, for flexible diaphragm, the horizontal shear stress within the slab is close to 12MPa.  I thought the flexible diaphragm is a better represenation of the real structure, is that correct?  The floor slab thickness is about 250mm.

Thanks
Tom

Answer
Hi Tom,

This could be due to a number of reasons, one of which is if your tower is not uniformly stiff (un-symmetrical) in orthoganal directions. Also, you may find that the shear will reduce if you used cracked sections (stiffness modifiers).

The shear in columns will reduce away from the beltwall because the flexibility of the columns allows them to deflect and hence attract less and less shear as you move away, either up or down.

I agree that semi-rigid diaphragm is the correct and more representative approach.

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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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