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Civil Engineering/Structural design of flat slab

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Expert: - 2/14/2010


Question
Dear Sir,
Deflection of flat slabs,
1.How do i take or measure the critical span length  to
 calculate for deflection if the flat slab in consideration is not rectangular in shape(means columns are put in staggred manner)
2.Suppose i am designing my flat slab using SAFE software, i have
 fixed my thickness of slab to L/33(ACI)and design the slab.
 The result will show me a deflection values.Am i going to compare these values with L/240 or L/480 , considering creep & shrinkage are not in theit worst condition.

3.To fix the thickness of slabs BS8110 & ACI gives different  values , can you give me a comment on this thing.
Thanks.

Answer
Hi Dada,

1. You will need to use your engineering judgement on that- you can always choose 2 or 3 alternative spans as critical and compare the results.

2. You can take long term creep/cracking effects by modifying the bending stiffness of the slab. Then you can compare the results with the codified criteria.

3. I suggest that depending on your local authority requirements, use either BS8110 or ACI, not a combination. Part of the difference may arise from different load combinations or rebar ultimate stress values. (ACI uses slightly lower ultimate stress for HY rebar).

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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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