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Civil Engineering/RAFT SLAB ON DOWNSTAND BEAMS ON THE GROUND SURFACE

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Expert: - 6/9/2011


Question
Dear Arshad,
I have a raft slab that is to be supported by ground beams 1.2m deep. The beams bear directly on the ground (with a bearing capacity of about 39-40KN/m2), hence suspending the slab above the natural ground level. Will it be proper to still arrange the main reinforcements in the slab as top bars or will the influence zone create hogging moments at the bottom of the slab, thus requiring slab reinforcements to be arranged at the bottom of the slab?

Answer
Hi Dennis,

Firstly what you describe is not a raft slab, but a ground slab suspended between ground or grade beams. In this case, the slab is (as you say), a suspended slab and should bedesigned with both bottom (sagging) rebar and top (hogging) rebar at the supports- assuming that there can be some fixity at the supporting beams.

Incidently, the bearing capacity of the soil seems rather low to safely carry any significant span of slabs- so I would re-check the bearing capacity against applied loads.

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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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