Civil Engineering/raft slabs

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Expert: - 12/21/2009


Question
Good Morning from sunny South africa Sir.

per my subject, please suggest where may download a drawing detailing a basic raft.
the reason for a raft is certainly not because of unstable ground conditions but rather for the sake of the time constraint.
I require an 8mx 8m foundation slab in order to construct a unit consisting of 16000 bricks. the time constraint is due to the upcoming FIFA Soccer world Cup that will bring a possible rental income to our proposed unit in June 2010.
Cost is therefore a factor as well.

I have followed many of your answers and trust you will be able to advise on this and other cost saving measures.
I operate a construction business in SA and have built rafts under instruction from Engineers previously and believe I will therefore be able to expedite this with relative ease.

Thank you for a good site.

Answer
Hi Kay,

The basic detailing of a raft is like that of a suspended flat slab, but upside-down. So, if you have a column in the centre of the raft, you will have concentrated rebar around its footprint in the bottom layers.

I do not have a link for an image, but in any case each raft is different depending on the loading pattern, ground conditions, etc. For a 8x8 m raft, I would imagine it needs to be between 600-800mm thick, or you can make it more economical by having some parts of it thinner- for example, if all the loading is around the periphery, then that can be thicker and the central portion can be reduced to say 400mm thickness, assuming that you do not have much uplift (basement situation with high water table- in this case the whole raft will need to be thick).

Here is some useful reference:

http://books.google.com/books?id=wu_e-KnkBPgC&pg=PT256&lpg=PT256&dq=raft+slab+de...  

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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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