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Civil Engineering/Formula for calculating raft foundation thickness

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Expert: - 2/22/2010


Question
Hi Arshad,
I am trying to determine if there is a standard formula/calculation to determine concrete raft foundation thickness for coursework on my civil engineering course.

We are designing an 8 story commercial building with ground surface area of 1664.46m2. We are not required to design the complete structure so cannot state what the total load is so was thinking that I could produce a table stating if load was 10,000kN then raft thickness needs to be X mm thick, if load is 20,000kN then raft is Y mm thick and so on. Can you advise if a formula is available?
Thanks

Answer
Hi Paul,

I am afraid there is no such standard formula, as the raft thickness is determined by various factors such as:

1. Ground conditions and limits for differential settlement.
2. Distances between supported walls, columns, etc.
3. Loading on columns.
4. Overall loading and any uplift pressures due to high groundwater level.
5. Grade of concrete and percentage of rebar.

As far as 3. above is concerend, if you know the size and loading of the most heavily loaded column then you can determine the minimum raft thickness based on the concrete resistance to punching shear. (similar procedure to designing a flat slab, but turned up-side down).
As a rule of thumb, apart from above considerations, the raft should be between span/7 to span/6, where span is the maximu, distance between 2 heavily loaded columns or walls.

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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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