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Civil Engineering/point load on raft footing

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Expert: - 4/4/2011


Question
Dear Arshad,
I am designing a raft footing for a three storey building. The soil is very weak (loose sand).Therefore i have gone for raft.I am designing it in more conventional way-supposing that raft is rigid.load from each columns is almost same. Therefore I have combined all loads from the columns and self weight of raft(supposed value) and divided it by bearing capacity of soil to get the area of footing. It seems that the raft covers entire building area. I have designed the raft as solid slab with beams under the soil pressure. There is a load coming from lift core and lift on top of slab. I am clueless as how to take in account the load from lift core to analyse portion of slab bay and beams coming on that slab.Please help me in this matter.

Answer
Hi Ananta,

You should only consider the raft as "rigid", if the thickness is more than about 0.2 to 0.25 times the longest plan dimension, otherwise it will deflect under load and your bearing pressure underneath will not be uniform.

So, if your raft is still behaving as rigid, you may just add the loading from the lift shaft (not point load, but rather a line load, under the walls) to the overall load and work out the UDL bearing pressure. If it is semi-rigid, the only way to accurately analyse the deflection and pressure is to do a 3-D FEA analysis, using spring supports to simulate the soil moduli.

If you browse my past answers using keywords "raft" or "matt", you will find some further guidance on this subject.

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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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