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Civil Engineering/Buried Aeration Tank Floor Slab


I am designing a 60x20x5m aeration tank. The tank is buried and sits on bedrock. The ultimate moment at mid span is around 2500kNm when assuming the slab is supported at each end by the wall which is far too large.
My question is whether...

My assumption that the beam is supported at the ends is incorrect and that the slab is infact supported throughout its span by the bedrock(compacted shale). Therefor the moment throughout the slab is just the moment that is present in the toe of the wall due to overturning  which is 450kNm which is then much more acceptable.

Or is the moment in the slab just due to the force created by the weight of the wall over the length of the span?

Your help would be greatly appreciated


Hi Sean,

Assuming that there is no uplift pressure on the floor slab (due to external groundwater, when tank is empty), then yes you can assume no moments in the slab span and just cater for the localised moments due to wall base forces.

If you have uplift (which can result in flotation!), then you will have higher negative moments and will need to find some other solutions (thicker slab or anchors into the rock at regular intervals).

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Arshad Khan


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.


Employment history: 40 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.

.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)

•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 •2013: IEK International Conference, Kisumu, Kenya

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 2010)

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