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Civil Engineering/Relation between the thickness of a wall and its length

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Expert: - 11/2/2009


Question
If I consider a dam that is 10 meters in height then the pressure at the bottom of the dam will be around 98kN/m2. The material I select to build the wall should have a compressive? strength exceeding this pressure.

Now what I don't know is how the the length of wall influences its thickness. That is what other forces are acting on it?

Also am I right to say that the pressure on the wall is compressive rather than tensile as it is towards it? I am confused because the the pressure of the water is resisted by the bond formed by the wall with the ground at the bottom.

Thank You.

Answer
Hi John,

This depends on which type of dam wall you are designing- an arch dam or a gravity dam or a cantilever dam. In the former, the concrete wall arches in a horizontal plane against the sides of the valley(and can also arch across the  vertical plane to make it a double-arched dam), and all forces are more or less compressive across the section of the wall. Of course you also need to cater for the shear stresses.

If the dam is gravity, then more or less all concrete remains in compression and stability is provided by self-weight resisting over-turning and sliding.

If the dam is cantilever, then there will be compression on the dry side and tension on the wet side, so you will need rebar to resist the latter. Sliding will be resisted by anchorage into a deep foundation block. You will also need to cater for the shear stress.

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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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