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Civil Engineering/Temperature Shrinkage

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Question
QUESTION: Sir,
When we say temperature shrinkage (assume water tank thick walls restraining top slab) does it mean that shrinkage is more critical then expansion? Why we dont consider temperature expansion?

In my view when slab contracts, there will be cracks near wall supports and
When slab expands, there will be cracks at center of slab?

ANSWER: Hi Waseem,

Yes, both shrinkage and expansion are critical, as you say. However, usually expansion creates compression within the RC structure, which the concrete can resist better than tension- hence in some cases shrinkage is more critical.

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QUESTION: So for example shrinkage is more critical because in my water tank it creates tension is slab. Now assume flexural cracks because of gravity loads on slab and then we add temperature load, which produces tension on slab, dont you think tension will help flexure cracks to close up?

Because right now we are analysing crack widths of slab based on flexural+axial tension, but I think my above statement counteracts this logic. Also I am asked to put modifiers for water tanks where as I think if I am restricting the crack widths to be 0.1mm then no need for cracked modifiers for walls. Whats your say sir?

ANSWER: Hi Waseem,

First of all you should not use modifiers for tanks, as the crack widths are to be restricted. As for the combination of bending and temperature shrinkage, the bending will only be beneficial in closing the cracks in compression zones (i.e., soffit near supports, and top surface at mid-spans).

The most critical areas (for cracks)will thereore be the soffit at mid-spans.

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QUESTION: Sir, Assume a deflected shape due to flexure. There are cracks in soffit at mid-span. Now assume 2 tension forces at either ends of this beam or slab. As we start pulling on either ends, the cracks generated due to flexure will start closing and reducing even at soffit of slab at mid-span.
Is this theory ok?

Answer
Hi Waseem,

Actually if you think about it, the opposite is how it will act. Assume that the beams are restraining any movement and the slab concrete is shrinking- this will make the cracks wider.

Of course if there is no restraint to the movement then temperature increase or decrease will have no structural effect on the slabs- such is the case where slabs are designed and detailed as sliding over the walls.

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Arshad Khan

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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: 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.

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 •2013: IEK International Conference, Kisumu, Kenya

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 2010)

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