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Civil Engineering/In some portion of beam decreasing the effective Cover will effect the strength of beam?

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Expert: - 11/9/2009


Question
Here in our project some portion of roof inverted beam has to chipped off to fix the purlins(Inorder to get proper level).Top cover of beam is 40mm and we chipped off 20mm from top concrete surface.means effective cover is reduced to 20mm so i want to know wheather this will effect the strength of beam? For design purpose we are taking the effective depth(C/C distance of bar) Am i right?Waiting for your reply...........

Answer
Hi Javed,

If the portion of concrete you have chipped off is in the compression zone (e.g. mid-span of a simply supported beam), then yes you have effectivly reduced the compression zone and the effective depth of the beam, so its capacity will be reduced. You can do a simple calculation to work out the reduced bending and shear capacity.

In addition, you will have reduced the durability and fire resistance of the beam.

If the cover chipped off is in the tension zone (say bottom of SS beam, or top of beam near supports for a continuous beam), then the strength is not affected but the durability/fire resistance is.

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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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