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Civil Engineering/-ve reinforcement in cont. beams.

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Expert: - 7/28/2011


Question
QUESTION: Sir, As you know, -ve moments are generally much more than the +ve moments in a continuos beam in a framed structure. So, we provide more -ve reinforcements to counter these -ve moments. My question is that" HOW THE BEAM WILL FAIL IF NO EXTRA  TOP REINFORCEMENT IS PROVIDED"?
The cracks are sure to develop at upper parts of beam. But what will happen after that? No sagging can happen because +ve reinforcement is proper, No shear failure can happen, then HOW?
Manoj Jain

ANSWER: Hi Manoj,

As you may be aware, you are allowed to redistribute between 15-25% of support moments to the span, so long as the overall summation of moments (support+span) remains the same. With that, you will get some top cracking at supports, but the shear force remains the same and you need to design for shear as normal. If you reduce rebar at supports, you have to add extra rebar at the spans to make up for that, so that the beam moments/capacity remain safe.

I do not understand the last bit of your question... "then HOW".

---------- FOLLOW-UP ----------

QUESTION: Dear Sir,
The -ve reinforcement being insufficiant to resist -ve momements will cause cracks at sopports. The question is that HOW (practically) the beam fails. Will it drop down suddenly due to shear failure? Pl. guide.
Manoj

Answer
Hi Manoj,

What happens is that the top steel yields a bit, so beam rotates very slightly, with cracks in the concrete for the top half (or less)depth only, and this immediately sheds the moments to the span. The remaining concrete section of beam that is un-cracked is sufficient to provide shear resistance, together with the shear links (stirrups).

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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.

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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

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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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