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Civil Engineering/Concept of Unit width Design

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Expert: - 3/17/2009


Question
QUESTION: Hi Arshad,

we have multiple cell reinforced concrete box section(tunnel) containing a huge fill of varying depth along length of 22.5m. For the purpose of analysis, we make 2D planar model of unit width and apply a load of fill on the basis of "average depth". Since the unit width concept is applicable to sections having uniform loads, can you please verify our design approach?. in case, if this is non-conservative than can you please advise on the other approach?

Extremely Urgent! many thanks for your support.

Kind Regards

ANSWER: Hi Agha,

Yes, this is a reasonable approximation, but do ensure that you have sufficient rebar along the longitudinal axis of the tunnel as well, to take up differential stresses due to different loading depths.

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

QUESTION: Excellant, on the longitudinal direction we have high rebar from early thermal cracking requirements. can you please advise how can we calculate differnetial stresses? or is there any thumb check we can make quickly to cross check proportion of differential stresses against existing rebar.
Manu thanks. (Urgent)


Answer
Hi Agha,

One way to check this would be to take the loadings for 2 different unit widths, say 4 or 5m apart and then analysing the section longitudinally for the differential loading/stresses. From what you are saying, probably this will not be a problem.

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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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