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Civil Engineering/Reinforce Concrete Thickness-Please Help

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Expert: - 11/14/2008


Question
QUESTION: Dear Sir,
  I have a problem in which I know the forces on a suspended reinforced concrete slab and the width and length, but need to determine the thickness.  Is there a formula for this?  The width is 13', the length is 20', the Imposed load is 520 kN and hte dead load is 1730 kN.  There is a uniform imposed load of 5kN/m2 (100 lbf/ft2).  Finally, the allowable bearing pressure on stiff gravely clay is 250 kN/m2 (2.5Tons f/ft2).  I really appreciate your help.  I am also expected to determine the reinforcement required for the slab.

ANSWER: Hi Aaron,

Yes, there are formulae for this, which you can see in the following website:


http://bccw.cityu.edu.hk/rc.design/slab_design.asp

The thickness of the slab will be around span/25, so in your case it will be about 6-8 inches, depending on which way it is spanning.

I am not sure why you have mentioned the ground bearing pressure, as that is only required for design of foundations or ground bearing slabs.


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

QUESTION: Dear Sir,
  Thank you for your response.  As I am new to this, I need to actually figure out what formulae I would use for these calculations.  I need to know what "span/25" means and where it comes from.  Also, how would I figure out what reinforcement would be needed for these slabs.  Finally, is there a different formula for thickness of a slab that acts as a ramp?  Is there one for Shear and moment? I'm guessing the forces change.

Answer
Hi Aaron,

The fonmulae are quite complex, so you will need to refer to guides such as BS 8110, or visit the website I gave you last time and see the examples:

http://bccw.cityu.edu.hk/rc.design/example_10.asp

This has all the details of how to calculate rebar, the formulae, etc.

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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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