Civil Engineering/Raft foundations

Advertisement

Expert: - 1/20/2008


Question
Dear Mr. Khan,
I would like to know how to calculate the thickness of the raft I require for a single storey restaurant that will be 25mx30m with a 4m deep basement into Soft brown CLAY (BC of less than 75kN/m2) with a water table from 1.5m - 8.5m below G.L. I have chosen a cellular lidded raft foundation but don't know how to calculate the thickness of the raft required or how to calculate the thickness for the inverted 'L' and inverted 'T' beams which will make up the cellular design. The walls will be of brick outer skin and blockwork inner skin. (Normal live and dead loadings assumed for the entire structure).This is for a section of my 2nd year Civil Eng. assignment. Thank you in advance for your help.
Regards,
Paul.

Answer
Hi Paul,

The best way for this is as follows:

1. Work out the total load down, and divide by the area of the raft- this gives a pressure of X kN/m2.

2. Either use a two-way slab design for determining the bending and shear distributions- then allocating these to the L or T beams according to their widths, OR...

3. Use a grillage analysis, whereby each of the lines of the grillage resist their own moments and shears, taking deflection compatibility into account.

I hope you understand!  

Civil Engineering

All Answers

Answers by Expert:

Ask Experts

Volunteer

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)

©2012 About.com, a part of The New York Times Company. All rights reserved.