Civil Engineering/Foundations

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Expert: - 8/11/2010


Question
dear sir,
if i have 25' by 14' bay and i want to design a footing for one of its columns (assume all columns are edge cols.and footing cannot go beyond property line)...total storeys are 4...col size is 14"x18" and beams are 12"x24". from my etabs model the load at base is approx...200kips...(yes masonary 9" walls on beams also)...
200 kips is very small load...soil capacity is say 3ksf...if my footing is concentric then footing size is 66.67 sq. ft. but the case is my footing eccentric due to property line restriction...now if i provide strap beam for 25' span it becomes 3 feet deep...if i provide combined footing it becomes 3' deep by 6 feet wide and if i try to design found as inverted t beam the depth of web exceeds more than 60"...my conc strength is 3ksi...steel 60ksi...

i want to ask...from ur experience, for this span and loading....what u think rougly the footing sizes will be...and which type is preferable....by the way seismic zone is 2B...'

this buidling was previously design by another engineer and he provided just 36 square feet area for each footing and connected the footing with 1meter deep straps....which was only 220mm wide....(he used bs codes)...

pls help

Answer
Hi Waseem,

This question is a bit too long and complex to help you in detail, but form my experience an eccentric footing with a strap beam should work best- you can also place the footing a bit deeper and take advantage of the weight of the soil and ground-floor slab above to mitigate the out-of-balance moment. I trust you have also taken account of the dead weight moment caused by the frame beams fixed to one side of the edge column?

Another thing you can do in order to equalise the pressure is to design a trapazoidal footing, with the wider edge next to the property line.

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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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