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Civil Engineering/Seismic Effect on Foundation


Dear sir!
According to ASCE-05 12.13.4 25% reduction in overturning moment is allowed. IBC states that its valid only for strength design level combinations. My question is for foundations we use 2 sets of combinations, first for design of reinforcement and it is ultimate. Second for geometric stability and that is service. So actually we are using both sets. Now i dont understand IBC refers to which case of combination. Can i reduce overturning moments by 25%? If yes then for design of reinforcement or for stability investigation only? or for both?

Hi Waseem,

See this article, which explains the intent:

In the past what I have done for foundation design is to check maximum ground bearing stresses during overturning load-cases and permit up to 25% over-stress at service load, should this be needed. This is on the rationale that seismic actions are rare and short-duration, so over-stress can be accommodated without any failure or excessive settlement.

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Arshad Khan


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.


Employment history: 40 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.

.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)

•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 •2013: IEK International Conference, Kisumu, Kenya

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 2010)

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