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Civil Engineering/SAFE Punching Modelling


An isolated foundation is modelled in SAFE with basement slab, raft thickness is different than isolated footing. SAFE checks for punching at location d/2 and at the junction where thickness changes. What are the assumptions for applying stiffness to these both areas. We have experienced that if spring stiffness is more under isolated foundation as compared with raft then it passes punching and vice versa. What are the practical assumptions to model this behaviour.

Hi Waseem,

The usual procedure is to reduce the spring stiffness around the periphery of a raft (or individual pad) to allow for the fact that the soil there is no as well "contained" against lateral/slip movement as it is below the foundation.

Extending that argument for bases within a raft, there is no logical reason for reducing the spring stiffness as you have indicated. So, you should either design the bases to support all column loading, or design the whole raft of uniform thickness, to spread the loading- I appreciate that punching shear may govern, but you can relieve the net punching force by deducting the ULS reaction within the 45 degree spread under each column.

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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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