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QUESTION: My Question is
If a same building is designed for three different altrernates, which alternate would be the best adopted.
Building consists of large span halls at ground floor, spanning upto 70ft in one direction and 29 ft in the other direction. Only ground floor provides clear height of 16 ft. Upper floor have apartments and provide clear height of 10ft.
Structural Alternates opted were:
1-Providing deep beams at ground floor (for load of upper floors) and erecting pickup column from it for upper floors
2-Designing each floor for its own load as a waffle slab.
3- Provinding post tension beams at each floor
Please tell me what are the advantages and disadvantages of using these three alternates. And which alternate would be the best adopted based on safety and economy.

ANSWER: Hi Salma,

The best option depends on local market conditions- cost of materials and manpower and also local expertise in post-tensioning.

If the latter is available, it would be best to provide PT beams in the 70 ft direction for all floors. If not, then the 2nd best option would be a waffle slab for all floors. The option of planted columns at 1st floor level is the least desireable, as that should not be used in conjunction with PT beams due to complications with incremental increase in loads as the floors are built one after another above.

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

QUESTION: What would be max depth of beam, if we go for PT beams in the 70 ft direction for all floors?
and also please tell me that does the column size in case of waffle slab is more as compared to ordinary beam column slab? Please give me the reson?

Hi Salma,

The maximym depth depends partly on width, loading and materials strength, but as a rue of thumb, for a continuous PT beam you can assume a depth of around 4-5 feet.

The column size depends on loading, so whichever option is lighter will result in a smaller sized 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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