Civil Engineering/etabs modelling


       Normally in ETABS in beam and slab system we model slab as membrane. For most cases it holds good. And in slab design we assume slab panels supported between the beams. Is this a correct approach since long span beams will be divided by secondary beams and we consider slab panels supported between those secondary beams,which actually will be a sinking support.

      So can we go by modelling slab as shell elements and design for the slab moments generated by etabs. which may also show sagging moment at the beam supports  depending on the beam stiffness and concentration of moments at the column supports Which I feel to be more realistic. Please correct me.

ANSWER: Hi Ouseppachan,

If you have one-way spanning slabs, then just model main beams and secondary beams and use the special one-way membrane elements for the slabs.

If you have 2-way spanning slabs, then use shell elements but make sure you mesh the elements into at least 6 sub-elements in each direction, otherwise the load distribution on to the beams will be incorrect.

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

Thanks for your reply

But In one way spanning slabs also slabs will deflect along with the beam creating more moments in the longer direction than the shorter direction if we go with shell modelling.
If we design with this output will it be correct instead of going with codal procedures and fully depending on FEM.

Hi Ouseppachan,

Computer modelling is always an attempt to simulate as best as possible actual behaviour and is never 100% accurate. In your case, it will depend much on relative stiffness of beams and slabs, so I suggest you do some test models and look at the upper and lower bound results before deciding how to proceed.

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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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