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Civil Engineering/Simply Support Shell elements


QUESTION: Assalam o alaikum,
Dear Sir,
I am to model a basement in etabs. Basement has 4 side walls and a slab at top. If I model slab as it as at top of walls, it behaves as partially fix support and produces moment at connections between slab and walls. But i want to release this moment.I want my slab to behave as Simply Supported and want my walls acting as Propped Cantilevers instead of both ends fixed. Also suggest me how to provide same connection in real by detailing of reinforcement. I will be very thankful to you.
Jazak Allah.
Thanks and Regards,
Waqas Haider

ANSWER: Hi Waqas,

If you do not want the slab to have moments at the edges, then model it as a  membrane, rather than shell element. That way, the slab has no out-of-plane stiffness but will have in-plane stiffness to provide propping to the walls.

In practical terms, you can provide either a concrete lip to the top of the wall or a bench, so that the slab can sit on the wall but restrains it from lateral movement. An alternative method is to provide vertical dowel bars at regular intervals, and possibly a rubber pad between the top of the wall and soffit of the slab, to permit slight rotation of the slab in the vertical plane.

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

QUESTION: Hello Sir,
Thank you so much for your quick response and detailed answer. I furthur need to ask will it be ok to model a shell element like a membrane because i want my slab to take moments so that i may read span moments (not edge moments) from ETABS and design steel of slab accordingly. Moreover, can i provide same detail of steel for pinned connection as we provide in case of footings?
Thank and regard,
Waqas Haider

Hi Waqas,

In that case you will need to do 2 models, the second one with slab as shell and you can reduce the m11, m22, and m12 stiffnesses of the walls to a value of say 0.01, so that they do not provide much edge restraint (moments) to the slab.

Yes, you can detail the slab edge like you say.

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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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