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Civil Engineering/Slab behaviour in etabs


QUESTION: dear sir, i am putting slabs on beams as membrane and flat slabs as shell in etabs...and design the slabs only on gravity in SAFE...but in actual case i think the slabs have out of plane stiffness too and it will help with drifts..and even in SAFE slab is not memrane rather its what would you recommend...if shell behaviour is more suitable then what is the need for a membrane slab option in ETABS?

ANSWER: Hi Waseem,

Yes, slabs can help in drifts but they tend to crack, so you should apply stiffness modifier of 0.25 if you adopt shell slabs. For membrane there is no need to apply modifiers- membrane is useful if you have very limited slab stiffness (such as ribbed RC or hollow-rib steel decking slabs) in one direction (i.e. at right angles to span of the ribs).

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QUESTION: means we should apply a shell for considerable stiff slab.. Such as 200mm thick.. With crack modifiers.. But sir If I am designing the slab only for gravity SAFE then what do you think I should model the slab in tabs or not? Because in ETABS I have lateral loads and slab act as a diaphragm so should carry the axial forces from lateral loads too...? But in SAFE I am designing only for gravity.. What you wills say?

Hi Waseem,

If it is a flat slab then additional moments/shears from lateral deformation should be designed for even if using SAFE, unless all lateral loading is resisited by bracing or shear walls. If it is a framed structure, then you can use SAFE to design the slab for gravity loads only, especially if the slab in ETABS has been modelled as a membrane.

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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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