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Civil Engineering/Ribbed Slab Cross-rib

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Expert: - 12/1/2009


Question
QUESTION: hi ...  i want to ask what is exactly the "cross rib" ? how it works ? and when we put it ??  but to clear the question i mean by cross rib that rib we put that rib which crosses all ribs in one way ribbed slab that they said it will less deflection and i want your oponion in it ... please quickly ...    thx

ANSWER: Hi Aladdin,

Yes, one cross-rib at mid-span of a ribbed slab helps to distribite concentrated loads that otherwise may only be carried by one rib. So this helps in making the slab more robust and also reduces localised exessive deflection. If the span is relatively small, then a cross rib may not be justified.

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

QUESTION: thanx for that ...
but if the one way ribbed slab will disribute the loads between two beams ... how will it less deflection if it is willt carry any load ...
and if it is carrying a load ... what will be the ratio of load will be taken by it ...

with thankfull

Answer
Hi Aladdin,

Perhaps you have not understood what I meant. A ribbed slab comprises ribs at say 450 to 600mm centres, connected by a relatively thin flange (topping). So, if you have a concentrated load along one rib (e.g. a wall parallel to the rib), without a cross-rib, most of the load will be carried by one rib closest to the wall load and this will deflect more than the adjacent ribs. With a cross-rib, the load will be shared by one or two ribs either side of the rib under the wall, and hence the overall deflection of the slab will be reduced.

The loading in the cross-rib can be determined by a detailed 3-D FEA model, but generally the moments are fairly nominal, with most of the load distribution conducted by way of shear action.

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Arshad Khan

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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.

Experience

Employment history: 36 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.

Organizations
.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)

Publications
•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

Education/Credentials
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 date)

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