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Civil Engineering/ETABS column reinforcement

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Question
Sir,
I am a structural engineer just joined recently in a firm, so new to this field, I have some doubts about the following.please clarify.
1.when i run the design  of a RCC building in ETABS,the model is designed and it displays the longitudinal Reinforcement area of all beams and columns, suppose i select and double click on a column and if click the flexure details or summary option it displays the re-bar area required for that particular element and other details such as moments and shear details etc.Now, when i click on the Envelope option after selecting a column and double clicking on it the new window displayed is showing 2 types of Reinforcement bar areas saying for example:

COLUMN END   Re-bar area:
TOP          4375 mm2
BOTTOM         10369.641 mm2

my doubts is what exactly should be taken as the actual reinforcement of the section, the maximum of the two or the sum of the two reinforcements? because flexure details say it is 10369.641 mm2?

2. my second doubt is if any of the section in ETABS DISPLAYS o/s but still displays the Design SHEET WHEN DOUBLE CLICKED ON THE Particular Item, DOES IT MEAN THAT THE SECTION HAS FAILED DUE TO OVER reinforcement ,but how can it fail due to over reinforcement when i have not specified he reinforcement the section should have..so what does o/s actually mean and what is its significance..?

Answer
Hi Shoyeb,

1. The reason you are getting different rebar at top and bottom must be because the moments must be very different. You can decide to detail bottom half with higher rebar of the full height, depending on how the rebar is working out above that level too.

2. For design, you have to choose the code to design to- for example if you select BS 8110, the maximum rebar percentage is 6%. So if the design rebar comes out higher, ETABS will report o/s. The same applies if the shear stress or links %age is higher than permitted (that usually applies to beams).  

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Arshad Khan

Expertise

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: 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.

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 •2013: IEK International Conference, Kisumu, Kenya

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 2010)

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