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Civil Engineering/structural design + moment redistribution

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Question
QUESTION: sir..
i am new to e tabs and dont know how to model and analyse ramps and basement walls..
can you please tell me the steps involved in ramps and basement wall modelling and analysis..please..

ANSWER: Hi Hani,

Please go to the ETABS help menu and study the watch and learn tutorials- they are very good indeed about these subjects.



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QUESTION: Can you also tell me why buildings with basement are given raft footings always? Cant isolated footings be given in any case?

ANSWER: Hi Hani,

Usually for a basement you need an integral waterproof floor slab, so sometimes that doubles as a raft foundation. However, if the soil conditions are good you can also use strip footings for the walls and isolated pads for the columns.

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QUESTION: thanks a lot sir..i dont know how to repay you for taking out time from your busy shedule and helping us beginners.

i would also like to know about the concept of moment redistribution.what is it actually.Does it become an advantage while designing?

Answer
Hi Hani,

If you browse my past Q&A with keywords "moment distribution redistribution", you will get lots of answers. One such is copied/pasted below:

Questioner: hassan
Subject: moment redistribution
Date Asked: 2013-11-22 11:16:17
Date Answered: today

Question:
QUESTION: Hello eng.Arshad

First, I hope you forgive me if there are any mistakes in my English and I hope you to  response all my questions because they all are very important for me :

1 - On the subject of redistribution of moments , as we use reinforcement  - at  support - to resistant the moment which we got by elastic analysis ,in this case , are cracks formed ?! And if no cracks are formed Do I need to re- distribution of moment at that?

2 - After analysis in Etabs ,  if we design without redistribution of moments , the results will be acceptable ?

Note : I did not understand the sentence -"you can just analyse and design without it, so long as the joints are ((buildable)) " - which you already said in an earlier response ,for the same question .

3 - Is not it very difficult to re- design the distribution of moment for a full tower - as example - and design manually after analyzing in Etabs ? ! Is not there certain conditions and considerations enable me to design in etabs without redistribution of moments ?

4 - If it is necessary to redistribute moments , what is the  range of percentage of  moment  distribution which I have to chose ? Am I free to any chosen value within the allowable limits ? Is there a certain way or reference by which I can calculate the percentage accurately ?


Sorry for the inconvenience and  my long questions Thank you very much Best Regards

ANSWER: Hi Hassan,

1. Cracks are always formed because when there is stress in rebar it strains (stretches) and the concrete cannot strain the same amount without cracking (unless the section is much over-reinforced). Remember that the  modular ratio between rebar and concrete is around 12.

2. Yes, but usually that leads to higher rebar percentage at supports, and so long as there is enough room to accommodate all the rebar at the joint and have it properly anchored past the columns rebar (i.e. it is buildable), then that is fine.

3. You need to be careful for a towers that only moments from gravity loading are redistributed, not moments from lateral loading (which are reversible). For MRF the maximum redistribution allowed is 10%.

4. The usual range is between 10 to 25% but there are rules and restrictions according to different codes- for example, in BS8110 refer to clauses 3.2.2, 3.4.4.4, 3.5.3.6,  and 3.12.11.2

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QUESTION: Thank you very much eng.Arshad and also I have only two questions .

1-  which from  (etabs or sap or safe) has "redistribution" of moment in its options , and if one has it , please tell me where is this option in the program .

2- in towers  -I know that only moments from gravity loading are redistributed- but my question was : Is not it very difficult to redistribute the moment for all stories , especially in case different stories ? usually do engineers do it in towers ?

Answer:
Hi Hassan,

1. I do not use SAP or SAFE much, so am not sure about that. I know ETABS does not have that option in the design modules, but for gravity loading alone (i.e. without moment reversals at supports) you could simulate redistribution by assigning partial moment releases at the supports.

2. It is not too difficult if you use a spread sheet, and I am not sure how many engineers do this- I certainly do, for reasons mentioned earlier.

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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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