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Civil Engineering/Column and Beam Modifier

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Expert: - 1/8/2011


Question
QUESTION: Sir,
My question is that in Acia code 318-02-8.5.1(b) modifier is there for column,beams & slab.
in Etabs for beam axis 3 0.35 ,Axis 2 0.35 and torsion is 0.01 shoud we used this value for analysis and desisgn or if we take a result from etabs for anlysi and simply design a beam and check for deflection.my Question is that why we used this modifer in etabs for simple g+1 beam column structure and what is the purpose of use this value.
Thanks

ANSWER: Hi Hamood,

You are right- there is no need to use modifiers for a G+1 building. These are required more for tall buildings or long spans, where cracked sections have a significant global effect.

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

QUESTION: sir if for high building why we uses this modifier, if we use etabs simply for analysis and design manuuly means get moments from etsb and then design mannuly. is ti true or not and 2nd question what is the purpoese of this modifier, my openion is that if we use for deflection then we use cracking moment of inertia so we use this modifier value other wise no need.
second is that if we use modifer in model it means that etabs not consider fully section for design of steel it conside cracked section for steel which is wrong.kindly guide me
Thnaks a lots for your nice cooperation.

Answer
Hi Hamood,

You are correct- for strength, or ultimate limit state, you should use the results from the uncracked analysis. The cracked analysis is mainly for servicibility checks- such as lateral sway or vertical deflections and also for dynamic analysis when considering occupants comfort (acceleration limits), which become significant for tall and/or slender buildings.

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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: 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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