You are here:

Civil Engineering/Stifness modifier

Advertisement

Expert: - 3/30/2011


Question
Dear Sir,
I am bit confuse in using stiffness modifier when to use and what value to use, i am designing by ETAB model mainly RCC buildings, can you please elaborate on this topic, Thanks.
I think we should use modifier for drift check only and specify value as col 0.7, beam 0.35, slab 0.25, wall 0.7/0.35 or these values can be readjusted but on what basis. the Other for designing directly from Etab model we should make onother model without stiffness modifier that is col,beam,slab,wall are 1.0 or these values can be readjusted to reduce percentage of reinforcement, is it allowed. Thanks.

Answer
Hi Farooq,

For drift limits, you are right about the factors quoted, although even here if the walls are shown to be un-cracked during initial analysis, you do not need to apply the walls factor.

There is, however, divided opinion about this subject for design. Some designers advocate using 1.0 for columns/beams (but slabs should always be 0.25, as they greatly influence beams/columns design if the full stiffness is used- another way is to define slabs as membranes), whilst others advocate using reduced stiffness if the elastic analysis shows that the members are likely to be cracked.

Please also visit the website http://www.eng-tips.com/viewthread.cfm?qid=78527

Civil Engineering

All Answers

Answers by Expert:

Ask Experts

Volunteer

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)

©2012 About.com, a part of The New York Times Company. All rights reserved.