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Civil Engineering/Boundary element length in Etabs

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Expert: - 1/7/2011


Question
Hello Sir,
When we design shear walls for boundary elements, the criteria we follow as per ACI 318 Chapter 21 clause 21.7.6.3, if extreme fibre compressive stress is greater than 0.2f'c. We then provide a length of boundary element larger of (c-0.1lw) or c/2 as per 21.7.6.4, where c=depth of neutral axis from extreme compressive fibre, and lw is length of wall.
In my case the extreme compressive stress is greater than 0.2f'c, but is almost constant throughout the c/s of wall. According to clause I have to provide Boundary element, but in this case 'c' would be very large as the neutral axis would be much outside the wall c/s. In this case how do we calculate length of boundary element.

Answer
Hi Shekhar,

In this case ensure that you have sufficient rebar to cater for all vertical stress throughout the wall, and then just treat the outer edges as boundary elements (with extra vertical rebar and extra closely-spaced links; the width of boundary elements can be taken as 2T, where T is thickness of the wall.

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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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