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Civil Engineering/Diaphragms extent and wind loading in ETABS

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Question
Dear sir,
I understand diaphragms are for in-plane stiffness when we talk about lateral loads and to transfer lateral loads. I am talking about this concept rather....

Here I want to ask about the wind loading applied to stories in ETABS...For that you have to have a diaphragm for ETABS to calculate the exposure widths...

My question is if i have a rectangular structure...down to top full concrete wall at perimeter...but inside that the actual structural is only at the middle...in the center...this is like an idealized structure to ask the question!

How would we then apply wind loading in ETABS...if we apply diaphragm only on the main structure and leave the perimeter wall  then how we will load perimeter wall with wind?

If we apply diaphragm(in this case semi rigid may be) to all the area including the open space between the perimeter wall and the middle structure...is it more realistic.?

Thank you!

Answer
Hi Waseem,

As you correctly note, for ETABS auto-wind calculations, you need to have diaphragms at each level. In cases where the structure has voids (or say you have a double volume atrium), you need to work out the wind loading manually and apply it at each level- either as a line load on beams or point loads at the nodes.

Alternatively you can introduce vertical area objects (with reduced stiffness)along the building periphery and select that option when defining the wind loading within ETABS.

It is not advisable to introduce diaphragms where there are no floor slabs, but you can include points within your defined diaphragm.  

Civil Engineering

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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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