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Civil Engineering/Center of mass in Etabs

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Expert: - 1/3/2010


Question
QUESTION: Dear Sir,

Can u let know the difference between centre of mass and centre of rigidity.Also how to see these parameters in Etabs.

  Thankig You.


ANSWER: Hi Tanveer,

If you have a completely symmetrical structure in terms of slabs, columns, lift-core walls, slab thickness, etc, in both directions, then the centre of mass will be at the same place as centre of regidity. If you shift the core wall to one side, then both centres of mass and regidity will shift in that diection, but by different values, so that you will end up with an eccentricity, which leads to torsion when the building is subjected to lateral loading.

In ETABS, after analysis, go to Display>Show Tables> Analysis Results, then go to Displacemnets and Building Outputs and you will get the required info.

This forum below also gives further info, although it is for another software:

http://communities.bentley.com/forums/thread/39332.aspx

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

QUESTION: Thanks alot for ur informative answer.Now i would like to know on what factors do these parameters(centre of mass & centre of rigidity) depends.


With Regards.


Answer
Hi Tanveer,

Centre of Mass location depends on the geometry of the building, as well as distribution of weight of structure- for example if there is a thicker slab on one side (or an atrium opening on the other side), then the centre of mass will shift towards the heavier side.

Centre of regidily depends on the location of your moment resisting frames or shear walls or core walls or X-bracing. The centre of regidity will shift towards the stiffer side of the building.

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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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