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Civil Engineering/Etabs and SAP torsional mode

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Question
Floor plan
Floor plan  
QUESTION: Hello Mr. Khan.

I am fairly new to both SAP2000 and Etabs. So far I have followed CSI tutorials and videos, so I am pretty confident that I am modelling my structures correctly.
My model is an 8 story building with shear walls and a concrete core. It is Rectangular, 48X30 m but symmetrical about both axis.
In both programs, the 1st mode shape is bending in y-dir but the 2nd mode is torsional rather than bending in x-dir, which I find odd.
My fundamental periods seem O.K (about 0.8 sec).
I've checked higher mode shapes looking for unconnected elements, I've verified base reactions and total mass. The loads are uniform and all stories are identical.

I would appreciate your opinion on this matter.
Thank you in advance.

ANSWER: Hi Itzik,

The only reason I can guess is that you may not have any peripheral stiff beams connecting the outer shear walls, so they are un-coupled and hence torsionally not very stiff?

Try introducing the beams and see if that makes any difference. Alos make sure you have modelled the lintol beams across the lift shaft doors.

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

Mass source
Mass source  
QUESTION: Thank you for your quick reply.

I have added the beams but their effect was only a minor change in the fundamental periods. The lintel beams in the cores already appear since the core is modeled as a shell element with openings.

Since this phenomenon also applies to my other etabs models, including 3d-frame structures, I wonder if the problem lies with my mass source definitions.
I've tried to define mass once from elements, and once from a static load case for "Self weight" but saw no change in the results.

Perhaps there are other definitions I'm missing?

Again, thanks in advance.

ANSWER: Hi Itzik,

Try making the walls stiffer and the slabs lighter (if possible)and evaluate the effect of that. I suspect that you have too much mass far away from the torsionally stiff core.

Just to verify that, you could try and place the corner walls without a gap (i.e. forming an L on plan at each corner). I am sure that will have a major impact!

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

2nd mode after changes
2nd mode after changes  
QUESTION: Again, thank you for your quick reply.

I have tried making the slabs thinner and the walls thicker. I have also moved the walls to the corner and added new walls along the core's axis.
All of these changes make the model stiffer, however its behaviour is still unchanged: 2nd mode is the torsional mode.

Answer
Hi Itzik,

This is very strange indeed! One other thing to check on is your connectivity between the floor slabs/beams and the core, and check that your diaphragm is defined as semi-rigid.

Finally, just to check a hunch I have, add say 4-5 extra floors and check the modes after that.

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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