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Civil Engineering/Dual System on Etabs

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QUESTION: Sir,
I have to model a dual system in ETABS.In this regard,I have to assign 25% lateral forces to columns using same dynamic characteristics of the system that include shear walls.While doing this I control the seismic forces to 25% by seismic coeff and use actual "t" of original system.Such modeling is performed to distribute all forces in columns eliminating stiffness of shear walls,but I found that elimination of shear wall's shear and flexural stiffness only is not possible as they are coupled with axial stiffness that cannot be eliminated.
Therefore,I am alternatively using a link element with axial stiffness only (equivalent to AE/L of walls)in place of walls.
Is this approach sounds correct?or do you have any better strategy for modeling of dual systems on ETABS?

ANSWER: Hi Syed,

You cannot "force" a building structure to behave as a dual system if the relative stiffness of the shear walls and the MRF (moment resisting frame) is not conducive to that. So basically you need to model the building as accurately as you can, analyse it and study the base shear and how this is apportioned between the shear walls and the columns. If the sum of the columns base shear is less than 25% of the total, you have two options:

1. Make the shear walls less stiff or the MRF more stiff (deeper beams or bigger columns, or better connectivity into the shear walls)

2. Design the building as a shear wall/bearing wall system.

Your approach of assigning 25% of lateral loading to columns is not correct, as the lateral loading will be distributed to shear walls too through diaphragm action.

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QUESTION: Sir,
I have adopted forced distribution approach from "SEAOC seismic design manual"(images attached).They proposed to analyze the frame independently
for 0.25V regardless of its proportion in actual system and finaly consider the frame design forces maximum of either the independent frame analysis or actual system analysis.Their approach seems to make frame capable of resisting 0.25V regardless of its actual proportion that can be achieved by forced distribution only.
As long as ETABS modeling is concerned,I am replacing shear walls with link elements (axial stiffness only)to account for walls support for axial forces having no contribution in seismic shear.
Kindly go through the document (attached)again and make a conclusive decision.I shall be so grateful to you.

Answer
Hi Syed,

I guess the approach mentioned there can be used, but you should also read the caveat in the commentary at the end.

If you want to proceed then yes, use the links method, but then you will not be able to accurately model the MRF interaction with the shear walls- in other words if the beams framing into the links are connected as hinges then your MRF will be too flexible, and if it is fully fixed, even then it will be too flexible as the links have no bending stiffness along the plane of the frame?

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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: 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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