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Civil Engineering/Mass Vs. Weight in ETABS

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Expert: - 4/11/2010


Question
I am using ETABS for modeling an irregular multi-story structure. Seismic load lateral system for the structure is a dual system consisting of special moment resisting frame and special concrete shear walls.
I am using automatically generated seismic loads as per UBC 94 for static loads and I am also conducting response spectrum analysis for the structure. I have faced a problem.
Actually, the weight that is used by the software to calculate base shear for the structure is not consistent with the mass of the system that is reported. The mass is much less than what it should be compared to the weight of the structure. I have noticed that the problem arises from shear walls. Indeed, mass of the walls are not correctly incorporated in the calculation of mass of each diaphragm.
I would be grateful if any comment can solve my problem.

Answer
Hi Arman,

The discrepancy could be due to several reasons, such as definition of mass source (menu>Define>Mass Source, where you should have the dead load plus 25% of live load), whether you have sufficient modes in order to capture at least 90% of the mass as per code requirements. Another problem could be due to any walls or ramps that are sloping, as sometimes ETABS does not pick these up when defining mass. Check out this forum:

http://www.eng-tips.com/viewthread.cfm?qid=150764

http://www.eng-tips.com/viewthread.cfm?qid=197253  

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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: 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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