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Civil Engineering/Reponse Scaling for Drift


Dear sir, I know for response spectrum analysis we make the initial scale = Ig/R according to csi in ETABS. According to ASCE drift values should be multiplied by Cd/I. My confusion is I multiply Cd/I by the drift i get from etabs analysis or I multiply Cd/I again in the response scale factor in settings? Both the procedures give me different resutls. Please guide.

Hi Waseem,

For RSA you need to apply scaling factors so that the base shear is within 90% of that from a static analysis. This scaling can be higher or lower than 1g/R, but you should start with that factor and adjust as necessary.

Once you obtain the analysis outputs, the sway deflections should be multiplied by Cd/I to arrive at inelastic maximum drifts to compare against code limits.

In ASCE 7-05, table 12.2-1 you will note that in most types of structures the value of Cd and R is the same. R reduces the base shear by taking ductility into account for energy dissipation, but of course the sways need to be considered for inelastic cases, hence you increase them by the Cd factor.  

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Arshad Khan


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.


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.

.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)

•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

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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