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Civil Engineering/high rise building primary time period


QUESTION: hello dear expert
i would like to ask about the range of seismic time period of high rise building
i am modeling a concrete building (tall building 26 story =83.25m)
the lateral resisting system consists of 2 internal cores,
when i calculate the time period using Rayleigh formula the period = 7.3 sec , and when i calculate the period from response spectrum analysis the period is equal to 7.6 sec .
this is too long period more than 2 sec
what should i do??????
where is the mistake ????
best regards

ANSWER: Hi Farah,

For your building the primary period should be between 2.5 and 3 seconds, so I would check your formulae and calculations. Have you taken account of the difference between weight and mass? You need mass (not weight) for the calculations for time period.

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

Rayleigh formula
Rayleigh formula  
QUESTION: Hello dear expert
thank you for your attention
Rayleigh formula which i used is as the attached image
it involves g
i still do not know where is the mistake

Hi Farah,

I am not sure where the problem is, but normally the manual calculation for working out T using UBC is as follows (this is from ETABS, for a 34 storey building):


  Direction: X + EccY
  Typical Eccentricity = 5%
  Eccentricity Overrides: No
  Period Calculation: Program Calculated
  Ct = 0.02 (in feet units)
  Top Story: LEVEL34
  Bottom Story: GFL00R
  R = 4.5
  I = 1
  hn = 140.775 (Building Height)
  Soil Profile Type = SB
  Z = 0.2
  Ca = 0.2000
  Cv = 0.2000


  Ta = Ct (hn^(3/4))
  If Z >= 0.35 (Zone 4) then:        If Tetabs <= 1.30 Ta then T = Tetabs, else T = 1.30 Ta
  If Z < 0.35 (Zone 1, 2 or 3) then: If Tetabs <= 1.40 Ta then T = Tetabs, else T = 1.40 Ta
  V = (Cv I W) / (R T)          (Eqn. 1)
  V <= 2.5 Ca I W / R          (Eqn. 2)
  V >= 0.11 Ca I W          (Eqn. 3)
  If T <= 0.7 sec, then Ft = 0
  If T > 0.7 sec, then Ft = 0.07 T V <= 0.25 V


  Ta = 1.9926 sec
  T Used = 2.7896 sec
  W Used = 468492.50
  V (Eqn 1) = 0.0159W
  V (Eqn 2) = 0.1111W
  V (Eqn 3) = 0.0220W
  V (Eqn 4) = 0.0569W
  V Used = 0.0220W = 10306.83
  Ft Used = 2012.63

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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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