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# Civil Engineering/high rise building primary time period

Question
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
why???????
what should i do??????
where is the mistake ????
best regards

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.

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

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):

AUTO SEISMIC INPUT DATA

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

AUTO SEISMIC CALCULATION FORMULAS

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

AUTO SEISMIC CALCULATION RESULTS

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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