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Hussain Abid

Calculate T in ETABS

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First-off could you please phrase your question (typing) properly?


As in UBC, Ta = Ct(hn)^0.75

And Tb comes from dynamic analysis

(see UBC-97 Chapter 16)

For drift you dont need to satisfy limit of 1.4 or 1.3 times Ta.


Is this you are referring to?

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First Thanx 4reply.i thought Tb is calculated by computer itself more accurately coz Ta is an approximate method. My query was abt that point of view as to how to see results of either Ta or Tb.To make u n myself correct T is calculated 1.3 times Ta for drift also.see book WIND and Earthquake Resistant Buildings by Taranath Bunglae who is considered an authority 

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Yes, Tb is calculated by computer based on stiffness of building. The book you are referring to clearly explains what are you are asking.

1. Choose method of analysis: Static or Dynamic?

A. Static

Calculate Ta

Calculate base shear V based on Ta

Calculate Drifts based on Ta


2. Dynamic

Calculate Ta

Calculate Tb

For base shear V, use Tb but Tb should not be more than 1.4Ta (or 1.3)

For drifts disregard the upper limit and just use Tb without 1.3/1.4 limit.

Hope that clarifies.

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Yes sir i also meant the same that Tb cant b used in static method. Whereas i also got it clear that for drift the limit of 1. 3 or 1. 4 T shudnt b considered. Thnx

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Method A is an approximation to find out the time period of the structure without taking into account of structural properties, method B takes into account of structural properties and deformation characteristics.

In this regard, ETABS will use the loop of If _  and Else to check the upper limit of Time period, please see the attached summary.

For example your building height is z =32.81ft using ct= 0.03,

Method A gives Ta = 0.411 sec

When Etabs run analysis, it gives T = 0.86 seconds

Check the loop in the report of ETABS, because time period of structure can not be more than Tmax, so ETABS will use  Tused = 1.3 Ta = 0.411 x 1.3 = 0.534 seconds  (For Zone 4)

Hope You understand






time period.png

12-18-2017 2-16-48 PM.png

Edited by SALMAN CH

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I noticed that usually dynamic time period is always bigger than (1.4Ta or 1.3TA) so thats why Etab always take (1.4Ta or 1.3TA) and we  get more base shear  through eqivalent static method for normal regular buildings than dynamic analysis.  "what I understand is that in equivalent static method , code allow us to make ecnomical design of building by not taking method A time period  and on the same side it is making conservative design by limiting method b(dynamic T) time period to 1.4 times of method A. plz correct me If I am wrong."

Actually I was confused with csi manual lines shown in image I attached below. why it is saying that if "Tmode>1.4Ta then T=TA". Isn't it the contradiction to what Etab actually doing.?


My deep Thanks and regards. 

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@ZOHAIB SATTAR NAGRA.. Your first paragraph is not clear to me.

I feel your confusion is valid point, for UBC code,  ETABS takes T=1.4 Ta, If Tmode >1.4 Ta, and CSI manual tell T= Ta, which is contrast to ETABS.

However i have checked this problem does not exists for IBC/ASCE provisions. I think you should switch to ASCE, no one uses UBC now. 



12-26-2017 11-02-04 AM.png

Edited by SALMAN CH

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@SALMAN CH what I have explained in paragraph 1 is that when you got less time period then you will got high base shear basically time period is inversely propportional to base shear.and ubc bound us to not take time period greater than 1.4 Ta thus leading us to conservative design .


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On 12/19/2017 at 10:56 AM, ZOHAIB SATTAR NAGRA said:

I noticed that usually dynamic time period is always bigger than (1.4Ta or 1.3TA)

Who said so? For majority of the buildings (low rise) dynamic time period is way lesser than 1.4Ta. Read code to understand about these limits.


On 12/19/2017 at 10:56 AM, ZOHAIB SATTAR NAGRA said:

why it is saying that if "Tmode>1.4Ta then T=TA"

It should have been; if Tmode>1.4Ta then T=1.4Ta. Just a typo mistake i think, but inside ETABS it takes it correctly for both UBC and IBC/ASCE codes. Don't worry.


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On 1/10/2018 at 6:31 PM, Rana said:

Who said so? For majority of the buildings (low rise) dynamic time period is way lesser than 1.4Ta. Read code to understand about these limits.

this is my observation , I usually found first mode (eign vector) time period  to be greater than (1.4 x ELP Time period) for low rise buildings.SO thats what I was saying in previous discussion that code make conservative design by taking larger time period hence large base shear....mean while when we consider dynamic time period as in case of RS analysis we get less base shear so we get ecnomical design. @Rana @SALMAN CH.

I noted that (in case of ubc 97 in etab we always get 1.4 x method A time period in low rise.)

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