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Fundamental Natural Period

 
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thirumalaichettiar
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PostPosted: Wed Dec 01, 2010 1:09 pm    Post subject: Fundamental Natural Period Reply with quote

7.6 Fundamental Natural Period

7.6.1 The approximate fundamental natural period of vibration ( T, ), in seconds, of a moment-resisting frame building without brick in.fd panels may be estimated by the empirical expression:
T. = 0,075 h07s for RC frame building
= 0.085 h075 for steel frame building

7.6.2 The approximate fundamental natural period of vibration ( T, ), in seconds, of all other buildings, including moment-resisting fimne buildings with brick infill panels, may be estimated by the empirical expression:
Ta=0.09h/sq.root of d
Will these expressions hold good for the MRF for the structures shown in the attachments?

If different please furnish.

T.Rangarajan.



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Kumar_Abhishek_Singh
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PostPosted: Wed Dec 01, 2010 2:03 pm    Post subject: Reply with quote

Dear Mr. Rangarajan,

The equations of IS-1893 that you have reproduced here are emperical equations(as I have been educated).

The first one is solely dependent on the height of the structure and the second one is dependent on the height as well as the lateral dimension  (in the direction of earthquake).

As far as the configurations you have furnished, as per code, all the buildings of same height have the same time period no matter what other parameters are except for the presence of retaining walls of basements uptill ground floor or not .This I say for the first equation.

ALTHOUGH I PERSONALLY DO NOT ENDORSE ANY OF THESE TWO EQUATIONS, I have to follow them as they are a part of our code.

As per the paper by Dr. Sudhir Jain Dr. Arlekar (1997) the actual time period of buildings of same height differ with the presence or absence of the following:

a. shear core
b. brick walls in different configurations.
c. column orientation.
d. story stiffness

So what i perceive is that if you have to design as per code then you will have to use either of the two equations depending on whether you want to take the resistance provided by the infill walls to deflection or not.

As far as the true time period is concerned( which can be evaluated using various software packages), they all will differ. Secondly, the building in hilly area that you have shown is very dicey, but what "I" think is that the drift on the right hand side, that is towards the hill face will be lesser(UNDER LATERAL LOADS) as compared to the drift on the other side as, the supports are at different elevations, thereby providing restraint at successive storeys. I think it somewhat same like a portal frame with fixed legs with one leg smaller and other larger. The frame tends to deflect more towards the longer leg,(as the longer leg is more slender) if a point load is applied at the center of the beam which is supported by these two legs. How the time period will be evaluated, I am not very sure.

All of the above are not COMMENTS but OPINIONS.


REGARDS,


KAS
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thirumalaichettiar
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PostPosted: Wed Dec 01, 2010 4:11 pm    Post subject: Reply with quote

Er.Kumar_Abhishek_Singh,

I appreciate your quick response on the above posting.

I have noticed that the above equations are valid only for regular and almost regular and symmetrical both in plan and elevation.
I hope after getting more response I shall give one reference where it is elegantly described about it.

Also I noticed in doing an analysis using STAADPRO software for a shear wall frame the value of Ah of Vb=AhW is not even close the manually calculated value and as well as the time periods.

We are simply follow the software with no knowledge how it was programed. I feel as always we recommend to check the validity of the out put of the software by checking some value manually. In the above case the total weight W calculated is exact but the time period and Ah are different. So I tried to figure out and then came with this doubt.


T.Rangaarajan.
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sukanta.adhikari
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PostPosted: Thu Dec 02, 2010 4:30 am    Post subject: Re: Fundamental Natural Period Reply with quote

Respected Rangarajan sir,

The approximate fundamental time period of vibration as per IS 1893(part 1)-2002 are for regular structures only.

However IS 1893(Part 4)-2005 provides  some guidelines for calculation of time period of structures..which can be tried for the structures attached in ur post.

As per 1893(Part 4)-2005..

9.1       Time Period Estimation

The time period of different industrial structures would vary considerably depending on the type of soil, span and height of the structure, distribution of load in the structure and the type of structure (concrete, steel and aluminum). It would be difficult to give one or two generalized formulae to cover all such structures.Accordingly, no simple guidelines can “be given for estimation of time periods of industrial structures.

9.3.1 The time period -shall be estimated based on Eigen value analysis of the structural mathematical model developed in accordance with 9.1 and 9.2.


9.1    Modelling Requirements

The mathematical model of the physical structure shall include all elements of the lateral force-resisting system.


In STAAD instead of using ST (type of structure) command..Px and Pz can be used.


For obtaining Px and Pz....a free vibration analysis can be carried out in STAAD using the command " Modal calculation requested" and the result in form of frequency,ie, eigen values (thereby its inverse as time period)  corresponding to highest mass participation in that particular mode can be used as the values of Px and Pz.

I think this can be tried sir.


Regards,

S.Adhikari
thirumalaichettiar wrote:
7.6 Fundamental Natural Period

7.6.1 The approximate fundamental natural period of vibration ( T, ), in seconds, of a moment-resisting frame building without brick in.fd panels may be estimated by the empirical expression:
T. = 0,075 h07s for RC frame building
= 0.085 h075 for steel frame building

7.6.2 The approximate fundamental natural period of vibration ( T, ), in seconds, of all other buildings, including moment-resisting fimne buildings with brick infill panels, may be estimated by the empirical expression:
Ta=0.09h/sq.root of d
Will these expressions hold good for the MRF for the structures shown in the attachments?

If different please furnish.

T.Rangarajan.

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thirumalaichettiar
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PostPosted: Thu Dec 02, 2010 4:39 am    Post subject: Reply with quote

Dear Er.S.Adhikari,
Thanks for your comments.
I will collect the formula which I informed in my previous posting and post it since we have to check the period calculated by software through some manual calculation. As a rough guide need a formula which may be useful for office work.

T.Rangarajan.
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Kumar_Abhishek_Singh
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PostPosted: Thu Dec 02, 2010 8:59 am    Post subject: Reply with quote

Dear Mr. Rangarajan,

Could you kindly upload the "time period" reference document you were talking about??

rgds

KAS
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thirumalaichettiar
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PostPosted: Thu Dec 02, 2010 2:58 pm    Post subject: Reply with quote

Dear Er.
Kumar_Abhishek_Singh and Er.S.Adhikari
,

Please find attached the formula for the fundamental time period and notes . May be useful to those interested.

T.Rangarajan.



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thirumalaichettiar
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PostPosted: Sat Dec 04, 2010 5:50 am    Post subject: Odd Shaped Buildings. Reply with quote

Need experts to help to find the

a. Funamantal period  for the strcture attached for seismic analysis?

b. Whether we can use the formulae furnished in IS 1893-2002 as given in my first posting under this topic?

c. If not shall we use the second one in my last posting? What is the depth d (base dimensions) to be considered in the formulae?

d. We have to use Response spectrum method.  By this analysis let the software calculate the time period?

e. How can we check manually the Base shear Vb?

f. I will be posting some more odd shapes for thinking.d

Hope this will help  to solve for  seismic analysis and design for odd shaped buildings like this?

T.Rangarajan



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thirumalaichettiar
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PostPosted: Sun Dec 05, 2010 12:01 am    Post subject: Reply with quote

No reply from anybody!!!

I attach more odd shapes for thinking.
Post your comments pleas!

T.Rangarajan.



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