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Time Period of the Structure
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Arunbabu.S
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Joined: 23 Apr 2008
Posts: 19

PostPosted: Tue Mar 30, 2010 9:01 am    Post subject: Time Period of the Structure Reply with quote

Dear Sefians  
I am analyzing a 33 storied building with dual system (Shear wall Framed Structure) in Etabs. In plan, the structure looks like an inverted T with base dimensions of about 40 m in X & Y. The height of the building is about 100 m. My queries are
<if>1. <endif>As my structure is with both Shear wall & Frame, As per IS 1893 which relation has to be followed to calculate Time period of the structure. [T=0.075h^0.75 (or) T=0.09h/Sqrt(D)]. At present I have taken an average of both the values considering my structure as partially infilled, is that correct.
<if>2. <endif>After analysis I have got the time period of about 6.8 seconds from Etabs. Is there any steps to reduce the Time period of my structure or any precautionary measures to be considered while designing the structural elements.  
Regards,
Arunbabu. S





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akjhacpwd
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PostPosted: Tue Mar 30, 2010 4:39 pm    Post subject: Time Period of the Structure Reply with quote

Dear Arun Babu

I feel it should be the second one because of presence of shear walls.

Secondly i am afraid the T shaped building in plan will face enormous torsion.


Best Wishes,
A K JHA

--- On Tue, 30/3/10, Arunbabu.S <forum@sefindia.org> wrote:

Quote:

From: Arunbabu.S <forum@sefindia.org>
Subject: [SEFI] Time Period of the Structure
To: general@sefindia.org
Date: Tuesday, 30 March, 2010, 8:53 PM

Dear Sefians
I am analyzing a 33 storied building with dual system (Shear wall – Framed Structure) in Etabs. In plan, the structure looks like an inverted T with base dimensions of about 40 m in X & Y. The height of the building is about 100 m. My queries are
1. As my structure is with both Shear wall & Frame, As per IS 1893 which relation has to be followed to calculate Time period of the structure. [T=0.075h^0.75 (or) T=0.09h/Sqrt(D)]. At present I have taken an average of both the values considering my structure as partially infilled, is that correct.
2. After analysis I have got the time period of about 6.8 seconds from Etabs. Is there any steps to reduce the Time period of my structure or any precautionary measures to be considered while designing the structural elements.
Regards,
Arunbabu. S





"This Message and its contents is intended solely for the addressee and is proprietary.Information in this mail is for L&T Business Usage only. Any Use to other than the addressee is misuse and infringement to Proprietorship of L&T ECC.If you are not the addressee please return the mail to the sender.L&T ECC DIVISION"







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prof.arc
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Joined: 26 Jan 2003
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PostPosted: Wed Mar 31, 2010 12:25 am    Post subject: Time Period of the Structure Reply with quote

Dear Mr. Arun Babu,
Such a tall building of 33 stories should be treated as a special
structure and analysed carefully. The empirical values of period do
not cater to such special structures. The fundamental period of the
structure must be quite long - at the tail end
of the response spectra. You have not mentioned the country or its seismic zone.
The earthquake forces must be smaller than that of wind.
Also, design could be governed by the sway - P-delta effect and so on.
You should try different modelling and find out the variation.
Definitely, more than one person or agency must analyze it
independently to be sure that nothing is amiss.
I am glad that you got the oppurtunity to analyse a special structure
and wishing you the best..
Sincerely
Prof. A R Chandrasekaran

On 3/30/10, Arunbabu.S <forum@sefindia.org> wrote:
Quote:
Dear Sefians
I am analyzing a 33 storied building with dual system (Shear wall   Framed
Structure) in Etabs. In plan, the structure looks like an inverted T with
base dimensions of about 40 m in X & Y. The height of the building is about
100 m. My queries are
<![if !supportLists]>1. <![endif]>As my structure is with both Shear wall &
Frame, As per IS 1893 which relation has to be followed to calculate Time
period of the structure. [T=0.075h^0.75 (or) T=0.09h/Sqrt(D)]. At present I
have taken an average of both the values considering my structure as
partially infilled, is that correct.
<![if !supportLists]>2. <![endif]>After analysis I have got the time period
of about 6.8 seconds from Etabs. Is there any steps to reduce the Time
period of my structure or any precautionary measures to be considered while
designing the structural elements.
Regards,
Arunbabu. S


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btgprasad
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Joined: 05 Aug 2008
Posts: 125

PostPosted: Wed Mar 31, 2010 6:57 am    Post subject: Reply with quote

If you consider slab elements as shell element, time periodwill be get reduced thus increases base shear. but ultimately reduce pt in swbecause sw will get stiff lateral supports at every levels. Program calculatedtime period dependent upon mass and stiffness so input decent pt in shear wallsin overwrite command before analysis(default 10mm dia@250mm).check if too many releases aregiven. try out with different combination/codes and study.
<o> </o>
I hope you considered following points in analysis
Proper load combination for p-delta,
live load reduction factors,
response spectra cases , ETABS scale factor for base shearcorrection in dynamic analysis,
gust
  
factor for windanalysis (H/B or H/L is less than 5),
avoid torsion mode in first few modes in considered numberof modes.
Overall stability checks, EQ with +_5% eccentricity, drift,slab diaphragm requirements .and lot.
<o> </o>
Remember very small issues in etabs sw-design beingrectified by developers so prefer strict manual check.
if you could send e2k file I can suggest some more .(mailbtg@gmail.com).
<o> </o>
Time period is all time issue for me.
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sakumar79
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PostPosted: Wed Mar 31, 2010 8:08 am    Post subject: Reply with quote

Dear Sir,
  If the building is T shaped, will it not make good sense to provide a seismic separation joint at the T interface and convert them into two rectangular structures? If not, you have to take into account the torsion generated due to earthquakes which can and will lead to much heavier reinforcement...

Arun
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Dr. N. Subramanian
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PostPosted: Wed Mar 31, 2010 2:26 pm    Post subject: Re: Time Period of the Structure Reply with quote

Dear Er Arunbabu,

As expressed by Prof. ARC, these formulae were derived for rectangular, regular buildings (FEMA 450 2001) and hence can not be applied to T-shaped buildings.
you may try the following UBC Formula:
T = 2π (Σwiδi2 /gΣfiδi)0.5
Where δi = Static elastic deflection at level i due to the forces fi
            fi = Lateral force at level i
            wi = gravity load at level i
            g = acceleration due to gravity

Best wishes and wishing you success in this project,
Subramanian

Arunbabu.S wrote:
Dear Sefians  
I am analyzing a 33 storied building with dual system (Shear wall Framed Structure) in Etabs. In plan, the structure looks like an inverted T with base dimensions of about 40 m in X & Y. The height of the building is about 100 m. My queries are
<if>1. <endif>As my structure is with both Shear wall & Frame, As per IS 1893 which relation has to be followed to calculate Time period of the structure. [T=0.075h^0.75 (or) T=0.09h/Sqrt(D)]. At present I have taken an average of both the values considering my structure as partially infilled, is that correct.
<if>2. <endif>After analysis I have got the time period of about 6.8 seconds from Etabs. Is there any steps to reduce the Time period of my structure or any precautionary measures to be considered while designing the structural elements.  
Regards,
Arunbabu. S





"This Message and its contents is intended solely for the addressee and is proprietary.Information in this mail is for L&T Business Usage only. Any Use to other than the addressee is misuse and infringement to Proprietorship of L&T ECC.If you are not the addressee please return the mail to the sender.L&T ECC DIVISION"

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Arunbabu.S
SEFI Member
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Joined: 23 Apr 2008
Posts: 19

PostPosted: Thu Apr 01, 2010 4:02 am    Post subject: Time Period of the Structure Reply with quote

Dear Sir

The Proposed Structure is located at Mumbai. I would like you to suggest some details regarding analysis for sequential loading, Creep & Temperature effects also.  

Thank you for your suggestions and wishes.

Regards,
Arunbabu.S


From: prof.arc [mailto:forum@sefindia.org]
Sent: Wednesday, March 31, 2010 10:01 AM
To: general@sefindia.org
Subject: [SEFI] Re: Time Period of the Structure



Dear Mr. Arun Babu,
Such a tall building of 33 stories should be treated as a special
structure and analysed carefully. The empirical values of period do
not cater to such special structures. The fundamental period of the
structure must be quite long - at the tail end
of the response spectra. You have not mentioned the country or its seismic zone.
The earthquake forces must be smaller than that of wind.
Also, design could be governed by the sway - P-delta effect and so on.
You should try different modelling and find out the variation.
Definitely, more than one person or agency must analyze it
independently to be sure that nothing is amiss.
I am glad that you got the oppurtunity to analyse a special structure
and wishing you the best..
Sincerely
Prof. A R Chandrasekaran

On 3/30/10, Arunbabu.S wrote:      
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MANOJ MITTAL
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Joined: 18 Jun 2008
Posts: 75

PostPosted: Thu Apr 01, 2010 5:45 am    Post subject: Time Period of the Structure Reply with quote

T shape block can be split in to two rectangular blocks. It will improve its behaviour and may be cost effective as well .
regards





MANOJ KUMAR MITTAL
BE,MSc,MS,CEng(I),MIE,FIAStructE,FIV

A-64, Sector-35, NOIDA-201307
09810039081 , 0120-2504381
manoj_shelter@yahoo.com (manoj_shelter@yahoo.com)
http://www.structures-india.com
Follow me on http://twitter.com/manoj_mittal

SHELTER CONSULTING ENGINEERS

*Structural Design & Retofitting * Construction Project Management * Green Buildings * Valuation *

This correspondence is from SHELTER Consulting Engineers and is intended
solely for the use of the recipient(s) named herein within the scope of
authorization, and may contain privileged, proprietary and/or confidential
information protected by laws and regulations. If you are not the intended
recipient(s) of this message, please notify the sender immediately, and
permanently delete the original, copies of it, any attachment and all
related printouts without reading or copying its contents.
This electronic message is not to be construed as contractual and/or legal
commitments by SHELTER Consulting Engineers,NOIDA.






--- On Wed, 31/3/10, akjhacpwd <forum@sefindia.org> wrote:
[quote]
From: akjhacpwd <forum@sefindia.org>
Subject: [SEFI] Re: Time Period of the Structure
To: general@sefindia.org
Date: Wednesday, 31 March, 2010, 1:30 PM

Dear Arun Babu

I feel it should be the second one because of presence of shear walls.

Secondly i am afraid the T shaped building in plan will face enormous torsion.


Best Wishes,
A K JHA

--- On Tue, 30/3/10, Arunbabu.S wrote:

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sarfaraj.husain
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Joined: 26 Jan 2003
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PostPosted: Thu Apr 01, 2010 6:00 am    Post subject: Time Period of the Structure Reply with quote

dear arun

your time pd. form etab is very high making structure very flexible.... please make your structure somewhat rigid to bring down the tp........as per 1893 your tp comes around 2.2 -2.4 secs......so you have to scale up the shear as per 1893 provisions....as building ht is around 100 m so serviceability may become one of the main criteria(making discomfort to persons living at upper stories) , so plan the frame size accordingly along with the other design criteria.


regards..
sarfraj


[SEFI] Re: Time Period of the Structure
akjhacpwd to: general  03/31/10 10:49 AM

Please respond to general  






Dear Arun Babu

I feel it should be the second one because of presence of shear walls.

Secondly i am afraid the T shaped building in plan will face enormous torsion.


Best Wishes,
A K JHA

--- On Tue, 30/3/10, Arunbabu.S wrote:
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akjhacpwd
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PostPosted: Fri Apr 02, 2010 2:57 am    Post subject: Time Period of the Structure Reply with quote

That is what IS 4326 says.

Best Wishes,
A K JHA

--- On Thu, 1/4/10, MANOJ MITTAL <forum@sefindia.org> wrote:
Quote:

From: MANOJ MITTAL <forum@sefindia.org>
Subject: [SEFI] Re: Time Period of the Structure
To: general@sefindia.org
Date: Thursday, 1 April, 2010, 11:36 AM

T shape block can be split in to two rectangular blocks. It will improve its behaviour and may be cost effective as well .
regards





MANOJ KUMAR MITTAL
BE,MSc,MS,CEng(I),MIE,FIAStructE,FIV

A-64, Sector-35, NOIDA-201307
09810039081 , 0120-2504381
manoj_shelter@yahoo.com (manoj_shelter@yahoo.com (manoj_shelter@yahoo.com))
http://www.structures-india.com
Follow me on http://twitter.com/manoj_mittal

SHELTER CONSULTING ENGINEERS

*Structural Design & Retofitting * Construction Project Management * Green Buildings * Valuation *

This correspondence is from SHELTER Consulting Engineers and is intended
solely for the use of the recipient(s) named herein within the scope of
authorization, and may contain privileged, proprietary and/or confidential
information protected by laws and regulations. If you are not the intended
recipient(s) of this message, please notify the sender immediately, and
permanently delete the original, copies of it, any attachment and all
related printouts without reading or copying its contents.
This electronic message is not to be construed as contractual and/or legal
commitments by SHELTER Consulting Engineers,NOIDA.






--- On Wed, 31/3/10, akjhacpwd wrote:
Quote:

From: akjhacpwd
Subject: [SEFI] Re: Time Period of the Structure
To: general@sefindia.org (general@sefindia.org)
Date: Wednesday, 31 March, 2010, 1:30 PM

Dear Arun Babu

I feel it should be the second one because of presence of shear walls.

Secondly i am afraid the T shaped building in plan will face enormous torsion.


Best Wishes,
A K JHA

--- On Tue, 30/3/10, Arunbabu.S wrote:

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