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STIFFNESS OF A RC FRAMED STRUCTURE
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nnsivakumar
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PostPosted: Wed Mar 16, 2011 5:28 am    Post subject: STIFFNESS OF A RC FRAMED STRUCTURE Reply with quote

I am modelling a 40 storyey RC framed service suite building. The period of the structure comes to 6s in Z direction and 4s in X direction. I wish to reduce the period to less than 3s as per clients requirements. I tried many possibilities like introducing additional shear walls, increase the column and beam sizes but the period does not change much. I wish to know how to increase the stiffness of the structure so as to reduce period. Seek suggestions.
Thanks
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ankurbajoria
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PostPosted: Wed Mar 16, 2011 11:53 am    Post subject: STIFFNESS OF A RC FRAMED STRUCTURE Reply with quote

Fellow Sefian,


from myunderstanding of dynamics of structure,
  • Time period of 6s in Z indicate structure is flexible probably due to higher slenderness ratio. The 4s in X is fine. what is the basis for 3s requirement by client? The fundamental time period calculated using 7.6.1 (IS code:1893-1) for 3.5m floor to floor height is around 3s, but actual value depends on it depends on stiffness.
  • for a test case to understand increase in stiffness required to achieve 3s time period model the building as exoskeleton building. If that works you will have an idea of how much stiffness is required. If that stilldoesn'twork, achieving 3s period might not bearchitecturallyfeasible.


use of active/ passive damper will help to reduce the response of building but wont change fundamental period. Hence increasing the stiffness with in design permissibility seems only way. Rest more discussion by other sefian's might give better solution to same.





Ankur Bajoria



On Wed, Mar 16, 2011 at 10:58 AM, nnsivakumar <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
           I am modelling a 40 storyey RC framed service suite building. The period of the structure comes to 6s in Z direction and 4s in X direction. I wish to reduce the period to less than 3s as per clients requirements. I tried many possibilities like introducing additional shear walls, increase the column and beam sizes but the period does not change much. I wish to know how to increase the stiffness of the structure so as to reduce period. Seek suggestions.
Thanks
     



     


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jiwaji
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PostPosted: Wed Mar 16, 2011 12:32 pm    Post subject: STIFFNESS OF A RC FRAMED STRUCTURE Reply with quote

The planar wall element won't contribute significantly, as in all probability its length to width ration is too high. Both flexural and torsional deflections will contribute. A tubular, or a tube-in-tube type wall structure, suitably cross-stiffened by floor-diphragms is a possible solution. These tubes shall be placed as far, or as outward, from the core as feasible.The sizes of these will have to be correlated to the building aspect ratios of ht-to-width of the building in the two principle directions.
Regards
Jiwaji Desai
Jamshedpur



"nnsivakumar" <forum@sefindia.org>  
03/16/2011 04:47 PM    Please respond to
general@sefindia.org

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[SEFI] STIFFNESS OF A RC FRAMED STRUCTURE




I am modelling a 40 storyey RC framed service suite building. The period of the structure comes to 6s in Z direction and 4s in X direction. I wish to reduce the period to less than 3s as per clients requirements. I tried many possibilities like introducing additional shear walls, increase the column and beam sizes but the period does not change much. I wish to know how to increase the stiffness of the structure so as to reduce period. Seek suggestions.
Thanks







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Arvind Kumar Jaiswal
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PostPosted: Wed Mar 16, 2011 1:54 pm    Post subject: Reply with quote

Dear Friend,


Please reduce the weight of building by using light weight material, reduce thickness of slab, etc  the fundamental time period will automatically reduce.
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nachko
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PostPosted: Wed Mar 16, 2011 3:26 pm    Post subject: STIFFNESS OF A RC FRAMED STRUCTURE Reply with quote

Increasing the mass would help. Well, I know that increasing the mass is not exactly what we want, but technically this would help.



-------- Оригинално писмо --------
От: "nnsivakumar"
Относно: [SEFI] STIFFNESS OF A RC FRAMED STRUCTURE
До: general@sefindia.org
Изпратено на: Сряда, 2011, Март 16 07:28:23 EET
     I am modelling a 40 storyey RC framed service suite building. The period of the structure comes to 6s in Z direction and 4s in X direction. I wish to reduce the period to less than 3s as per clients requirements. I tried many possibilities like introducing additional shear walls, increase the column and beam sizes but the period does not change much. I wish to know how to increase the stiffness of the structure so as to reduce period. Seek suggestions.
Thanks

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anshugoel
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PostPosted: Wed Mar 16, 2011 4:03 pm    Post subject: Reply with quote

I think you are saying first mode and second mode time period is 6 sec and 4 sec or is it some thing different ?
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anshugoel
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PostPosted: Wed Mar 16, 2011 5:35 pm    Post subject: Reply with quote

anshugoel wrote:
I think you are saying first mode and second mode time period is 6 sec and 4 sec or is it some thing different ?


Is it time period from modal analysis ?

May be you can upload your file.
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hemal
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PostPosted: Thu Mar 17, 2011 5:00 am    Post subject: STIFFNESS OF A RC FRAMED STRUCTURE Reply with quote

I think it is also important that how each frame is connected with each other. Stiffness of beams connecting column is also important. Size and stiffness of Link beam in lift core. Sizes of beams (Link beams) connecting shear walls and their configuration is also important. In short, we shall find a structural system (vertically and in plan) which can resist the torsion very well.

Regards
Hemal

--- On Wed, 16/3/11, Arvind Jaiswal <forum@sefindia.org> wrote:
Quote:

From: Arvind Jaiswal <forum@sefindia.org>
Subject: [SEFI] Re: STIFFNESS OF A RC FRAMED STRUCTURE
To: general@sefindia.org
Date: Wednesday, 16 March, 2011, 8:54 AM

Dear Friend,


Please reduce the weight of building by using light weight material, reduce thickness of slab, etc the fundamental time period will automatically reduce.
     



     


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yashupal
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PostPosted: Thu Mar 17, 2011 5:58 am    Post subject: Hello Reply with quote

Change the orientation in some the columns and check the time period..
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nrajkumar
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PostPosted: Thu Mar 17, 2011 8:00 am    Post subject: STIFFNESS OF A RC FRAMED STRUCTURE Reply with quote

sivakumar,
i also faced same problem like time period exceeds more than 4secondswhile modlling 32 storey building by ETABS. if you modelled slab as a membrane, please change to shell element and check time period, this leads to more top (-ve) reinforcement of slab when compared to conventional slab design.if you change membrane to shell, then dont do the slab design by conventional method.For slab design take Shell stresses M11,M22,M12 and design it accordingly.you can allow the time period upto 4sec as per IS1893:2002.

Rajkumar
Mumbai


On Wed, Mar 16, 2011 at 10:58 AM, nnsivakumar <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
  I am modelling a 40 storyey RC framed service suite building. The period of the structure comes to 6s in Z direction and 4s in X direction. I wish to reduce the period to less than 3s as per clients requirements. I tried many possibilities like introducing additional shear walls, increase the column and beam sizes but the period does not change much. I wish to know how to increase the stiffness of the structure so as to reduce period. Seek suggestions.
Thanks







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