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STIFFNESS OF A RC FRAMED STRUCTURE
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vnacharya
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PostPosted: Fri Mar 18, 2011 8:59 am    Post subject: STIFFNESS OF A RC FRAMED STRUCTURE Reply with quote

Dear Friend,
 
It is well known that to decrease the period of the structure following options u can try:
 
1. Increase the stiffnes of the structure and stiffness distribution shall be uniform to reduce torsional response.
2. Optimize the loads and avoid heavy loads at upper floors.
If all these options are not working means, I think u have to go for dampners..


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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vikram.jeet
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PostPosted: Fri Mar 18, 2011 10:11 am    Post subject: STIFFNESS OF A RC FRAMED STRUCTURE Reply with quote

A general viewpoint:

Flexibility is always helpful whether in reference to structural engg
or day to day life of an individual.

More stiff a structure ,more the effect of EQ .

But higher Flexibility of structure under EQ may lead to excessive drift  
and effects of P-delta come into play
Infills in the structural frames enhance the stiffness of structure for dynamic
response , reducing the time period.

As per rational formulas for working out time period of buildings:

RCC building without infills

Ta = 0.075*H^0.75 (as per IS 1893-2002)

Steel building without infills

Ta = 0.085*H^0.75 (as per IS 1893-2002)

Steel buildings are more flexible than rcc with higher time
period(about 13.33% as per comparision from above  
formulas ) but their damping is much less.

RCC building with Bk infills

For moment resisting frames without bracing/shear wall
(As per IS 800-1984)
Ta = 0.1*n
This was earlier formula which is not there in new code.

For all others(As per IS 800-1984)  
All buildings with Bk infills (as per IS 1893-2002)
Ta = 0.09*H/d^0.5  

Logically speaking these formula must give a lower time period
compared to buildings without infills  

Time period of 40 storied building (H=40*3.2=128m)

RCC building without infills Ta=0.075*128^0.75 = 2.86 s
Steel building without infills Ta=0.085*128^0.75 = 3.23 s

All building with infills
Slender building H/d=9 d=128/9 =14.2
Ta=0.09*128/14.2^0.5 =3.05 s

Average Bldg H/d=6 d=128/6=21.3
Ta=0.09*128/21.3^0.5 =2.49 s

Broader building H/d=3 d=128/3=42.7
Ta=0.09*128/42.7^0.5 =1.76 s

The dimensions of the building wrt to its height are important
The earlier simplified formula Ta= 0.1n =0.1*40=4.0 s This value is very
much off from the above values for building with infills.This formula
is deleted from new code.

Values of Ta for Average and Broad Bldgs with infills is less than that
of building without infills and OK. But for slender bldgs this is higher than that which
may be contradictory.I think formula without infills also need to
incorporate the plan dimension d.

regds

vikramjeet






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nalini4anand
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PostPosted: Fri Mar 18, 2011 3:35 pm    Post subject: STIFFNESS OF A RC FRAMED STRUCTURE Reply with quote

Dear friend,
 A 40 story tall building will govern for for wind as a design crieteria and not exactly for EQ. and i think if you arrange shear walls in such a way that   C.G and C.M almost coinside with each other it will work.

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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fugeeo
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PostPosted: Sat Mar 26, 2011 1:34 am    Post subject: STIFFNESS OF A RC FRAMED STRUCTURE Reply with quote

X Cabling may generate an influence. and or introduce a dampot systemfurther, there is the base isolation example.

On Tue, Mar 15, 2011 at 10:28 PM, 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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nnsivakumar
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PostPosted: Fri Apr 08, 2011 4:08 am    Post subject: Reply with quote

Dear All,
Many thanks for posting your valuable comments and sorry for the delay in my reply. I have attached my updated model here in which I did the analysis for windload. Wind load was calculated as per ASCE 7.02 with a frequency of 0.101 Hz which was obtained by calculating the Eigen frequency on the same  model. The damping ratio was taken as 0.01 whereas it can be from 0.03 to 0.05 for RC structure so as to get  more gust factor. I have also attached the .anl file for the Eigen frequency calculation. The frequency in Z direction is 0.101 HZ with a participation of 59%. I took this value as the wind load is critical for the model in Z direction.
I got a displacement of 726 mm at one of the top most nodes. This is more than the allowable limit. Now I seek your opinion on the following points:
1. Is my frequency value of 0.101 correct in wind load calculations?
2. Are my  entries for wind load and the model correct?
3. What is the limit for the natural frequency for a framed structure? If it is beyond the limit, can I accept the structure and design it based on the actual frequency value?
4. If I should not accept the natural frequency beyond the range , what changes are to be made to bring it to within the range?
5. What is the best option to reduce the deflection in the model to within limits?
Thanks in advance for your valuable suggestions and replies.
Anticipating your response,
Thanks
Siva



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nnsivakumar
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PostPosted: Tue Apr 12, 2011 12:54 am    Post subject: Reply with quote

Dear friends,
Please offer your thoughts on my posting earlier.
Thanks
siva
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nnsivakumar
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PostPosted: Fri Apr 22, 2011 1:45 am    Post subject: Reply with quote

Hi,
I seek experts to help me with my query posted here.
Thanks
Siva
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patiilgr
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PostPosted: Tue Aug 28, 2012 8:25 am    Post subject: STIFFNESS OF A RC FRAMED STRUCTURE Reply with quote

Dear Arvind,
R U the same guy studying in MSc in IISc during 1998?

Regards G R Patil

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, 7:24 PM

           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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