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AXIAL DEFORMATION OF COLUMN IN TALL STRUCTURES
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P.K.Mallick
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PostPosted: Mon Nov 26, 2012 1:35 am    Post subject: Reply with quote

METHOD DEVELOPED BY RAED M.SAMRA


This method is due to Prof.Raed M. Samra and was published in Journal of Structural Engineering in March 1995 issue. This is an improvement over his previous study of creep model which requires the use of an iterative procedure for the solution of the creep strain and creep stress under sustained load, published in Journal of American Concrete Institute (1998) under the title “Creep model for Reinforced Concrete Columns”. The new approach has a great advantage from practical point of view since the results for axially loaded column are identical to those obtained by using the iterative procedure, but can derived from a direct calculation. The best advantage of the current procedure is that it requires little input, including creep data and section and material properties ,which makes it useful in most case of commonly encountered design problems. The brief outline of the procedure is as follows:
To evaluate the change in stress and deformation arising in reinforced concrete section under sustained loads, the procedure has been evolved based on the algebraic constitutive law to describe the creep deformation of the concrete. The law is expressed as follows:

Єct    =   fci /Ec    (1+Øt  ) - ∆ ft /Ec   (1+ 0.8 Øt   )

Where ,

Єct  = concrete strain at time “t”.
fci  = Initial concrete stress.
∆ ft  = stress decrement in concrete at time “t”.
Øt = Creep coefficient at time “t”.
Ec = Modulus of elasticity of concrete.
The law is particular case of well-known age-adjusted effective modulus method.
For axially loaded columns, the requirements of strain compatibility, equilibrium and stress-strain relationship apply at any time under load. These requirements are written at time t > to, a long time after load application.

From the strain compatibility and equilibrium the stress in steel ,the stress in steel can be expressed by the formula:

Fst = n Fci {(1- 0.8 ) *Øt + Fct/Fci (1+0.8 Øt)}

Hence strain in steel due to creep= Fst/ Єct

The above equation completely solves the problem in closed form. A comparison between results of tests conducted by Troxell etal. on concrete columns and results predicted by the analytical procedure presented in the Prof.Samra’s paper shows a good correlation between measured and computed value.
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P.K.Mallick
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PostPosted: Mon Nov 26, 2012 1:12 pm    Post subject: Reply with quote

METHOD DEVELOPED BY RAED M.SAMRA-CONTD



Although this paper only deals with creep, the most cases of  practical interest it is important to superpose the results of a shrinkage model proposed by Park and Pauley with the outcome of the Samra’s creep model, because the final stresses and strains under the combined effect of creep and shrinkage are normally of interest.

Since the equilibrium and compatibility are satisfied in the individual models, they will be also satisfied in the final superposed model. The shrinkage model is simple to use and is based on sound theoretical basis in which it is assumed that the restraint of shrinkage concrete by reinforcement will induce tensile stresses in concrete accompanied by compressive stress in steel. Since the shrinkage can occur even in the absence of any external load, the requirement of equilibrium would dictate that total force induced in concrete is equal and opposite of the total force in steel. Simple mathematical derivation yields that the stress in steel, Fs, at any time equal to
Fs=Fc*Ac/As


Where Fs=Steel compressive stress due to shrinkage.
Fc =X/Y

X=єsh

Y= (1+Øt)/Ec +( Ac/As*Es)



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thirumalaichettiar
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PostPosted: Mon Nov 26, 2012 3:06 pm    Post subject: Reply with quote

As Er.Bijay sarkar has asked in his posting on 25-11-12 may  I request you on behalf of all Sefians the following:

At the end of your write up can you put the concluding remarks in a nutshell with the notes:

a. What is the Creep and shrinkage effect on Column?

b. When it is important i.e from how many floor level it is important?

c. Is this phenomenon is applicable to all materials?

d. How to calculate with example specially with respect to R.C.C with the latest ( new method as per your previous statement)method using the necessary equations in SI units?

e. Is there any software as per your knowledge available?

f. If not is there any method that can be used with any of the available commercial software?

g. Can you list out the few important reference articles and books on the above subject?

Hope you will do for the sake of interested Sefians?

T.RangaRajan.
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P.K.Mallick
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PostPosted: Mon Nov 26, 2012 3:19 pm    Post subject: Reply with quote

thirumalaichettiar wrote:
As Er.Bijay sarkar has asked in his posting on 25-11-12 may  I request you on behalf of all Sefians the following:

At the end of your write up can you put the concluding remarks in a nutshell with the notes:

a. What is the Creep and shrinkage effect on Column?


b. When it is important i.e from how many floor level it is important?

c. Is this phenomenon is applicable to all materials?

d. How to calculate with example specially with respect to R.C.C with the latest ( new method as per your previous statement)method using the necessary equations in SI units?

e. Is there any software as per your knowledge available?

f. If not is there any method that can be used with any of the available commercial software?

g. Can you list out the few important reference articles and books on the above subject?

Hope you will do for the sake of interested Sefians?

T.RangaRajan.


Respected RangaRajan Sir
By the time I finish on this subject ,I will automatically answer all your questions including Software.I will try my best to deal as much as I know.
Warm Regards.

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Dr. N. Subramanian
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PostPosted: Mon Nov 26, 2012 3:30 pm    Post subject: Reply with quote

Dear Er Rangarajanji,

The paper New Analysis for Creep Behavior in Concrete Columns by R.M. Samra,  Journal of Structural Engineering, Vol. 121, No. 3, March 1995, pp. 399-407, referred by Er Mallick contains an example. The abstract of the paper is given below:
     This paper presents a new rational approach for the evaluation of the effects of creep on reinforced-concrete axially loaded columns at sustained service stresses. The analysis involves a straightforward computation based on a closed form procedure and the assumption of linear elastic materials for both concrete and steel. The analysis may be easily extended to cover the case of reinforcement at yield. The results of the proposed approach may be superposed with those from a shrinkage model presented by Park and Paulay in 1975, and the overall behavior of column axial shortening and stress transfer from concrete to steel may be described using the combined approach. The process involved is very convenient to use from an engineering view point since it requires few input parameters, which are easy to estimate or measure experimentally, such as the modulus of elasticity of concrete and the creep coefficient. The results of the theoretical approach correlate well with experimental tests conducted on specimens in the laboratory and with deformations of columns measured in the Water Tower Place and Lake Point Tower in Chicago.

As I already informed Dr Taranath discusses these effects in his book- These effects should be considered when the no. of stories exceed 30. Taranath discusses about steel columns, the same can be applied to concrete also, which creeps more than steel. Prof. Samra's method is more refined for RCC.

Regards,
NS
thirumalaichettiar wrote:
As Er.Bijay sarkar has asked in his posting on 25-11-12 may  I request you on behalf of all Sefians the following:

At the end of your write up can you put the concluding remarks in a nutshell with the notes:

a. What is the Creep and shrinkage effect on Column?

b. When it is important i.e from how many floor level it is important?

c. Is this phenomenon is applicable to all materials?

d. How to calculate with example specially with respect to R.C.C with the latest ( new method as per your previous statement)method using the necessary equations in SI units?

e. Is there any software as per your knowledge available?

f. If not is there any method that can be used with any of the available commercial software?

g. Can you list out the few important reference articles and books on the above subject?

Hope you will do for the sake of interested Sefians?

T.RangaRajan.
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thirumalaichettiar
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PostPosted: Mon Nov 26, 2012 3:39 pm    Post subject: Reply with quote

Dear Er.P.K.Mallick,

Please find all your write up in the attached PDF file. Check this if anything is missing so Sefians can have all in one file.

Further write ups will be uploaded in the same file.

If you do not like please inform so that it will be deleted.

With Best Regards,
T.RangaRajan



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P.K.Mallick
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PostPosted: Mon Nov 26, 2012 4:17 pm    Post subject: Reply with quote

Analysis of Method developed by Prof. Samra.


Basically this method consists of two parts.

1) Prediction of creep coefficient and ultimate shrinkage.
2) Calculation of axial deformation.

The beauty of Prof. Samra method is that the prediction of creep coefficient  and ultimate shrinkage has been separated from the main problem and therefore latest codal practices in the field of creep and shrinkage can be accounted for where as in earlier two methods (Method developed by M.Fintel and F.R.Khan and subsequent method by M.Fintel,H.Iyenger & S.K.Ghosh) do not take the help of latest developments in the field of Creep and Shrinkage and still depend on research data of late sixties.

It is mentioned in the article of Prof. Samra that accuracy of proposed method has been checked and found to be comparable with actual measurement taken on water tower place and lake point tower of U.S.A.

Rest of the beauties of Prof.Samra method have already been highlighted by Dr. N. Subramanian in his comment.
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P.K.Mallick
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PostPosted: Mon Nov 26, 2012 4:18 pm    Post subject: Reply with quote

thirumalaichettiar wrote:
Dear Er.P.K.Mallick,

Please find all your write up in the attached PDF file. Check this if anything is missing so Sefians can have all in one file.

Further write ups will be uploaded in the same file.

If you do not like please inform so that it will be deleted.

With Best Regards,
T.RangaRajan


Thanks a lot ,Sir. My salute goes to you.

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P.K.Mallick
p.k.mallick1962@gmail.com
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thirumalaichettiar
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PostPosted: Mon Nov 26, 2012 4:22 pm    Post subject: Reply with quote

P.K.Mallick wrote:
thirumalaichettiar wrote:
Dear Er.P.K.Mallick,

Please find all your write up in the attached PDF file. Check this if anything is missing so Sefians can have all in one file.

Further write ups will be uploaded in the same file.

If you do not like please inform so that it will be deleted.

With Best Regards,
T.RangaRajan


Thanks a lot ,Sir. My salute goes to you.

Dear Er.Mallick,
Continue your write up and I will up date it and place PDF .

T.RangaRajan.
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Dr. N. Subramanian
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PostPosted: Mon Nov 26, 2012 4:52 pm    Post subject: Reply with quote

Thanks for your efforts Er Mallick and Er Rangarajan. Hope the document will be of use to many engineers.

Best wishes,
NS
thirumalaichettiar wrote:
P.K.Mallick wrote:
thirumalaichettiar wrote:
Dear Er.P.K.Mallick,

Please find all your write up in the attached PDF file. Check this if anything is missing so Sefians can have all in one file.

Further write ups will be uploaded in the same file.

If you do not like please inform so that it will be deleted.

With Best Regards,
T.RangaRajan


Thanks a lot ,Sir. My salute goes to you.

Dear Er.Mallick,
Continue your write up and I will up date it and place PDF .

T.RangaRajan.
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