Time Period of Unretrofitted & Retrofitted column

[zsaeed]

I modeled a circular column of Reinforced concrete, performed Eigenvalue analysis on it and found the natural period. After that i applied a single CFRP layer (specifying appropriate FRP Jactet ratio) on half length of column and performed eigenvalue analysis again ..the time period was found to be decreased. Lastly i applied two CFRP layer by further increasing the FRP jacket ratio and evaluated the natural periods.
I was amazed that the time period didnt change this time (i.e for a single CFRP wrapped and Double layer of CFRP wrapped column , the time period remained same ) ..Although experimental results for Single and Double CFRP layered column have different time period..what could be the reason ??? Do i need to change some other values to simulate the effect of Single and Double CFRP layers ??

the Tabular values can bee seen in following table
https://drive.google.com/file/d/0B_mBNM ... sp=sharing


Thanks
Zain Saeed

[huffte]

An eigenvalue analysis is, by it's very nature, an analysis which depends on the mass distribution and on the initial stiffness. There is no inelastic behavior modeled in an eigenvalue analysis (unless one used secant stiffness values to perform some sort of equivalent linear analysis).

So, zsaeed, I believe that you have likely employed elastic frame elements to define the member. In this case the confining effects of the jacketing are unaccounted for. You may see this in the EA and EI values - they will be identical for the two sections when elfrm element types are used.

If you will change to infrmFB element types, then I think you will see a period shift.

Best of luck zsaeed.

[fakharifar.mostafa]

zsaeed, as stated by huffte initial stiffness affects eigen value analysis results. Therefore, for the case of FRP wrapping, the initial stiffness of RC columns before cracking would be approxiamtely equal for the case of as-built and jacketed column, where FRP jacketing contributes upon cross section cracking. Thus, it is no surprise what you observed.

For more information in this regard below paper will help:

El-Sokkary, H., and Galal, K. (2009). “Analytical investigation of the seismic performance of RC frames rehabilitated using different rehabilitation techniques.” Engineering Structures Journal, Elsevier, 31(9), 1955-1966.

[zsaeed]

Thanks for the reply Sir. I first used elastic frame element..but then changed modelling technique and used inelastic element for each of the three sample( 1. column without CFRP 2. Col with Single CFRP Layer 3. Column with double CFRP layer )
As i move from case 1 to 2 the results of time period were changing and matching the experimental data but while moving from case 2 to case 3 i didnt found any change in Time period during Eigenvalue analysis..
For single CFRP layer i used a 0.0071 and 0.0143 for double layer of CFRP on a circular column of 300mm dia.
(do i upload the Model for more clearance of this issue ? )

Thanks

[zsaeed]

quote:Originally posted by fakharifar.mostafa

zsaeed, as stated by huffte initial stiffness affects eigen value analysis results. Therefore, for the case of FRP wrapping, the initial stiffness of RC columns before cracking would be approxiamtely equal for the case of as-built and jacketed column, where FRP jacketing contributes upon cross section cracking. Thus, it is no surprise what you observed.

For more information in this regard below paper will help:

El-Sokkary, H., and Galal, K. (2009). “Analytical investigation of the seismic performance of RC frames rehabilitated using different rehabilitation techniques.” Engineering Structures Journal, Elsevier, 31(9), 1955-1966.

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Many many thanks for your valuable comments and the reference paper..i gone through the paper you referred in the post and it was clearly mention in that "for the same building height, the existing structure and the rehabilitated one using FRP wrapping have the same natural period" ...
In my case we didnt failed or craked the column .In first phase we wrapped CFRP on undamaged circular column and compared its natural period with the same undamaged model but without a CFRP layer.

In second phase we wrapped 2 CFRP layers on undamaged column and subjected it to free vibration and ultimately compared its natural period with the Undamaged model with single CFRP wrap.

The experimental results for the above mentioned cases showed variation in the time periods at each phase ..why the same cant be simulated in software ??

[fakharifar.mostafa]

I am not able to answer this question about your experiment and your simulation. However, for further insight you can use the below reference to calculate the effective stiffness of your column. This work is indeed incorporated in ASCE 41-06 (2007) provisions.

Elwood, K. J., and M. O. Eberhard. "Effective Stiffness of Reinforced Concrete Columns." ACI Structural Journal, 2009: 476-484.

[huffte]

I do believe that the variation may be simulated in SeimoStruct.

I created a very simple model and used identical sections in two separate eigenvalue analyses, changing only the jacket ratio in the second run, and the period did shift.

Also, note from the Help System for SeismoStruct:

"Concrete confinement will increase the compressive strength of the material, and hence the stiffness of the member, leading thus to shorter periods of vibration."

So, I cannot say why you have not been able to obtain a period shift. Could it be that even though you doubled the ratio, both values are very small to start with and the period shift is small enough that it shows up only in the 4th or 5th decimal point?

Best of luck zsaeed.

[zsaeed]

quote:Originally posted by huffte

I do believe that the variation may be simulated in SeimoStruct.

I created a very simple model and used identical sections in two separate eigenvalue analyses, changing only the jacket ratio in the second run, and the period did shift.

Also, note from the Help System for SeismoStruct:

"Concrete confinement will increase the compressive strength of the material, and hence the stiffness of the member, leading thus to shorter periods of vibration."

So, I cannot say why you have not been able to obtain a period shift. Could it be that even though you doubled the ratio, both values are very small to start with and the period shift is small enough that it shows up only in the 4th or 5th decimal point?

Best of luck zsaeed.


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Sure i doubled the value for Double CFRP layer and as you said that there will be change in time period but in 4-5 decimal place which is true for my case as well ..But the experimental results showed deviation in 1 decimal place ..thats why i was confused and thinking that there might be some minor mistake i have been doing which is causing this issue...I have attached the model for more clarification of the problem
https://drive.google.com/file/d/0B7rekW ... sp=sharing

[zsaeed]

Sometimes i feel that i am using some wrong formulae for calulation of Mx and My while simulating Eigenvalue analysis..
I used the following formula to calculate value of I and put this value in Mx and My respectively ..Kindly suggest some other formulae for calculation of Mx My and Mz ...
https://drive.google.com/file/d/0B7rekW ... sp=sharing

Need assistance on this issue of finding the natural time period using eigenvalue analysis


Thanks

[zsaeed]

For more clarification of the problem i stated above, the following image shows Tabular results of Experimental time periods and also the one found using eigenvalue analysis in Seismostruct.

https://drive.google.com/file/d/0B7rekW ... sp=sharing