CFRP Confined-Strength Degradation

[anantparghi]

Hi there,
I am modelling CFRP-jacketed circular RC cantilever column. I applied quasi-static cyclic time history load at the tip of the column along with constant lumped mass at the top of the column. I used the Taked link element for the stiffness gradation.

The as built column experimental and SeismoStruct results are quite similar, but for CFRP-jacketed column, the experimental results shows that the load carrying capacity is increased up to 3% to 4% drift and then it is decreasing, while the SeismoStruct result shows that the strength and stiffness is not decreasing after 30% to 4% drift, and it is constant which means confinement is governing. But, the experimental results shows that the stiffness and load carrying capacity is increased in up to 4% drift and the it is decreasing.
Anybody, could please suggest me what is wrong with my model?
Thanks
Anant

[seismosoft]

The two reasons that we could identify are the following:
(1) the axial loads are not modelled correctly, ie. they are smaller in the model than in the experiment
(2) the active confinement factor in the test is smaller than the one in the computer model, ie. either the computed confinement factor is larger than it should be, or the confinement in the test is smaller (e.g. the FRP wraps were not applied correctly or were not anchored correctly)

Note that from the two, the second option is not very likely, since the max. capacities in the test and the computer model are very close. Which is the value of the calculated confinement factor in SeismoStruct?

Seismosoft Support

[anantparghi]

Dear SeismoSoft Support Team,
Thank you very much for your kind response to my earlier query.
I would like to ask you that, I calculated the confinement pressure (fl)for FRP using this equation (for entire circumference of circular section)

fl= tfrp.n.Efrp.efrp/d,

where tfrp = thickness of FRP jacket (0.111mm), n = layer of FRP jacket, Efrp =stiffness of FRP jacket (266000 MPa), efrp = rapture strain of FRP jacket (0.0163)and d = dia. of circular column (400mm).

fl = 0.111*1*266000*0.0163/400
fl = 1.2031 MPa

and I calculated the confinement pressure by steel stirrups using the equation given by Mander et al. (1998) paper. which is = 0.06336
After that I add the FRP confinement plus the steel stirrups confinement which is = (1.2031+ 0.06336) = 1.27 MPa,

But, the Seismostruct shows the confinement factor = 1.4694
Therefore, I was just wondering that why there is difference between the confinement for the manual calculation and the software calculation?
Could you please let me know, how the software is calculating the confinement factor?
I doubt due to this different confinement factor, there might be difference between the experimental and Seismostruct results, Please share your opinion.
Thanks
Anant

[z.gronti]

Dear anantparghi,

In SeismoStruct the confinement pressure applied by FRP continuous sheet, for the case of circular cross-sections is calculated, according to the Annex of Eurocode 8-Part 3, from the following expression:

f1=2tf.Ef.åju/D

In SeismoStruct the confinement pressure applied by FRP is not added to the one applied by stirrups, but the larger one of the two is used in the confinement factor calculations.

Finally, from what i can understand, you compare the value you have calculated for the confinement pressure with the one you get from the program for the confinement factor. After defining the confinement pressure for the section you should use the no.29 equation given in Mander et al.[1989] paper in order to calculate the section's confinement factor.