Effect of Bond-Slip on Dynamic Response of FRP-Confined RC Columns with Non-Linear Damping

Round 1

Reviewer 1 Report

The paper is very well written. The approach is clear and the methods are exhaustively detailed. Nevertheless, there are some issues (many of them, editorial) that should be addressed before the paper may be fully considered valid for publication.

1. Introduction - many articles are cited, ut often in a "collective" manner. It is not clear who had contributed to what, and whom the authors refer to. So it is asked to expand the Introduction, by detailing a bit more the articles cited, even at the cost of reducing the citations.
2. Despite the clear representation of the problem, the authors miss to point out what is their original contribution. It s understood they play a role in assessing a model... or even in creating a model. But details are missing to make the reader know what is the novelty they bring in unless the reader him(herself makes assumptions.  This is a pity because the authors neglect to cite the best part fo their own work :-)
3. Figure 6. For increasing its readability, captions reported in the label should be instead placed in the pictures themselves.
4. Figure 7-10. As they are, the graphs and curves are not distinguishable. Would the author think to a way to make evident the different curves, for instance by making some targeted zoom over certain diagram parts. Meaning of VD, BS and CD should be reported as labels in the pictures., to increase readability.
5. The only technical comment (5.2-5.4).
   1. Figure 7-10 report the behaviour of the column under different kinds of load. Now, before the curves, only the 7 and 8 are cited. After the curves, all the figures are recalled. This misprint shall be corrected.
   2. It is believed that the comparison of the max displacement cannot be a sufficient index to determine the "quality" of the results, being linked to many parameters like the modal frequency, the modal shape (linear structure model, excluding damping as it is large), the load time history (and then the load spectrum) and so on. A comparison could take place on a sufficient number of external loads, so to have a representative statistical base.
   3. A nice trial could be to report the FFT of the spectrum. In fact,  irrespectively of the nonlinear characteristic of damping: till it is "small", it should not have a tremendous impact on the system response. Furthermore, the frequency content and the associated amplitudes could be of some interest.
   4. In the time domain, other "global" parameters could be considered as indexes for better reporting a comparison among the different responses, like the "decay time", the integral of the squared signals (or even its time, space derivatives and so on), etc.
   5. Authors' discussion about the points 2.4 is therefore expected to some extent.
6. Conclusions paragraph is adequate.

Author Response

Thank you very much for giving us an opportunity to revise our manuscript, we appreciate editor and reviewers very much for their positive and constructive comments and suggestions on our manuscript. These comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researches. We have studied comments carefully and have made correction which we hope meet with approval. Revised portion are marked in red in the paper. Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

The topic discussed in this manuscript is significant, since it is not dealt in literature and, for this reason, present work is of interest for journal readers. However introduction should be improved since n the past there were some attempts to improve the evaluation of nonlinear lateral response of confined columns, e.g. by considering concrete cover spalling and reinforcement buckling (a common failure mode for hollow members, e.g. DOI: 10.1016/j.compstruct.2008.10.001 and this can be cited as a different improvement compared to present valuable proposal).

Furthermore, some issues of FRP confinement of RC members, making the evaluation even more complex, are not cited at all in the introduction. I suggest to clarify at least that stress-strain relationships of confined concrete rely on an assumed FRP ultimate strain, however effective experimental hoop strain at failure in FRP jackets is lower than the FRP ultimate strain in flat coupon tests (e.g. DOI: 10.1016/j.compositesb.2012.04.007); potential reasons and governing parameters for the premature failure of the strengthening system are discussed for instance in DOI: 10.1016/j.compositesb.2010.07.003 ).

Discussion about zero-length section element having a unit length is confusing, as it becomes like a spring element, where the main parameter is rotation and not curvature… Please clarify

It should be clarified that slip occurs between steel bar nodes and concrete nodes along the bar length (as a physical phenomenon) and not at the base section only; howsoever it can be calibrated as a concentrated effect, since slip occurs where the stress gradient is higher (hence where shear stress is higher between steel and concrete materials, e.g. at the base).

Please note that description of coefficient μ is missing after equation 3.

I suppose that symbol ~ at line 122 means a range, however it should be clarified.

If figure 3 is verbatim taken from reference 31, permission should be asked.

Symbols maxrl rsvg minrl in equation 10 should be described.

At line 142, please clarify which one of the three methods you used: Newton, Modified Newton, or Newton with Line Search ?

Adding to figure 4 the simulations without your proposed model for bond slip will add major emphasis to your contribution to knowledge.

In table 1 and text “Lateral rate” can be revised as “drift ratio”?

Please clarify how did you evaluate energy dissipation in section 2.2.3.

At line 189, please provide a reference or define directly longitudinal reinforcement ratio and FRP volume ratio.

I suggest to move lines 191 to 249 to an appendix, as they are not strictly related with the flow of the paper. Readers are waiting for the discussion about effect of bond-slip (section 4).

Please provide deeper explanation for equations 20, or a reference.

At line 277 please revise “The loss factor η IS calculated”…

At line 291 “neglected” should be “neglecting”.

Section 4.3 (Seismic Simulation Results and Discussion) is not fully clear. How do you state that it is inaccurate to simply adopt constant damping ratio (at line 328), only because the response is different from other two?

At line 335, what do you mean with “various FRP constraints”?

Author Response

Thank you very much for giving us an opportunity to revise our manuscript, we appreciate editor and reviewers very much for their positive and constructive comments and suggestions on our manuscript. These comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researches. We have studied comments carefully and have made correction which we hope meet with approval. Revised portion are marked in red in the paper. Please see the attachment.

Author Response File: Author Response.docx