Test on Compressive Performance of Hollow Concrete-Filled Sandwich Circular Steel Tubes Connected by Thread

Round 1

Reviewer 1 Report

The work developed presents a contribution to understand the behaviour of concrete filled sandwich circular steel tube connected by thread.

Some comments are provided below, with the aim of clarifying some aspects:

1. Abstract.

Comment: Do a full revision of all the text of the Abstract. Text is incomprehensible and not at all clear about the work developed.

2. Section 2 – Specimen design.

Comment 1: The parameters variation imposed on the specimens tested are not contextualized nor justified. It is important to state what variables are going to be studied and why. All the dimensions chosen for the specimens aren´t justified either: why 399 mm length? Why the welds and the thread positions chosen? …

This is not a standardized test, so it is recommended3.1 to clearly present how the design of the specimens was developed.

3. Section 2 – Specimen design.

Comment 2: No information is given on the production of the specimens. This type of assembly seems difficult to execute and it is important to show the technology involved in the process of fabrication and also the necessities and difficulties of the process.

Section 2.1 (lines 116-119)

Comment: Check the sentence: “The length of the specimen L = 399 mm, the diameter of the outer steel tube Dos = 133 mm, the diameter of inner steel tube Dis = 76 mm, the wall thickness of the outer steel tube tos and wall thickness of the inner steel tube tis are 6 mm, the wall thickness of outer tube of inner lining tube tois and the wall thickness of inner tube of inner lining tube tiis are 8mm.”

From this paragraph, I understand that the thicknesses of the inner tube and the outer tube are both 6 mm. However, all the pictures show an outer tube with much larger thickness than the inner tube. Please clarify and correct.

4.     Section 2.1 (Table 1)

Comment: The parameter h, which values 0.6 or 0.9 mm is not defined anywhere. What does it stand for? Add the information in the text and also in a figure, so that it is clear.

5. Section 2 - Figure 2.

Comment: This figure should also include the cross section. The geometry of the specimens is not totally clear. A representation of the concrete filling should also be included, as well as a more precise representation of the inner lining tubes. At first sight, it is difficult to fully understand the figure and the needed information to read the paper.

6. Section 2.4 – test set-up

Comment: The positions and orientations of the strain gauges are not clear.

7. Section 2.4 – test set-up, lines 147-151

Comment: The loading sequence is not clear. What is the velocity in the initial phase of loading? The test is load controlled or displacement controlled?

8. Section 3.1”Failure modes”

Comment: The description of the three failure modes identified should be improved for clarity. Text in this part also needs revision.

9. Section 4.1

Comment: Use the same load scale in Figure 13, Figure 14.a and Figure 14.b, so that it is easier to establish a comparison between the results obtained. Also, check the steps of the load scale, they have a variation of 900 kN, which makes is difficult to read the graph with enough precision. In Figure 14.a, it is impossible to compare specimens M6 to M9 and in Figure 14.b, it is impossible to compare specimens E6 to E9. I suggest a major revision on how these graphs and the corresponding information is presented.

10. Section 5.2 “thread position” (lines 278-279)

Comment: Check the sentence: “End connections have a greater impact on the stiffness and bearing capacity of the specimen.” As far as I can see, end connections always present higher load capacity, but stiffness seems quite similar between End connections and middle connections. Check the text and improve the analysis with some calculations that can justify the affirmations made. Again, I suggest a revision on the load scale of Figure 17.

11. Section 6 Calculation of Load Bearing Capacity

Comment: The load capacity is calculated corresponding to a strain of 6500 microstrains. There isn´t a clear reason for assuming this value and the authors need to justify properly their assumptions, because, as far as I can see in Figure 15, at 6500 microstrains the specimens are well in the plastic stage.

12. Section 6 – Table 3

Comment: The equations used to prepare Table 3 are just referenced and not presented in the paper. Then, it makes it difficult to understand what is presented

13. Section 7 – Conclusions

Comment: Conclusions seem short and do not cover some of the main aspects of the paper that deals with the threaded connections. A full revision of conclusions is recommended.

The quality of english language should be improved, in overall. However, the quality of english text is not the same along the paper. A major revision is needed in the Abstract. Section 1 is much better and then Section 2 and the following sections need carefull revision.

Author Response

We have responded to the questions raised by the reviewers one by one, please refer to document Response to Reviewer Comments

Author Response File: Author Response.pdf

Reviewer 2 Report

1. The introduction is very long, but it did little to help me understand the content of the article. An extreme example are long descriptions of selected articles [15, 16], without presenting the purpose of this article.

2. Please indicate clearly in the manuscript its purpose and scientific significance; the potential impact of the manuscript on the development of science.

3. The introduction presents the advantages of the described structures, but in my opinion this is not an objective view. There is a lack of a critical look at the presented solutions. There is no information about the behavior of structures during fire, no information about technological and assembly difficulties, etc.

4. Section 2 should definitely be improved. The description is unclear. The description of the examined elements was limited to one paragraph (page 2, 112-121), and figure 2 is poor and incomprehensible. There is also no detailed discussion of the selection of sample geometry.

5. Table 2: "inner inner lining tube"?

6. Under table 2, it is stated that the Young's modulus of concrete is 36 GPa. At the level of a scientific article, I would expect greater precision in how the modulus is defined, especially since the modulus stress-strain relationship in concrete is non-linear.

7. Page 6. I don't understand the sentence "During the entire loading process, within the elastic range, the preload amplitude is 1/10 of the estimated bearing capacity until it reaches 60% of the estimated bearing capacity."

8. Page 6, line 155: "The first mode corresponds to C, MW and EW."  I can't find anywhere where it is described what the C, MW and EW samples are characterized by.

9. Pictures and charts are provided later in the manuscript. The whole was concluded with two points that contain more or less obvious observations. Thus, the poorly defined purpose of the manuscript was highlighted in the conclusions.

1. I have doubts about the correctness of the title (from a linguistic point of view). Of course, I finally understood what the article was about, but the title itself was not clear and did not help in understanding the content.

2. The authors use terms such as "the lined threaded connection", although they do not really explain what they mean (in the initial phase of the article). This does not help in understanding the article and also raises some linguistic doubts.

Author Response

We have responded to the questions raised by the reviewers one by one, please refer to document Response to Reviewer Comments-1

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The authors introduced significant corrections that improve the readability of the work and ease of understanding it. There are additional photos that explain how to make the described connection. The authors also added sentences specifying the problem or explaining the observed phenomena. In the current version, the manuscript is suitable for further processing. The paper requires a minor editorial correction.