Evaluation of the Ultimate Collapse Load of a High-Voltage Transmission Tower under Excessive Wind Loads

Round 1

Reviewer 1 Report

1. Please write some points about why this research is important in the introduction and about transmission towers and what they are used for.

2. How did you determine that transmission towers are only impacted by excessive wind? It may also deteriorate due to precipitation, and the steel's quality will likely decline, correct?

3. Please rephrase the abstract (which looks ordinary) to make it more professional in order to improve the quality of the paper.

4. Please cite the source for Figure 1, and the introduction reads more like a general literature review without any flow; therefore, it is suggested that the introduction  be rewritten to meet journal standards for flow.

5. How wind load was applied to the tower until its ultimate failure progressively or immediately

6. Materials and methods look very unexceptional and are not structured properly, so it is advised to use more technical language to improve quality.

7. How were the input parameters for the Abaqus model determined, whether based on previous full-scale experiments or the code book and available resources?

8. Adding a future scope and recommendations section after the discussion will enhance the quality of the article even further.

Author Response

Comment 1: Please write some points about why this research is important in the introduction and about transmission towers and what they are used for.

Response: We have tried to improve our introduction, in order to address the mentioned points.

 

Comment 2: How did you determine that transmission towers are only impacted by the excessive wind? It may also deteriorate due to precipitation, and the steel's quality will likely decline, correct?

Response: The site of the collapse could be visited immediately after the event, and there was no icing or snow load found on the conductor cables. This is mentioned in the manuscript.

The material was analysed in a separate study, but as the material examinations did not give rise to any concerns, we didn't want to mention it here.

 

Comment 3: Please rephrase the abstract (which looks ordinary) to make it more professional in order to improve the quality of the paper.

Response: Thank you for your feedback. We have tried to improve the abstract.

 

Comment 4: Please cite the source for Figure 1, and the introduction reads more like a general literature review without any flow; therefore, it is suggested that the introduction be rewritten to meet journal standards for flow.

Response:  Thank you for the comment. Fig. 1 is our own work (or material from our partners, who don't want to be mentioned by name), therefore we cannot cite it more. We had the opportunity to do the inspection on-site personally just after the event had occurred. Most of the pictures were taken by the authors.

We have tried to improve the introduction.

 

Comment 5: How wind load was applied to the tower until its ultimate failure progressively or immediately?

Response: In the LBA the load (and so also the wind) was applied constantly, in RIKS the wind load is progressively increased. (Imperfections was calculated based in a LBA), line 507-509, first paragraph in Section 4.3.

 

Comment 6: Materials and methods look very unexceptional and are not structured properly, so it is advised to use more technical language to improve quality.

Response: We have tried to improve this paragraph.

 

Comment 7: How were the input parameters for the Abaqus model determined, whether based on previous full-scale experiments or the code book and available resources?

Response: Thank you for your comment. Most of the parameters were developed in the small-scale models described in the first part of the paper, i.e. in the single truss and in the planar lattice sub-assembly. Partly they were verified by data from literature.

 

Comment 8: Adding a future scope and recommendations section after the discussion will enhance the quality of the article even further.

Response: Thank you for the comment, we have tried to amend the discussion, and address some points in the conclusion part.

Reviewer 2 Report

The manuscript reports an evaluation of the ultimate collapse load of a high-voltage transmission tower subjected to excessive wind loads. The study proposes modeling methods for lattice towers under wind loading which are affected by structural instability, with the final purpose of being applied to a failure analysis case, specifically in the estimation of ultimate load capacity and failure position. The topic is very interesting since very limited literature are devoted to this area. The manuscript is well-written in general, however, the following points should be carefully addressed before any consideration for publication.

 

 

1.     In the abstract, please highlight the major findings more clearly.

2.     The literature review can be updated. No paper that published in the last year is cited in the manuscript. Please update the literature to reflect the current state of the art. Moreover, for this type of study a focus on progressive collapse is insightful. The manuscript can be enriched in both literature review and discussion. The following references can be consulted:

Hughes, W., Zhang, W., & Lu, Q. (2022). Fragility Analysis of Transmission Tower-Line System under Multiple Environmental Loadings. In Electrical Transmission and Substation Structures 2022: Innovating for Critical Global Infrastructure (pp. 207-222). Reston, VA: American Society of Civil Engineers.

Kiakojouri, F., De Biagi, V., Chiaia, B., & Sheidaii, M. R. (2022). Strengthening and retrofitting techniques to mitigate progressive collapse: A critical review and future research agenda. Engineering Structures, 262, 114274.

Russell, J. M., Sagaseta, J., Cormie, D., & Jones, A. E. K. (2019). Historical review of prescriptive design rules for robustness after the collapse of Ronan Point. In Structures, 20, 365-373.

3.     At the end of the Section 1 please focus more clearly on the challenging ideas and the main contribution of the current study.

4.     In technical literature, in bi-linear curve for steel usually hardening behavior is also included. The "Bi-linear" model in the current study is actually an elastic-perfectly plastic model. Anyway, which material model is adopted in the Section 4? Such details shoul be clearly provided.

5.     In the Section 2, please just focus on the methods and technics you actually used in the current study, not a literature review and over-focus on the solver's options.

6.     The entire methodology (which analyses on which models and why) of the manuscript is a little clumsy. It's simply because the manuscript mainly devoted to different modeling methods, and for such type of study it’s a common issue. However, adding a flowchart can clarify this and enhance the readability of the manuscript.

7.     The huge impact of mesh size, element type and different imperfections are well-known facts. Please be more specific in the conclusions and avoid general comments.

8.     In comparison of numerical results to their experimental equivalent a well-known pattern can usually be traced; numerical models show stiffer response. It is mainly due to the fact that a numerical model is a perfect model, while different types of imperfections, especially in material, are always involved in experiments. Considering the results reported in the Figures 7 and 9, can the Authors explain why the initial stiffness of experimental model is higher than all of the numerical models?

9.     Can we use the legend in the Figure 10 for all three numerical models?

10.  Discussing a structural collapse behavior, without understanding its design details almost impossible. In this regards, some backgrounds referring to the primary design of towers, code requirements and design loads are necessary.

11.  Since no dynamic analysis is performed in the numerical study, and wind load is essentially a dynamic load, do the Authors consider dynamic effects directly or indirectly in the analyses? For example by a dynamic increase factor or based on the code recommendations?

Author Response

General comments: The manuscript reports an evaluation of the ultimate collapse load of a high-voltage transmission tower subjected to excessive wind loads. The study proposes modelling methods for lattice towers under wind loading which are affected by structural instability, with the final purpose of being applied to a failure analysis case, specifically in the estimation of ultimate load capacity and failure position. The topic is very interesting since very limited literature are devoted to this area. The manuscript is well-written in general, however, the following points should be carefully addressed before any consideration for publication.

Response: Grateful appreciation for your positive comments on our manuscript and thoughtful suggestions. Following your advice, we have carefully revised the manuscript. By doing so we feel that the manuscript has been strengthened considerably. Hereby we provide our response to your comments point-by-point as follows:

Comment 1: In the abstract, please highlight the major findings more clearly

Response: Thank you for the comments. We have partly rewritten the Abstract and tried to improve it.

 

Comment 2: The literature review can be updated. No paper that published in the last year is cited in the manuscript. Please update the literature to reflect the current state of the art. Moreover, for this type of study a focus on progressive collapse is insightful. The manuscript can be enriched in both literature review and discussion. The following references can be consulted:

• Hughes, W., Zhang, W., & Lu, Q. (2022). Fragility Analysis of Transmission Tower-Line System under Multiple Environmental Loadings. In Electrical Transmission and Substation Structures 2022: Innovating for Critical Global Infrastructure (pp. 207-222). Reston, VA: American Society of Civil Engineers.
• Kiakojouri, F., De Biagi, V., Chiaia, B., & Sheidaii, M. R. (2022). Strengthening and retrofitting techniques to mitigate progressive collapse: A critical review and future research agenda. Engineering Structures, 262, 114274.
• Russell, J. M., Sagaseta, J., Cormie, D., & Jones, A. E. K. (2019). Historical review of prescriptive design rules for robustness after the collapse of Ronan Point. In Structures, 20, 365-373.

Response: Thank you for the comments. Hughes et al. is certainly a valid reference to complement the discussion and suggest further improvements in the design process. The other papers, however, seem less relevant to the authors, as the lattice tower is not in the same category as high-bay warehouses or multi-story buildings (it is in a way an extreme case of the latter). It is more difficult to build redundancies or measures to prevent progressive collapse. The tower by its nature just stands on 4 main legs or pillars. If one of the main legs is affected by buckling during the collapse, the total failure is programmed, as the centre of gravity will move to the edge of the remaining support triangle. Hence, only secondary lattice elements (trusses and beams) may fail without total collapse, and these secondary failures may be protracted.

 

Comment 3: At the end of the Section 1 please focus more clearly on the challenging ideas and the main contribution of the current study.

Response: Thank you for the comments. We have tried to amend the end of Section 1.

 

Comment 4: In technical literature, in bi-linear curve for steel usually hardening behavior is also included. The "Bi-linear" model in the current study is actually an elastic-perfectly plastic model. Anyway, which material model is adopted in the Section 4? Such details should be clearly provided

Response: Thank you for the comments. Elastic-perfectly plastic might be considered a special case of bilinear. But we have clarified this in the text.

 

Comment 5: In the Section 2, please just focus on the methods and technics you actually used in the current study, not a literature review and over-focus on the solver's options.

Response: Thank you for the comments. We have shortened this part accordingly.

 

Comment 6: The entire methodology (which analyses on which models and why) of the manuscript is a little clumsy. It's simply because the manuscript mainly devoted to different modeling methods, and for such type of study it’s a common issue. However, adding a flowchart can clarify this and enhance the readability of the manuscript?

Response: Thank you for the comments. Unfortunately the time for review was too short to amend the manuscript with such a certainly helpful chart. The presented technique is not standardised for the design phase. We have improved it in the last paragraph of Chapter 2.

 

Comment 7: The huge impact of mesh size, element type and different imperfections are well-known facts. Please be more specific in the conclusions and avoid general comments.

Response: Thank you for the comments. We have tried to address this point. As shown in Table 1 / Fig. 6 not all the buckling modes can be correctly mapped and represented with the various element types. Also the mesh size has to be determined for the specific analysis task.

 

Comment 8: In a comparison of numerical results to their experimental equivalent a well-known pattern can usually be traced; numerical models show a stiffer response. It is mainly due to the fact that a numerical model is a perfect model, while different types of imperfections, especially in material, are always involved in experiments. Considering the results reported in the Figures 7 and 9, can the Authors explain why the initial stiffness of experimental model is higher than all of the numerical models?

Response: Thank you for the comments. Unfortunately this could not be fully explained. We only used Kettler's experimental results [10] (and [11]) for validation purpose. Kettler shows the same discrepancy with his own models, and we could not explain it with these papers [10, 11] either (we have tried to contact the author).

 

Comment 9: Can we use the legend in the Figure 10 for all three numerical models?

Response: Thank you for the comments. Yes this is correct. Only in the figure caption it is phrased "From left to right:   " [we have omitted (a), (b), (c) notations]

Comment 10: Discussing a structural collapse behaviour, without understanding its design details almost impossible. In this regards, some backgrounds referring to the primary design of towers, code requirements and design loads are necessary.

Response: Thank you for the comments. We have tried to mention the relevant standards, and we have provided a general overview on the wind loads estimation. It's correct they are not clearly listed anywhere in the paper. Finally, a Failure Analysis is not primarily a verification of standards, but a proof of plausibility (however, by applying the state-of-the-art in analysis and mechanical design). Furthermore, we are not experts in the Design of such structures either. It was not our ambition to improve the design process.

 

Comment 11: Since no dynamic analysis is performed in the numerical study, and wind load is essentially a dynamic load, do the Authors consider dynamic effects directly or indirectly in the analyses? For example by a dynamic increase factor or based on the code recommendations?

Response: Thank you for the comments. We just considered constantly blowing wind, as we didn't have any information or measured data on wind gusts and turbulences. Factors accounting for dynamic spectrum of the winds are indeed used in the designing phase. It is also correct that numerical models are developed where dynamic wind is considered. But in this failure analysis case, the purpose would be to prove that the collapse occurred just with the lower range of plausible wind scenarios, i.e. constant static wind acc. to the lowest assumptions in the standard. Hence, additional turbulences just would have speeded up the collapse. In the manuscript, we rewrote the statement as "Note that for EN 1991-1-4, the load is much larger if the peak dynamic value is considered. For this study, the mean wind pressure value based on this standard is used instead, which represents the lower value of the realistic range" line 434-437.

Round 2

Reviewer 1 Report

The authors addressed all the comments and the article can be accepted in present form. 

Reviewer 2 Report

The manuscript is sufficiently improved to warrant publication.