Study on Wind Loads of Different Height Transmission Towers under Downbursts with Different Parameters

Round 1

Reviewer 1 Report

The topic is of relevance to the journal of Buildings. The influence of different parameters in relation to downburst on the wind load of five transmission towers with different heights has been investigated. A comparison study between the downburst and normal wind has been carried out as well. Moreover, the responses of the transmission towers under steady-state and unsteady-state downbursts have been calculated and compared. The manuscript is clearly representing the research performed by the authors. However, some minor remarks should be resolved before proceeding to publication.

1. In Abstract, it is concluded that the responses from the normal wind load on the transmission tower become larger than those from the downburst when the height of the tower is not lower than 81.5 m in the last two sentences. Can this conclusion be generalized? Or is it only appropriate for this case (with the specific parameters of the wind and the structure)? Please clarify in the text. Moreover, the rations 0.95-1.05 are from Tables 3, 4, 5 and 6 if it is correct. If so, the rations should be in the range 0.91-1.01. Please confirm.
2. In Introduction, please state the novelties of this research in a more clear manner. It is interesting to see that no other studies have investigated the influence of the height of the transmission tower.
3. In section 4.1 (lines 210-215), it is not clear how the conclusions are obtained from Figure 4. I guess that the legend in Figure 4 is not correct. Please confirm. Please also confirm the content in the paragraph in lines 218-229, as well as the legend in Figure 5.
4. In section 4.1 (lines 218-229), it is concluded that the most unfavorable wind direction angle is 45 degree. It is important to clearly understand how the transmission tower in Figure 2 are placed in Figure 3a. Please clarify. Moreover, as only four points are presented in Figure 5, how does it be determined that the most unfavorable value is 45 degree but not other values (e.g. 30 degree)?
5. In section 4.1, the influence of wind field parameters are investigated. The maximum internal forces in the structural system are presented. Could you provide the information about the locations where the maximum internal forces are recorded? As different heights for the transmission towers are taken into account and therefore different geometrical dimensions (i.e. different locations for the five different towers in a same wind field), could you give some comments on the influence of these effects?
6. In section 4.2 (in line 253), the wind direction angle of 0 degree is adopted. However, 45 degree has been found to be the most unfavorable condition in the previous section. Therefore, why does not use the value of 45 degree？
7. In section 4.2, the responses in case of downburst are larger than those from normal wind. Could you add more discussions on this? For example, as shown in Figure 1, the normal wind is first smaller than the downburst but becomes larger after a certain height, which shows a same trend comparing to the responses in this section. Is it possible that there are some links between the two situations? The height of tower 5 is 81.5 m, where different results are obtained. Can this value be generalized? Or there exists a specific height which is a function of wind field parameters?
8. In section 4.3 (line 320), should ‘Fig. 9(a)’ be ‘Fig. 8(a)’? Please also check the caption of Figure 8. Moreover, what does ‘100/200 point smoothing’ mean in Figure 8(b) as no explanations can be found in text? Please confirm.
9. In section 4.3 (lines 336-352), why tower 2 has different response comparing to tower 1, 3, and 4? Which is different from the conclusion obtained in previous sections.
10. The texts in the figures (e.g. Figures 1, 3b, 4, 5, 6, 7, 8, and 9) are too small, which make readers hard to read. Please revise and present the figures in a better way.
11. In section 4, it is weird that the last two rows in Tables 3, 4, 5, and 6 are in the same row, which are different to other rows. Is this in purpose?

Author Response

1.When the transmission tower is more than 81.5m, it is debatable whether the effect of the boundary layer wind is greater than the downburst. The conclusion conducted in this article based on this case. But it can be a suggestion for the design load of the tower under downburst. Regarding the ratio, the original intention of the conclusion given in the article was only to give a reference value of an approximate range. Here should still be scientifically rigorously revised to 0.91-1.01

2.I have revised it in the new revision.

3. The number of the tower was marked in the figure. Different tower number means different height, so that the height was directly used in the analysis. The meaning is unclear. Here I have revised in the new vision.

4.I’ve added some descriptions of the location of the transmission tower to this paper. And I deleted the analysis of the wind direction angle. There was not enough information about the wind direction angle to draw conclusions. In next stage I’ll add more direction angle to analysis.

5.In section 4.1, I have added more details about the internal forces. For second question in this point, the effects that different towers om different positions in the same wind field are exactly what this paper ask for. This study was order to find out the law between different size towers and the response of tower under downburst.

6. I endorse your suggestion. The analysis about the wind angle was not enough to get the conclusion that the most unfavorable wind angle is 45 degrees. Like point 4, I’ll add more direction angle to analysis in next stage. In section 4.2, the wind direction angle of 0 degree is adopted. This is not a problem. The main purpose of this study is to explore the parameters of the most adverse downburst. The most unfavorable levels of other factors can also be obtained while keeping the wind direction angle constant.

7.This is a really great advice. The wind profile shown in Figure 1 reflects changes in velocity of wind, which often directly affect the magnitude of the wind load on the transmission tower. Therefore, the response value here will also increase directly as the tower height went up. Finally, the fact that the wind speed of the boundary layer will be greater than that of the downburst after reaching a certain height, because the transmission tower height studied in this paper was limited and had not reached that height. So, there was no way to explore this point further. In fact, transmission towers rarely reach that height.

8.I have revised it in the new revision. And I have added more details about the ‘100/200 point smoothing’ in section 4.3.

9.In section 4.3, the wind field was replaced with the moving downburst. So that there was same different compared with previous section. The overall trend of the curve of tower 2’s response was unchanged though there was difference between tower 2 and the others.

10.I have adjusted in the new revision.

11.It was mistake in my work. There wasn’t any special meaning about this.

Reviewer 2 Report

The paper is focused on the study of wind load on transmission towers under downburst. The paper is really interesting and well-written. Some minor comments are :

• some typos have been found along the text. Please revise it carefully.
• What is the first period of vibration of the towers ? could dynamic instability affect the safety of the towers ? discuss in the manuscript.
• The transmission towers are made by L profiles. When you model this kind of members, you must be sure that the sotware is able to include also the warping deformation on the results (i.e. you need a 7dof beam element for the analyses). If you do not consider this aspect, internal forces and displacements may be  sligthly wrong. You should mention this in the paper. Please make reference to : 10.5937/jaes18-24459 and 10.1016/j.tws.2019.106290
•  How the foundation and the soil type affect the results ? discuss in the paper 

Author Response

1.Thanks for your checking. I have revised it in the new version.

2.This paper was mainly a suggestion for the design load of the tower under downburst. Dynamic instability is the ultimate bearing capacity of the tower. It’s beyond the consideration of this paper.

3.The transmission tower rods were simulated by the BEAM188 unit, which was a three-dimensional linear unit and was suitable for analyzing slender to medium-thick beam structures. BEAM188 has 6 or 7 degrees of freedom, and the change in the number of (degrees of freedom) is controlled by KEYOPT(1). When KEYOPT(1) = 0 (default), there are 6 degrees of freedom per node. The displacement along x, y, z and rotation around them are respectively. When KEYOPT(1) = 1, a seventh degree of freedom (warpage) is added. This paper was order to find out how the size parameters of the downburst and the size of the towers affect the wind loads to tower. Maintaining control variables can also get the most unfavorable level of factors.

4.The foundation and the soil type are not studied in this article. May we will analysis in the future.

Round 2

Reviewer 2 Report

Some parts of the text have been improved accordly to prvious comments. However, the Authors should better specify, directly in the text, if they considered or not the warping in the finite element analyses 

Author Response

Thanks for your suggestions! I have revised it in the new vision. 

The details about whether the warping in the finite element analyses were taken into account I have added to the modeling section of the article. The study was carried out based on the elastic analysis, and the warpage was not considered. But I carefully studied the two references you provided. For engineering, displacements and stresses should be calculated accurately. The main purpose of this study is to consider the influence of the parameters of the transmission tower and the downburst on the resistance of towers to the downburst critical current. Therefore, I think this point can be ignored in this article.