Damping Assessment of Lightweight Timber Floors Under Human Walking Excitations

Round 1

Reviewer 1 Report

This paper aims at assessing damping ratios for timber floors subject to human walk excitations. Several experimental tests are carried out using two well-known methods – namely, the Enhanced Frequency Domain Decomposition (EFDD) and the Subspace Stochastic Identification (SSI) methods – which, as such, do not constitute any true scientific novelty. Hence, I don’t think that the paper deserves a publication as a research article.

Also, the scientific rigorousness of the paper seems questionable to me, i.e.:

(1) Page 2, line 69: the sentence “damping ratios induced by … excitations” which is wrong given that damping ratios are intrinsic properties of structures, i.e., they do not depend on the excitations.

(2) Page 8, line 214: the white noise assumption for the excitations – which is that used by EFDD and SSI – which, from my point of view, does not correspond to a human walk.

(3) Figure 4 (space discretization issue): the small number of sensors/accelerometers used which can penalize the determination of the mode shapes (EFDD), and further the determination of the damping ratios.

(4) The fact that the underlying mathematical models of the EFDD and SSI are based on linear equations (which is an assumption), which might not be true for timber structures subject to human walk excitations.

(5) Section 2.4: the so-called “dynamic serviceability parameter” whose physical meaning does not appear straightforward to me.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript examined the damping of lightweight timber floors built in Chile through lab and in-situ experimental tests. It is no wonder that the damping of the floors with human walking is higher than that exited by an impact hammer or a shaker. The experimental results are validated but the reason for the increasing damping effect is not very well explained.

The reviewer is not familiar with the floor vibration design regulation in Chile, however, both EU and NA use different approaches to deal with timber floor vibration by limiting the frequency and deflection. Damping is important but cannot be assessed. The acceleration based design method require too much effort for practical design. Can the authors justify the objective of this study? How can the results help developing design methods?

During the tests, were there any subjective evaluation? The floor spans are relative short. It is a question whether they had vibration issues.

The methods part emphasizes too much on the two methods for evaluating damping ratios. 

Table 6 and Figure 13 are presenting the same information.

The authors may have to look into human structural interaction for a better explanation of the increased damping.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

This is a comprehensive study although the topic is not very new. The contents are well presentation. Some improvements in language is desired.

Author Response

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Author Response File: Author Response.pdf

Reviewer 4 Report

introduction refers disorderly to different papers. A better consequential and logic construction would be preferred. For instance, it starts with a huge "collective" reference [2-19]. Then, it cites again [2-13] (without any specialization) and move to [14-19] giving details and opinions only on three of them (16, 18 and 19 to come back to 14 in the end).

The Introduction shall be therefore rewritten, dedicating some details to the description of the references, why they are important for the presented work and what are the weak points (if any) that are aimed at being overcome in the paper.

The more, it should be clearly stated "what is the novelty brought by the paper" or "what is the contribution to the state of the art" of this paper.

The experiments description is very well detailed. In spite of that a logical scheme of the instrumentation deployed is missing. It is suggested to complete the description by inserting a proper block diagram.

The sentence "there is usually a high scattering in the values reported by other researchers" is almost trivial. damping is a parameter that is strictly correlated to the specific characteristics of the construction, the installation and even the surroundings to certain extent, so that it is practically intrinsic to have different damping values for similar constructions in different sites or labs!

"Therefore, to compare results, it is always advisable to use more than one technique to estimate damping following different mathematical approaches." This sentence is not correlated with the former. Using different approaches to estimate the damping value (or any other physical parameter) is always wise. But this is not "because different researchers have different estimates" but because the evaluation methods do not guarantee the fully correctness of the results. Did the authors estimate the "confidence"  (percentage scattering) of the methods they applied? It should be interesting to know.

Is "the first resonant mode" the authors studied, the first with relevant participation of the timber floor or the first of the whole system? What is the modal density? Is there any numerical prediction that sustain the hypothesis "it is the one that mostly contributed to general dissipation"?

Such kind of sentences are repeated a lot all over the text. It is recommended to review critically the paper and:

give evidence and substance to any single statement;

limit the assumption to what is reasonably certain.

Lines 172-182 refer to a logic process that would be better illustrated by a logic diagram.

"In addition, considerable attention must be paid when a frequency resolution is selected. It has been shown that estimated damping ratios decrease and converge to a reasonable value when the frequency resolution improves."  This is due to the fact that damping is derived by the FRF shape, with specific reference to its poles. therefore, the higher is the resolution, the better (and the lower) the damping estimation will be. In this case, the statement would have sound better like "...because the damping estimates improves with the frequency resolution, [reference(s)], its value waas set to xx Hz, (depending on the reference structure characteristics... generally modal frequencies and modal density)..."

It is always bad to see screenshots in a paper (Figure 8).

From this point over I just viewed the other part of the text fast.

In line with what seen before, te description of the experiment is accurate, in spite some details are completely missing... as the layout of the accelerometers, the block diagram of the instrumentation, pictures of the experimental set-up, and so on.

Is there a numerical model of the system? what were its results?

Conclusions are ok.

In synthesis, I would say that the paper shows good material for being presented again to the journal, once it is deeply reviewed as contents and form.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Again, I don’t recommend the publication of the paper for the same reasons as those given in my previous review report:

- The sole novelty consisting in (only) applying the EFDD and SSI methods to analyze timber floors does not motivate, from my point of view, a research paper, but rather a technical report.

- Again, I don’t agree with the authors’ response when it is said that “different types of excitations could activate different sources of damping”. I think they confuse the modal properties of structures (eigenfrequencies, mode shapes, modal dampings), which do not depend on the excitations, with the dynamic response analysis when the excitations “activate” one or several mode shapes (mode superposition).

- Regarding white noise assumption, this would be the case (although I’m not 100% sure) if many people were randomly walking (random frequencies) on a floor at the same time. I don’t think this matches the kind of applications proposed in the paper (e.g., Figure 4(a)).

- …

Author Response

Dear Editor,

Regarding to Point 1, we thank you for considering as valuable the experimental and practical contribution of our paper. We hope it help other researchers to increase the knowledge in our field.

Your commentaries related to Point 2, has been addressed in the paper (manuscript´s line 45-50 highlighted in green) as follow:

“Under human walking operational conditions, damping can be generated due to the combined effect of different small sources of non-linearity, such as, joints friction, interaction between structural and non-structural elements, and the effect of floor content. Hence, operational damping is not only generated by the intrinsic modal damping of the structure, but also because of all these other energy dissipation mechanisms. In terms of structural performance, operational damping is more interesting to analyze than intrinsic modal damping, because this is the effect perceived by people”.

Your suggestions related to Point 3, has been included in the paper (manuscript´s line 217-221 highlighted in green) as follow:

“According to [14], mobile loads produced by many people randomly walking on a floor at the same time can be classified as multiple-input loads, which are an excellent alternative to meet the assumptions of white noise for excitations. The walking paths showed in Figure 4(a) and Figure 7 is not exactly a random excitation, but it is suitable to satisfy the white noise excitation assumption in the context of fast in-situ testing”.

We expect that these commentaries we have added, adequately satisfied your requirements and now the article would ready for publication. We appreciate all your comments and they surely help to improve the quality of our paper.

PS: We have not prepared a proper reply to each reviewer, because we assume we need to respond only to your commentaries. In case it is necessary, please let us know and we will prepare these documents.

Reviewer 4 Report

The authors duly took into account the suggested recommendations. In this reviewer's opinion, the paper is now ready for publication.

A particular appreciation for the rapidity of the answer.

A fast review of the text should help in overcoming some minor typos or English misuse (for instance: The judgment made is of the probability that the  determined vibration dose might result in an adverse comment by those who experienced it... inn this and other cases, a reformulation is necessary).

As a general recommendation: please would you keep your statements simple.

Author Response

Dear Editor,

Regarding to Point 1, we thank you for considering as valuable the experimental and practical contribution of our paper. We hope it help other researchers to increase the knowledge in our field.

Your commentaries related to Point 2, has been addressed in the paper (manuscript´s line 45-50 highlighted in green) as follow:

“Under human walking operational conditions, damping can be generated due to the combined effect of different small sources of non-linearity, such as, joints friction, interaction between structural and non-structural elements, and the effect of floor content. Hence, operational damping is not only generated by the intrinsic modal damping of the structure, but also because of all these other energy dissipation mechanisms. In terms of structural performance, operational damping is more interesting to analyze than intrinsic modal damping, because this is the effect perceived by people”.

Your suggestions related to Point 3, has been included in the paper (manuscript´s line 217-221 highlighted in green) as follow:

“According to [14], mobile loads produced by many people randomly walking on a floor at the same time can be classified as multiple-input loads, which are an excellent alternative to meet the assumptions of white noise for excitations. The walking paths showed in Figure 4(a) and Figure 7 is not exactly a random excitation, but it is suitable to satisfy the white noise excitation assumption in the context of fast in-situ testing”.

We expect that these commentaries we have added, adequately satisfied your requirements and now the article would ready for publication. We appreciate all your comments and they surely help to improve the quality of our paper.

PS: We have not prepared a proper reply to each reviewer, because we assume we need to respond only to your commentaries. In case it is necessary, please let us know and we will prepare these documents.