Mechanics of Screw Joints Solved as Beams Placed in a Tangential Elastic Foundation

Round 1

Reviewer 1 Report

The paper describes an analytical approach to study the behavior of screw joints under tension. The research adopts a tangential elastic foundation model to predict the interaction between screw and base material (wood), in terms of force transmission, deformation and stress states. In any case, the proposed solution requires the prior performance of a tensile test. The results of the same are used both to configure the analytical model and to subsequently evaluate its accuracy.

The methods are not new, but the approach adopted can be considered innovative. Additionally, and as the authors point out, the solution entails an important simplification compared to other methods (FEM).

Both the title and the abstract provide an appropriate description of the research. The methodology used is correct and, in general terms, the conclusions conveniently synthesize the results obtained.

Consequently, our opinion is favorable to the publication of the paper. However, minor revisions and several suggestions are provided in order to improve the final writing of the article.

GENERAL COMMENTS

The paper focuses mainly on the behavior of screw joints in pieces of wood (Picea abies). As indicated (page 17), “the issue of bones is not the main subject of this article and therefore the results of bone screw joints are presented very briefly”. Indeed, the associated description is extraordinarily concise, far from the usual academic soundness. However, figures 2, 12 and 13 and references 3, 7, 13, 14, 16, 23 and 26 are explicitly linked to biomechanical issues. This question seems excessive, in particular given that some relevant references are missing, as well as illustrative images, in both cases specifically related to the part of the research described.

It is evident that the doctoral thesis included as reference [20] can be used to expand the information contained in the paper. However, in our opinion, the document should present all the relevant information without the need to repeatedly point to that reference (pp. 1, 2, 5, 6, 8, 9, 10, 12, 13 and 14).

Figures 3 and 12 do not seem to correspond directly to the exposed research. Consequently, it is worth asking if they have been made by the authors or come from an external reference (in which case it should be indicated, as it has been done in figure 2). Additionally, the set of figures can be improved (resolution, font size, ...).

Table 2 shows the results of the 13 tests carried out with the wood specimens. They correspond to 8 configurations of screw type, screw material and pre-drilled hole diameter. However, it is not clear why two or three tests are done in some configurations and only one test in others. In any case, the application of the proposed method requires the performance of a previous test to determine the peak values ​​(F_E, u_E) (section 12, result number 6). Therefore, the study carried out can show a certain accuracy in order to reproduce the behavior of the joint, and obtain useful conclusions, but always starting from this preliminary test. It should be clear that this cannot be omitted in practical cases, given the small number of tests of each configuration, and the scattering that usually accompanies this type of problems.

The authors understand that the proposed analytical model is suitable in isotropic / orthotropic / anisotropic materials (pages 12 and 13). However, it should be noted that the research only addresses two types of screws in Picea abies wood samples (as indicated, the information on bone tests is very limited). It would be interesting to complement the experimental campaign with other materials and configurations: the authors correctly point out that the screw - wood interaction is influenced by multiple factors (page 6), which affect the results of the experiments.

SPECIFIC COMMENTS

The title of the first section (introduction) is provided in lower case, while the rest are in upper case. It would be convenient to standardize criteria according to the journal.

In expression (2), it is advisable to separate the first part (N/EA) with respect to the comma (,) to facilitate readability and not to be confused with (A'). Similar considerations can be applied to expression (6) (page 9).

It is recommended to complete the information on the screws used (characteristics and photographs), regardless of whether the reference [20] or the supplier's website can be consulted (page 5). Something similar can be pointed out with regard to the universal testing machine and the wood used, beyond the fact that it presents a standard quality and has been purchased at the supermarket (page 6).

It is recommended to supplement the information on the extraction test (page 6), for example with reference to the regulations used.

Figures 7 to 9 present results in (F-u_B) terms. The first shows the values ​​of test number 1, the eighth expands the initial part of the graph (in a generic way, without relation to any specific test), and the last one presents, again in relation to test 1, the results superimposed on the regression analysis. It would be advisable to reconsider these figures to show the results associated with the remaining categories of unions. Likewise, we consider that the precision of the model should be statistically estimated in relation to each of the tests carried out.

HB 6.5 screws are mentioned on page 5. However, both table 2 (page 8) and the conclusions (page 15) indicate 6,5. It would be convenient to unify the designation (dot instead of comma).

Figure 11 can be redrawn to facilitate its interpretation (with the current configuration the plots are almost horizontal). It is suggested to modify the width-height ratio of the chart, for example.

It is also recommended that sections 12 (results and discussion) and 13 (conclusions) be revised in light of possible changes derived from previous comments. The last paragraph of the conclusions, in particular, is excessively generic.

In reference [1] "Gluedin" should be replaced by "Glued-in".

Unless the editors show different criteria, it is not necessary to provide ISBN / ISSN in the references.

In reference [26] “van” must start with a capital letter (“Van”).

Author Response

Dear reviewer no. 1

Thank you for your work. I am sending my response to your Review.

With regards

Author Response File: Author Response.pdf

Reviewer 2 Report

My congratulations for the very interesting work. The pullout behavior of screws inserted in wood have been one of the subject most addressed in Timber engineering so any contribution in this topic is welcome. Moreover, there is a great practical value because those kind of joints are very common o timber construction.

I have just some few comments, and all of them addressed to the timber "part". 

• How the "fixed end" of the specimens (see Figure 6) was materialized? glued? clamp?
• Wood is very sensible to some parameters as for example moisture, wood density and grain direction. Could it be possible to have the moisture content and density of the wood specimens tested? About the grain direction, it is necessary to make clear that the screw is insert in the grain direction. Please this into account in all schemes, and figures.
•  It is important to note that the number of tests on wood specimens (13) is short.
• It would be interesting, but not fundamental, to compare the ultimate value (Fe)with the expressions proposed by the Eurocode 5 for the design of such screwed joints. 
• Can this model can be apply to predict the Fe (pull-out resistance or, the withdrawal resistance as mentioned in the design codes)?

Author Response

Dear reviewer no. 2

Thank you for your work. I am sending my response to your Review.

With regards

Author Response File: Author Response.pdf

Reviewer 3 Report

The paper deals with a new original analytical solution of deformation, force and stress states in wood screw joints up to the limit values of pulling-out/breaking the screw. The experimental verification of the proposed models using pull-out tests (i.e. pulling-outs crews from dry spruce wood in laboratory conditions) confirms the correctness of the proposed models of the elastic linear/non-linear foundation. The validity of the model is also analytically and experimentally verified in the biomechanical model of pulling out screws from the femur of a bovine/human cadaver, which confirms and expands the validity of newly designed screw joint models outside the timber structure area.

The paper topic is interesting and the manuscript is comprehensive. However, some points should be reviewed before publication:

1. The introduction of the paper is superficial. This should be developed and documented based on several bibliographical references. I propose to the authors that from sections 1. Introduction and 2. Motivation to make a single section (Introduction).
2. In the opinion of the reviewer, the paper is structured on many sections (13). Some sections could be merged. The authors should review this issue.
3. The conclusions are simplistic, observational without revealing findings of generic value. Based on the results obtained some generic and fundamental conclusions need to be drawn.

Author Response

Dear reviewer no. 1

Thank you for your work. I am sending my response to your Review.

With regards

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

Authors did not address all the comments that I did in my first review. For example, the point 2: In the opinion of the reviewer, the paper is structured on many sections (13). Some sections could be merged. The authors should review this issue. The revised paper includes 12 sections.

Other comments did not receive the expected response. For example, the introduction was not developed as required. The authors limited themselves to adding more references, but the comparison with these studies is missing.  Also, the comments in point 3 did not receive the expected response. 

Due to these reasons, I consider that the paper is not acceptable for publication.