Ductile Moment-Resisting Timber Connections: A Review

Round 1

Reviewer 1 Report

This study covers a number of previous studies.But it is only a summary of the previous research, why the previous research is this phenomenon or the reasons for the conclusion did not involve, low ductility and other related issues to improve the method does not say.This article will be a good summary if we can sum up the research rules and put forward the solutions through the previous research.

Author Response

This study covers a number of previous studies.But it is only a summary of the previous research, why the previous research is this phenomenon or the reasons for the conclusion did not involve, low ductility and other related issues to improve the method does not say. This article will be a good summary if we can sum up the research rules and put forward the solutions through the previous research.

Reply: The authors agree that the manuscript must go further than to be a summary of the existing research. Existing research must be analysed and clear guidelines and recommendations for the next steps should be propose. For that, the manuscript presents a ductility assessment of the most relevant semi-rigid connections based on selected ductility factors and on the geometric properties of the connections. In this updated version of the manuscript, a new topic 5.1 Recommendations, was added:

“From the experimental data collected and analysed within this work, it is possible to trace a practical path to analyse the ductility of a timber connection. First, it is necessary to know the moment-rotation curve of the joint. From it, the initial stiffness and the yield point must be defined. Moment-rotation curves need to reach a clear plateau for yield point definition and should reach maximum rotation values close to 0.15rad (monotonic) and 0.10rad (cyclic), without significant loss of moment resistance at this ultimate point. Moreover, when tested, ductile connections should preferably have a ductile failure mode.

Even if different definitions of ductility exists, in the light of the above parameters, considering the two different expressions for the ductility (equations 2 and 13) it was possible to obtain coherent values for the ductility ratio (see figure 25). Although several semi-rigid connections have been studied and tested in order to evaluate their ductility, few comparisons between the different types of ductile timber connections have been made. From the state-of-the-art review performed, it was possible to identified the most used parameters for the definition of bolted slotted-in steel plates connections with common geometry (approximately 300mm for columns and beams height), obtaining as a reference the use of 8 bolts per connection, as well as STS close to the bolts, but only in sufficient quantities to prevent splitting and increase the rotational capacity of the connection.

In glued-in rods connections, it is recommended to apply a steel profile between the timber elements to obtain greater ductility and energy dissipation, with a small number of rods (four in most of the studies carried out). These connections can be the key point for the development of high and mid-rise timber building projects, using moment-resisting timber frames to enable more open and versatile architectural set-ups. The knowledge about the glued-in rods connections also contributes to the development of analytical procedures to calculate it for the determination of non-linear moment-rotation curves, as already have been applied in steel and concrete structures.

The analysis of the current state-of-the-art allows to indicate the need to study timber frames and/or buildings under lateral loading with the aim to assess displacement and stiffness responses of the connections.”

Author Response File: Author Response.pdf

Reviewer 2 Report

The paper is much more a description of the tests and results of other researchers than the original paper. The purpose of the work is not defined.

1. The paper is much more a description of the tests and results of other researchers than the original paper. The novelty of this research need to be addressed.

2. The purpose of the work is not defined.

3. The duplication with previous published works seems to be a little bit high. For example, please compare:

- “moment-resistant joint was developed by Dr. Bryan Walford 206 at NZ Forest Research Institute, in 1970, by employing multiple nails with” with “moment-resisting joint was developed by Dr. Bryan Walford at NZ Forest Research Institute in 1970 by employing multiple nails with” (Komatsu 2017);
- “Timber frame experienced large deformation due to the absence of brace or infill material” with “timber frame experienced large deformation due to the absence of brace or infill material” (Cao et al. 2021);
- “the dynamic behaviour of a semi-rigid moment-resisting dowel-type connection between timber beam and column” with “dynamic behaviour of a semi-rigid moment-resisting dowel-type connection between timber beam and column” (Solarino et al. 2017);
- “if a certain target displacement can only be achieved with excessive strength loss, the connection is not ductile” with “If a certain target displacement can only be achieved with excessive strength loss, the connection is not ductile” (Ottenhaus et al. 2021).

4. The citation style is different in Tables 5 and 6 and Figures 26 and 27 than in the rest of the article.

 

Author Response

The paper is much more a description of the tests and results of other researchers than the original paper. The purpose of the work is not defined.

1. The paper is much more a description of the tests and results of other researchers than the original paper. The novelty of this research need to be addressed

Reply: In fact, this manuscript consists in a state-of-the-art review. This is assumed in the abstract, along the text and in the title. This type of work is fundamental as one of the first steps of a wide research on semi-rigid timber joints. Despite being a recurrent topic for publications and research, most of the existing works focus only one type of connection, addressing a particular geometry and/loading, etc. The main strength of this manuscript is the novel discussion focused on ductility presented about the main semi-rigid connections used in timber frames, analysing their geometric and mechanical characteristics, their failure mode and the ductility factor according to two different methodologies. The manuscript also provides a summary of the values of the most important design parameters for designing a connection, which can serve as reference parameters for designers and researchers. This added information to the existing state-of-the-art is mainly addressed in the topic “5.1 Recommendations”.

2. The purpose of the work is not defined.

Reply: The authors agree that the purpose of the manuscript was not so clear. Therefore, some parts were changed in the revised manuscript to improve this information, in line 51:

“Despite their relevance, the ductile behaviour of MRTFs connections have not been in-depth discussed and explored. This paper presents a review of the main types of timber beam-column moment connections with improved ductility to study more about its mechanical behaviour and to identify gaps in some aspects that have not been studied. The main objective of this work is to evaluate the ductility of the selected connections based on the most relevant recommendations provided by different standards and guidelines. It is intended to provide a detailed comparison between the most common types of semi-rigid timber joints with improved ductility. Therefore, geometric parameters are presented and the connections behaviour under cyclic and monotonic load are described, identifying the failure modes obtained. This study of the existing knowledge is essential to evaluate the potential associated to semi-rigid timber joints within frame structural systems and allows to identify the research gaps for their implementation in practice, through design guidelines and recommendations.”

3. The duplication with previous published works seems to be a little bit high. For example, please compare:

- “moment-resistant joint was developed by Dr. Bryan Walford 206 at NZ Forest Research Institute, in 1970, by employing multiple nails with” with “moment-resisting joint was developed by Dr. Bryan Walford at NZ Forest Research Institute in 1970 by employing multiple nails with” (Komatsu 2017);
- “Timber frame experienced large deformation due to the absence of brace or infill material” with “timber frame experienced large deformation due to the absence of brace or infill material” (Cao et al. 2021);
- “the dynamic behaviour of a semi-rigid moment-resisting dowel-type connection between timber beam and column” with “dynamic behaviour of a semi-rigid moment-resisting dowel-type connection between timber beam and column” (Solarino et al. 2017);
- “if a certain target displacement can only be achieved with excessive strength loss, the connection is not ductile” with “If a certain target displacement can only be achieved with excessive strength loss, the connection is not ductile” (Ottenhaus et al. 2021).

Reply: The authors recognize that these statements are somehow, very close to the original cited work and, therefore, corrections have been introduced in the revised manuscript, as show below:

“… the first moment-resistant connection was built at NZ Forest Research Institute by employing nails with diameter of 6.35mm, with steel side plates with thickness of 3.175mm….”

“…The frame presented large lateral displacements and local deformations probably due the removed bracing or lack of infill material…”

“…Reference [35] analysed the vibration and dynamic response of a semi-rigid moment-resisting beam-to-column dowel-type connection…”

“…studied the validity of these propositions based in four criteria: i) A connection will not be considered ductile if maximum displacement or rotation values are reached with a high loss of resistance; ii) Definitions that are directly related to the calculation of energy dissipation by the area under the curve are impractical; iii) The definitions must consider the post-peak behavior to be able to properly compose the connection displacement amplification ability; and, iv) when the definition produces vastly different ductilities for variations in initial stiffness while the load–displacement curves look very similar and achieve the same final displacement, it is not applicable”

4. The citation style is different in Tables 5 and 6 and Figures 26 and 27 than in the rest of the article.

Reply: The authors recognize that citation style is different in these tables and figures and, therefore, the revised manuscript includes updated tables and figures.

Table 5. Comparison of the collected experimental results for bolted connections with slotted-in steel plates

Table 6. Comparison of the collected experimental results for glued-in rods connections

Figure 24. Geometric configurations and failure modes of glued-in rods ductile connections

Figure 25. Comparative ductility performances of moment-resisting connections based on different Ductility factors.

Author Response File: Author Response.pdf

Reviewer 3 Report

Review of Buildings Journal Manuscript

Ductile moment-resisting timber connections: a review

 

This paper presents a review of the most important research studies that have focused on the ductile beam-to-column connections in moment resisting timber frame structural system.  Two types of connections namely glued-in and the bolted connections with slotted-in steel plates were reviewed. It was reported that the ductility of the former type is higher. 

The addressed topic is interesting in the sense that little has been performed in this area which should be regarded as a topic still open to research and, as such, would have merited publication in Buildings Journal. However, in the opinion of the reviewer, the paper, in the present form, suffers from several major flaws and therefore, cannot be accepted for publication in the present form. By the way, some comments that can improve the quality of the manuscript are as follows.

- English should be majorly revised. There are several grammatically incorrect and typos in the manuscript.

- The idea behind this study was to concentrate the plasticity in the connections to achieve a desired level of ductility in the structural level. Although the authors brought few examples of the design codes that recommend ductile connections, this approach is not welcomed by some building design codes in which it is preferred to transfer the plastic joints away from the connection zone with the aim of improving the redundancy. Please comment. 

- Relation 1~12; Please refer to each relation.

- Line 137; What is meant by “shearing of the columns”? and in Line 139, “was found bigger values in frames with 0.02° during the …” and in Line 163 “premature splitting at the joints” and “very strong deformation capability”.

- Table 1; There is no need to bring this table as it is completely the results of another study. The same goes for Fig. 3.

- Fig. 4; this is exactly the same as the figure in the original manuscript published elsewhere. Have the authors acquired the necessary permission related to copyright from the publisher? This is true for Fig. 5~23 and Table 4.

- Figures with more than one part, should have their own caption and labeling.

- Why do Table 5 and 6 have different parameters? Foe example, Table 6 does not contain parameters related to yielding?

- Why it was claimed that glued-in connections have significantly higher ductility that those of the bolted connections with slotted-in steel plates? Please justify this. As it is obvious, ductility depends on several parameters e.g., the boundary conditions, the number of screws/rods, material properties, dominant behavior mode, etc. Are there any studies reviewed by the authors that two identical series of specimens were tested, one as glued-in and the other the corresponding bolted connections with slotted-in steel plates? Only in that way the response characteristics of the connections can be compared.

- There are no deep conclusions about the results in terms of correlation between the response characteristics and the design variables. No practical use can be found in the output of the review. If the most important conclusion of this review is that the glued-in connections are better in terms of ductility, there are several exceptions too that these types of connection shoed  almost unity ductility.   

 

The manuscript requires major revision for possible publication in the Buildings Journal. The reviewer accepts reviewing the revised version of the manuscript provided that point-by-point replies are provided in the response letter which addresses each modification in the revised manuscript.  

Author Response

This paper presents a review of the most important research studies that have focused on the ductile beam-to-column connections in moment resisting timber frame structural system.  Two types of connections namely glued-in and the bolted connections with slotted-in steel plates were reviewed. It was reported that the ductility of the former type is higher. 

The addressed topic is interesting in the sense that little has been performed in this area which should be regarded as a topic still open to research and, as such, would have merited publication in Buildings Journal. However, in the opinion of the reviewer, the paper, in the present form, suffers from several major flaws and therefore, cannot be accepted for publication in the present form. By the way, some comments that can improve the quality of the manuscript are as follows.

- English should be majorly revised. There are several grammatically incorrect and typos in the manuscript.

Reply: A final reading by a native speaker was performed to improve the quality and the clearness of the writing.

- The idea behind this study was to concentrate the plasticity in the connections to achieve a desired level of ductility in the structural level. Although the authors brought few examples of the design codes that recommend ductile connections, this approach is not welcomed by some building design codes in which it is preferred to transfer the plastic joints away from the connection zone with the aim of improving the redundancy. Please comment. 

Reply: Ductility is important from an earthquake resistance point of view, to ensure that the structure survives an earthquake without excessive damage. In practice, the seismic design loads depend on the ductility class that can be assumed for the building, in which, the ductility of the joint is crucial. The Eurocode 8, as all of the seismic resistant codes for timber, recommends that dissipative zones are located at the connections, while the elements must behave elastically. On the other hand, even in non-earthquakes prone regions, the use of ductile connections, and in particular their plastic behaviour, promotes a better redistribution of stresses essential to ensure economic structures with the additional benefit to contribute for the structural robustness.

 

- Relation 1~12; Please refer to each relation.

Reply: The authors cited each relation in the revised manuscript in line 81:

“The (1) to (7) are relative definitions, while (8) to (12) are absolute definitions of ductility. Definition (2) is cited in both EN 12512 [7] and in the Swiss timber code for timber structures SIA265 [6]”

- Line 137; What is meant by “shearing of the columns”? and in Line 139, “was found bigger values in frames with 0.02° during the …” and in Line 163 “premature splitting at the joints” and “very strong deformation capability”.

Reply: The authors agree that these statements were unsuitable and corrected them in the revised manuscript:

Line 145: “The beam-to-column connection did not presented enough ductility during the extreme event simulation. Comparing the maximum rotations in the beam-to-column testing and the rotations measured in the frames, with 0.02° during the first seismic test, 0.72° in the second test, and 1.41° in the third seismic test.”

Line 172: “During the test, the moment-rotation curves did not presented significant load drop, but simple frame specimen showed premature splitting around the bolts on the tension side of the beam member at the rotation of approximately 6°”

- Table 1; There is no need to bring this table as it is completely the results of another study. The same goes for Fig. 3.

Reply: The authors understand the reviewer’s recommendation, but Table 1 summarizes the a proposal made by [15] for a ductility classification being important to demonstrates that despite definitions suggested by codes, in this case, Eurocode 8, there are different definitions for that parameter. On the other hand, figure 3, now figure 2, is important to emphasize that in a timber frame submitted to lateral loading, plastic deformations and damage are concentrated in the joints and therefore, the importance to study in detail the behaviour of the joints.

- Fig. 4; this is exactly the same as the figure in the original manuscript published elsewhere. Have the authors acquired the necessary permission related to copyright from the publisher? This is true for Fig. 5~23 and Table 4.

Reply: The authors recognize that these figures and tables need attention, therefore, the necessary adjustments were made in the revised manuscript.

- Figures with more than one part, should have their own caption and labeling.

Reply: The authors recognize that these figures need attention, therefore, the necessary adjustments were made in the revised manuscript.

- Why do Table 5 and 6 have different parameters? For example, Table 6 does not contain parameters related to yielding?

Reply: The authors agree that both tables should have the same parameters for better comparative analysis. But for the selected studies, it was not possible to obtain data related to yielding because most of them were related to an experimental load-deformation curve. This is in fact a gap in the past research.

- Why it was claimed that glued-in connections have significantly higher ductility that those of the bolted connections with slotted-in steel plates? Please justify this. As it is obvious, ductility depends on several parameters e.g., the boundary conditions, the number of screws/rods, material properties, dominant behavior mode, etc. Are there any studies reviewed by the authors that two identical series of specimens were tested, one as glued-in and the other the corresponding bolted connections with slotted-in steel plates? Only in that way the response characteristics of the connections can be compared.

Reply: Glued-in rods have presented several advantages over bolted connections with slotted-in steel plates. The most important advantage is that the first ones have a ductile failure mode, even after high rotations. Moreover, this type of joint makes possible to develop design routines that can enable the development of effective analytical methods for predicting the nonlinear behaviour of ductile connections.

As identified by this work, studies comparing the performance of different semi-rigid timber joints are missing. The available works are focused in one specific type of joint, with a defined geometry, and therefore, comparisons does not exist. The identification of this gap is, for sure, one of the most important outcomes of the current work. Based on the available data, and using the most representative ductility definitions, a comprehensive comparison between the most important typologies of semi-rigid joints has been presented. This is a first step essential for a wide research. This manuscript results from this earlier analysis and will be essential to plan the further steps of the research that will include experimental campaigns with the aim to obtain this missing comparison.

- There are no deep conclusions about the results in terms of correlation between the response characteristics and the design variables. No practical use can be found in the output of the review. If the most important conclusion of this review is that the glued-in connections are better in terms of ductility, there are several exceptions too that these types of connection shoed  almost unity ductility.   

Reply: The authors agree that, based on the review performed, the present manuscript must identify clear contributions to the current knowledge and practical recommendations and conclusions should be given. Therefore, the updated manuscript includes a new section, 5.1. Recommendations, where an effort to point out clear contributions to the current knowledge are made.

“From the experimental data collected and analysed within this work, it is possible to trace a practical path to analyse the ductility of a timber connection. First, it is necessary to know the moment-rotation curve of the joint. From it, the initial stiffness and the yield point must be defined. Moment-rotation curves need to reach a clear plateau for yield point definition and should reach maximum rotation values close to 0.15rad (monotonic) and 0.10rad (cyclic), without significant loss of moment resistance at this ultimate point. Moreover, when tested, ductile connections should preferably have a ductile failure mode.

Even if different definitions of ductility exists, in the light of the above parameters, considering the two different expressions for the ductility (equations 2 and 13) it was possible to obtain coherent values for the ductility ratio (see figure 25). Although several semi-rigid connections have been studied and tested in order to evaluate their ductility, few comparisons between the different types of ductile timber connections have been made. From the state-of-the-art review performed, it was possible to identified the most used parameters for the definition of bolted slotted-in steel plates connections with common geometry (approximately 300mm for columns and beams height), obtaining as a reference the use of 8 bolts per connection, as well as STS close to the bolts, but only in sufficient quantities to prevent splitting and increase the rotational capacity of the connection.

In glued-in rods connections, it is recommended to apply a steel profile between the timber elements to obtain greater ductility and energy dissipation, with a small number of rods (four in most of the studies carried out). These connections can be the key point for the development of high and mid-rise timber building projects, using moment-resisting timber frames to enable more open and versatile architectural set-ups. The knowledge about the glued-in rods connections also contributes to the development of analytical procedures to calculate it for the determination of non-linear moment-rotation curves, as already have been applied in steel and concrete structures.

The analysis of the current state-of-the-art allows to indicate the need to study timber frames and/or buildings under lateral loading with the aim to assess displacement and stiffness responses of the connections.”

The manuscript requires major revision for possible publication in the Buildings Journal. The reviewer accepts reviewing the revised version of the manuscript provided that point-by-point replies are provided in the response letter which addresses each modification in the revised manuscript.  

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The author has made a number of revisions, which I personally believe can be published. It is suggested that the clarity of the pictures and tables in the paper be further improved.

Author Response

The authors would like to sincerely thank the reviewers for their comments, which certainly increased the quality of the manuscript by being considered in the revised version. The authors hope that the new version of the paper can be accepted for publication.

Reviewer 2 Report

All comments have been addressed.

Author Response

The authors would like to sincerely thank the reviewers for their comments, which certainly increased the quality of the manuscript by being considered in the revised version. The authors hope that the new version of the paper can be accepted for publication.

Reviewer 3 Report

Although the authors have tried to improve the manuscript and replied to most of the comments/concerns, there are several remaining issues regarding the revised manuscript including the typos, and unattended figures and tables which had been required to deal with. The authors also tried to make the results of the their review study more practical by adding a section entitled "recommendation"; however, this part added little to the practicality of the findings and the statements were not fully supported by the findings of this study.    

Author Response

The authors agree that these figures need attention, therefore, the adjustments were made in the revised manuscript. The authors recognize that some statements were not directly answered. Therefore, the updated manuscript expanded the recommendations section, in order to make them more practical and direct, including a greater number of contributions and their relationships with the main works presented in the literature. In addition, the article included a new section 5.3 “Challenges”, that brings practical questions about the most relevant applications for ductile connections in moment-resisting timber frame system.

Author Response File: Author Response.pdf