Experimental studies on bolted glubam connections

Abstract

This paper reports tensile and compressive test results of bolted glubam (glued laminated bamboo) connections. The tensile tests were carried out with two types of specimens designed for tensile loadings in the longitudinal and transverse directions in relevance to the orientations of the bidirectional bamboo strips (fibers). In each direction, the specimens were further divided into nine groups according to different combination conditions of end and edge spacings. Compressive tests were executed for three groups of bolted glubam connections, with varying thickness of the main board and bolting conditions. The tensile experiments show that the failure of the specimens is strongly influenced by the loading directions. Recommended end distance and side distance are provided, whereas the load carrying capacity is analyzed. Based on the compressive testing results, failure modes and load displacement relationships are analyzed, in which the yield bearing capacity is shown to be close to that given by the equations in existing design specifications for timber structure.

Keywords

bolted connection compressive testing embedment glubam tensile testing

Introduction

The performance of a structure always involves safety and serviceability, which is frequently influenced by the stiffness and strength of connections and joints of the structural members. Many different types of connections are used in timber structures, including traditional mortise and tenon type connections, bolted connections, glued connections, and large-finger connections, etc. In 2013, Aicher et al. (2013) led a great effort organizing an international conference, which covered wide topics related to the joints in timber structures. The bolted connections are more often used for joining components in modern timber structures because of their diversity and on-site assembly capability. Numerous studies on bolted timber joints have been performed on their load-slip characteristics subjected to a lateral force, which are governed by the embedment stress and deformation characteristics of timber, the bending stress and deformation characteristics of the bolt, and the geometric properties and types of joints (DeJong, 1977; Echavarría et al., 2007; Santos et al., 2010; Sawata and Kei, 2015; Tlustochowicz et al., 2011). Numerical studies were also carried out to calibrate the experimental results and provide better modeling to the embedment behavior and the bolted connections (Liu and Xiong, 2018; Schoenmakers et al., 2010; Xu et al., 2013; Zhou and Guan, 2006).

The second author’s research group has been involved in developing a bamboo-based glulam or glubam (glubam is glued laminated bamboo) for more than ten years (Xiao et al., 2008a,b, 2010, 2013). The glulam can be categorized as the thin layer strip lamination and thick layer strip lamination. The thick layer laminated glubam is made by pressure gluing a few layers of relatively thicker bamboo strips (5–8 mm thick and about 20 mm wide). The thin layer laminated glubam sheets typically have a thickness of about 10–30 mm, and are made by laminating approximately 2 mm thick 20 mm bamboo strip mats. The glubam sheets are manufactured using a hot-pressing procedure, with a typical plate size of about 2500 mm long and 1200 mm wide. The sheets can be stacked for easy transportation. Based on the design needs, the glubam elements can be produced by a cold-lamination process, similar to timber-based glulam. In recent years, following the flourish of the cross laminated timber (CLT) structures, research attentions are also directed towards the connections of CLT structures, particularly for seismic design (Brandner et al., 2016; Ringhofer et al., 2018).

The utilization of bamboo materials in timber-timber like structures are becoming more and more popular in recently years, due to the expectation of the eco-friendly, fast-mature and excellent mechanical property in bamboo (Sun et al., 2020; Xiao et al., 2008a, 2019; Xiao and Shan, 2013). Several types of engineered bamboo structures have been developed. The second author’s group has developed a structural system glubam, or glued laminated bamboo, which is a bamboo-based glulam made with two step lamination procedures (Xiao et al., 2008b, 2013). Construction methods including details of glubam structures, such as trusses and frames can be largely following the practice of timber structures (Feng et al., 2014; Wu and Xiao, 2018). With different material properties, the failure patterns and geometric properties of glubam bolted connections are expected different than timber ones, thus need to be investigated. Research on behaviors of connections made of bamboo or bamboo-based materials is very limited. For round bamboo culm structures, Disén and Clouston (2013) presented a literature review and summarized various types of connections. An earlier research on dowel-type steel rods bolted glubam connection was reported by Zhang et al. (2008). He et al. (2016) recently presented research on dowel type bolted connections of bamboo scrimbers. Hong et al. (2020) reviewed connections for engineered bamboo structures. The second author’s research group has recently reported axial tension or pull-out test results on steel rods glued-in glubam connections (He and Xiao, 2020; Li et al., 2020).

This paper reports tensile and compressive testing results of bolted glubam connections. The study was conducted to provide basic understanding of bolted glubam connections, and to acquire the necessary experimental evidence and data for the development of design guidelines in near future.

Experimental programs

Tensile testing

Bolted joints subjected to tension may behave differently than those in compression. Different failure modes, such as the rupture of the base material need to be addressed. In order to simplify the research problems, the testing program was focused on the single bolt connections and selected steel as the side plate whereas the glubam as the main board of consideration. As shown in Figure 1, the experimental parameters included the edge and end distances (b and e) measured from the center of the bolt hole.

Figure 1.

Single bolt pinned connection.

The thin-strip bidirectional glubam board measured with a nominal thickness of 28 mm was adopted as the main board in the single pin joint specimen. The ratio of the bamboo fibers in the longitudinal grain orientation and transverse orientation was 4:1. As shown in Figure 2, the testing side of the glubam element is the single pin joint with a bolt of 12 mm diameter, whereas the other end is connected to three 16 mm diameter bolts for fixing the specimen to the testing machine. The holes in the glubam elements was about 1 mm larger than the bolt diameter.

Figure 2.

Glubam tensile specimen details (all dimensions in mm).

In the experimental program, the specimens were designed for longitudinal tensile loading and transverse tensile loading. For each loading direction, the specimens were further divided into nine groups according to different end and edge spacing. The number of specimens in each group was five, with a total of 90 specimens tested. Table 1 summarizes the testing parameters and matrix of the specimens.

Table 1.

Tensile testing matrix for pin joints.

Longitudinal loading				Transverse loading
Specimen name	b/2 (mm)	e (mm)	Number	Specimen name	b/2 (mm)	e (mm)	Number
LAa-1~5	12	24	5	TAa-1~5	12	24	5
LAb-1~5	12	36	5	TAb-1~5	12	36	5
LAc-1~5	12	48	5	TAc-1~5	12	48	5
LBa-1~5	18	24	5	TBa-1~5	18	24	5
LBb-1~5	18	36	5	TBb-1~5	18	36	5
LBc-1~5	18	48	5	TBc-1~5	18	48	5
LCa-1~5	24	24	5	TCa-1~5	24	24	5
LCb-1~5	24	36	5	TCb-1~5	24	36	5
LCc-1~5*	24	48	5	TCc-1~5*	24	48	5

Specimens in LCc and TCc groups are also used for embedment strength evaluation.

The tests were conducted in accordance with ASTM A. D5652-15 (2015). The loading equipment was universal test machine with a maximum loading capacity of 200 kN, as shown in Figure 3(a). In the test, displacement control was adopted to apply the load, and the loading speed was 3 mm/min, to ensure that the time from loading to failure was no less than 5 min but no more than 20 min. Figure 3(b) shows the specially designed testing fixture for loading the specimen. The upper end is the tensile loading point, and the lower end is the end to hold the specimen.

Figure 3.

Test setup: (a) test setup and (b) specimen fixtures.

Compressive tests

One of the simpler and reliable way to study the group effects of bolted connections in timber structure is to test the joint in compression. A pilot study on testing three groups of bolted glubam under compressive loading was carried out, as shown in Table 2.

Table 2.

Specimens of glued bamboo bolt joints.

Groups	Number of specimens	Thickness of main board (mm)	Bolting conditions
BGJ group 1	3	28	Tight
BGJ group 2	3	28	Snag-tight
BGJ group 3	3	56	Snag-tight

Specimens: Details of the specimens are presented in Figure 4(a) and (b), which were designed in reference to the design specifications of wood structures. The diameter d of the bolts was 12.0 mm. The end distance of 90 mm along the main bamboo fiber direction (grain direction), was larger than the required minimum end distance of 7d = 84mm. The 90 mm pitch was set larger than the minimum pitch of 7d = 84 mm. The 40 mm edge distance exceeded the required minimum edge distance of 3d = 36 mm. The gauge is designed with 70 mm, larger than the minimum gauge of 3.5d = 42 mm.

Figure 4.

Specimen details: (a) for first and second groups of specimens and (b) for third group specimens.

As shown in Table 2, the research parameters of the first and second groups of specimens were whether the bolts were tightened using wrench or only snug-tight with hand. In the tightened case, the slippage between the center and the side plates was essentially prohibited during the initial stage of loading, while the snag-tight allows such slippage. The testing parameters of the second and third groups of specimens were the thickness of the main board. Three specimens were tested for each testing parameter. All the specimens were prepared by professional carpenters, with particular attentions given to plane the bamboo board surfaces and the loading ends. The hole diameter was drilled with 1mm larger than the diameter of the bolts. The drilling speed was less than 120 mm/min. Typically, the rotation speed of the electric drill should not be too slow, and it was taken as 300 r/min, to avoid unnecessary over-enlargement of the holes.

Loading methods: A 1000 kN universal testing machine was used for testing the bolted glubam joints. As shown in Figure 5, a dial indicator attached on each side of the specimen was used to measure the relative slip between the main board and the side boards. Based on the Chinese standard for testing timber structures (GB/T50329, 2012), the loading procedure shown in Figure 6 was adopted. The specimen was first loaded to 0.3 of the pre-test estimated loading capacity, F, for 30 s, then was unloaded to 0.1 F for another 30 s. After that, the load was increased every 30 s, and the increment per stage was 0.1 F. After exceeding 0.7 F, the loading speed was slowed down to 0.1 F per min., until the failure of the specimen, as shown in Figure 6. The pre-test loading capacity, F was taken as 67.0 kN, based on the following equation from Chinese Standard for Design of Timber Structures (GB50005, 2017),

N_{v} = k_{v} d^{2} \sqrt{f_{c}}

(1)

where, N_v is the design value of the bearing capacity of each shear surface of a bolt; k_v is the calculation coefficient of the bearing capacity of the bolt connection, taken as 5.5; d is the bolt diameter (mm); f_c is the designed value of the compressive strength of the wood (in current case, the glubam) used.

Figure 5.

Test setup.

Figure 6.

Loading process.

Experimental results of tensile tests

Failure modes and load-displacement relationships from tensile tests

In the tests, the failure of the specimens was strongly influenced by the loading directions. During the tests with longitudinal tensile loading in the direction of the 80% bamboo strips (fibers), the specimens typically failed in the so-called plug shear mode with the pin being pulled out along with the based material at the end of the specimen, as shown in Figure 7(a). When the edge distance increases, the plug shear failure tended to occur after the significant enlargement of the hole along the loading direction due to embedment deformation. Similar trends could also be seen in the specimens with an edge of 24 mm and an end distances of 48 mm under the transverse loading, in which failure occurred after some enlargement deformation. However, most specimens subjected to tensile loading in the direction along the 20% bamboo fiber arrangement appeared to have complex failure mode with rupture initiated typically from the net section on one side of the bolt hole, as shown in Figure 7(b). In both cases, the 12 mm diameter bolts did not develop sufficient yield deformation, therefore, the specimens are deemed to have failed in the base material.

Figure 7.

Typical failure modes: (a) longitudinal loading; (b) transverse loading.

The applied load and deformation relationships for the specimens under longitudinal and transverse loadings are summarized in Figures 8 and 9, respectively. For each testing group, only the curve of the specimen that represents the average curve of the group is shown.

Figure 8.

Tensile load versus deformation relationships for longitudinal loading tests: (a) specimens with b/2 = 12 mm and different end distances, (b) specimens with b/2 = 18 mm and different end distances, and (c) b/2 = 24 mm and different end distances.

Figure 9.

Tensile load versus deformation relationships for transverse loading tests: (a) specimens with b/2 = 12 mm and different end distances, (b) specimens with b/2 = 18 mm and different end distances, and (c) b/2 = 24 mm and different end distances.

As shown in Figure 8, for the same edge distance b/2, increasing the end distance e results in significant improvement of the load and deformation behavior with increased load carrying capacity and deformability. By comparing Figure 8(a)–(c), one can notice that the curves with the same end distance e, but different edge distance b/2 are essentially similar. This is the evidence that the end distance controls the failure pattern for specimens subjected to the longitudinal loading. Both the AWC (2018) NDS specification and the Chinese GB code (2017) require an end distance (7d for soft wood and 5d for hardwood) for dowel type bolted joints. Based on the test results of glubam, a 4d (48 mm) end distance is enough to effectively delay the plug shear failure in the main bamboo fiber direction. For the edge distance, a 1.0d (b/2 = 12 mm) seems to be sufficient to prevent rupture of the net area across the main bamboo fiber direction.

As shown in Figure 9(a), for an edge distance of b/2 = d (12 mm), increase of end distance e does not result in any change in the behavior of the bolted joint subjected to loading in the transverse direction, indicating that the failure is controlled by the rupture of the net area perpendicular to the tensile force. By comparing the red curves in Figure 9(a)–(c), it can be seen that the increase in the edge distance does not result in distinct strength enhancement for specimens with smallest end distance e = 2d. This is considered due to the small end distance, the area between the hole and the end edge is more flexible, therefore the rupture is localized near the hole. When the end distance is larger, the area between the hole and the end edge is stiffer, as the result, more materials can be engaged in resisting the rupture in the net area. This is evidenced in Figure 9(b) and (c), in which increase in the end distance is shown to be effective to enhance the tensile capacity. Echavarría et al. (2007) developed an analytical approach to explain the failure modes in bolted timber joints. Their research findings indicate that the shorter end distance may result in more intensive stress concentration around the bolt hole enhancing the tendency of failure, particularly for e is less than 4d. The observation from this research appears to be in line with Echavarría et al.’s analysis.

Discussions on strength of bolted connection in tension

The 5% off-set yield capacity and the maximum load are obtained for the specimens and shown in Table 3. The following calculated capacities are also shown in Table 3.

Table 3.

Tensile test capacities of bolted joints.

	b/2	e	Yield load	Max. load	Net tension capacity (kN)	Plug shear capacity (kN)
	(mm)	(mm)	(kN)	(kN)
LAa	12	24	13.5	15	27.6 (1.84)	9.9 (0.66)
LAb	12	36	21	20.2	27.6 (1.37)	14.9 (0.74)
LAc	12	48	23.8	26.3	27.6 (1.05)	19.9 (0.76)
LBa	18	24	12	13.9	55.1 (3.96)	9.9 (0.71)
LBb	18	36	22.8	24.1	55.1 (2.29)	14.9 (0.62)
LBc	18	48	26.5	28.9	55.1 (1.91)	19.9 (0.69)
LCa	24	24	16.3	17.5	82.7 (4.73)	9.9 (0.57)
LCb	24	36	25.1	25.9	82.7 (3.19)	14.9 (0.58)
LCc	24	48	23.2	26.9	82.7 (3.07)	19.9 (0.74)
TAa	12	24	10.6	12.3	5.7 (0.46)	9.9 (0.80)
TAb	12	36	11.6	13.1	5.7 (0.44)	14.9 (1.14)
TAc	12	48	10.8	12.3	5.7 (0.46)	19.9 (1.62)
TBa	18	24	11.1	12.1	11.4 (0.94)	9.9 (0.82)
TBb	18	36	14.3	15.1	11.4 (0.75)	14.9 (0.99)
TBc	18	48	17.8	18.6	11.4 (0.61)	19.9 (1.07)
TCa	24	24	11.8	12.6	17 (1.35)	9.9 (0.79)
TCb	24	36	14.5	16.3	17 (1.04)	14.9 (0.91)
TCc	24	48	17.5	20.6	17 (0.83)	19.9 (0.97)

Tensile capacity of net section, $T_{nt}$ :

T_{nt} = (b - d) t f_{t}

(2)

Plug-shear capacity, $T_{ps}$ :

T_{ps} = 2 et f_{v}

(3)

where, $f_{t}$ , and $f_{v}$ are the tensile strength along the direction of loading, and shear strength in section of shear parallel to the loading direction, respectively.

The test results indicate that the edge distance b/2 = d = 12 mm is sufficient to provide enough tensile strength for the net section to prevent rupture. This is reflected by the much larger calculated net tensile capacities compared with the experimentally obtained yield force and maximum load carrying capacities of the specimens subjected to the longitudinal loading in the direction along the main bamboo fiber direction. On the other hand, the calculated plug shear capacities yield conservative predictions to the yield and maximum capacities of the specimens subjected to longitudinal loading.

The net section rupture capacities calculated based on equation (2) can predict conservatively the capacities of most of the specimens subjected to the transverse tension in the less bamboo fiber direction, except for specimens with b/2 = 2d = 24 mm with an end distance of 2d or 3d. For the specimens in these two cases, the plug shear equation equation (3) provides a closer but slightly lower prediction to their capacities. As a result, the following minimum equation can be adopted to provide the conservative predictions to the test results.

T = min (b - d) t f_{t}, 2 et f_{v}

(4)

It should be pointed out that the cross-grain loading (transvers direction to the wood fibers) is not permitted for structural element made of timber. The thick layer glubam is typically laminated with bamboo fibers in the longitudinal direction, thus the transverse loading should be strictly avoided, unless special reinforcement is considered. For bi-directionally configured thin-strip glubam used in structural elements, the typical bamboo fiber ratio is 4:1 to 7:1 in the longitudinal and transverse directions. Therefore, the pinned connection with loading in the transverse direction can resist certain level of tension force. However, unless for special circumstances, pinned glubam connection in transverse loading is not recommended.

Embedment strength

The data of the glubam specimens with 12 mm diameter bolts and large edge and end distances (48 mm for both) were also used for discussing the embedment behavior of dowel type joints. Five tests were executed, for the main bamboo fiber direction (f_h;xz) and the transverse direction (f_h;yz). The results of 5% offset strength are shown in Table 4.

Table 4.

Embedment strength of thin strip glubam with 12.0 mm bolts.

Loading direction	Specimen	5% Yield load (kN)	Embedment strength (MPa)	Average embedment strength (MPa)	Standard deviation (MPa)	Characteristic embedment strength (MPa)
f_h;xz	1–1	22.69	63.0	64.2	2.9	57.1
	1–2	21.88	60.8
	1–3	24.60	68.3
	1–4	23.71	65.9
	1–5	22.77	63.3
f_h;yz	2–1	19.62	54.5	51.0	6.0	36.2
	2–2	18.92	52.6
	2–3	17.70	49.2
	2–4	15.00	41.7
	2–5	20.63	57.3

The average embedment strength and standard deviation are also shown in Table 4. The characteristic embedment strength values can be calculated and shown in Table 4, using the following equation,

f_{k} = m - kS

(5)

where, m is the average values of each test results, S is the standard deviation of strength values, k is the characteristic factor with 75% confidence level, taken as 2.464, based on ASTM 2915 (2010).

Experimental results of compression tests

Failure patterns

During the compressive testing, the bolts were pushed down in the middle and supported by the two side plates, as schematically shown in Figure 10, in which the bolt is simplified as a non-dimensional beam and the pressures along the holes of the three elements are assumed as uniformly distributed. Bending of the bolts results in the slippage of the middle plates relative to the side plates (shown by the red curve in Figure 10). If the relative slippage exceeded 15 mm, the joint was judged as failure. The bending of the bolt may cause an upturn deformation of the bolt ends and the holes of the side plates are pressed upward (shown by the green curve in Figure 10). In this case, if the relative slippage deformation was over 10 mm, failure was considered. All the specimens in the second and third groups had the deformation pattern with the bolt ends being pushed upward, as the example illustrated in Figure 11. During loading, sharp sounds indicating the cracking of the boards or rupture of bamboo fibers were audible and become intensive corresponding to the increase of the load. For specimens in Group 1, due to the restriction of slippage by tightening the bolts, the target slippage of 10.0 mm could not be reached, and the specimens essentially failed in local crushing in compression in one of the side boards.

Figure 10.

Schematic loading condition of bolt.

Figure 11.

Push up of bolt ends due to bending.

Compressive load deformation relationships

The load and slippage deformation relationships for all the bolted connection specimens are shown in Figure 12. As shown in Figure 12(a), two of the three specimens had high initial stiffness during preloading to 0.3F, indicating the effect of bolt-tightening. However, one of them had lower initial stiffness. Such discrepancy reveals the randomness and undependable control of the tightness of the bolts. The specimens failed essentially due to local crushing of one of the side boards in compression, so the load was terminated without reaching the target failure deformation of 10.0 mm. For Group 2 testing, one of the specimens failed prematurely at a load level of 100 kN, due to the local crushing of one of the glubam side plates. The other two specimens behaved well, and the loading was terminated when the deformation exceeded the target failure deformation at 10.0 mm. Specimens in Group 3 behaved reasonably consistently, reaching the target failure deformation 10.0 mm, as shown in Figure 12(c). In the discussions of the load carrying capacities and yielding mechanism of the bolted connection, the specimens with premature compression failure are not used.

Figure 12.

Load and slippage deformation relationships of specimens in: (a) group 1, (b) group 2, and (c) group 3.

Analysis of bolted glubam connections in compression

Based on the 5% offset yield strength definition, the capacities of compressive specimens are calculated. The average yield capacity of the two specimens with slippage deformation reaching 10.0 mm in Group 2 is calculated as 134.4 kN, whereas, the average yield capacity of the three specimens in Group 3 is 125.0 kN, which are higher than the capacity F, estimated based on shear failure according to the Chinese GB code (2017). Therefore, other failure mechanisms need to be considered.

Yield capacities of dowel type bolted joints can be analyzed based on the mechanisms originally proposed by Johansen (1949). Based on Johansen’s approach, four types of failure mechanisms are considered, for the double shear joints: Mode-I: failure is caused by the damage to the center main board, while the side boards and the bolt remain intact; Mode-II: failure is caused by the damage to the side boards, while the center main board and bolt remain intact; Mode-III: failure is in the boards with the yielding in the middle portion of the bolt; Mode-IV: bearing failure in the boards with the yielding of the bolt in three locations.

The yield capacities of the connection based on the four failure modes have been established in design codes, based essentially on the Johansen’s (1949) theory, and the refinements [McLain and Thangjitham, 1983; Soltis, 1986), however, with some modifications. In the literature, a similar research on connections using bamboo boards was carried out by Zhang et al. (2008). They conducted compressive loading tests on similar engineered bamboo joints but with eight bolts configured in a staggered fashion. They observed the mode-IV failure patterns for the specimens and concluded that the NDS specification could obtain the design bearing capacity of bolted laminated bamboo joints.

Based on the observations of the specimens in Group-2 and Group-3, the mode-IV failure patterns are assumed for these specimens, therefore the capacities can be calculated based on the AWC NDS approach [NDS Table 12.3.1A, and B in Error! Bookmark not defined.], however, without using the reduction factor R_d,

\begin{matrix} Z_{4} = 2 D^{2} \sqrt{\frac{2 F_{em} F_{yb}}{3 (1 + R_{e})}} = 2 \times 12^{2} \\ \times \sqrt{\frac{2 \times 59 \times 235}{3 (1 + \frac{50}{235})}} = 24.76 kN \end{matrix}

Since the bolt group satisfies the requirements for all the spacing, the capacity can be calculated as 6 × 24.76 = 148.6 kN. The result is reasonably close to the 5% offset yield capacities, 134.4 kN and 125.0 kN of the specimens in Group-2 and Group-3, respectively. Further studies might be needed to modify the analysis in order to reduce the degree of the over-estimation.

Conclusions

Tensile and compressive testing results of bolted glubam connections were presented and analyzed, through which the following conclusions can be obtained:

When the bi-directionally configured thin-strip glubam connection was subjected to tension, its behavior was strongly influenced by the loading direction relevant to the fiber orientation. Tensioning of the pinned glubam connections in the longitudinal direction with 80% bamboo strips (fibers) resulted in much higher capacities compared to that in the transverse direction.

Two failure modes, the rupture of net area and the end plug-shear failure were observed for the specimens. The corresponding capacities can be estimated using equations (2) and (3) in the paper, respectively. The smaller of the values calculated based on equations (2) and (3) can be used to estimate the capacity of the connection.

Based on the test results of the longitudinal loading, a 4d (48 mm) end distance is enough to effectively delay the plug shear failure. For the edge distance, a 1.0d (b/2 = 12 mm) is sufficient to avoid rupture of the net area across the main bamboo fiber direction. For the transverse loading, the failure was mainly controlled by the net area rupture. The results also show that larger end distance makes the area between the hole and the end edge stiffer, which engages more materials in resisting the rupture in the net area and finally enhances the tensile capacity.

In the compressive testing, tightening the bolts, as the case in the Group-1 specimens, could increase initial stiffness, however, the effects are found to be undependable due to difficulties in controlling the pretention force.

The yield capacity based on the 5% offset deformation was defined for the compressive specimens. The AWC (2018) NDS equation based on the type IV mode of the Johassen’s failure mechanisms can provide a closer however overestimation to the experimentally obtained capacities of the specimens.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The authors are grateful for the financial support of China MOST National Key Research and Development Project for Developing eco-friendly structural systems for prefabricated residential buildings in rural areas (2019YFD1101002), National Natural Science Foundation of China National Key Project, the Training Program for Excellent Young Innovators of Changsha (kq1802023), Chaired professorship of the Zhejiang University.

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