Mechanical behavior of laminated bamboo lumber dowel-type connection

Abstract

Bamboo is a kind of green material with high specific strength, while the hollow tubular structure makes it rather hard to be utilized in structure. Glue and hot-pressing processes make laminated bamboo lumber rectangular cross section and with high strength properties. The dowel-type connection can be used in I-joist instead of the costly adhesive, while the behaviors of which are extremely complicated. European yield model is confirmed to be an effective method to estimate loading capacity of connection and is adopted by various standard and design codes. This article focused on a kind of connection innovatively with laminated bamboo lumber dowel. The embedment tests were carried out to study the embedment strength of laminated bamboo lumber members. Connection tests under lateral load were conducted to investigate the performance and loading capacity. Finally, theoretical results determined by design rules in current codes were compared with experimental results

Keywords

dowel-type connection embedment strength European Yield Model laminated bamboo lumber loading capacity

Introduction

Bamboo is a kind of fast growing material with renewable, natural, and biodegradable properties (Bonilla et al., 2010), which makes it an exceptional economic resource for wide use. Moreover, studies indicated that the environmental performance of bamboo was favorable compared to timber in energy consumption and environmental costs (Van der Lugt et al., 2003).

The tensile strengths of outer part of bamboo can be up to 580 MPa (Shao et al., 2010), which is stronger than structural steel for 350 MPa. While as hollow tubular structure, the tensile strength of the entire bamboo is still close to 90 MPa (Correal et al., 2010). The specific modulus is close to steel and the specific tensile strength is about four times that of steel (Lakkad and Patel, 1981). However, the round shape and small in diameter of raw bamboo are the largest impediments to manufacture large section members, design connections, and either to achieve the preassembling. Engineered bamboo composite, such as laminated bamboo lumber (LBL), is a perfect solution and thus is increasingly becoming popular in the green building community. LBL is made by laminating the bamboo chips together with resin and hot pressing to form rectangular cross section.

I-joist is popularly used in pre-assembled constructions. For engineered bamboo I-joist, the adhesive was used in connecting members despite its relatively high cost and limitation on strength (Aschheim et al., 2010). Thus, dowel-type connection could be considerable. Studies on the dowel connections with timber members indicated that the low embedment strength, shear strength, and tension strength in perpendicular to grain direction of wood limited connections to be higher effective (Izzi et al., 2018; Jensen, 2005; Jorissen, 1998; Larsen, 2003; Mohammad and Quenneville, 1999; Rammer and Winistorfer, 2007; Schoenmakers et al., 2010; Sjödin and Serrano, 2008; Soltis and Wilkinson, 1987). As with high strength, LBL is considered to be a favorable choice for dowel material.

Various methods such as the linear or non-linear beam on foundation, fracture mechanics concepts and finite element method were used to model the behaviors of connection, such as strength and stiffness (Chen et al., 2003; Daudeville et al., 1999; Daudeville and Yasumura, 1996; Dias et al., 2007; Kharouf et al., 2003; Moses and Prion, 2003; Patton-Mallory et al., 1997; Racher and Bocquet, 2005; Sawata and Yasumura, 2003). But the most common method is the European Yield Model (EYM). EYM was originally proposed by Johansen (1949) and continually developed by Larsen (1973). By experimental investigations, Möller (1950), Meyer (1957), McLain and Thangjitham (1983), Brungraber (1986), and Soltis et al. (1986) verified the theory for both parallel- and perpendicular-to-grain loading. Since then, EYM became commonly used in timber to timber connection with steel dowel and adopted by standards and design codes worldwide.

This article deals with the analysis and description of the load-carrying characteristics of LBL dowel-type connections. The focus is placed on the experimental investigations of embedment and single dowel connection tests under lateral load. The embedment test was conducted for studying the bearing capacity and also obtaining the embedment strength for estimating the connnection capacity. Moreover, this article lists the design rules from current design codes and compares the theoretical values calculated by design rules to the experimental values. The main objective is not only to present test data for embedment and dowel-type connection but also to check the available for various design rules in predicting load-carrying capacity with accuracy.

Material and methods

Embedment test

All LBL materials in this paper were manufactured from Phyllostachys pubescens (Moso species bamboo) in China, harvested at 3–5 years of age. After grading, cutting and splitting, grinding, and anti-septic treatment, LBL was shaped with phenol formaldehyde adhesive and hot pressing. The moisture content (MC) of LBL was 7.5% and the density was 1.3 g/cm³.

The embedment test was conducted according to ASTM D5764 (2013) standard, and 12 specimens for each group were considered. As shown in Figure 1(a), the specimen is a hexahedron with curved groove. The groove sections are semicircles with dimensions of 6, 8, 10, and 12 mm for each group. The dimensions were unified to be 50 mm by 50 mm by 38 mm, for width (W), length (L), and thickness (T) separately.

Figure 1.

Schematic of (a) specimen dimensions and (b) testing setup.

Figure 1(b) shows the setup of embedment test. A linear variable differential transformer (LVDT) was used to record the displacement of crosshead. The loading speed was 1 mm/min. The judgment for termination was reaching either the maximum load or 5 mm deformation according to BS EN383 (2007) standard while D5764 standard did not mention.

Connection test under lateral load

The connection test under lateral load was conducted according to ASTM D5652 (2015) standard. The test dowel diameters were 6, 8, 10, and 12 mm. Five specimens in each group were adopted. As shown in Figure 2, the main member and side members were assembled by LBL dowel and there is no adhesive on the interface.

Figure 2.

Specimens dimensions.

As shown in Figure 3, the specimen was put onto the testing machine horizontally, and the loading block pressed on the main member. A fixture assembled with rods and angle brackets was used in the bottom to avoid the rotation of side members. Two LVDT devices were used to record the displacement. The test speed was 1.0 mm/min ± 50% and determined with the principle that the maximum load should be reached in approximately 10 min.

Figure 3.

Schematic of testing setup.

Theory

Most of the existing standards and codes, which prescribe some different design rules for dowel-type connections, are resting on EYM theory. As shown in Figure 4, the failure modes of double shear connection can be divided into four modes. Failure modes I_m and I_s refer to main and side member embedment failure, respectively. Modes III_s and IV respect one hinge and two hinges on dowel, respectively. The formulas of EYM correspond to each mode are given by equation (1)

F_{V, R k} = \min {\begin{matrix} 0.5 f_{h, m, k} t_{m} d & M o d e I_{m} \\ f_{h, s, k} t_{s} d & M o d e I_{s} \\ \frac{f_{h, s, k} t_{s} d}{2 + β} [\sqrt{2 β (1 + β) + \frac{4 β (2 + β) M_{y, R k}}{f_{h, s, k} d \cdot t_{s}^{2}}} - β] & M o d e I I I_{s} \\ \sqrt{\frac{4 β M_{y, R k} f_{h, s, k} d}{1 + β}} & M o d e I V \end{matrix}

(1)

where $F_{V, Rk}$ is the load-carrying capacity per shear plane; $f_{h, s, k}$ and $f_{h, m, k}$ are the embedment strength of side member and main member, respectively; t_s and t_m are the thickness of side member and main member, respectively; d is the dowel diameter; $M_{y, Rk}$ is the dowel yield moment; and $β$ is the ratio of member embedment strength which can be obtained by equation (2)

β = \frac{f_{h, m, k}}{f_{h, s, k}}

(2)

Figure 4.

Push-out failure modes.

In this article, the material of side and main member is the same; hence, $β$ is equal to one. Thus, equation (1) can be simpified as follows

F_{V, R k} = \min {\begin{matrix} 0.5 f_{h} t_{m} d & M o d e I_{m} \\ f_{h} t_{s} d & M o d e I_{s} \\ \frac{1}{3} f_{h} t_{s} d [\sqrt{4 + \frac{12 M_{y, R k}}{f_{h} d \cdot t_{s}^{2}}} - 1] & M o d e I I I_{s} \\ \sqrt{2 M_{y, R k} f_{h} d} & M o d e I V \end{matrix}

(3)

BS EN 1995-1-1 (2004), also known as EC5, provides the similar design method

F_{V, R k} = \min {\begin{matrix} 0.5 f_{h, m, k} t_{m} d & M o d e I_{m} \\ f_{h, s, k} t_{s} d & M o d e I_{s} \\ 1.05 \frac{f_{h, s, k} t_{s} d}{2 + β} [\sqrt{2 β (1 + β) + \frac{4 β (2 + β) M_{y, R k}}{f_{h, s, k} d \cdot t_{s}^{2}}} - β] & M o d e I I I_{s} \\ 1.15 \sqrt{\frac{2 β}{1 + β}} \sqrt{2 M_{y, R k} f_{h, s, k} d} & M o d e I V \end{matrix}

(4)

where the symbols are the same as defined in equation (1), and $M_{y, Rk}$ and $β$ can be calculated by equations (5) and (6), respectively

M_{y, Rk} = 0.3 f_{u, k} d^{2.6}

(5)

β = \frac{f_{h, m, k}}{f_{h, s, k}}

(6)

where $f_{u, k}$ is the axial tensile strength. In this article, equation (4) can be simpified as equation (7).

F_{V, R k} = \min {\begin{matrix} 0.5 f_{h} t_{m} d & M o d e I_{m} \\ f_{h} t_{s} d & M o d e I_{s} \\ 0.35 f_{h} t_{s} d [\sqrt{4 + \frac{12 M_{y, R k}}{f_{h} d \cdot t_{s}^{2}}} - 1] & M o d e I I I_{s} \\ 1.15 \sqrt{2 M_{y, R k} f_{h} d} & M o d e I V \end{matrix}

(7)

ANSI/American Forest & Paper Association NDS (2015) published to help designing wood structure. The design method is presented as equation (8)

F_{v} = \min {\begin{matrix} \frac{D l_{m} F_{e m}}{R_{d}} & M o d e I_{m} \\ \frac{2 D l_{s} F_{e s}}{R_{d}} & M o d e I_{s} \\ \frac{2 k D l_{s} F_{e m}}{(2 + R_{e}) R_{d}} & M o d e I I I_{s} \\ \frac{2 D^{2}}{R_{d}} \sqrt{\frac{2 F_{e m} F_{y b}}{3 (1 + R_{e})}} & M o d e I V \end{matrix}

(8)

where F_v is reference lateral design values; D is diameter; l_m and l_s are dowel bearing length of main and side member respectively; $F_{em}$ and $F_{es}$ are dowel bearing strength of main and side member, respectively; $F_{yb}$ is dowel bending yield strength (herein equal to tensile strength); R_d is reduction term; and k is factor, which can be determined by equations (9) and (10)

k = - 1 + \sqrt{\frac{2 (1 + R_{e})}{R_{e}} + \frac{2 F_{yb} (2 + R_{e}) D^{2}}{3 F_{em} l_{s}^{2}}}

(9)

R_{e} = \frac{F_{em}}{F_{es}}

(10)

According to the rules in NDS, the reduction terms are listed in Table 1.

Table 1.

The reduction terms from NDS.

Dowel diameter	Yield mode	Reduction term, R_d
6	I_m, I_s, III_s, IV	2.862
8	I_m, I_s	4
	III_s, IV	3.2
10	I_m, I_s	4
	III_s, IV	3.2
12	I_m, I_s	4
	III_s, IV	3.2

NDS: National Design Specification.

Canadian code CSA O86 (2009) gives the method below

N_{u} = \min {\begin{matrix} \frac{1}{2} f_{m} d t_{m} & M o d e I_{m} \\ f_{s} d t_{s} & M o d e I_{s} \\ f_{s} d^{2} (\sqrt{\frac{1}{6} \frac{f_{m}}{f_{m} + f_{s}} \frac{f_{y}}{f_{s}}} + \frac{1}{5} \frac{t_{s}}{d}) & M o d e I I I_{s} \\ f_{s} d^{2} \sqrt{\frac{2}{3} \frac{f_{m}}{f_{m} + f_{s}} \frac{f_{y}}{f_{s}}} & M o d e I V \end{matrix}

(11)

where N_u is unit lateral yielding resistance; f_m and f_s are embedment strength of main and side member, respectively; d is diameter of dowel; t_m and t_s are thickness of main and side member, respectively; f_y is yield strength of fastener in bending. According to clause 10.4.4.3.3.3 in CSA O86, f_y can be calculated by equation (12)

f_{y} = \frac{f_{ym} + f_{um}}{2}

(12)

where $f_{ym}$ is tensile yield strength of dowels and $f_{um}$ is tensile ultimate strength of dowel.

The method from Chinese standard GB 50005 (2017) is as follows

F_{V, Rk} = k_{\min} t_{s} d f_{h, s, k}

(13)

where $k_{\min}$ is the minimum length factor and can be determined by the equations below. Other parameters are the same as defined in equation (1)

k_{\min} = \min [k_{I}, k_{III}, k_{IV}]

(14)

k_{I} = \frac{R_{e} R_{t}}{r_{I}}

(15)

k_{III} = \frac{k_{s, III}}{r_{III}}

(16)

\begin{matrix} k_{s, III} = \frac{R_{t} R_{e}}{1 + 2 R_{e}} \\ [\sqrt{2 (1 + R_{e}) + \frac{1.647 (1 + 2 R_{e}) k_{ep} f_{yk} d^{2}}{3 R_{e} R_{t}^{2} f_{h, s, k} t_{s}^{2}}} - 1] \end{matrix}

(17)

k_{IV} = \frac{k_{s, IV}}{r_{IV}}

(18)

k_{s, IV} = \frac{d}{t_{s}} \sqrt{\frac{1.647 R_{e} k_{ep} f_{yk}}{3 (1 + R_{e}) f_{h, s, k}}}

(19)

where R_e is same as $β$ in equation (1); R_t is the ratio between the thickness of main member and side member; r_I, $r_{III}$ , and $r_{IV}$ are the resistance factors which is equal to 4.38, 2.22, and 1.88, respectively; $k_{ep}$ is the intensified coefficient and equal to one herein; $f_{yk}$ is the yielding strength of dowel and equals to tensile strength.

Results and discussion

Embedment test

Failure mode

The typical load–slip curves are shown in Figure 5. The curves are all with two stages. At first, the curves are almost linear and then the slope tends to be smooth gradually. Once the ultimate strength has been reached, the load declines sharply and the failure happens suddenly.

Figure 5.

Typical load–deformation curve of embedment tests.

The failure modes are shown in Figure 6. The failure of specimens is both brittle and with two features: one is local bearing around the contact area, and another is splitting. When loading, the area contacted with pressure head crushed regional first. As LBL was laminar, few imperceptible initial cracks along fiber direction existed and were the start of failure. After some amounts of deformation, the cracks propagated through the entire specimen gradually and finally leaded to brittle failure and the sudden drop of loading. Some buckling happened in the parts isolated by cracks because the high slenderness ratio relatively.

Figure 6.

The typical failure mode of group with diameter of (a) 6 mm, (b) 8 mm, (c) 10 mm, and (d) 12 mm.

Strength and stiffness

All strengths, deformations and stiffnesses, are determined by 5%d offset method in ASTM D5764 (2013). The mean values (mean), as well as the coefficient of variation (CV) of the embedment strength and stiffness, are summarized in Table 2. The ultimate strength is 66.191, 62.547, 61.127, and 60.870 MPa for 6, 8, 10, and 12 mm diameter respectively. The ultimate strength was used in estimating the connection capacity by the design rules mentioned above.

Table 2.

Embedment strength and stiffness.

Dowel diameter (mm)	6		8		10		12
Dowel diameter (mm)	Mean	CV (%)	Mean	CV (%)	Mean	CV (%)	Mean	CV (%)
Number of specimens	12	–	12	–	12	–	12	–
Yield strength (MPa)	60.014	11.55	58.568	7.91	61.026	9.6	60.577	5.28
Yield seformation (mm)	1.378	27.93	1.695	23.93	2.084	43.34	2.614	15.6
Ultimate strength (MPa)	66.191	7.70	62.547	9.34	61.127	9.70	60.87	5.05
Ultimate seformation (mm)	3.829	31.98	2.769	15.65	2.28	49.47	2.911	22.61
Proportional limit strength (MPa)	44.501	12.16	42.49	12.13	46.465	20.33	40.153	9.57
Proportional limit deformation (mm)	0.841	41.86	1.003	36.05	1.464	50.54	1.546	21.71
Stiffness (kN mm^–1)	16.493	34.66	18.795	33.94	23.271	55.47	17.618	12.55

CV: coefficient of variation.

Connection test under lateral load

Failure mode

Two phenomena of dowel bearing and member embedment are shown in Figure 7(a) and (b) separately. The dowel bended and the outer layer ruptured at the interface of members, where meanwhile the embedment happened. Figure 7(c) has shown the main cause of dowel failure. The dowel yielded with secondary bending and two plastic hinges appeared on right and left interfaces separately. The failure modes observed in the test were the ones indicated as EYM failure modes IV.

Figure 7.

The typical failure mode of (a) dowel bearing, (b) hole embedment, and (c) the yielding with secondary bending failure of dowel.

Load and deformation

The typical load–slip curves are shown in Figure 8. Generally, the dowel-type joints indicate an initial consolidation in the first phase of load–slip curves which is a typical non-linear beginning response. But that was not observed here according to illustrated load–slip curves, which was also observed by Kunecký et al. (2015) and Milch et al. (2017). This behavior may be explained by the fact that the dowels were inserted into the pre-drilled holes without clearances and the contact between the dowel and components was perfect. The global behavior could be deemed practically as linear elastic behavior before failure, which occurred suddenly and in a brittle manner.

Figure 8.

The typical load curve of connection test under lateral load.

The mean values of ultimate loads and deformations are listed in Table 3. The ultimate load for each group is 1.870, 3.321, 4.070, and 5.858 kN separately.

Table 3.

Results of connection tests under lateral loads.

Specimen number	Ultimate load (kN)	Ultimate deformation (mm)
6-1	1.978	1.873
6-2	1.235	1.615
6-3	2.596	2.960
6-4	1.991	1.613
6-5	1.550	4.633
Mean	1.870	2.539
8-1	2.407	5.505
8-2	3.805	1.455
8-3	3.566	5.355
8-4	3.364	5.623
8-5	3.465	4.775
Mean	3.321	4.543
10-1	4.022	4.685
10-2	4.246	6.730
10-3	3.314	3.345
10-4	4.990	5.133
10-5	3.780	5.075
Mean	4.070	4.994
12-1	4.266	5.213
12-2	5.632	4.403
12-3	8.253	4.713
12-4	6.401	6.640
12-5	4.738	4.703
Mean	5.858	5.134

Comparison with existing methods

Tensile tests were conducted according to ASTM D143 (2014) standard. Strain gauge was used to collect the strain. Ten specimens were tested and the mean value and CV of tensile properties in parallel to grain direction are listed in Table 4. As the tensile and bearing mechanical properties of LBL are brittle, the tensile strength and dowel yielding strength are the ultimate strength.

Table 4.

The tensile properties of LBL.

Properties	Modulus ofelasticity (MPa)	Ultimate strength (MPa)	Ultimatestrain (%)
Mean value	7631	71.95	1.04
CV	12.65%	15.87%	12.99%

LBL: laminated bamboo lumber; CV: coefficient of variation.

The experimental and theoretical results are listed in Table 5. The failure modes predicted by design codes and observed in tests were mode IV. The results illustrate that the deviations of EYM, EC5, and CSA O86 are large while those of NDS and GB 50005 are with less variation. NDS and GB 50005 seem to be preferable for predicting capacity of LBL dowel-type connection. But it has to be noted that EC5 and CSA O86 design methods were only for metallic fasterens, and NDS and GB 50005 were preferred to be used in metallic fasterens. Although the failure modes and the mechanical behaviors are extremely the same, further researches are needed to determine the adjustment factors or develop appropriate methods.

Table 5.

The experimental and theoretical results of connection tests under lateral loads.

Dowel diameter (mm)		6	8	10	12
Experimental results (kN)		1.870	3.321	4.070	5.858
EYM	Theoretical results (kN)	2.561	4.246	6.477	8.960
EYM	Error (%)	36.96	27.86	59.14	52.95
EC5	Theoretical results (kN)	2.945	4.883	7.449	10.304
EC5	Error (%)	57.49	47.03	83.02	75.90
NDS	Theoretical results (kN)	1.909	2.998	4.782	6.861
NDS	Error (%)	2.09	9.73	17.49	17.12
CSA O86	Theoretical results (kN)	2.732	4.797	7.651	10.977
CSA O86	Error (%)	46.10	44.44	87.99	87.38
GB 50005	Theoretical results (kN)	1.700	2.986	4.762	6.788
GB 50005	Error (%)	9.09	10.10	17.00	15.87

EYM: European yield model; EC: Europe code; NDS:National Design Specification; CSA: Canadian Standards Association.

Conclusion

This article presented a novel LBL dowel-type connection. The embedment property and the connection performance under lateral load were investigated. The conclusions can be summarized as follows:

The embedment failure mode was crushing around the pressure head and spliting along the fiber direction. The ultimate embedment strength for each group is tightly distributed in the range of 60–67 MPa.

The failure mode of connection test under lateral load was the dowel yielding with two plastic hinges, which was the same as mode IV of EYM. The global behavior was almost linear elastic behavior, and the failure was brittle.

The design rules from Europe, United States, Canada, and China, including EYM, EC5, NDS, CSA O86, and GB 50005, were collected and listed in this article. The comparison between experimental results and theoretical results indicated the NDS and GB 50005 were more suitable for predicting LBL connection capacity. Considering most codes were applied to metallic fasteners, adjustments for existing models or appropriate methods are necessary in future.

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: This study was supported by the National Natural Science Foundation of China (51778299), National Key Research Project (2017YFF0207203-04), and the Priority Academic Program Development of Jiangsu Higher Education Insitutions (PAPD) and the National First-class Disciplines (PNFD).

ORCID iD

Aiping Zhou

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