Structural design and mechanical properties analysis of bamboo-wood cross-laminated timber

Abstract

To explore the overall mechanical properties of bamboo-wood composite cross-laminated timber (BCLT), a simulation model of BCLT mechanical behavior based on the solid element was established using the finite element software ABAQUS. The actual four-point bending experiment was compared and analyzed with the finite element numerical simulation. The total curve error coefficient of the BCLT specimen at 18-mm displacement was 0.2988 while the interval was 0.5 mm. The error coefficient was 0.0178 when the maximum load was reached, and the minimum error coefficient was 0.0015 at 12 mm of displacement. Analysis of the influence of material parameters, meshing density, and material arrangement on the final stress distribution indicate that the difference in the elastic parameters of the material greatly influence the final stress distribution, and the arrangement and combination of materials also have an effect on the overall mechanical properties of the BCLT board. The combination CLT1-2-1 (i.e., the upper and lower layers of the bamboo are Arrangement 1 and the hemlock is Arrangement 2) have a maximum load of 57682 Ν and a maximum stress of 103.9 MPa.

Full Article

Structural Design and Mechanical Properties Analysis of Bamboo-wood Cross-laminated Timber

Chao Li, Xilong Wang, and Yizhuo Zhang *

To explore the overall mechanical properties of bamboo-wood composite cross-laminated timber (BCLT), a simulation model of BCLT mechanical behavior based on the solid element was established using the finite element software ABAQUS. The actual four-point bending experiment was compared and analyzed with the finite element numerical simulation. The total curve error coefficient of the BCLT specimen at 18-mm displacement was 0.2988 while the interval was 0.5 mm. The error coefficient was 0.0178 when the maximum load was reached, and the minimum error coefficient was 0.0015 at 12 mm of displacement. Analysis of the influence of material parameters, meshing density, and material arrangement on the final stress distribution indicate that the difference in the elastic parameters of the material greatly influence the final stress distribution, and the arrangement and combination of materials also have an effect on the overall mechanical properties of the BCLT board. The combination CLT1-2-1 (i.e., the upper and lower layers of the bamboo are Arrangement 1 and the hemlock is Arrangement 2) have a maximum load of 57682 Ν and a maximum stress of 103.9 MPa.

Keywords: Cross-laminated timber; Bamboo and wood composite; Bending behavior analysis; Four-point bending experiment; Finite element simulation

Contact information: Northeast Forestry University, Harbin, 150040, China;

* Corresponding author: zhangyz@nefu.edu.cn

INTRODUCTION

Cross-laminated timber (CLT) is a new type of wood structural material. It is a solid wood board made of timber or structural composite sheets with an orthogonal (90°) staggered assembly and is pressed by structural adhesives (FPInnovations 2011). It has the potential of substituting cement or steel (Bejtka 2011; Brandner et al. 2016). In addition, CLT has excellent fire resistance, similar to the characteristics of the structure of heavy wood. In the event of fire, the surface layer of CLT wood carbonizes slowly at a certain rate, and at the same time, it can maintain its internal original structural strength for a long time (Buchanan 2000; Frangi et al. 2010). As a new type of green building material, CLT has excellent performance and a relatively low cost. It can be used as roofs, floors, doors, windows, partition walls, and many other applications (Chen 2009). The research and development of CLT has important practical significance and application prospects for promoting the efficient utilization of wood resources and realizing the rational allocation of resources in China’s plantations.

The modulus of elasticity (MOE), the modulus of rupture (MOR), and shear strength are important properties of CLT plates, and they have an important influence on the structural design of CLT floors or roofs. The bending test of CLT plates in practical experiments as well as the modeling and simulation of CLT plates by finite element software can better analyze and predict the overall mechanical properties of CLT plates (Schneider et al. 2015; Sebera et al. 2015; Pina et al. 2019).

He et al. (2018) studied the out-of-plane bending properties and compression properties of the Canadian hemlock CLT plate, where the mechanical properties of the CLT plate in the primary and secondary strength directions were obtained through experimental tests, and a numerical model was established to predict the bending stiffness of the CLT plate. The results showed that the performance of Canadian hemlock CLT board is the same as other common wood varieties (Spruce-pine-fir (SPF) lumber or Douglas fir-Larch lumber) (He et al. 2018). Sikora performed bending and shear tests on the spruce CLT plate to obtain the effect of the plate thickness on the mechanical properties of the spruce CLT plate. The overall trend is that the bending strength and shear strength will decrease as the plate thickness increases (Sikora et al. 2016). Gong et al. (2018) calculated the equivalent bending stiffness and moment of CLT based on mechanical connection theory, composite laminate theory, shear analogy theory, and simple design method. Their study verified the accuracy of theoretical predictions through actual measurements, and screened suitable prediction methods (Gong et al. 2018). Yao predicted the mechanical properties of the effective bending stiffness and bending strength of the three-layer CLT plate of hemlock using the mechanical composite beam theory and studied the deflection and bearing of the three-layer CLT plate of hemlock in practical engineering applications. The structural properties, including bending moment and natural frequency of vibration, show that the two-level CLT effective bending stiffness, bending strength, and vibration performance can meet the needs of practical engineering applications (Yao et al. 2018). Wang et al. (2014) studied the mechanical properties of composite laminated plywood of different tree species of Pinus citigensis, Pinus radiata, and Poplar. It was concluded that poplar is placed in the core layer, and the composite tree CLT with good mechanical properties, such as Citi pine, is formed on the surface layer. The flexural modulus and shear strength of poplar CLT can be significantly improved by this composite method. However, few studies have focused on the bending properties of CLT boards made of bamboo (Wang et al. 2014).

Bamboo is widely distributed in southern China. It is a type of natural material for sustainable development. It can be harvested in 3 to 5 years. It is widely used in southern China. Bamboo has the characteristics of high strength, high hardness, good toughness, wear resistance, and biodegradability, but it also has the disadvantages of a small diameter, low yield, and low processing efficiency. Bamboo wood-based panels in China, including bamboo-wood composite panels, have a low utilization ratio and low added value in the production process. Statistics show that the bamboo direct utilization ratio of bamboo floor is only 20% to 25%, and bamboo utilization ratio of bamboo laminated board is 50%. The low utilization rate of bamboo is due to the structural properties of bamboo pole itself as well as the unreasonable product structure and improper processing technology (Jiang et al. 2002). Flattened bamboo is a new type of engineering product. The bamboo tube is cut off, mechanically processed to remove the inner and outer sections (bamboo green and bamboo yellow), and the seam is opened. After high temperature softening (90°, 5-10 mins), it is sent to the unfolding mold and flattened to obtain the unfolded flat bamboo, and the unfolded surface has no visible cracks. Compared with the flat-paneled semi-bamboo deployment method, the obtained bamboo sheet width is doubled or more (Fang et al. 2018).

Bamboo composite CLT (BWCLT) is a new product that is made of flattened bamboo and structural boards with orthogonal (90) staggered assembly and pressed by structural adhesives. Through scientifically determining the combination form and gluing process, the product’s internal and external quality could be improved, and the bamboo utilization ratio could be increased. The purpose of this study is to investigate the influence of element elastic parameters on the overall mechanical properties of BCLT, and to obtain the optimal design model of BCLT.

EXPERIMENTAL

Materials

Bamboo

Phyllostachys pubescens with an age of 6 years, was produced in Chongyi County, Jiangxi Province, China. It was made of bamboo board formed by the flattening bamboo process (see Fig. 1). Its length was 1280 mm, width was 135 mm, and thickness was 8 mm. After being planed, the thickness of the flat bamboo changed from 8 mm to 6 mm. It had good physical and mechanical properties, including a small coefficient of water absorption and low expansion, and exhibited non-cracking, and non-deformation. According to the requirements of GB/T1931 (2009) and GB/T1933 (2009), its moisture content and density were 6.56% and 0.617 g/cm³, respectively. The average tensile strength of the bamboo sheets was 107.40 MPa, and the compressive strength was 81.64 MPa. The shrinkage of bamboo was small, but the elasticity and toughness were high. The tensile strength and compressive strength (i.e., the strength limit) of the grain were 2.5 times and 1.5 times that of the Chinese fir, respectively.

Fig. 1. Flattening bamboo production process

Hemlock

Canadian hemlock has an air-dry density of 0.47 g/cm³ and an average moisture content of 15%. Canadian iron fir is fine and moderate in hardness, with high flexural strength and durability. The average compressive strength of the grain is 55 MPa. After processing, the size of Canadian hemlock was 1280 mm × 140 mm × 20 mm. After being planed, its thickness changed from 20 mm to 15 mm.

Methods

The schematic diagram of the research process is shown in Fig. 2. BCLT specimens were prepared, and bending experiments were carried out. The results show that the performance of bamboo-wood-bamboo CLT specimens (BWBCLT) was better than that of wood-bamboo-wood CLT specimens (WBWCLT). So only BWBCLT was used in finite element simulations.

Fig. 2. Schematic diagram of research process

Mechanical Properties of BCLT Samples

Samples bending experiment

Each of the initial specimens was cut into seven resulting CLT specimens, as shown in Fig. 3. According to the Chinese national standard GB/T 17657 (2013), the four-point bending test method was adopted in the bending test process, as shown in Fig. 4. The height of the BWB sample was 28 mm, and the length of the sample was 644 mm, which was 23 times the height. The distance between the two supporting rolls was 588 mm, which was 21 times the height of the samples, and the two loading rolls took 1/3 of the spacing of the supporting rollers (i.e., 196 mm). The thickness of the WBW sample was 36 mm, the length of the sample was 828 mm, the spacing of the supporting rolls was 756 mm, and the spacing of the loading rolls was 252 mm.

A TENSON micro-computer controlled electronic universal mechanical testing machine (WDW-50; TIANCHEN Testing Machine Co., JiNan, China) was used in the bending test. A 50 kN load cell was used in the universal mechanical testing machine to record the load during testing.

The loading process was performed by a load controller, and load was applied to the samples until samples were damaged and failed (when the load decreased 40%) with a loading speed of 10 mm/min. The samples started to break when the maximum load was applied. During the measurements, the load–displacement data at mid-span was recorded using a data logger connected to a computer with 10 acquisitions per second (10 Hz). It took 2 to 3 min to test each sample.