At this stage, only a few institutes and universities have conducted some research of the CLT in China concentrating on process optimization, basic panel performance, and use of local species for the CLT manufacturing. And some valuable results were obtained.
In conformation with the North American CLT standard, Prof. Wang from SCLC led his team to build the China's first CLT pilot production line, and subsequently developed the world's first large-scale prefabricated CLT panel made from Canadian western hemlock lumber (https://mp.weixin.qq.com/s?__biz=MjM5NTg5NDI4Mg==&mid=2651228420&idx=1&sn=027306c8724ef5c02e24d4238efd33d9#rd). Wang et al. (2018) prepared Canadian western hemlock CLT using two types of adhesive and two levels of applied pressure. The results of block shear and delamination test showed that the type of adhesive and the level of applied pressure had a significant effect on the bond quality, wood failure and durability of the CLT. Other team members made the full-scale hemlock CLT panel and systematically evaluated and analyzed the basic mechanical properties in major strength direction and durability of the CLT (Wang Y L et al., 2017; Li M M et al., 2018; Lu Y et al., 2018a; Xie et al., 2018). The interlaminar shear strength of the hemlock CLT made in the SCLC was measured using a short-span three-point bending method with a span-to-depth ratio of 6 to validate the theory. The cooperative research team from Tongji University conducted a study on the flatwise bending and compressive performance of the Canadian hemlock CLT and obtained the mechanical properties of the hemlock CLT in the major strength and minor strength directions through the test, which provided the basic data for the engineering applications of hemlock CLT (He et al., 2018)
Based on the CLT pilot production line of the SCLC, the CAF used domestic Japanese larch lumber to make the CLT and tested its bending performance, inter-laminar shear performance, rolling shear performance, and compressive performance (Gong et al., 2018a; Gong et al., 2018b; Gong et al., 2018c; Gong et al., 2018d). They also explored the influence of process, lumber modulus of elasticity, assembly direction, lumber thickness and number of layer on its mechanical properties (Gong et al., 2018a; 2018b; 2018c; 2018d). Thus, for the domestic Japanese larch, the key process parameters and basic mechanical performance of the CLT were successfully established for engineered applications.
The research team from South China Agricultural University (SCAU) studied the feasibility of using the fast-growing small-diameter eucalyptus wood to make the CLT and optimization of relevant process parameters (Liao et al., 2017; Lu Z H et al., 2018). The results of eucalyptus CLT bending and shear tests proved that the CLT had acceptable mechanical properties.
Scientists from NFU studied the hybrid CLT fabricated with lumber and laminated veneer lumber (LVL) or different wood species. And the test data of rolling shear, bending, and inter-laminar shear helped improve the rolling shear performance of the CLT and provided a reference for engineering applications (Wang et al., 2014; Wang et al., 2016; Wang Z Q et al., 2017). Wang (2017) studied the effect of macroscopic characteristics of sawn timber such as density, annual ring, pith distance, pith and wood species on rolling shear properties of cross layer of softwood CLT. Jiang (2016) studied mechanical properties of the CLT made from Chinese fir with different thickness and analyzed its failure modes.
To compare some above-mentioned domestic R & D teams' research (Wang et al., 2016; Liao et al., 2017; Gong et al., 2018a; Gong et al., 2018b; Lu Y et al., 2018b; Xie et al., 2018), bending and shear performance of the CLT made from different wood species are listed in Table 1 and Table 2. It should be note that the size of the CLT has a neglectable effect on its mechanical performance. However, the data referred to different publications still has to some extent value to discuss with the same span-depth ratio, especially for the CLT with large difference of mechanical properties resulting in the relative reduce of size effect. As shown in Table 1, eucalyptus is one of the representational tree species in China. The small-diameter eucalyptus CLT had a higher modulus of elasticity (MOE) but lower flexural strength. And its flexural strength was much lower than that of the softwood (Canadian western hemlock and Japanese larch) CLT. Composite the CLT, Douglas fir LVL as core layer, had a lower MOE and flexural strength than hybrid CLT made from SPF lumber. While taking Douglas fir LVL as the surface layers of composite CLT, its MOE and MOR increased about 29% and 17%, respectively. According to Table 2, the shear strength of small-diameter eucalyptus CLT was lower than that of the softwood (Canadian western hemlock and Japanese larch) CLT and composite CLT. Spruce-Pine-Fir (SPF) was a mixture of three wood species, commonly used for the CLT manufacturing in North America. Obviously, the mechanical properties of the CLT made from Canadian western hemlock and domestic Japanese larch were comparable to and even superior to SPF CLT.
CLT type Specimen dimension (mm) Span-depth ratio MOE (MPa) MOR (MPa) Canadian western hemlock CLT 3150×305×105 30.0 10 668–11 966 34.78–41.51 Domestic Japanese larch CLT 2700×305×75 30.0 10 340–16 760 40.52–59.76 Small-diameter eucalyptus CLT 1820×305×54 30.0 11 466 (Mean) 24.5 (mean) Hybrid CLT (SPF-SPF-SPF) 2000×89×114 16.4 7908 (Mean) 28.6 (mean) Composite CLT (SPF-LVL*-SPF) 2000×89×114 16.4 7222 (Mean) 25.7 (mean) Composite CLT (LVL*-SPF-LVL*) 2000×89×114 16.4 9299 (Mean) 30.1 (mean) *Note: LVL was made from Douglas fir veneer.
Table 1. Bending properties of CLT
CLT type Specimen dimension (mm) Span-depth ratio Inter-laminar shear strength (MPa) Canadian western hemlock CLT 735×305×105 6 1.88–2.23 Domestic Japanese larch CLT 510×305×75 6 2.28–2.84 Small-diameter eucalyptus CLT 400×150×54 6 1.30 (mean) Composite CLT (SPF-SPF-SPF) 610×305×114 5 2.25 (mean) Composite CLT (SPF-LVL-SPF) 610×305×114 5 2.11 (mean) Composite CLT (LVL-SPF-LVL) 610×305×114 5 2.40 (mean)
Table 2. Inter-laminar shear properties of CLT
Wang (2014) used poplar and Douglas fir as raw materials to prepare evenly layered and homogeneous CLT panels and evenly layered and hybrid CLT panels. By the test of three- and five-layer CLT panels, an optimization design method based on strength was proposed. In addition, Mao (2015) studied the in-plane mechanical properties (ultimate bearing capacity and in-plane equivalent stiffness) of seven groups of 21 CLT wall members, and the results showed that the in-plane performance of the CLT panel was mainly in the elastic stage and the failure was generally brittle.
Fu (2012) of the CHD analyzed the bending and dynamic performance of the CLT bridge deck through experiment, and discussed the influence of number of layer and connection mode on the mechanical performance of the CLT bridge deck.
Domestic scholars had also carried out the relevant research on the connection performance of the CLT members. Sun et al. (2018) revised the hysteresis curve of finite element simulation based on the node and wall test of the CLT structure, and developed a finite element model of the CLT structural node and wall. The numerical simulation method was also applied to analyze the influence of damper parameters on the energy consumption of the shear wall system for the pre-stressed shear CLT wall system and the dual effects of pre-tension on the energy dissipation and self-reset (Hu and He, 2018).
Dong et al. (2018) selected self-tapping screws to connect the CLT. Through the monotonic loading and gradual loading tests of the connected test pieces, the results revealed that increasing the length and diameter of the self-tapping screws can significantly improve the shear capacity of the CLT connected pieces and when the nailing angle was 45°, and the shear bearing capacity achieved its maximum. And a good connection of butt joint of main side material and oblique-nailed joint of the STS on both left and right directions was proposed. In order to evaluate the influence of laminate materials on the lateral performance of the CLT shear wall, they tested single lumber CLT and hybrid CLT of lumber and laminated veneer lumber (LVL) under the unidirectional and low-cycle repeated load and the hybrid CLT shear wall exhibited good performance (Wang Z Q et al., 2017).
Zhang (2015) conducted a study on the integrity and energy dissipation capacity of steel-wood buckling restraint support. It was found that the CLT structure had a good ability to restrain the steel tube. Therefore, the design method and flow of steel-wood buckling restraint support was presented.
The research on the seismic performance of semi- rigid connections of the CLT was carried out by USTB. It was found that the failure of all fasteners was pulled out as the ideal ductile failure mode, and the hysteresis curve showed high nonlinearity and degradation of stiffness and strength and pinching phenomenon (Shen et al., 2015). Based on three flexible joint tests of CLT, the team used OpenSees to simulate the high nonlinearity, degradation of strength and stiffness and pinching and then carried out damage analysis of the CLT connection (Shen et al., 2016). And they also numerically studied the mechanical performance of steel-frame and CLT infill wall structure under the monotonous and cyclic loading. The effect of cooperative performance between the CLT wall and the steel frame and number of connections on overall structure was analyzed (Shen et al., 2017).
Considering the characteristics of the structure and section of the wood bridge deck, Gao (2010) proposed a new bridge deck system based on the combination of the CLT panel and steel beam. They found that the number of layer and the connection method will affect the CLT bridge deck. Jia et al. (2018), from BFU, conducted a study on the shear performance of the T-connector at the CLT wall-floor joint. The connectors were able to withstand the shear force of the wall-slab joints and the CLT wall showed good stiffness.
Owing to the incompleteness of China's specifications and the public's lack of awareness of multi- and high-rise wood buildings, the CLT were predominantly used in low-rise buildings in China, and those buildings were mainly built for demonstration purpose.
In March 2014, a two-storey wood construction building with a hybrid light-frame and the CLT structure in China was reported for demonstration in Qian'an City, Hebei Province (Fig. 3a). And its CLT materials were manufactured by Qian'an City Big Tree Industry Co., Ltd., which was the first producer of the CLT in China (Chen et al., 2018). In the same year, the 5-storey CLT wood construction building, the first multi-storey CLT building in Asia, located in Taiwan Province, China was completed. In this building, the architects made full use of the CLT's good cantilever performance, and set the cantilevered balcony on the second, third and fourth floors (Fig. 3b1). In addition, the CLT was left exposed inside the building, which largely reflected the natural features of wood construction (Fig. 3b2) (Strobel, 2016).
Figure 3. The CLT wood construction buildings in China. (a) CLT wood construction building in Qian'an, Hebei; (b) CLT wood construction building in Taiwan; (c) OTTO Café in Ninghai, Zhejiang; (d) All-ecological CLT demonstration house in Ninghai, Zhejiang
Figure 3c shows the China's first public CLT demonstration building (OTTO Café), which was jointly built by SCLC and Tongji University (Xiong et al., 2018). All CLT panels were prefabricated. The prefabrication method made the OTTO Café simple and convenient to build meeting the requirements of green, low-carbon, energy saving, environmental protection and sustainability. Cooperating with Portugal Amerin Group, SCLC successfully applied the insulated cork board as insulation and cladding to build China's first two-storey ecological CLT house (Fig. 3d). Note that the OTTO Café and the all-ecological CLT demonstration house (Fig. 3d) were demolished and reconstructed after one year and it took only three days to dissemble and reassemble, which fully reflected the flexibility and adaptability of the CLT buildings. Therefore, those types of the CLT buildings can be widely applied for China's tourism real estate, urbanization and rural industrialization.
In addition, Zhongyi Scientech Timber Structure Co., Ltd., one manufacturer of the CLT located in Shandong Province, also carried out some engineering applications of the CLT. The business building of the Binzhou Administrative Center was one case, which was designed to use composite CLT elements combined with wooden beams and columns.
Cross-Laminated Timber (LY/T3039- 2018), a National Forestry and Grassland Administration forestry industry standard, completed by SCAU and SCLC etc., was officially released at the end of 2018 and will be officially implemented in May 2019. Although the newly promulgated Standard for design of timber structures (GB 50005-2017) and Technical standard for multi-story and high rise timber buildings (GB/T 51226-2017) provided specifications for the CLT component design and connector design, etc., The construction and acceptance specifications have not yet been completed (Li Z et al., 2018; Liu and Yang, 2019). Especially, at present, the lack of regulations in the area of fire prevention and building inspection resulted in some obstacles for promoting the residential and non-residential CLT buildings.