Abstract
Three kinds of typical Chinese traditional mortise-tenon joints were tested. The effects of sparging on the deformation, hysteretic behavior, strength and stiffness degradation, and energy dissipation of the mortise-tenon joints were studied via low-cycle reversed loading tests with and without a ‘Que-Ti’ component. The results showed the following: the bearing capacity of the straight-tenon joint was the strongest, and the hysteretic loop of the through-tenon joint and half-tenon joint were asymmetric due to the asymmetry of the tenon form. The half-tenon joint was most likely to pull out the tenon, and the tenon pulling condition of the half-tenon joint can be effectively alleviated by adding a ‘Que-Ti’ component. From the perspective of energy consumption, it was found that the energy consumption capacity of the mortise-tenon joints after adding a ‘Que-Ti’ component was stronger than the joints without a ‘Que-Ti’ component. This shows that the ‘Que-Ti’ can be used as an effective component in terms of enhancing the mechanical properties of the mortise-tenon joints.
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Comparative Analysis on the Mechanical Properties of Mortise-Tenon Joints in Heritage Timber Buildings with and without a ‘Que-Ti’ Component
Yuanhe Li, Lihong Yao,* Yu Guo, Ruijng Liu, Yueqi Wu, Honglei Jia, Xia Yu,
Ce Wang, Zhibang Hu, and Chang Chen
Three kinds of typical Chinese traditional mortise-tenon joints were tested. The effects of sparging on the deformation, hysteretic behavior, strength and stiffness degradation, and energy dissipation of the mortise-tenon joints were studied via low-cycle reversed loading tests with and without a ‘Que-Ti’ component. The results showed the following: the bearing capacity of the straight-tenon joint was the strongest, and the hysteretic loop of the through-tenon joint and half-tenon joint were asymmetric due to the asymmetry of the tenon form. The half-tenon joint was most likely to pull out the tenon, and the tenon pulling condition of the half-tenon joint can be effectively alleviated by adding a ‘Que-Ti’ component. From the perspective of energy consumption, it was found that the energy consumption capacity of the mortise-tenon joints after adding a ‘Que-Ti’ component was stronger than the joints without a ‘Que-Ti’ component. This shows that the ‘Que-Ti’ can be used as an effective component in terms of enhancing the mechanical properties of the mortise-tenon joints.
DOI: 10.15376/biores.17.3.4116-4135
Keywords: Heritage timber buildings; ‘Que-Ti’ component; Mortise-tenon joints; Hysteretic behavior
Contact information: College of Material Science and Art Design, Inner Mongolia Agricultural University, Hohhot 010018 P.R. China; *Corresponding author: yaolihong82@163.com
INTRODUCTION
A ‘Que-Ti’ is a special component in heritage timber buildings that plays the role of both a mechanical and decorative component. It is a short wood piece placed under a beam and at the intersection of a column (Lyu et al. 2016). A ‘Que-Ti’ is usually placed at the intersection of the horizontal material (beam, i.e., Fang) and the vertical material (column) of heritage timber buildings, and generally, it has three roles: the first is to shorten the net span of the beam to enhance the bearing capacity (Lyu et al. 2017); the second is to reduce the downward shear force between the beam and column (Dai 2020); and the third is to prevent the angle tilt between the horizontal and vertical wooden components (Yang et al. 2020). The development of the configuration of the ‘Que-Ti’ began to take shape during the Northern Wei Dynasty, and it was not until the Ming Dynasty that it began to be widely used and became a unique heritage timber building component in the Qing Dynasty (Chun et al. 2019).
A ‘Que-Ti’ component is assembled using the mortise-tenon connection technology in heritage timber buildings. It is the combination of structural mechanics and aesthetics, for it reinforces the mechanical properties of the mortise-tenon joints connecting the Fang and the column in terms of the structural mechanics as well as the shaping of the wings attached to both sides of the column cap with interesting contour curves, paints, and carvings at an artistic level (as shown in Fig. 1) (Li et al. 2020).
Fig. 1. A ‘Que-Ti’ component in heritage timber building
The mortise-tenon joint in heritage timber architecture is usually considered as a semi-rigid joint in modern structural engineering, and the primary reason it is a research focus by many scholars is due to its excellent seismic performance (Xue et al. 2018, 2019). Taking the bending moment capacity of the mortise-tenon joint as the starting point for research is an important means to measure the strength of the mortise-tenon joint and evaluate its seismic performance. Generally, the factors affecting the bending moment capacity and bending stiffness of mortise-tenon joints are the wood species, growth ring, adhesive type, load type, tenon size, tenon geometry, etc. (Záborský et al. 2017; Hu and Liu 2020). Considering the effects of the wood species, adhesive type, and tenon size on the bending strength and flexibility of a T-shaped mortise-tenon joint, the test showed that the stiffness of the mortise-tenon joint was positively correlated with the tenon length and width, and the influence of the tenon width was more obvious than that of the length (Erdil et al. 2005). Taking into account the function of the tenon geometry, grain orientation, length, and shoulder fit, the bending moment capacity of the mortise-tenon joint with complete insertion of the tenon was 54% larger than the bending moment capacity of the joint without complete insertion. Additionally, a rectangular tenon has the largest bending moment capacity under the same conditions (Likos et al. 2012). In terms of the influence of the joint dimensions on the mechanical properties of the mortise-tenon joint, regression functions in the form of a second power polynomial with interactions or in the form of a power functions product can be used to express the relationship, and the influence on the joint strength from strong to weak is the tenon length, tenon width, and tenon thickness, respectively (Wilczyński and Warmbier 2003; Tankut and Tankut 2005). In addition, experiments have shown that the shape of the adhesive has a strong influence on the strength of the tenon, and non-dilatational deformation can considerably limit the pressure of the tenon on the mortise, thereby reducing the level of dangerous shear stresses (Prekrat and Smardzewski 2010).
The mortise-tenon joint is the weakest part and has a great influence on the mechanical properties of heritage timber buildings; therefore, analyzing the damage of mortise-tenon joint under seismic waves in terms of their strength, stiffness, and energy dissipation is an important method to evaluate the seismic performance of heritage timber buildings (Poletti et al. 2019). A specific research method is to use finite element analysis software or physical tests to analyze the dynamic response of a mortise-tenon joint under different seismic intensities to extract the moment-rotation (M-θ) curve of the mortise-tenon joint before and after damage, and to measure the energy dissipation of the mortise-tenon joint with the area of the largest hysteresis loop as the quantitative indicator (Wang and Jin 2014; Xie et al. 2020). To effectively simulate the seismic performance of mortise-tenon joints, it is necessary to accurately evaluate the load-deformation hysteretic performance of the joints (Xie et al. 2019). To achieve this evaluation, it is necessary to establish the hysteretic models of the mortise-tenon joints, where the existing hysteretic models of mortise-tenon joints can be divided into three categories, i.e., finite element (FE) models, physical models, and phenomenological models (Xue and Xu 2018; Meng et al. 2019). The finite element models and physical models can effectively predict the hysteretic performance of mortise-tenon joints but are not suitable for the structural model with a large number of joints (Loss et al. 2018). However, the phenomenological models are controlled by a set of mathematical equations with parameters to specify the envelope path and hysteresis so that the application scope is wider and can be used in combination with open-source software (Dong et al. 2021).
When utilizing mortise-tenon technology to connect the components of heritage timber structures, the mortise-tenon joint is formed by inserting different shapes of tenons into the corresponding mortise, and the typical mortise-tenon joints concluded from various joint forms include straight-tenon, through-tenon, half-tenon, swallowtail-tenon, etc. (Huang et al. 2017). Long-term stress at the mortise-tenon joints will lead to joint loosening and tenon pull-out, which could moderately weaken the stiffness, bearing capacity, and energy dissipation capacity of heritage timber structures. Therefore, it is of great scientific importance to compare the mechanical properties of different types of mortise-tenon joints (Yue 2014). In this paper, three typical mortise-tenon joints, i.e., straight-tenon, through-tenon, and half-tenon, were selected as the research objects. Each mortise-tenon joint pattern was made into two types: one type assembled with the ‘Que-Ti’ component and the other type without. The influence of the ‘Que-Ti’ component on the mechanical properties of the mortise-tenon joint was compared and analyzed from four aspects, i.e., the hysteretic curve, skeleton curve, force and stiffness, and energy dissipation (Chun et al. 2011).
EXPERIMENTAL
Mechanical Property Tests of Pinus sylvestris var. mongolica
The standard values of the bending strength, modulus of elasticity, compressive strength parallel to the grain, full surface compressive strength perpendicular to the grain (tangential direction and radial direction), and sectional compressive strength perpendicular to the grain (tangential direction and radial direction) were obtained by testing the mechanical properties of Pinus sylvestris var. mongolica. The experiments were carried out according to the test specifications outlined by GB/T standard 1936.1 (2009), GB/T standard 1935 (2009), and GB/T standard 1939 (2009). In this part of the test, the measured values of each index were obtained according to the average values obtained after the test of 15 specimens.
The specific processing of the material test is shown in Fig. 2, where (a) is the test process of the modulus of rupture, (b) is the test process of the compressive strength parallel to the grain, (c) is the test process of the full surface compressive strength perpendicular to the grain, and (d) is the test process of the sectional compressive strength perpendicular to the grain. The standard values of all the mechanical properties tested are shown in Table 1.
Fig. 2. The specific test process of Pinus sylvestris var. mongolica: (a) test process of the modulus of rupture; (b) test process of the compressive strength parallel to the grain; (c) test process of the full surface compressive strength perpendicular to the grain; and (d) test process of the sectional compressive strength perpendicular to the grain
Table 1. Physical and Mechanical Parameters of Pinus sylvestris var. mongolica
Design and Fabrication of the Specimens
The fabrication of the specimens was based on the Cai and Fèn system (1 Cai = 15 Fèn) stipulated in Ying Zao Fa Shi, Song Dynasty. In the Cai and Fèn system, there are eight grades (Grade Ⅰ to Grade Ⅷ) of Cai, and this paper takes Grade Ⅲ, in which a Fèn is approximately equal to 16 mm. The specimens were designed at a 1 to 3.2 scale ratio to ensure both moderate size and integral geometric dimensions of the components. In the experiment, three types of mortise-tenon joint specimens, i.e., straight-tenon joints, through-tenon joints, and half-tenon joints, were made. Two specimens were made for each type of mortise-tenon joint, one of which was supported by a ‘Que-Ti’ component and the other was not, for a total of six specimens. All specimens were made of Pinus sylvestris var. mongolica, and the basic mechanical properties of Pinus sylvestris var. mongolica were tested.
The mortise-tenon joint specimens without a ‘Que-Ti’ component are composed of two parts: the column and the Fang, while the mortise-tenon joint specimens with a‘Que-Ti’ component were composed of three parts: the column, the Fang and the ‘Que-Ti’ (Fig. 3.). The basic dimensions of the ‘Que-Ti’ component were 63 mm x 180 mm x 450 mm (width, height, and length), and the tenon of the ‘Que-Ti’ component was 15 mm x 135 mm x 75 mm (width, height, and length), while the other dimensions of each mortise-tenon joint are shown in Table 2.