Abstract
To evaluate the shear-resistant behavior of wooden dowels used in Blockhaus shear walls under cyclic load, 19 specimens under ten groups of conditions were prepared and tested. The failure modes, hysteresis curves, mechanical properties, stiffness degradations, and energy dissipation capacities of the specimens were studied. The test results showed that with the increase in the number of dowels, the initial stiffness and peak load of the specimens increased greatly. The diameter of the dowels had little influence on the mechanical properties of the specimens. Furthermore, the test findings demonstrated that the pretension load between the walls greatly enhanced the initial stiffness and energy dissipation capacity of specimens. A simplified finite element model was established in Opensees. Considering the effect of material variability, the parameters of single dowel shear spring and friction spring were identified by Genetic Algorithm with modified objective function in Matlab. The identified parameters were applied to the finite element model of the multi-dowel specimens. The simulation results were in good agreement with the test results, and the validity of the numerical model and parameter identification method was verified.
Download PDF
Full Article
Parameter Identification Procedure for Hysteretic Shear-Resistant Properties of Beech Wood Dowels
Jiwei Liu,a,* Huifeng Yang,b,* Yutao Zhou,b Benkai Shi,b and Haotian Tao a
To evaluate the shear-resistant behavior of wooden dowels used in Blockhaus shear walls under cyclic load, 19 specimens under ten groups of conditions were prepared and tested. The failure modes, hysteresis curves, mechanical properties, stiffness degradations, and energy dissipation capacities of the specimens were studied. The test results showed that with the increase in the number of dowels, the initial stiffness and peak load of the specimens increased greatly. The diameter of the dowels had little influence on the mechanical properties of the specimens. Furthermore, the test findings demonstrated that the pretension load between the walls greatly enhanced the initial stiffness and energy dissipation capacity of specimens. A simplified finite element model was established in Opensees. Considering the effect of material variability, the parameters of single dowel shear spring and friction spring were identified by Genetic Algorithm with modified objective function in Matlab. The identified parameters were applied to the finite element model of the multi-dowel specimens. The simulation results were in good agreement with the test results, and the validity of the numerical model and parameter identification method was verified.
DOI: 10.15376/biores.19.2.3681-3698
Keywords: Wooden dowels; Blockhaus shear walls; Hysteretic behavior; Parameter identification
Contact information: a: School of Civil Engineering, Southeast University, Nanjing, 211189, China; b: College of Civil Engineering, Nanjing Tech University, Nanjing, 211816, China; * Corresponding authors: 945290530@qq.com; hfyang@njtech.edu.cn
INTRODUCTION
This paper considered the shear performance of wooden dowels in Blockhaus shear walls under cyclic load. Blockhaus structure is a timber structure made by overlapping square timbers, as shown in Fig. 1. It has been shown that Blockhaus shear walls exhibit greater lateral resistance than light wood shear walls and similar hysteretic properties to concrete shear walls (Graham et al. 2010; Branco and Araújo 2012). Additionally, Blockhaus timber structures show great seismic performance during both the experiments and actual usage (Branco et al. 2013; Tomasi and Piazza 2013). Therefore, Blockhaus structures are widely built in areas of the world having a high percentage of forest cover (Bedon and Fragiacomo 2019; Sciomenta et al. 2020).
Square timber shear walls show an obvious load resistance mechanism. The vertical load-resistant ability of traditional Blockhaus wall is determined by the compression strength of timber perpendicular to the grain and by the contact area. The ability to resist horizontal load is mostly dependent on the friction between the timber pieces, the interlock between orthogonal walls, and dowel-type members between the timbers (Hirai et al. 2004).
Although the Blockhaus building has been shown to have satisfactory seismic performance, the non-structural damage brought on by sliding between logs is still substantial. Thus, in a Blockhaus structure, a sensible and secure anti-lateral load mechanism must be implemented. The following conclusions have been drawn through the research: (i) The lateral load-resistant ability of Blockhaus shear walls can be calculated by the superposition of each lateral load-resistant element; (ii) The primary energy dissipation capability of the structure is provided by friction between interfaces; and (iii) If the wooden dowels are properly designed, the lateral load resistance ability offered by the dowels will be substantially greater than that offered by the interlock between orthogonal log walls (Hirai et al. 2004; Scott et al. 2005a,b). The horizontal resistance of the Blockhaus shear wall is significantly influenced by the friction between the timber pieces and the dowel-type shear components. The combination of friction and dowel-type shear components effectively reduces the relative displacement between the timbers and the non-structural damage caused by sliding between them if the dowel-type components are designed and built properly. However, under normal design conditions or under specific pressures such as earthquakes, the currently existing European standards do not include an analytical model for the shear resistance of wooden dowels in the Blockhaus shear wall (EN 1995-1-1 2004; EN 1998-1 2004). Nevertheless, the vast majority of studies on the shear contribution element ignore the impact of friction and wooden dowels in favor of concentrating on interlock between orthogonal log walls of the shear walls. As shown in Fig. 1, orthogonal timber walls involve cross-bite connections among the timbers for adjoining walls (Branco and Araújo 2010; Giovannini et al. 2014; Grossi et al. 2016; Sciomenta et al. 2018). Therefore, it is difficult to accurately predict and evaluate the load bearing capacity and seismic performance of the Blockhaus structure, which limits its design and application.
As for the numerical model, it turns out that it is challenging to precisely model the solid finite element simulation of dowel-type components in timber structures concerning the use of dowel-type components in other timber systems. The primary reason is that macroscopic timber property tests are typically used to determine the timber mechanical properties used in the finite element model. In contrast, the early stage of timber damage between the dowel-type components and timber material in the push-out test is miniscule. As a result, the shear components of dowel-type connectors frequently have an excessively high initial stiffness estimate using solid finite element simulation (Wang et al. 2018). Furthermore, the mechanical properties of wooden dowels are influenced by the properties of timber materials and the randomness of defects. Therefore, there are some differences in the shear performance of wooden dowels with the same geometric parameters (Lam et al. 2008; Wang et al. 2016, 2019).
Considering the limitations of previous research, in this paper the influence of the number and diameter of the wooden dowels and vertical pretension load on the shear mechanical properties of the dowels was analyzed. Considering the limitation of the initial stiffness analysis of the solid element numerical model of the shear components in timber structures, a simplified numerical model was established in the Opensees environment. Multiple groups of test data under the same experimental conditions were applied to the parameter identification program (Genetic Algorithm), considering the randomness of timber properties and defects. To determine the parameters for the single dowel and friction spring, the modified objective function is the square sum of the differences between each group of test data and the model data. The identified parameters were then applied to the multi-dowel specimens to compare the test results with the results of the numerical model. The validity of the numerical model and the parameter identification method were confirmed.
Fig. 1. Blockhaus shear walls and orthogonal log walls
EXPERIMENTAL TEST
Test Specimens
The goal was to evaluate and compare the effects of wooden dowel diameter, number, and pretension load on shear-resistant performance. The primary design information of the specimens is shown in Table 1. Pinus sylvestris glulam (glued laminated timber) was used to manufacture rectangular timber pieces. The dimensions of timber pieces used in test structures were 105(T) mm × 850(L) mm × 180(R) mm. The dowels were made of beech. The length of each dowel was 540 mm, and the diameter was selected according to different working conditions, as shown in Table 1. The primary properties of timber material were obtained by material testing according to European code (EN 408:2010+A1 2012), as shown in Table 2.
Table 1. Test Information of Hysteretic Shear Test of Wooden Dowels
Table 2. Physical and Mechanical Properties of Timber Material
Grade 235 steel with a yield strength of 235 MPa and elastic modulus of 200 GPa was used to manufacture the steel brackets and plates. The yield strength of the steel bars used to apply the pretension load was 640 MPa and the diameter of steel rods was 14 mm. The detailed geometries and specimen sizes are shown in Fig. 2.
Test Setup and Loading Procedure
The loading device and measurement instruments of the specimens are shown in Fig. 4, including (a) Test setup; (b) Loading procedure; and (c) Preliminary test.
Fig. 2. Geometry of test specimens
Fig. 3. Pretension load application diagram and timber logs
(a) Test setup (b) Loading procedure (c) Preliminary test
Fig. 4. Test set-up and loading procedure
Figure 4(a) shows the two sides of the walls fastened to the reaction frame by the steel plates and screws. Linear variable displacement transformers (LVDTs) 1 to 4 were used to measure the relative displacement between walls. The LVDTs 5 and 6 arranged at the bottom of the middle walls recorded the absolute displacements of the specimens. Two force sensors recorded the changes in pretension load in steel rods.
The specimen designed with pretension load was preloaded through the core jack and the screw-thread steel bars, as shown in Fig. 3(a). When the nut was screwed well on one side, the special steel member and the core jack were used to stretch the steel bars on the other side. Finally, the nut at the tension section was tightened to realize the pretension force. The pressure sensor was used to test whether the pretension force reached the design value, and the hysteresis cycle test was carried out immediately after the end of the tension. In order to prevent the screw from being sheared during the loading process, the reserved hole for the screw in timber logs was enlarged (an oval hole with a length of 60 mm) according to the ultimate displacement obtained from the monotonic test results, as shown in Fig. 3(b). The steel plate was positioned to control the central position of the screw in the reserved hole, to ensure that the screw did not shear during the loading process.
A 300 kN actuator was fastened to the middle wall using steel plates and screws to apply cyclic loads. The loading protocol refers to the loading method in European code (EN 12512 2001), as illustrated in Fig. 4(b), where the loading displacement uses a multiple of the yield displacement. The peak displacement of the first two cycles was 0.25 and 0.5 times the yield displacement, respectively. The remaining loading steps used 0.75, 1, 2, and 4 times the yield displacement, respectively. Each step was performed for three cycles. The yield displacements of the specimens were selected according to the preliminary monotonic tests, as shown in Fig. 4(c) (Wang 2018).
SIMULATION METHODS
Failure Modes
The failure mode of the specimen is shown in Fig. 5. Local crushing damage can be found around the reserved holes in the timber walls due to the compression of wooden dowels. Shear failure of the dowels due to cyclic load can be observed in Fig. 5(a).
Fig. 5. Failure modes of specimens
Hysteresis Curves and Mechanical Properties
The hysteretic curves of load-relative displacement of the specimen recorded from the cyclic tests are shown in Fig. 6. Hysteretic behavior of the specimen with pretension load and without dowels is illustrated in Fig. 6(a). The response force of this specimen is generated only by the friction of the contact surface. The hysteresis curve of one group is shown in Fig. 6(a), because the results of the three groups were similar. The specimen’s hysteretic curve was full and almost identical to the specimen with ideal elastoplastic behavior. Before the sliding of the test specimens, static friction provided anti-shear force, as shown in the elastic stage in Fig. 6(a). After the sliding of the test specimens, dynamic friction provided shear force, as shown in the yield stage in Fig. 6(a). The relative displacement can be observed in Fig. 5(b). Considering the short-term test, the creep behavior of timber under long-term compression was not considered in this paper. The following conclusions can be drawn from the hysteresis curves in Figs. 6(b) to 6(n): (i) Specimens with pretension load had the fuller shape of hysteretic curves; (ii) With the increase of the number of dowels, the peak load of the specimens increased greatly; (iii) The peak load of each specimen with a single dowel without pretension load was similar. However, the load-decreasing section and the ultimate displacement were quite different among the above-mentioned specimens due to the randomness of properties and defects of timber material.
