Abstract
In this study, the finite element method (FEM) was used to determine the effect of the optimal angle of the thread and double thread application among self-tapping screw (STS) design information on the improvement of the withdrawal capacity of the connection. It was modeled by reflecting the design information of an Italian STS distributed in the domestic wooden building market, and the stress distribution of the connections was compared according to the change in the thread angle. A cross laminated timber (CLT) composed of five layers was modeled as a member. The STS modeling was centered on the threaded area, and two threaded angles were applied: 90° and 95°. Additionally, the stress changes were compared when double threads located in the middle of the thread pitch in the screw pitch were applied to improve the withdrawal capacity of the connection. The domestic STSs were manufactured using four materials and two shapes. The finite element analysis and strength performance tests of the STS types indicated that the material properties, angle of the screw thread, and shape of the screw thread affect the Korean CLT withdrawal capacity.
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Improved Design of Self-tapping Screw (STS) for Korean Larch and Red Pine Cross Laminated Timber (CLT)
In-Hwan Lee,a Sang-min Lee,a and Keon-Ho Kim b,*
In this study, the finite element method (FEM) was used to determine the effect of the optimal angle of the thread and double thread application among self-tapping screw (STS) design information on the improvement of the withdrawal capacity of the connection. It was modeled by reflecting the design information of an Italian STS distributed in the domestic wooden building market, and the stress distribution of the connections was compared according to the change in the thread angle. A cross laminated timber (CLT) composed of five layers was modeled as a member. The STS modeling was centered on the threaded area, and two threaded angles were applied: 90° and 95°. Additionally, the stress changes were compared when double threads located in the middle of the thread pitch in the screw pitch were applied to improve the withdrawal capacity of the connection. The domestic STSs were manufactured using four materials and two shapes. The finite element analysis and strength performance tests of the STS types indicated that the material properties, angle of the screw thread, and shape of the screw thread affect the Korean CLT withdrawal capacity.
DOI: 10.15376/biores.19.2.3353-3361
Keywords: Self-tapping screw; Cross laminated timber; FEM; Connections; Withdrawal; Capacity; Larch; Red pine; CLT
Contact information: a: Wood Engineering Division, Forest Products and Industry Department, National Institute of Forest Science, 57 Hoegi-ro, Dongdaemun-gu, Seoul 02455, Republic of Korea; b: Wood Industry Division, Forest Products and Industry Department, National Institute of Forest Science; *Corresponding author: keon@korea.kr
INTRODUCTION
In order to respond to the climate change crisis, many countries have been establishing a carbon-neutral plan. IPCC (2018) reported that greenhouse gas emissions in the construction sector account for a significant proportion. Limiting global warming to 2° requires achieving global carbon neutrality by 2070, and reaching the 1.5° target requires achieving global carbon neutrality by 2050. Wooden construction using timber, a carbon storage material used as an eco-friendly building material, has been recognized as an important part of realizing carbon neutrality. With the recent development of high-rise wooden construction technology, construction projects using cross-laminated timber (CLTs) have increased (Lee et al. 2022; Muñoz et al. 2022). In the CLT structure, the performances of the CLT and CLT fasteners have been found to be the most crucial factors. Self-tapping screws (STS) are attracting increasing attention as CLT fasteners owing to their excellent performance and high fastening force (Ringhofer et al. 2015; Sullivan et al. 2018; Srivaro et al. 2021). In Korea, STS research is ongoing (Lee and Kim 2023; Lee and Kim 2024).
The specific gravity of the species mainly used in Europe, North America, or Japan is approximately 0.35 to 0.45 (Unterwieser and Schickhofer 2013; Yusoh et al. 2021). However, larch (356,208 m3, 64.3 %) and red pine (131,873 m3, 23.8 %), with an average weight of 0.52 and 0.45, comprise the majority of softwood lumber producers in Korea (Wood Utilization Survey 2021). Therefore, various studies on larch and red pine CLT have been conducted in Korea (Song and Hong 2016; Choi et al. 2018; Pang and Jeong 2019; Song et al. 2019; Lee and Kim 2021). Larch CLT requires higher performance for fasteners owing to their superior physical properties and relatively high specific gravity. Therefore, high-performance STS must be developed.
To improve the performance of a self-tapping screw used as a fastener for CLT, the design conditions must be determined. The strength performance according to the type of fastener and the mechanical performance of the member are crucial factors for predicting the shear performance of a CLT connection.
The variables in the withdrawal capacity performance of STS suitable for larch and red pine were angle, material, and shape. The design information of the Italian STS for its angle was analyzed using FEM (Finite element analysis) to compare the stress distribution according to the angle change. The STS material was reviewed in terms of bending strength performance by varying the type of material and heat treatment conditions. The withdrawal resistance strength performance of the STS shape was compared by applying double-screw threads. However, the new STS must be reviewed for availability as a fastener through a joint shear test if it shows good withdrawal capacity performance on the arch.
EXPERIMENTAL
Material and Methods
FEM by angle of thread
FEM was performed on the withdrawal resistance capability according to the thread angle of the STS fastener and the CLT fiber direction. The FEM model implemented the cross-section of the STS and that of the larch, as shown in Fig. 1. The withdrawal resistance strength performance of both types were evaluated with different thread angles. The FEM model was analyzed using the static structure of the ANSYS 2023 R2 program. The physical properties of the wood were set as listed in Table 1, and those of the STS were set as basic structural steel. The contact condition between the STS and Lac was set as no separator. The larch timber part without contact surface with the STS was fixed. To measure the detailed behavior, the mesh was set to 0.5 mm. Certain information was applied upward to the STS to observe similar behavior as that in the withdrawal strength test. The data used in the analytical model were obtained from the literature and are listed in Table 1.
Table 1. Orthotropic Material Properties Used in FE Simulations taken from Wood-Handbook (2010) and Warguła (2020) (at approximately 12% moisture content)
Fig. 1. Schematic diagram of FEM cross section model and thread angle using STS
Material Properties of STS
Because the strength of the screw thread is related to the strength characteristics of the material, the STS material was changed to reinforce the stress-intensive screw thread, and an STS bending strength test was conducted. Four types of Korean STS are produced depending on the material, and the number of heat treatments, methods, and materials differs for each type. HBS 10 (Rothoblaas, Italy) was produced in Europe.
Table 2. Material Properties of STS
A bending performance test based on the STS material was performed under a concentrated load with a span of 80 mm.
Shape of Screw Threads
The thread interval of the STS was wider than that of the normal thread, increasing the volume of timber because that of the thread interval supports the thread when the thread is drawn. Despite the additional thread being processed, the timing would not be crushed and supported owing to the higher specific gravity of the Larch CLT and red pine CLT than the species commonly used in Europe or the United States. In addition, the resistance will increase when STS is inserted into the timber if the thread size remains the same. Therefore, additional processing of small threads between those of the existing STS improves the withdrawal capacity. The shape of the thread was divided into the basic shapes of the European STS and double thread. A new thread was machined in the space between the existing thread and the thread (Fig. 2). A withdrawal resistance strength test according to the shape of the screw thread was performed according to KS F ISO9087.
Fig. 2. Korea STS design for double threaded applications
Performance Evaluation of Larch CLT and Pine CLT Withdrawal Capacity
The Japanese larch (Larix kaempferi (Lamb.) Carriere) and Korean red pine (Pinus densiflora) were used for the withdrawal capacity test drawn after STS was inserted into the CLT 3ply. The average specific gravities of Japanese larch are 0.5, and Korean red pine are 0.44. Two types of STS, HBS 10 (Rothoblaas, Italy) and an STS developed based on the experiments in this study were used. The withdrawal capability test was performed by fixing the lower part of the CLT to the lower jig of the instrument and the STS head to the upper jig (Fig. 3). The withdrawal speed was set to fail within 1.0 to 3 min. The speed was 4 mm/min.
Fig. 3. Images of withdrawal specimens and strength test method
RESULTS AND DISCUSSION
Effect of Angle of Thread on Withdrawal properties of STS
The strength applied to the FEM model was 7226 N, calculated based on the maximum withdrawal resistance of the STS inserted into the larch CLT.
Fig. 4. FEM deformation distribution for each angle of screw thread (left: A type – Max. deformation 1.2175 mm, right: B type – Max. deformation 0.638 mm)
Fig. 5. FEM stress distribution of STS (top: A type – Max stress 5140 MPa, bottom: B type – Max. stress 3710 MPa)
From the FEM analysis, the STS with a thread angle of 90° was deformed by 0.638 mm at the same withdrawal load, but the STS with a thread angle of 95° was deformed by 1.218 mm (Fig. 4). In addition, the maximum stress that the STS receives was 38.6 % higher at 95° than at a thread angle of 90° (Fig. 5).
Maximum stress and deformation were observed under the thread. Therefore, the angle of the STS screw thread and verification experiments must be optimized and conducted in the future.
Improvement Effects of Material Properties of the Threads of STS
When a withdrawal load is applied, the resistance stress is concentrated on the screw thread; therefore, the strength characteristics of the STS material are crucial.
As shown in Table 2, the bending strength of the Type C steel was excellent. However, Type D has better ductility than Type C (Fig. 6). When STSs are used as CLT fasteners, the ductility of the connection is a crucial factor in securing structural stability.
Table 3. Bending Strength Performance According to STS Fabrication Method
Fig. 6. Load-deformation curves and failure modes of the STS bending strength specimen.
Comparison of Withdrawal Resistance by the Tip and Double Thread
The withdrawal resistance strength according to the groove of the tip for penetrating the STS, was compared. The groove of the STS had little effect on the STS withdrawal resistance of the CLT. However, many grooves were required to facilitate fastener insertion (Fig. 7).
Fig. 7. Processing tip of Korea STS fastener (Type D)
Table 4. Comparison of 3 CLT Withdrawal Resistance Loads of STS According to Tip Type
Withdrawal Capacity Performance of Korean CLT Using New STS
Figure 8 compares the STS of the final production and the existing HBS 10. Table 5 presents the dimensions of the two STS. In the figure, ds is the shank diameter, dc is the shank cutter diameter; d1 is the outer thread; d2 is the inner thread; d3 is the middle thread; and t2 is the pitch between threads. As shown in Table 6, the withdrawal resistance performance of the STS with double screws was 1.05 times greater for deciduous pine CLT and 1.06 times greater for pine CLT than for single screws. Because the shape of the screw affects the withdrawal performance, various FEM analyses are needed to optimize the STS shape, such as the internal diameter, external diameter, pitch, and double screw, in the future.
Fig. 8. Images of STS samples (top and bottom: HBS10, new STS)
Table 5. STS Dimensions (mm)
Table 6. Comparison of Withdrawal Resistance of 3ply CLT using Korea STS
Type A: single thread, Type B: double thread
CONCLUSIONS
In this study, various variables were applied to self-tapping screws (STS), and their performances were verified to develop a new type of STS, a connecting fastener suitable for Korean larch and red pine cross-laminated timber (CLT). The results are as follows:
- The additional machining of small threads between the threads of the STS improves the withdrawal capacity.
- The thread angle of the Italian STS, widely used as a CLT fastener, exhibits good withdrawal resistance strength when adjusted from 95 ° to 90 °.
- The STS with the best material was the STS that treated the SCM435 model once with heat treatment.
- The tip of the STS did not affect the withdrawal performance.
A new STS was produced under these conditions. The new STS was inserted into the larch CLT and red pine CLT to verify the withdrawal capacity. The larch CLT was improved by 0.8 %, and the red pine CLT was improved by 25.4 %.
ACKNOWLEDGMENTS
This research was supported by a Research Project (FP0200-2021-01-2023) through the National Institute of Forest Science (NIFoS), South Korea.
REFERENCES CITED
Choi, C., Kojima, E., Kim, K. J., Yamasaki, M., Sasaki, Y., and Kang, S. G. (2018). “Analysis of mechanical properties of cross-laminated timber (CLT) with plywood using Korean larch,” BioResources 13(2), 2715-2726. DOI: 10.15376/biores.13.2.2715-2726
IPCC (2018). Intergovernmental Panel on Climate Change: Global warming of 1.5°C, V. Masson-Delmotte et al., Eds. (IPCC Special Report, IPCC, 2018); www.ipcc.ch/sr15. Accessed on 04 February 2021
Korea Forest Service (2021). 2021 Report on Wood Utilization Survey, Korea Forest Service, Daejeon, Korea.
Lee, I. H., and Kim, K. H. (2021). “Influence of adhesive and layer composition on compressive strength of mixed cross-laminated timber,” BioResources 16(4). DOI: 10.15376/biores.16.4.7461-7473
Lee, I. H., Kim, K., and Shim, K. B. (2022). “Evaluation of bearing strength of self-tapping screws according to the grain direction of domestic Pinus densiflora,” Journal of the Korean Wood Science and Technology 50(1), 1-11. DOI: 10.5658/WOOD.2022.50.1.1
Lee, I. H., and Kim, K. H. (2023). “Effect of physical properties of softwoods on embedment strength performance of self-tapping screws,” BioResources 18(2), 3576-3589. DOI: 10.15376/biores.18.2.3576-3589
Lee, I. H., and Kim, K. (2024). “Prediction of withdrawal resistance of self-tapping screws in softwood structural lumber,” BioResources 19(1), 1274-1282. DOI: 10.15376/biores.19.1.1274-1282
Muñoz, F., Tenorio, C., Moya, R., and Navarro-Mora, A. (2022). “CLT fabricated with Gmelina arborea and Tectona grandis wood from fast-growth forest plantations: Physical and mechanical properties,” Journal of Renewable Materials 10(1), 1-17. DOI: 10.32604/jrm.2022.017392
Pang, S. J., and Jeong, G. Y. (2019). “Effects of combinations of lamina grade and thickness, and span-to-depth ratios on bending properties of cross-laminated timber (CLT) floor,” Construction and Building Materials 222, 142-151. DOI: 10.1016/j.conbuildmat.2019.06.012
Ringhofer, A., Brandner, R., and Schickhofer, G. (2015). “Withdrawal resistance of self-tapping screws in unidirectional and orthogonal layered timber products,” Materials and Structures, 48(5), 1435-1447. DOI: 10.1617/s11527-013-0244-9
Song, Y. J., and Hong, S. I. (2016). “Evaluation of bonding strength of larch cross-laminated timber,” Journal of the Korean Wood Science and Technology 44(4), 607-615. DOI: 10.5658/WOOD.2016.44.4.607
Song, Y. J., Lee, I. H., and Hong, S. I. (2019). “Evaluation of horizontal shear performance of larch CLT walls according to the edge connection shape,” Wood Research 64(2), 213-222. DOI:10.15376/biores.13.2.2715-2726
Srivaro, S., Pásztory, Z., Le Duong, H. A., Lim, H., Jantawee, S., and Tomad, J. (2021). “Physical, mechanical and thermal properties of cross laminated timber made with coconut wood,” European Journal of Wood and Wood Products 79(6), 1519-1529. DOI: 10.1007/s00107-021-01741-y
Sullivan, K., Miller, T. H., and Gupta, R. (2018). “Behavior of cross-laminated timber diaphragm connections with self-tapping screws,” Engineering Structures 168, 505-524. DOI: 10.1016/j.engstruct.2018.04.094
Unterwieser, H., and Schickhofer, G. (2013). “Characteristic values and test configurations of CLT with focus on selected properties,” in: Report, Focus Solid Timber Solutions–European Conference on Cross Laminated Timber (CLT), COST Action FP1004, Graz, pp. 53-73.
Warguła, Ł., Wojtkowiak, D., Kukla, M., and Talaśka, K. (2020). “Symmetric nature of stress distribution in the elastic-plastic range of Pinus L. pine wood samples determined experimentally and using the finite element method (FEM),” Symmetry, 13(1), 39. DOI: 10.3390/sym13010039
Yusoh, A. S., Tahir, P. M., Uyup, M. K. A., Lee, S. H., Husain, H., and Khaidzir, M. O. (2021). “Effect of wood species, clamping pressure and glue spread rate on the bonding properties of cross-laminated timber (CLT) manufactured from tropical hardwoods,” Construction and Building Materials 273, article 121721. DOI: 10.1016/j.conbuildmat.2020.121721
Article submitted: October 19, 2023; Peer review completed: November 4, 2023; Revised version received: April 3, 2024; Accepted: April 5, 2024; Published: April 15, 2024.
DOI: 10.15376/biores.19.2.3353-3361