Abstract
A separable glued-laminated timber (GLT, Larix kaempferi Carr.)-steel beam system is presented in this work for easy recycling at the time of disposal. The minimum thickness of steel required to induce compressive GLT failure was assembled with GLT by inclined screws. In a total of 8 GLTs, 3 GLTs were not reinforced (control group), and 5 GLTs were reinforced with steel plates (comparison group). In the GLT in the comparison group, a steel plate (SPHC, yield strength: 227 MPa, modulus of elasticity 166.33 GPa) was installed with screws (∅9x160mm, 45°). The deflection and load of specimens were measured by a third-point bending test to derive their bending stiffness and load-carrying capacities. All specimens in the control group showed brittle tensile failure, but all specimens in the comparison group showed ductile behavior and maintained a load-carrying capacity of about 30 kN. After the compression failure of the GLT, there was no damage to the screw connection, while the steel plate was extended. Based on the behavior of the steel, a GLT-steel beam prediction model was developed, similar to the structural design method for reinforced concrete.
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Bending Behavior of Separable Glued-Laminated Timber (GLT)-Steel Beam Combined with Inclined Screws
Sung-Jun Pang,a and Jung-Kwon Oh a,b,*
A separable glued-laminated timber (GLT, Larix kaempferi Carr.)-steel beam system is presented in this work for easy recycling at the time of disposal. The minimum thickness of steel required to induce compressive GLT failure was assembled with GLT by inclined screws. In a total of 8 GLTs, 3 GLTs were not reinforced (control group), and 5 GLTs were reinforced with steel plates (comparison group). In the GLT in the comparison group, a steel plate (SPHC, yield strength: 227 MPa, modulus of elasticity 166.33 GPa) was installed with screws (∅9x160mm, 45°). The deflection and load of specimens were measured by a third-point bending test to derive their bending stiffness and load-carrying capacities. All specimens in the control group showed brittle tensile failure, but all specimens in the comparison group showed ductile behavior and maintained a load-carrying capacity of about 30 kN. After the compression failure of the GLT, there was no damage to the screw connection, while the steel plate was extended. Based on the behavior of the steel, a GLT-steel beam prediction model was developed, similar to the structural design method for reinforced concrete.
DOI: 10.15376/biores.18.2.3838-3855
Keywords: Glued-laminated timber; Steel; Composite beam; Bending performance; Reinforcement
Contact information: a: Department of Agriculture, Forestry and Bioresources, Seoul National University, Seoul, Republic of Korea; b: Research Institute of Agriculture and Life Sciences, Seoul National University, Seoul, Republic of Korea; * Corresponding author: jungoh@snu.ac.kr
INTRODUCTION
Carbon emission reduction has become a key topic in the construction industry. Timber construction has a low-carbon emission due to the light weight and easy cutting of the timber elements (Gerilla et al. 2007; Yan et al. 2010; Hafner and Schäfer 2018; Sandanayake et al. 2018; Li et al. 2019; Röck et al. 2020). In addition, timber retains carbon absorbed as the tree grows. Structural timber within a building will store the carbon as long as the building is maintained. Thus, the use of timber as a structural element contributes to the atmospheric carbon emissions reduction of buildings (Petersen Raymer 2006; Resch et al. 2021; Pang et al. 2022).
Structural timber is used as a beam due to its high bending strength compared to its weight. The design value of structural timber is based on the 5% lower limit, which is the 5th value from the lowest strength when 100 destructive tests are conducted (Pang et al. 2013, 2020). This is because a high safety factor is required for timber due to brittle failure. If the timber is subjected to ductile failure rather than brittle failure, then the safety factor can be lowered. Steel is a typical structural material with good ductility. The combination of steel and glued-laminated timber (GLT) is used in dry construction and the components are recyclable (Tsai and Le 2018). In addition, both materials are cut or drilled in the factory using CNC cutting machines (Lee et al. 2021). Thus, the machining precision is high and the machining error is small (Tolerances: ±1mm range). This means that the two materials have many advantages that can be used as a hybrid.
Several researchers tried to develop timber-steel hybrid beams (Moritani et al. 2021). André (2006) showed that when the GLT is reinforced by steel, the bending performance of the GLT beam is improved and the deviation is reduced. Riola Parada (2016) showed that the creep of a GLT beam reinforced by steel was around 85% (after one year) and 50% (after the fifth year) of the GLT beam. Loss et al. (2016a, b) used an I-shape steel beam to avoid the brittle failure of the timber beam and timber floor in the fabrication of modern buildings. Tsai and Le (2018) reinforced the web of an I-shaped steel beam with timber using screws, and the strength of the composite beam was improved by about 15%. Wu et al. (2021) developed an I-shaped wood-steel composite beam and reported that an elastic-plastic phase appeared as a result of the bending performance test. As mentioned above, researchers revealed the advantages of timber-steel composite beams. However, the structural behavior of the hybrid beam was designed and analyzed in the elastic range.
When a GLT beam is glued with other materials (steel or fiber-reinforced polymer (FRP)) by an adhesive, the risk of glueline delamination has been reported (Metelli et al. 2016; Schober et al. 2015), and it is hard to expect good durability. Tomasi et al. (2009) tried to develop a ductile bending behavior of GLT beams by inserting steel bars. They showed that steel reinforcement provides a simple and reliable solution for robust timber structures. Wang et al. (2021) developed a glulam beam mechanically reinforced by a steel rod. They showed the possibility of ductile failure due to the yielding of glulam in compression parallel to the grain or yielding of the steel rod.
As mentioned above, most GLT-steel beams were developed by gluing GLT and steel. However, this study developed a separable glued-laminated timber (GLT)-steel beam without gluing as an eco-friendly solution to building construction. The GLT-steel beam was assembled using inclined screws without gluing, which enabled the easy separation of the materials for recycling purposes. The minimum thickness of steel for inducing compressive GLT failure was investigated, and the reinforcing effect and bending behavior of the developed GLT-steel beam were analyzed. Based on the experimental test, a prediction model for the load-carrying capacity of the GLT-steel beam has been developed.
DESIGN OF GLT-STEEL BEAM
Figure 1 shows schematically the concept of the separable GLT-steel beam. In this study, the GLT-steel beam is designed to fail by a universal test machine with a load capacity of 100 kN, based on the previous study (Pang et al. 2018). The load-carrying capacity of the GLT-steel composite beam was designed by applying the composite beam theory, under the assumption that GLT and steel are completely combined and linear elasticity materials. Figure 2 shows the load-carrying capacity, neutral axis position, and total thickness according to the steel plate thickness when a GLT (80 mm (width) × 120 mm (thickness) × 2450 mm (length)) is reinforced with steel plates. As the thickness of the steel plate increases, the thickness of the composite beam increases and the neutral axis goes down toward the bottom.
In the case of load-carrying capacity, as the thickness of the steel plate increased, the load that the composite beam could support also increased, but the increase rate of the load decreased when the thickness was 4 mm or more. When GLT was reinforced with a steel thickness of 3 mm or less, the maximum load of the beam was governed by the tensile failure of the steel plate. However, when GLT was reinforced with a steel thickness of 4 mm or more, the maximum load of the beam was governed by compression failure of GLT. Therefore, in this study, the GLT was reinforced with a 4 mm thick steel plate to check whether compression fracture of GLT occurred.
Fig. 1. Separable glued-laminated timber-steel beam
Fig. 2. Load-carrying capacity, neutral axis position, and total thickness of beam according to the steel plate thickness
EXPERIMENTAL
Materials
Figure 3 shows the cross section of GLT and the combination of lamina. One lamina grade (E9) was used, and the modulus of elasticity (MOE) of that grade was greater than 9 GPa. The GLTs were manufactured using four layers of larch species (Larix kaempferi Carr., the density of 550 kg/m3) lamina according to the KS F3021 standard (KS F3021 2013). The size of GLT was 80 mm (width) × 120 mm (thickness) × 2450 mm (length). The GLT specimens were stored for one week in a bending test laboratory (20 °C, 65% R.H.). After the bending test, the moisture content of GLT was measured with a moisture tester (HM-530, Kett Electric Laboratory, Tokyo, Japan) and was about 6 ± 2%.
Table 1 shows the dimensions and mechanical properties of GLT and steel plate. Eight GLTs were prepared, and all of the actual bending stiffness of the GLTs were measured. GLTs were divided into two groups to have similar bending stiffness. One group (3 out of 8) was used to measure the bending strength of GLT, and the other group (5 out of 8) was reinforced with a steel plate.
Fig. 3. Cross section of GLT and GLT-steel beam
Table 1. Dimensions and Mechanical Properties of GLT and Steel Plate
1) Glued-laminated timber (KS F3021 2013)
2) Experimental value in this study
3) Steel Plate Hot Commercial (KS D3501 2018)
4) Experimental value from Suh et al. (Chang-Min and Hyun-Chul 2009)
5) Experimental value from mill test certificate
To reinforce the GLT, a 4 mm thick steel plate (Steel Plate Hot Commercial, SPHC (KS D3501 2018)) was installed at the bottom of the GLT. The steel plate and GLT were fixed with self-tapping screws (VGS ∅9 × 160mm (ETA-11/0030 2019)). Axial slip moduli of screws are significantly higher compared to slip moduli in shear (Dietsch and Brandner 2015). Thus, the screw was installed at an angle of 45° using a washer (HUS945, Rothoblaas) to minimize the slip between the GLT and the steel plate.
When the GLT is reinforced with a 4 mm steel plate, the maximum load of the composite beam is about 20 kN, and the shear force generated between the GLT and steel is 84.7 kN. The design value of the inclined screw used is 11.25 kN (Rotho Blaas Srl 2020) and 8 screws can support the shear force generated between the GLT and steel plate. However, 10 screws were installed at intervals of 60 mm to prevent the screw connector failure and to observe the bending failure of the GLT-steel beam. The axis of the screw was rotated 20 degrees toward the center of the GLT to prevent the GLT from being cracked by the screws, as shown in Fig. 4.
Fig. 4. Screw installation (45° inclined, 20° rotation)
Experimental Tests
The deflection and load resistance of specimens were measured by a third-point bending test to derive their bending stiffness and load-carrying capacities (Fig. 5). The load was applied using a universal test machine (Zwick GmbH & Co., Ltd., Ulm, Germany) according to ASTM D198 (2010). A yoke was installed on the neutral axis of the specimen to measure the pure deflection.
Fig. 5. Configuration of third-point loading test for the specimens
The actual displacement was measured at the center of the specimen using the linear variable displacement transducers (LVDT). The test span and the loading speed were 2,280 mm and 10 mm/min, respectively. The bending stiffness and bending moment of the specimens were calculated using Eq. 1 (BS EN 408:2010+A1:2012 2012; Pang et al. 2019; Pang and Jeong 2019) and Eq. 2, respectively,
(1)
(2)
where (EI)test is the experimental bending stiffness of the specimen (N·mm2), Le is thedistance between the load position and support positions (mm), L is thespan of the specimen (mm), P1 and P2 are theloads corresponding to 10% and 40% of the maximum load, Pmax, respectively (kN), d1 and d2 are thedeflections corresponding to P1 and P2, respectively (mm), and Mmeasured is the measured bending moment capacity (kN·m).
RESULTS
GLT Beam
Figure 6 shows the failure modes of the GLT beam. All specimens were broken around the knot in the tensile zone. At the time of failure, the load resistance dropped sharply, as shown in Fig. 7. This brittle failure is a typical failure mode of GLT or wood in bending tests (Pang et al. 2011, 2018, 2021; Pang and Jeong 2019).
Table 2 shows the bending properties of the GLT specimens. The average bending stiffness of the GLT in the failure test was 0.168 × 1012 N∙mm2, which was similar to the stiffness in the grading test. The average load-carrying capacity of the GLT was 28.2 kN, and the bending moment capacity was 10.7 kN∙m.
(a) Tensile failure of the bottom lamina
(b) Tensile failure around knot at the bottom lamina
Fig. 6. Failure mode of the GLT beam
Fig. 7. Load-displacement curve of the GLT beam
Table 2. Bending Properties and Failure Modes of GLT and Reinforced GLT
1) Effective bending stiffness by Eq. 1
2) Measured in grading test to divide the specimens into two groups.
3) Measured in failure test to evaluate the maximum load-carrying capacity of the specimen
4) (EIeff in grading test / EIeff in failure test) × 100
5) Maximum load
6) Bending moment resistance calculated by Eq. 2
7) Coefficient of variation
GLT-steel beam
Figures 8 through 10 show the failure mechanism of the GLT-steel beam. In two of the five specimens, tensile failure occurred first at the finger joint (Fig. 8a) or knot (Fig. 9a) of GLT. The finger-joint and knots are the main factors causing the strength reduction (Pang et al. 2011, 2018, 2021; Pang and Jeong 2019). Unlike the GLT beam, compression failure occurred at the top of the GLT (Fig. 8b and Fig. 9b) and large deformation occurred (Fig. 8c and Fig. 9c).
In three of the five specimens, compressive failure occurred first at the top of the GLT (Fig. 10a). After that, tensile failure occurred at the knot, but the tensile failure of the specimens did not lead to rapid brittle failure. The specimens showed ductile behavior due to the elongation of the steel plate. As described in the material section, the number of screws was calculated and installed to sufficiently support the expected shear force between GLT and steel plate. During the experiment, no screw was pulled out, and no damage was found between the screw and the steel plate. Therefore, the elongation of the steel plate shows that the screw connection sufficiently supported the shear force between GLT and the steel plate.
Figure 11 shows the load-displacement curve of the GLT-steel beams. Unlike the GLT beam, the GLT-steel beam did not lose its load-carrying capacity significantly after the initial failure and maintained a load-carrying capacity of approximately 30 kN. This may be because the steel plate is in a plastic state in which the stress does not increase even when the strain increases. The maximum load-carrying capacity was higher in the case where the compression failure occurred first (48 kN to 51 kN) than in the case where the tensile failure occurred first (32 to 38 kN). This shows that the tensile part (bottom of GLT) contributed to the load-carrying capacity of the GLT-steel beam until the tensile failure occurred.
(a) Tensile failure at finger joint of GLT
(b) Compression failure on top of GLT
(c) Large deformation
Fig. 8. Fracture mechanism of GLT-steel beam in which tension failure (finger joint) of GLT occurs first (RGLT-1)
(a) Tensile failure at knot of GLT
(b) Compression failure on top of GLT
(c) Large deformation
Fig. 9. Fracture mechanism of GLT-steel beam in which tension failure (knot) of GLT occurs first (RGLT-2)
(a) Compression failure on top of GLT
(b) Tension failure on bottom of GLT
(c) Large deformation
Fig. 10. Fracture mechanism of GLT-steel beam in which compression failure of GLT occurs first
Fig. 11. Load-displacement curve of the GLT-steel beam
DISCUSSION
By reinforcing the GLT with a 4 mm steel plate, the bending stiffness was improved by about 58.5% from 0.166×1012 to 0.263×1012 N/mm2, and the coefficient of variation (COV) decreased from 0.054 to 0.03 (Table 2). The bending moment resistance of the GLT-steel beam was increased by 56.4% from 10.702 to 16.740 kN∙m, and the COV decreased from 0.258 to 0.161. Therefore, the GLT-steel beam developed in this study has high bending performance compared to general GLT and can be produced with uniform quality.
Figure 12 shows the bending stress distribution of the GLT beam and GLT-steel beam at the maximum bending moment. The bending stress distribution in Fig. 12 was plotted by elastic analysis (Eq. 3). The maximum bending stress of the GLT beam was 55.7 MPa, which was similar to the bending strength (54 MPa) of a GLT grade with similar MOE in KS F3021 standard (KS F3021 2013). In the case of the GLT-steel beam, however, the maximum bending stress acting on the steel plate (414.6 MPa, Fig. 12(b)) exceeded the yield strength of the used steel plate (227 MPa). This means that the steel plate was in a plastic state, and plastic analysis is required.
(3)
where 
is the bending stress in the elastic analysis (MPa), n is the ratio of elastic modulus of GLT and steel in GLT-steel beam (1 for GLT beam), Mmeasured is the measured bending moment capacity (kN·m), Ieff is effective moment of inertia (mm4), y is the distance from the neutral axis (mm).
(a) GLT beam
(b) GLT-steel beam (stress acting on steel exceeds the yield strength (227 MPa) of steel)
Fig. 12. Bending stress distribution of GLT beam and GLT-steel beam by elastic analysis
Figure 13 shows the bending stress distribution of GLT-steel beam considering the plasticity of the GLT and steel. The maximum bending moment of the GLT-steel beam can be predicted by Eq. 4. Equation 4 consists of three terms. The first term is the plastic bending moment of compression zone in GLT, the second term is the elastic bending moment in GLT, and the third term is the plastic bending moment of the steel plate. In the compression test of wood, the load-displacement curve shows a ductile behavior in which the strength decreases slowly as the wood wrinkles after the maximum load (Pang and Jeong 2018). For plastic analysis of GLT-steel beam, the load-displacement curve of GLT in compressive stress was idealized as an elastic-plastic curve. Tomasi et al. (2009) and Wang et al. (2021) also assumed the compressive behavior for the fiber direction in a GLT reinforced by steel rods as an elasto-plastic behavior. Among the reported experimental data (Pang and Jeong 2018; Park et al. 2010), the compressive strength (58.8 MPa) of wood with similar elastic modulus in the same species was regarded as the yield strength of GLT,
(4)
where Mmax is the maximum bending moment (kN·m), c1is the distance between the top surface and the neutral axis of the composite section (mm),hc,GLT is the height of the compressive zone in GLT (mm), 
is the compressive strength of GLT (MPa), y is the distance from the neutral axis of the composite section (mm), c2 is the distance between the bottom surface and the neutral axis of the composite section (mm), tsteel is the thickness of steel plate (MPa), EGLT is the elastic modulus of GLT (MPa), 
is the strain of GLT, and fy is the yield strength of steel plate (MPa).
Fig. 13. Bending stress distribution of the GLT-steel beam at the maximum bending moment by elastic-plastic analysis
Figure 14 shows the bending stress distribution of the GLT-steel beam when the steel and timber in compressive zone reach plastic state. Assuming that the steel plate and the GLT are fixed and do not slide, the GLT-steel beam design method can be derived by considering the balance of compressive force and tensile force generated in the GLT-steel beam, similar to the reinforced concrete design method. The compressive force acting on the upper part of the GLT and the tensile force by the steel plate are equal according to the condition of equilibrium of compressive force and tensile force. Thus, the height of the compressive zone in the GLT-steel beam can be calculated by Eq. 5. Figures 14 (a) and (b) show when the maximum stress of steel reached the yield strength (227 MPa) and the maximum tensile strength (345 MPa) of the steel plate, respectively. When the maximum stress of the steel from yield strength to maximum tensile strength increased, the height of the compressive zone in the GLT-steel beam also increased.
The bending moment and the load-carrying capacity of the GLT-steel beam can be predicted by Eqs. 6, and 7, respectively. The predicted load-carrying capacity of GLT-steel beam was 21,845 N by applying yield strength of steel (227 MPa) and 32,035 N by applying yield strength of steel (345 MPa). Figure 11 shows the load-carrying capacity predicted by Eq.7 together with the experimentally measured load-displacement curve. The load capacity predicted by the maximum tensile strength of the steel was similar to the maximum load capacities at the plastic condition of the test specimens. This indicates that the steel plate almost reached its maximum tensile strength. In addition, all of the load capacities of the test specimens were higher than the load predicted by the yield strength of steel. Therefore, the bending moment of GLT-steel beam can be safely designed with the yield strength of steel.
(5)
is the the height of the compressive zone in GLT (mm), fy is the yield or tensile strength of steel plate (MPa), tsteel is the thickness of steel plate (MPa), and is the compressive strength of GLT (MPa).
(6)
In Eq. 6, Mpredict is the predicted bending moment (kN·m), fy is the yield or tensile strength of steel plate (MPa), w is the width of steel plate (mm), tsteel is the thickness of steel plate (mm), h is the height of GLT-steel beam (mm), and hc,GLT is the height of compressive zone in GLT (mm).
(7)
In Eq. 7, Ppredict is the predicted load-carrying capacity of GLT-steel beam (kN), Mpredict is the predicted bending moment (kN·m), and the distance between the load position and support position (mm).
(a) Bending stress distribution at the yield strength (227 MPa) of the steel plate
(b) Bending stress distribution at the tensile strength (345 MPa) of the steel plate
Fig. 14. Bending stress distribution of the reinforced GLT specimens at the plastic state
CONCLUSIONS
In this study, a separable GLT-steel beam was designed using the minimum steel plate thickness (4 mm) at which GLT experienced compressive failure, and the bending behavior was experimentally analyzed. The main findings are as follows.
- The bending moment resistance and bending stiffness of the GLT-steel beam were improved by 56.4% and 58.5%, respectively, compared to the GLT beam. Additionally, the COV of both properties decreased. Thus, the GLT-steel beam developed in this study has the uniform quality and high bending performance compared to GLT.
- The GLT-steel beam showed ductile behavior even after GLT failure and maintained a load resistance of about 30 kN during large deformation. The prediction model for the ductile behavior (plastic moment) was derived by considering the balance of the compressive forces and tensile forces in the composite beam, similar to the reinforced concrete design method. The experimentally measured load-carrying capacity of the beam was similar to the predicted load-carrying capacity. Therefore, the ductile behavior of the GLT-steel beam can be designed by the developed model.
- The GLT and steel plate were mechanically connected using only inclined screws without the use of any adhesives. The resistance of the screw sufficiently supported the shear forces so that the steel plate can perform plastic behavior after yielding. Therefore, this study shows the possibility of a non-adhesive GLT-steel beam with plastic behavior.
ACKNOWLEDGMENTS
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. NRF-2021R1I1A1A01045628).
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Article submitted: February 15, 2023; Peer review completed: March 11, 2023; Revised version received and accepted: April 7, 2023; Published: April 18, 2023.
DOI: 10.15376/biores.18.2.3838-3855