Abstract
Timber–concrete composite systems are widely used in Europe, North America, and Australasia, primarily due to their good mechanical performance in terms of statics, dynamics, and seismic response. In addition, the concrete slab provides excellent protection to the timber, making such systems suitable for outdoor application. The seismic performance of timber–concrete composites is normally governed by their ductility and energy dissipation capacity. However, few design codes address the ductility and energy dissipation capacity of timber–concrete composite systems, owing to a lack of reliable performance data. Therefore, further research on the hysteretic performance of timber–concrete composite systems is necessary. In this study, six timber–concrete composite specimens with an angle steel connection of the same size were investigated using reversed cyclic tests. The corresponding failure modes were observed, and the salient features of the connection, i.e., the stiffness, ductility, and energy dissipation, were computed from the test results. The force mechanism of the timber–concrete composite specimens under reversed cyclic load was analyzed. Equations were presented to calculate the yield force and negative force in the same load step. A comparison of the test results and the theoretical results indicated good agreement.
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Hysteretic Performance of Angle Steel Connections in a Timber-Concrete Composite System
Lan Xie,a Guojing He,b,* Xiaodong (Alice) Wang,c Xiao Tang,d and Roberto Crocetti e
Timber–concrete composite systems are widely used in Europe, North America, and Australasia, primarily due to their good mechanical performance in terms of statics, dynamics, and seismic response. In addition, the concrete slab provides excellent protection to the timber, making such systems suitable for outdoor application. The seismic performance of timber–concrete composites is normally governed by their ductility and energy dissipation capacity. However, few design codes address the ductility and energy dissipation capacity of timber–concrete composite systems, owing to a lack of reliable performance data. Therefore, further research on the hysteretic performance of timber–concrete composite systems is necessary. In this study, six timber–concrete composite specimens with an angle steel connection of the same size were investigated using reversed cyclic tests. The corresponding failure modes were observed, and the salient features of the connection, i.e., the stiffness, ductility, and energy dissipation, were computed from the test results. The force mechanism of the timber–concrete composite specimens under reversed cyclic load was analyzed. Equations were presented to calculate the yield force and negative force in the same load step. A comparison of the test results and the theoretical results indicated good agreement.
DOI: 10.15376/biores.17.1.1270-1284
Keywords: Timber–concrete composite structure; TCC; Hysteretic performance; Energy dissipation capacity; Ductility
Contact information: a: College of Civil Engineering, Changsha University, 98 Hongshan Road, Changsha, Hunan 410022 China; b: Department of Civil Engineering and Mechanics, Central South University of Forestry and Technology, 498 Shaoshan Road, Changsha, Hunan 410004 China; c: Department of Wood and Forest Sciences, Laval University, Quebec G1V 0A6 Canada; d: Changsha Urban Construction Secondary Vocational School, Nongyuan Rd, Changsha, Hunan 410126 China; e: Department of Civil and Architectural Engineering, KTH Royal Institute of Technology, Stockholm 10044 Sweden; *Corresponding authors: lily.csuft.xie@gmail.com; xieyuantaixielan@126.com
INTRODUCTION
Timber–concrete composite (TCC) systems are a type of composite system in which a timber member and a concrete slab are connected to exhibit composite behavior. Timber–concrete composite systems have been widely used in Europe, North America, South America, and Australasia for new buildings and new bridges as well as for upgrading existing timber floors (Calil and Monlina 2008; Yeoh et al. 2011; Molina et al. 2019; Wacker et al. 2020). This is due to their good mechanical performance in terms of statics, dynamics, and seismic response (Rijal et al. 2015), as well as their environmental friendliness (Rodrigues et al. 2013), acoustics (Martins et al. 2015), and fire-resistant properties (Frangi et al. 2009; Frangi et al. 2010; Skinner et al. 2014).
Although the static characteristics of TCC systems have been thoroughly investigated (Lamothe et al. 2020; Zhang et al. 2021), their influence on seismic performance is not clearly understood. The seismic performance of TCC systems is normally governed by its structural ductility (He et al. 2020). Thus, the measurement of its hysteretic characteristics, e.g., ductility and energy dissipation capacity, is necessary to investigate the seismic performance of TCC systems.
In a timber–concrete hybrid system, the hysteretic curve exhibits strength and stiffness degradation as well as a pinching effect (Isoda and Tesfamariam 2016; Kawai et al. 2016). Isoda and Tesfamariam (2016) found that the connection system has considerable influence on the hysteretic behaviour of a timber–concrete hybrid system. Similarly, Leijten et al. (2006) found that the deformation and energy dissipation of the timber frame structure depends on the nonlinear deformation of the connector. Thus, the connectors in the TCC system play a vital role in the shear capacity, seismic performance, and resistance to vertical uplift between the upper concrete slab and the bottom timber beam (Zhu et al. 2019). The shear strength, stiffness, and stress mechanisms of connectors have been investigated (Mascia and Soriano 2004; Dias et al. 2010; He et al. 2016; Xie et al. 2017). While the seismic performance of the connections in timber and hybrid systems have been studied (Izzi and Polastri 2019; Yang et al. 2020), only a few studies have addressed the hysteresis of the connections in TCC systems (Carrero et al. 2020). In addition, no research or code is available to guide the seismic design of TCC systems.
A TCC system with angle steel connection performs well under static loads (Chen 2016). However, its hysteresis property has not been thoroughly investigated. To investigate the hysteretic behaviour of angle steel connectors in a TCC system, six identical specimens were manufactured and subjected to reverse cyclic displacement testing. The failure modes were observed, and the characteristics of each connection, i.e., the stiffness, ductility, and energy dissipation, were computed from the experimental results. In addition, the force mechanism of the TCC specimens under a reverse cyclic load was analyzed. The research will be referable for the seismic design for the TCC systems.
EXPERIMENTAL
Design and Fabrication of the Reversed Cyclic Specimens
Tests were conducted on six TCC specimens of the same size with an angle steel connection. Each specimen had three components: a timber block, concrete blocks, and shear connectors.
Fig. 1. Dimensions of the timber–concrete composite reversed cyclic specimens (in mm)
Taking into account the Eurocode 5 (2004) and the study of He et al. (2016), the dimensions of the timber and concrete blocks were 200 mm × 120 mm × 400 mm and 60 mm × 300 mm × 400 mm, respectively. The dimensions of the specimens subjected to reversed cyclic load are shown in Fig. 1.
The TCC specimens were manufactured in three steps. First, two grooves were made on the left and right sides of the timber block. Second, one angle steel connector was spread with aqueous polyurethane adhesive and then glued in the bottom surface of groove on each side of the timber block. Finally, the concrete was poured into pre-assembled moulds and after 28 d, the moulds were disassembled. The manufacturing of the TCC specimens is shown in Fig. 2.
Fig. 2. Manufacturing of the TCC specimens
Material Properties
The timber blocks in this study were made from Xing’an larch, grown in the forest of the Great Khingan Mountains of China. The timber was tested according to GB/T standard 50708 (2012), as shown in Fig. 3a. The compressive strength of the timber was 44.8 MPa, and the experimental mean modulus of elasticity (MOE) of the timber was 13.84 GPa at a moisture content of 11.81%.
Normal–weight concrete with a characteristic compressive strength of 40 MPa was specified for the TCC specimens. The average compressive strength of three 150 mm × 150 mm × 150 mm concrete cubes (after curing for 28 d) was determined to be 45.20 MPa according to Code for design of concrete structures (GB50010-2010), as shown in Fig. 3b.
Fig. 3. Tests for the timber and concrete
The angle steel connector consisted of two steel plates with dimensions of 50 mm × 30 mm × 5 mm. The details of the angle steel connector are shown in Fig. 4. The yield strength of the angle steel connectors was 235 MPa, as reported by the manufacturer.
Fig. 4. Details of the angle steel connectors
The characteristic properties of the aqueous polyurethane adhesive were provided by the manufacturer. The shear strength was no less than 10 MPa for hardwood and no less than 6 MPa for softwood.
Test Apparatus and Procedure
A specially built apparatus was used for the reversed cyclic test, as shown in Fig. 5. The TCC specimen was placed in the apparatus with the concrete blocks fixed by a concrete fastening device. The timber block of the TCC specimen was fixed to an electrohydraulic servo universal testing machine with a capacity of 250 kN using a junction plate with four high-strength bolt connectors. The push and pull loads were applied to the top and bottom surfaces of the timber block of the TCC specimen.
Fig. 5. Testing apparatus for the reversed cyclic tests
The reversed cyclic tests performed on the TCC specimens were conducted according to ISO standard 16670 (2003). The load was applied using a displacement-controlled method with a slip rate of 0.1 mm/s, as shown in Fig. 6. The loading procedure consisted of ten loading steps. From steps 1 to 4, the load was applied once at each step; and from steps 5 to 10, the load was applied three times at each step. The displacement amplitudes for each step are shown in Fig. 6. Based on the mean value of the ultimate displacement in the monotonic tests, calculated according to BS EN standard 26891 (2003), vu was recorded as the displacement at 0.8 Fmax in the descending portion of the load–displacement curve (Chen 2016).
Fig. 6. Loading procedure in the tests according to ISO standard 16670 (2003)
Four displacement gauges were arranged symmetrically on both sides of the specimen to measure the slip between the concrete and the timber. The mean value was recorded as the slip value (s). The test apparatus and the arrangement of the displacement gauges are shown in Fig. 7.
Fig. 7. Test apparatus and the arrangement of the displacement gauges
Test Phenomenon
During the tests, cracks, deformation of the timber and concrete blocks, and deformation of the angle steel connectors were observed. The failure modes of the six TCC specimens were similar, with the following similarities:
At the initial stage of the positive (push) load, all specimens were in an elastic state. As the controlled displacement (∆) increased to 0.8 mm, minor fibre compression noise from the timber blocks was heard. As the ∆ in the negative (pull) direction reached -0.8 mm, the timber block separated from the concrete block, producing a gap between them. The gap size decreased during the unloading process until the timber and concrete blocks were in contact again during the positive loading process. The gap between the timber and concrete at different stages is shown in Fig. 8a.
After the tests, the failure of the connectors was observed to be the primary factor in terminating the loading procedure. The timber and concrete blocks were separated, and the angle steel connectors failed due to bending deformation (as shown in Fig. 9b). The angle steel connectors failed with one hinge in the vertical limb and another in the horizontal limb. No cracks merged in the timber and concrete blocks. However, there was some fibre compression around the angle steel connectors in the timber block. The concrete blocks after the test as well as the timber fibre compression are shown in Fig 9c and 9d, respectively. For all the specimens, there was no adhesive merged after testing.
Fig. 8. The TCC specimen test phenomena
Hysteretic Loops and Skeleton Curves
The specimens had failed when the controlled displacement reached 9 mm. The hysteretic (load–slip) curves are shown in Fig. 9, where the push force is plotted as positive, and the pull force is plotted as negative. As shown in Fig. 9, the hysteretic curves are shuttle-shaped, indicating a pinching effect caused by the slip between the timber and the concrete blocks during the loading process. As the load increased, the slip increased, which was due to the residual deformation during the compression process of the timber and angle connectors. The curves were sharp in the initial stage of unloading and flatten as the controlled displacement (absolute value) decreases.
The skeleton curves for the six specimens are presented in Fig. 10. Residual deformation was observed throughout the test, which indicated that the combination of timber and concrete was flexible to some extent. The TCC specimens failed after some plastic deformation, which indicated that the composite effect of the timber and concrete blocks may involve complex behaviour in terms of the angle steel connector yielding and the local embedding of the wood fibres in compression.
Fig. 9. Hysteretic loops for TCC specimens
As shown in Figs. 9 and 10, the hysteretic curves and skeleton curves were not symmetrical in the positive and negative directions, which may have occurred for two reasons. First, the TCC specimen was pushed and then pulled during the loading procedure. After the positive (push) load was applied, the timber was compressed by the connectors, producing an internal gap between the timber and connector. Thus, the force and corresponding slip in the negative direction were less than the force and corresponding slip in the positive direction. Second, the existing gap between the timber and concrete blocks opened as the timber was pulled and closed as the timber was pushed. Therefore, when the specimen was pulled, a smaller load was required to achieve the specified displacement.
The ultimate shear capacity (Fu), and the corresponding deformability (Su), slip modulus (Ks), and ductility factor (μ) obtained during the laboratory tests are shown in Table 1. The ultimate shear capacity (Fu) is the maximum load in the hysteretic loop in Fig. 9, and the corresponding deformability (Su) is the slip corresponding to the maximum load. In addition, the yield deformability (Sy) can also be found as the slip relate to the slope of the curve begin to change. The ductility factor equals the ratio of the ultimate deformability (Su) to the yield deformability (Sy). The ductility of the six specimens fluctuated; the ductility factor ranged from 6.00 to 7.78 with a mean value and a coefficient of variation of 6.75 and 0.09, respectively.
Fig. 10. Skeleton curves for TCC specimens
Table 1. Test Results
Force Mechanism
All specimens were first pushed down and then pulled up. To analyze the force mechanism, three specimen states under reversed cyclic loads were considered.
State I
With the controlled displacement in the range of 0 mm to 0.8 mm, the mean slip of the timber and concrete was 0.80 mm. The TCC specimens were in an elastic state, i.e., the timber, angle steel connectors, and concrete worked cooperatively (as shown in Fig. 12a). The yield load of the TCC specimen can be expressed as Eq. 1,
(1)
where Fy is the yield load, Ks is the slip stiffness, and δi is the initial slip.
State II
According to the test results, the specimens yielded and became elastic–plastic when the controlled displacement reached 0.8 mm. However, the angle connector did not yield; large plastic deformation was observed in the timber block around the angle connectors due to wood fibre compression. If the stress of the concrete did not exceed its tensile strength, the ductility of the TCC specimen provided by the timber, the strength provided by the connectors, the deformation of the connectors and timber was restrained by the concrete blocks.
Fig. 11. Mechanism of the TCC specimens under cyclic load
As shown in Fig. 11b, the TCC specimen changed from elastic to elastic–plastic under a downward controlled displacement of ∆i. The specimens exhibited a plastic deformation (sp) after the initial slip (δi). The slip value between the connector and the timber was 
and the load reached 
. The load of the TCC specimen in the elastic–plastic stage can be expressed as Eq. 2,
(2)
Where is the load corresponding to the controlled displacement (∆i), δi is the initial slip, sp is the plastic deformation corresponding to the controlled displacement ∆i, and K’ is the secant modulus during the elastic-plastic stage.
The TCC specimen behaviour under an upward controlled displacement (-∆i) is shown in Fig. 11c. When the controlled displacement was between 0 and –sp, there was a gap sp in the vertical direction between the timber and the connector. The timber block moved as a rigid body; the load was equal to the timber gravity plus the friction force between the timber and the concrete. When the controlled displacement was between –sp and -∆i, the load was equivalent to the load under a controlled displacement of 
and can be expressed as Eq. 3,
(3)
where 
is the load corresponding to controlled displacement -∆i and 
is the load corresponding to controlled displacement 
.
The load under the downward displacement (in the positive direction) was larger than the load under the upward displacement (in the negative direction). According to the load–slip curves for the specimens in this study, the controlled displacement ∆i was approximately equal to the slip between the timber and the concrete (as shown in Fig. 10). According to Eq. 3, 
is approximately equal to δi. Thus, the load under the upward controlled displacement -∆i only reached the yield strength.
To verify the validity of these equations, the test results for the six specimens were compared with the theoretical results according to these equations. The results indicated good agreement, as shown in Table 2. The Fu represents the ultimate shear capacity, and the Fy represents the yield shear force. The ultimate shear capacity Fu can be found as the maximum force in the hysteretic loops, and the yield force Fy can be found as the point when the slope of the loops begin to change.
Table 2. Comparison of the Theoretical and Test Results
Energy Dissipation Capacity
The energy dissipation curves, according to the first cycle for each loading step, are presented in Fig. 12a. As the controlled displacement increased, the dissipated energy (E) increased to a mean value of approximately 60.70 kJ.
To investigate the energy dissipation of the TCC specimens under reversed cyclic loads, the cumulative dissipated energy in the elastic stage and the elastic–plastic stage were compared, as shown in Fig. 12b. When the controlled displacement reached 0.8 mm, the cumulative dissipated energy (∑E) reached a mean value of 1.69 kJ, representing 3% of the total value. When the specimens were in the elastic stage, the gap between the timber and the concrete was small and unrecoverable; the energy dissipation capacity had not yet been realized. However, as the TCC specimens moved from the elastic stage to the elastic–plastic stage, the ∑E sharply increased, from less than 2 kJ to 57.23 kJ, reaching 97% of the total value. In this stage, the wood fibre was compressed back and forth by the angle connectors, causing plastic deformation of the wood fibres. The energy dissipation capacity rapidly increased, which indicated that the energy dissipation capacity of the TCC structure was primarily provided by the timber.
Fig. 12. Energy dissipation for TCC specimens
Fig. 13. Stiffness degradation curves
Stiffness Degradation
The stiffness degradation of the angle steel connectors in the TCC specimen was calculated according to Eq. 5,
(5)
where Fi and –Fi are the positive and negative peak loads corresponding to the ith loading displacement, respectively, and |∆i| and –|∆i| are the displacements corresponding to Fi and –F, respectively (the results are presented in Fig. 13). As shown in the figure, the stiffness rapidly decreased, which was related to the fibre compression and yielding of the angle steel connectors. In the later stage, the slopes of the curves tended to flatten. For specimen D20-6, the initial stiffness was higher owing to the good connection between timber and concrete, and it decreased to the same level with other specimens at load step 2.
CONCLUSIONS
This study investigated the angle steel connection used in a timber–concrete composite system. Six reverse-cyclic tests were conducted, and the corresponding failure modes were observed. The salient characteristics of each connection, i.e., stiffness, ductility, and energy dissipation, were calculated from the test results. In addition, the force mechanism of the TCC specimens under a reverse cyclic load was analyzed. Equations were presented for calculating the yield force and the negative force during the same load step. From these results, the following conclusions can be drawn:
- The failure mode for all specimens was the angle steel connector bending and the local embedding of the wood during compression. The shear capacity of the TCC specimens was primarily provided by the angle steel connectors. The ductility and energy dissipation capacity were primarily provided by the timber and connectors. The deformation of the timber and angle steel connectors was restrained by the concrete.
- The hysteretic loop had a shuttle shape, which indicated pinching, resulting in degraded strength and stiffness. The pinching effect was caused by the slip between the timber and the concrete. In the elastic–plastic stage, the hysteretic curves were not symmetrical in the positive and negative directions, and the negative force only reached the yield force. This was related to the compression of the wood fibres and the gap between the timber and concrete blocks during manufacturing.
- The TCC specimens with angle steel connectors performed well in terms of ductility, with a mean ductility factor of 7.78. The cumulative dissipated energy reached 57.23 kJ. Only 3% of the energy was dissipated in the elastic stage; however, 97% of the energy was dissipated in the elastic–plastic stage, primarily due to the plastic deformation of the wood fibres.
- Based on the force mechanism analysis, the negative force in the elastic–plastic stage was approximately equal to the yield force. The yield force equation was presented, and the theoretical results were verified via the experimental results, which indicated good agreement.
ACKNOWLEDGMENTS
This study was supported by the State Forestry Administration Project 948 (Project No. 2014-4-51), the National Natural Science Foundation of China (Grant No. 51478485), the Natural Science Foundation of Hunan Province (Grant No. 2020JJ5618), the Scientific Research Project of Education Department of Hunan Province (19C0169), and the Science Popularization Special Project on the Construction of Innovation-Oriented Provinces (No. 2021ZK4148), all of which are highly appreciated.
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Article submitted: June 9, 2021; Peer review completed: November 20, 2021; Revised version received and accepted: December 27, 2021; Published: January 5, 2022.
DOI: 10.15376/biores.17.1.1270-1284