Abstract
The strength performance of edge connections between the cross-laminated timber (CLT) panels, as currently applied to CLT construction, was compared to that of connections reinforced with glass fiber-reinforced plastic (GFRP) by means of a tensile-type shearing test. In this study, the reinforced half-lapped connection is intended to prevent CLT from coming apart due to failure of self-tapping screws (STS) by bonding GFRP sheets to connections between CLT panels. The end-distance and edge-distance of this reinforced half-lapped connection were designed to equal 5D (where D is the fastener diameter) and 4D, respectively, which is shorter than the 6D recommended by European Technical Approval (ETA). Nevertheless, the yield strength was increased by 7%, and the stiffness by 92%, compared to the non-reinforced half-lapped connection. In addition, the internal spline connections using GFRP-reinforced plywood were 57 and 36% higher than the connection made up of LVB or plywood, respectively, and the energy dissipation percentages were 400 and 76%, respectively. These results indicate that the reinforcement effect of the connection by the GFRP was very significant. On the other hand, the half-lapped connection of the larch CLT improved the strength performance as the end-distance increased, and the end-distance had a greater effect on the strength performance than the edge-distance.
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An Evaluation of Strength Performance of the Edge Connections between Cross-laminated Timber Panels Reinforced with Glass Fiber-reinforced Plastic
Yo-Jin Song, In-Hwan Lee, and Soon-Il Hong *
The strength performance of edge connections between the cross-laminated timber (CLT) panels, as currently applied to CLT construction, was compared to that of connections reinforced with glass fiber-reinforced plastic (GFRP) by means of a tensile-type shearing test. In this study, the reinforced half-lapped connection is intended to prevent CLT from coming apart due to failure of self-tapping screws (STS) by bonding GFRP sheets to connections between CLT panels. The end-distance and edge-distance of this reinforced half-lapped connection were designed to equal 5D (where D is the fastener diameter) and 4D, respectively, which is shorter than the 6D recommended by European Technical Approval (ETA). Nevertheless, the yield strength was increased by 7%, and the stiffness by 92%, compared to the non-reinforced half-lapped connection. In addition, the internal spline connections using GFRP-reinforced plywood were 57 and 36% higher than the connection made up of LVB or plywood, respectively, and the energy dissipation percentages were 400 and 76%, respectively. These results indicate that the reinforcement effect of the connection by the GFRP was very significant. On the other hand, the half-lapped connection of the larch CLT improved the strength performance as the end-distance increased, and the end-distance had a greater effect on the strength performance than the edge-distance.
Keywords: Cross-laminated timber; CLT edge connection; Self-tapping screw; Half-lapped; Spline; Shear test; Shear test; Glass fiber-reinforced plastic
Contact information: College of Forest & Environmental Sciences, Kangwon National University, Chun-Cheon 200-701, Republic of Korea; *Corresponding author: hongsi@kangwon.ac.kr
INTRODUCTION
Cross-laminated timber (CLT) panels contain longitudinally laminated laminae that are cross-laminated one by one. As they can be fabricated in an infinite size in theory, no edge connection between the panels is required. Because an extremely large CLT cannot be shipped, appropriate-sized CLTs are field-connected and expanded to the required size (Nakashima et al. 2012; Oh et al. 2017). In the CLT structural system, more connections between the panels result in increased construction cost and reduced strength against the horizontal load. As these connections are the main source of ductility and energy dissipation capability, however, the CLT buildings with connections demonstrate excellent seismic performance. Therefore, the behavior of the connection largely determines the overall performance of the structure, making it a key factor in maintaining the structural integrity, strength, and stability of the building (Ceccotti et al. 2006; Dujic and Zarnic 2006; Follesa et al. 2010).
Several studies have been carried out on the half-lapped and spline connections, which are the most representative of the CLT panel-edge connection, due mainly to their simple process, minimal waste of wood, and easy construction in the field. Sadeghi et al. (2015) reported that the half-lapped and single-surface spline connections have low strength relative to bending moments. Given the economics and ease of construction, however, they still have a sufficient potential to improve the design of the connection (Sadeghi et al. 2015). Gavric et al. (2012) reported that the half-lapped connections have higher stiffness than the single-surface spline connections. Brittle failure due to a plug shear was observed, however, in the half-lap of some specimens (Gavric et al. 2012). Follesa et al. (2010) studied the strength performance of the half-lapped, internal spline, and single-surface spline connections, as well as their ease of operation and further cost for the workforce. The study results showed that the actual strength of the connections was 1.5 times higher than the value calculated according to EN 1995-1-1 (2004), the criterion for solid wood and glulam, etc., due to the cross-lamination effect. In addition, the stiffness and difference were significant, as the formula according to EN 1995-1-1 (2004) is calculated without considering the thickness of the layer and the metal type of the fastener used. In terms of cost, nails are cheaper than screws, and the half-lapped connections are the least expensive (Follesa et al. 2010). Sullivan (2017) showed in a shear test for the half-lapped and single-surface spline connections that the connection strength increased in proportion to the diameter of the self-tapping screw (STS), but higher ductility was measured when using STSs with an 8 mm diameter rather than STSs with a 10 mm diameter.
On the other hand, cracks occurring along the end-distance or edge-distance were frequently found in the neck joint using metal joints such as STS, dowel, and bolt (Oh et al. 2017; Ottenhaus et al. 2018). Therefore, methods to increase the end-distance and edge-distance of the joints or to suppress the failure rate by reinforcing the joint were applied to suppress potential failure. The reinforcement of members and joints using fiber-reinforced plastic (FRP) in wood structure has been shown to be effective when applied to solid wood or glulam, prompting the authors to conclude that it would improve strength performance if it is applied to side joints between CLT panels (Kim et al 2013; Raftery and Harte 2011; Song et al 2017).
In this study, the performance of the lateral connections between CLT panels using glass fiber-reinforced plastic (GFRP) was evaluated for the reinforcement of the typical lateral connections between CLT panels.
EXPERIMENTAL
Material
Cross-laminated timber
In this study, a five-layer CLT (thickness: 130 mm) was fabricated using larch laminae (Larix kaempferi Carr.), which were classified by their respective modulus of elasticity using the visual stress grading method (KSF 3021 2016). The longitudinal layer consisted of grade 2 or higher-grade laminae, and the transverse layer consisted of grade 3 or lower-grade laminae. Phenol-resorcinol formaldehyde adhesive (PRF) was used as a bond between laminae, and the adhesive spread rate was set at 400 g/m2 (single spread) while the pressing pressure was set at 0.7 MPa.
Spline
For the spline, larch plywood and radiate pine LVB (Pinus radiate D. Don.), which satisfy the KS F 3101 criteria, were used, and a reinforced plywood in which a 6-mm-thick GFRP was laminated onto an 18-mm-thick plywood was also used. The GFRP consisted of a fabric-type glass cloth that was inserted in a glass fiber plastic sheet to suppress the potential cleavages generated in a general sheet along the direction of the glass fiber. Polyvinyl acetate resin (PVAc) was used as a bond between GFRP and plywood (Park et al. 2009). The splines were 24 mm thick, and their mean density were 553.4 kg/m3 (plywood), 530.0 kg/m3 (LVB), and 901.1 kg/m3 (reinforced plywood).
Fig. 1. Shape of reinforced plywood with GFRP plate
Fastener
The STS made by Wurth (Künzelsau, Germany) was used for the CLT edge connection. The diameter of the STS was less than 1/10 of the thickness of the CLT, and STSs with different diameters and lengths were used depending on the connection type (DIBT 2013). The shape and size of the fastener are shown in Table 1 and Fig. 2. According to Sheikhtabaghi (2015), the tensile strength and shear strength increased by 5% when a washer was inserted in the half-lapped connection in the case of the STS measuring 8 mm in diameter, while in the single-surface spline connection, the tensile strength increased by 41% and the shear strength increased by 61%. In this study, there was no need to insert a separate washer on the screw head, but an STS with a wider screw head was used.
Table 1. Dimensions and Strength of Self-Tapping Screws (CCMC 2013)
Fig. 2. Schematic diagram of the self-tapping screw (Würth 2016)
Methods
Fabrication of a tension-type shear test specimen
The European Technical Approval (ETA) specifies that the minimum end distance and minimum edge distance of the CLT should be 6D (D: Thread diameter of fastener) (DIBT 2013). In this study, the test specimens were fabricated according to the end distance, edge distance, and reinforcement of the half-lapped connections based on the 6D standard (Table 2). In Series-1, the end distance was made to 5D, 6D, and 7D, respectively, with the edge distance fixed to 6D, while in Series-2, the end distance was made to 4D, 5D, and 6D, respectively, with the edge distance fixed to 7D (Fig. 3). Series 3 was reinforced by applying GFRP to the top side of the half-lap after applying stringent design specifications: end distance to 5D and edge distance to 4D (Fig. 6). The half-lapped test specimens were connected together by STS measuring 8 mm (d) × 120 (l).
Table 2. Test Program Summary
The spline connections were designed to have the same end distance of 7D and the same groove height of 24 mm. The test specimen of the internal spline connections was fabricated by inserting LVB (Series-4), plywood (Series-5), and reinforced plywood (Series-6), and then fastening them using two STSs (6 mm (d) × 120 mm (l)) so that the edge distance would measure up to 5D (Fig. 4). The test specimen of the double-spline specimens was fastened together using reinforced plywood (Series-7) and four STSs (6mm (d) × 120mm (l)) so that the edge distance would measure up to 4D and the spacing perpendicular to a plane parallel to the grain would measure up to 2D (Fig. 5).
The STS head was nailed so it would not protrude out of the CLT surface, by drilling a hole with an area and a thickness corresponding to those of the STS head when inserting the STS into the all test specimens to prevent the wood from being damaged by the STS head during the test.
Fig. 3. Schematic diagrams of the half-lapped connection specimen
Fig. 4. Schematic diagrams of the internal spline connection specimen
Fig. 5. Schematic diagrams of the double-spline connection specimen
Fig. 6. Half-lapped connection reinforced with GFRP
Fig. 7. Schematic diagram of the tension test on the edge connections between the CLT panels
Tension-type shear test
A vertical load testing machine with a maximum capacity of 30 tons was used for the tension-type shear test. As shown in Fig. 7, two displacement transducers (CDP-50) with a maximum capacity of 50 mm were installed on the left and right sides of the CLT connection test specimen by fastening them using two 20 mm diameter bolts at the top mounts of the specimen and four 12 mm diameter bolts at the bottom mounts. In this study, only the monotonic test was carried out, and the loading rate was set to 5 mm/min according to ASTM D5652-07 (2007) so that the specimen would fail in between 5 and 20 min.
RESULTS AND DISCUSSION
Load and Deformation
Figures 8 and 9 are representative load-deformation curves of the half-lapped and spline connections, drawn using the mean strain values of the two displacement transducers and the load values of the load cell. Table 3 shows the maximum load and failure load of the test specimens. The failure load was assumed to be 80% when the failure of the test specimen was not clearly visible on the load-deformation curve or during the test.