Abstract
Laminated timber composed of small-diameter timbers reinforced with a steel bar and fiber-reinforced polymer (FRP) were fabricated to satisfy the seismic design performance level of wooden columns, and their compression strength performance was evaluated. The experimental results showed that the average compression strength of the specimen reinforced with a CFRP (Carbon FRP) bar increased by approximately 7% compared to that of the control. The average compression strengths of the specimens reinforced with a GFRP (Glass FRP) bar and a steel bar increased by 38 and 37% compared to that of the control, respectively. The unreinforced control column specimens showed a diagonal failure tendency due to the fiber slope angle, and the wood part of the reinforced specimens showed a failure mode with suppressed diagonal fracture. The average strength of the column reinforced with a CFRP plate increased by approximately 6%, but the average strength of the column reinforced with a GFRP plate decreased by approximately 5%. A comparison of the measured and predicted compression strengths of the specimens showed that the strength differences of all the specimens except the specimen reinforced with a GFRP plate were good (2 to 10.4%).
Download PDF
Full Article
Evaluation of the Compression Strength Performance of Fiber-reinforced Polymer (FRP) and Steel-reinforced Laminated Timber Composed of Small-diameter Timber
In-Hwan Lee, Yo-Jin Song, and Soon-Il Hong*
Laminated timber composed of small-diameter timbers reinforced with a steel bar and fiber-reinforced polymer (FRP) were fabricated to satisfy the seismic design performance level of wooden columns, and their compression strength performance was evaluated. The experimental results showed that the average compression strength of the specimen reinforced with a CFRP (Carbon FRP) bar increased by approximately 7% compared to that of the control. The average compression strengths of the specimens reinforced with a GFRP (Glass FRP) bar and a steel bar increased by 38 and 37% compared to that of the control, respectively. The unreinforced control column specimens showed a diagonal failure tendency due to the fiber slope angle, and the wood part of the reinforced specimens showed a failure mode with suppressed diagonal fracture. The average strength of the column reinforced with a CFRP plate increased by approximately 6%, but the average strength of the column reinforced with a GFRP plate decreased by approximately 5%. A comparison of the measured and predicted compression strengths of the specimens showed that the strength differences of all the specimens except the specimen reinforced with a GFRP plate were good (2 to 10.4%).
Keywords: CFRP; Column; Compression strength; FRP; GFRP; Reinforced; FRP-reinforced multi-layer glued column composite
Contact information: Department of Forest Biomaterials Engineering, College of Forest & Environmental Sciences, Kangwon National University, 1, kangwondaehak-gil, ChunCheon-si, Gangwon-do, 24341, Republic of Korea;
* Corresponding author: hongsi@kangwon.ac.kr
INTRODUCTION
Among extreme weather events, earthquakes are almost impossible to forecast, and the damage is mostly concentrated on building structures because the energy is transmitted through the ground. Therefore, seismic design plays a critical role as the only safety measure that can prevent damage from earthquakes. In seismic design, structures must satisfy the functional performance and collapse prevention levels. The functional performance level refers to a structural level where serious structural damage can be prevented and that will allow facilities and structures to maintain their normal functionality after an earthquake. The collapse prevention level, on the other hand, refers to a structural level where large-scale loss of lives and property damage can be prevented while allowing a considerable degree of damage to the structure.
The scope of seismic-design-obligated buildings has expanded recently. In addition, the standards for small-scale building structures are being revised, and methods for the seismic design and reinforcements of steel structures are being implemented (Cozmanciuc et al. 2009; Lam et al. 2009; Chang et al. 2012). The research on the seismic design and reinforcement of wooden buildings, however, is still insufficient (Attari et al. 2019).
Wood has many advantages as a construction material, such as its higher specific strength, elasticity, and shock-absorbing performance compared with other materials. The high specific strength of wood minimizes deformation by inertia during an earthquake, and wooden buildings have a better seismic performance than reinforced-concrete buildings. The column-beam wooden-structure method is the most widely used and oldest wooden building construction method. The column-beam wooden structure is a structure made of columns erected on a foundation and joined by beams as horizontal members. Seismic performance can be secured through appropriate structural forms. Columns are members that form a building space and support the load of the upper part and transmit it to the foundation. When an earthquake occurs, the damage of the column is directly related to the safety of the entire building. Fiber-reinforced polymers (FRPs) with excellent specific strength and durability are appropriate reinforcements for wooden building members and are actively being researched. Small-diameter timber has a low structural performance due to its small cross-sectional area and high juvenile wood content, but it can be used for various types of structural member by increasing the cross-sectional area through multi-layering, and its utilization as a building member is expected.
Larix in Korea takes up 73% of general lumbering industry. Medium-diameter and small-diameter logs produced in the course of thinning takes up the most part of it, or it is produced as logs or chips. Kwon et al. (2015) conducted an experimental evaluation of the seismic performance of columns reinforced with FRP seismic reinforcements and reported that the maximum strength and maximum displacement of the reinforced specimen increased by approximately 1.06 to 1.10 and 1.66 to 1.98 times, respectively, compared with the unreinforced specimen. Based on an analytical study on the applicability of fiber-reinforced composites for the improvement of the seismic performance of columns, Jang et al. (2012) reported that the carbon-fiber-reinforced polymer (CFRP) plate had higher strength than the glass-fiber-reinforced polymer (GFRP) plate and Hiper Glass. Xiong et al. (2016) reinforced timber columns by wrapping them with CFRP for improved strength, but it had a disadvantage of decreasing the aesthetic value. Lee et al. (2015) attempted to improve the utilization of a joint in which a GFRP plate was inserted and bonded, reporting that good performance was obtained at the GFP bar thickness and insertion depth of 5 times the diameter. Shin et al. (2011) experimented on the compression strength of a multi-layered joint column using domestic small-diameter timber and found that the measured maximum load was proportional to the cross-sectional area.
In this study, columns were fabricated through multi-layer bonding with four small-diameter domestic Larix timbers. For the reinforcements, a CFRP plate, a GFRP plate, a CFRP bar, a CFRP bar, and a steel bar were used. Thus, a total of six specimen types were fabricated, including an unreinforced control specimen. Compression strength tests were performed with the fabricated FRP and steel-reinforced glued laminated timber using small-diameter timber, and their strength performances were compared with that of the control. A test specimen was fabricated to increase seismic performance and enhance aesthetic properties. Furthermore, a strength estimation equation was proposed by considering the non-destructive modulus of elasticity of the small-diameter timbers as well as the properties of the reinforcements.
EXPERIMENTAL
Material
For the testing materials, small-diameter domestic Larix timbers (60 [T] × 60 [W] × 3600 [L] mm) with a 0.52 average air-dried specific gravity, 16% average water content, and no end-joint were used. The FRP reinforcement uses: a CFRP plate (T: 1.3 mm), a square CFRP bar (6 [T] × 13 [W] mm), a GFRP plate (T: 3.5 mm), and a circular GFRP bar (ϕ: 16 mm) were used. In addition, an SD 350 steel bar (ϕ: 16 mm) was also used as a type of reinforcement (Fig. 1). For wood-wood bonding, PRF (phenol-resorcinol-formaldehyde) adhesive was used. For the bonding of wood with the CFRP plate, GFRP plate, and steel bar, the epoxy (Sikardur-30), polyvinyl acetate (MPU-500), and polyurethane (Ottocoll P84) adhesives were used (Plevris and Triantafillou 1992; Lee et al. 2019). The mechanical strength characteristics of all the FRP testing materials were examined through compression and bending strength tests, and the steel bar complied with the strength characteristics of SD 350.
Fig. 1. Photographs of the reinforcements used in this study
Methods
Column specimen fabrication and test methods
To improve the bonding performances of the small-diameter timbers that were used for the columns, those whose adjacent faces were tangential were selected. The slenderness ratio of the specimen was set to 13, and the length was determined to be 450 mm. All specimens were fabricated as 120 × 120 × 450 mm columns with two additional bonds after bonding the reinforcements onto a single small-diameter timber. For the first bonding of the control specimen, 300 g/m2 PRF adhesive was applied to two small-diameter timbers, and they were hardened for 24 h at room temperature under 1.5 MPa pressure. The second-bonding surfaces of the first-bonded specimens were sanded, and the second bonding was performed under the same conditions as in the first bonding. The reinforced specimens were joined with the reinforcements before the first bonding, as follows (Fig. 2). The specimen bonded with a CFRP plate was hardened for 24 h at room temperature under 1.5 MPa pressure after being bonded with a single small-diameter timber using 600 g/m2 epoxy adhesive. The specimen bonded with a GFRP plate was hardened for 24 h at room temperature after bonding with a single small-diameter timber using 600 g/m2 polyvinyl acetate adhesive. The specimen reinforced with a CFRP bar was fabricated under the same bonding conditions as the CFRP plate after pre-processing one surface of a single small-diameter timber (7 x 15 mm) so that the bonding layer would be 1 mm thick. The specimens reinforced with a GFRP bar and a steel bar were bonded by machining 18x18mm square grooves on a single small-diameter timber and then filling them with an adhesive so that the bonding layer thickness between the reinforcement and the timber would be at least 1 mm. Polyvinyl acetate adhesive was used in the small-diameter timber reinforced with a GFRP bar, and the steel bar and timber were bonded using polyurethane adhesive. The adhesive coating amount and pressure were 300 g/m2 and 1.5 MPa, respectively.