COMPARATIVE STUDY OF FIBERGLASS REINFORCED TIMBER JOINTS VERSUS BAMBOO REINFORCED TIMBER JOINTS

Transcription

1 COMPARATIVE STUDY OF FIBERGLASS REINFORCED TIMBER JOINTS VERSUS BAMBOO REINFORCED TIMBER JOINTS César Echavarría 1, Claudia de la Cruz 2, Julio Sánchez 3 ABSTRACT: A research study was undertaken to investigate the mechanical performance of dowel-type timber joints reinforced by fiberglass fabrics or bamboo. Local reinforcement is proposed to improve the embedding strength and ductility and to prevent the splitting failure of timber joints. The reinforcements are glued to the side of the timber members. A series of fiberglass reinforced timber joints and bamboo reinforced timber joints were tested to determine their load-deformation characteristics. Experimental work to evaluate the reinforcing technique is reported here. According to experiment results, the fiberglass and bamboo reinforcements lead to a higher performance and provide a good security factor to the timber joints. The results show a considerable improved deformation behaviour of the reinforced over the non-reinforced specimens. KEYWORDS: bolted, timber, joints, fiberglass, bamboo, reinforcement 1 INTRODUCTION 123 For a typical bolted joint, smaller bolts signify lower bending capacity and higher ductility due to their ability to undertake large deformations. Joint stiffness can be enhanced by increasing bolt. However, the increase in stiffness cannot be accomplished without increasing the joint dimension. Brittle failure can occur in joints with insufficient end distance, edge distance, and (or) bolt spacing. To improve the behaviour of the dowel-type timber joints without excessively increasing the size of the connection, several techniques have been proposed in the past (Blass et al. [4-6], Chen et al. [7-9], Haller et al. [11-14], Jorissen [15], Rowlands et al. [16]). The joints are reinforced, such that wood strength is increased and the behaviour of the whole structure is consistently improved. Local reinforcement is proposed to guarantee some ductility and to prevent premature splitting. Local reinforcement eliminates the cracks for excess of perpendicular-to-grain stress. Such a joint is achieved by 1 César Echavarría, School of Construction, Universidad Nacional de Colombia. caechavarrial@unal.edu.co 2 Claudia de la Cruz, School of Civil Engineering, Universidad Nacional de Colombia. cjcruz@unal.edu.co 3 Julio Sánchez, School of Construction, Universidad Nacional de Colombia. jcsanche@unal.edu.co an optimization of the embedding strength and by the prevention of a brittle failure of the timber joint. Using fiberglass or bamboo as local reinforcement, it is possible to obtain a considerable increase in the timber tensile strength perpendicular-to-grain. Thus, the joint behaviour will be ductile, ultimate displacement will be high and ultimate strength will reach 100% of the available strength in its members. In this way, it is possible to increase the reliability of the timber structure. 2 REINFORCED TIMBER JOINTS For timber structures, joint design plays a decisive role. The use of reinforcement appears to be a very good possibility to face problems like local stress peaks. In this work, in order to increase the load capacity and the ductility of the bolted timber joints, fiberglass or bamboo are used as superficial covering (reinforcement) of the wood joint lateral faces. The enhanced capacity of these connections leads to a decrease in the transverse section of the connected timber members. It is possible to make compact joints characterized by high stiffness. This implies a reduction of weight and cost of the whole structure. 2.1 FIBERGLASS A composite fiber-reinforced polymer (FRP) is used in this study. FRP consists of a polymer matrix that is reinforced with fibers. The fibers are usually fiberglass, carbon, or aramid, while the polymer is usually an epoxy, vinylester or polyester thermosetting plastic. FRPs are commonly used in the aerospace, automobile, marine, and construction industries.

2 Figure 1 shows the fiberglass reinforcement. The reinforcement materials used here are the OCV roving fiberglass bi-directional fabrics woven in 800 gr/m2 (WR800) and the epoxy resin of Sikadur HEX300. Table 2: Mechanical properties of fiberglass reinforcement Plate thickness t [mm] σ x Tensile strength [MPa] 0,6 650 Figure 1: Fiberglass These reinforcements are glued laterally on the timber as shown in Figure 2. This reinforcement contributes to the improvement in the mechanical wood behaviour, giving the necessary strength to avoid brittle failures. In particular, shear and tensile strength perpendicular-tograin of reinforced wood increase considerably. 2.2 BAMBOO (GUADUA ANGUSTIFOLIA) The predominant bamboo in Colombia is the Guadua Angustifolia. Bamboo is mainly used in Colombia for household objects and roof systems. It is also employed for flooring and furniture. Other more industrial-scale uses are gradually being developed and investigated. Bamboo can be cut and laminated into sheets and planks. This process involves cutting stalks into thin strips, making them flat, and finally boiling and drying the strips. Products made from bamboo laminate include flooring, cabinetry, furniture and decorative objects. Thin flat strips of bamboo are used here as reinforcement to eliminate the fissures caused by excess of perpendicular-to-grain stress and to improve the behaviour of the bolted joint. Figure 3 shows the reinforcement, which is constituted by the bamboo (Guadua Angustifolia) and the epoxy resin of Sikadur HEX300. The bamboo strip reinforcements are glued laterally on the timber as shown in Figure 2. Figure 2: Reinforced timber joint High strength allows for the decrease in the end distance without increasing the possibility of failure. The elastic constants and material properties are summarized in Tables 1 and 2. Table 1: Elastic constants of fiberglass reinforcement [MPa] E x E y G xy ν yx ,23 Fiberglass becomes an alternative particularly useful of reinforcement into existing structures. Figure 3: Bamboo (Guadua Angustifolia) The bamboo reinforcement increases the load capacity of the joints and the ultimate strength reaches 100% of the available strength in wood members. Elastic constants and material properties are summarized in Tables 3 and 4. Table 3: Elastic constants of bamboo reinforcement [MPa] E x E y G xy

3 Table 4: Mechanical properties of bamboo reinforcement Plate thickness t [mm] σ x Tensile strength [MPa] The joints were tested with static load applied in tension parallel-to-grain using a universal testing machine in accordance with EN 26891:1991 [10]. Table 5: Elastic constants of White Cedar [MPa] 5,0 100 The bonding of fiberglass and bamboo reinforcements must be prepared very carefully: - The wood surface must be flat, clean, and not weathered to insure proper adhesion. - The wood surface must be dry when gluing. 3 EXPERIMENTAL VERIFICATION AND DISCUSSION By reinforcing the timber bolted joints with bamboo or fiberglass, it is possible to control the premature cracking due to concentration of perpendicular-to-grain stress. A more homogeneous mechanical behaviour is obtained, since load capacity, ultimate displacement and ductility are increased. The tensile strength perpendicular-to-grain must be mainly enhanced with the judicious use of reinforcement. Laboratory tests on non-reinforced and reinforced timber plates loaded parallel to grain by a single bolt, represented by the geometry shown in Figure 4, were performed for various end distances. One bolt was tested using various combinations of end distance e. The bolts were -mm (3/8-in.) in, made of low carbon steel conforming to ASTM A307, and edge distance was 44,5-mm. lengths were selected to ensure that threads were excluded from bearing against the wood. The ratio of the wood member thickness to bolt was small enough to induce failure in the wood, with minimum bending deformation of the bolt. Joint wood plates were cut from 38 by 89-mm (nominal 2 by 4-in.) White Cedar kiln-dry lumber so that the joint area was free of defects. Prior to testing, the specimens were conditioned to attain 12% equilibrium moisture content. Specific gravity based on ovendry mass and volume at 12% moisture content of the specimens varied from 0,42 to 0,50 as determined per ASTM D [2]. For each test, wood elastic properties were determined nondestructively using transverse vibration (E-computer) and visual grading criteria. Material shear strength parallel-to-grain and tensile strength perpendicular-to-grain were determined using ASTM D [1] test methods, with the exception that the thickness of the specimens was equal to the thickness of the plates (19 mm for the -mm bolts). The shearing surface dimensions were identical for all shear strength parallel-to-grain tests. The dowel embedding strength for each bolt was determined according to ASTM D a [3]. Elastic constants and material properties are summarized in Tables 5 and 6. E x E y G xy ν yx ,43 A hole-radial clearance of 1 mm was considered. The displacement-control rate of loading was applied to achieve failure in 5-15 min and the load-displacement curve was recorded. Tests were stopped when one of the following conditions was reached: when the load dropped with no recovery or when the displacement exceeded 10 mm. Table 6: Mechanical properties of White Cedar Plate thickness t [mm] σ x Tensile strength [MPa] σ y Embedding strength [MPa] τ xy Shear strength [MPa] 19,0 A total of 60 joints were tested in the course of this study. The number of replications and the summary of test results for each configuration are given in Tables 7, 8 and 9. Figure 4: Double shear connection Splitting was observed in the configurations having the smallest end distance for non-reinforced joints. For the same geometry with reinforced joints, the failure modes are replaced by a significant bearing failure. Figure 5 shows a non-reinforced timber joint

4 The mechanical properties of the reinforced timber increased appreciably. The crushing of wood beneath the bolt became more significant. Figure 7: Bamboo reinforced timber joint Table 7: Experimental average failure load in nonreinforced joints Figure 5: Non-reinforced timber joint Figure 6 shows a fiberglass reinforced timer joint. Figure 7 shows a bamboo reinforced timber joint. Both fiberglass and bamboo work well as reinforcements. The ratio of reinforced and nonreinforced load-carrying capacity using fiberglass as reinforcement varies between 45 % and 75 %. The increase using bamboo is among 54 % and 87 %. Number of replications Non-reinforced joints Average failure load F exp [kn] Standard deviation ,7 21, ,3 22,3 There is a noticeable increase of load capacity by reinforcing the joints. It is also evident that the results for reinforced bolted timber joints show a relatively small standard deviation (Tables 8 and 9). Table 8: Experimental average failure load in fiberglass reinforced joints Fiberglass reinforced joints Number of replications Average failure load F exp [kn] Standard deviation ,5 12, ,1 11,5 Figure 6: Fiberglass reinforced timber joint Joints are designed on the basis of their load capacity, which is a function of material properties, joint geometry, and the load and boundary conditions. Another important consideration is failure ductility. It is desirable that the joint continues to support load as it deforms. Figures 8, 9 and 10 are load displacement plots for connections of the same geometry.

5 Table 9: Experimental average failure load in bamboo reinforced joints Number of replications Bamboo reinforced joints Average failure load F exp [kn] Standard deviation ,0 23, ,7 13,4 The brittle failure (splitting and shear with plug-shearout) is avoided by reinforcing. Table 10 resumes the comparative results. Consequently, the mechanical behaviour of reinforced timber joints will be more homogeneous and the reliability of the whole structure will increase. Figure 10: Load displacement curve for bamboo reinforced joint Table 10: Relation between load-carrying capacity of reinforced and non-reinforced joints Ratio of reinforced and nonreinforced load-carrying capacity Fiberglass reinforcement Bamboo reinforcement 2 74,7 86,9 3 44,5 53,6 Figure 8: Load displacement curve for non-reinforced joint Figure 9: Load displacement curve for fiberglass reinforced joint 4 CONCLUSIONS There are many advantages to use fiberglass or bamboo reinforcements in timber joints: - Increase strength, ductility and stiffness. - Improve structural efficiency and reduce structural member size requirements, weight, and costs under certain conditions. - Allow upgrading structures for higher loads or restoring original strength. The improvements on mechanical performance of fiberglass and bamboo reinforced timber joints offer higher ductility and a higher failure load. The increased tensile strength perpendicular-to-grain transforms the brittle failure modes into less fragile failure modes. The brittle failure, in particular the splitting, is avoided. The critical end-distance as well as the edge distance can be optimized while the mechanical properties of reinforcements are well known. The load-carrying capacity of fiberglass reinforced timber joints and bamboo reinforced timber joints are identical. Bamboo could be a good alternative of reinforcement because it is less expensive than fiberglass.

6 ACKNOWLEDGEMENTS We thank the Universidad Nacional de Colombia as well as DIME for support of this project. Thanks to Jorge Rendón and César Lope (Sikadur HEX300, Sika-Colombia) and to Andrés Franco (OCV WR800 fiberglass, Andercol-Colombia). The constructive comments of Ph.D. Daniela Blessent (Université Laval, Canada) are greatly appreciated. REFERENCES [1] American society for testing and materials (ASTM). Standard methods of testing on small clear specimens of timber. D ASTM Annual Book of Standards. West Conshohocken, Pa., [2] American society for testing and materials (ASTM). Standard test methods for specific gravity of wood and wood-based materials. D ASTM Annual Book of Standards. West Conshohocken, Pa.,2006. [3] American society for testing and materials (ASTM). Standard test method for evaluating dowel bearing strength of wood and wood-base products. D a. ASTM Annual Book of Standards. West Conshohocken, Pa.,2006. [4] Blass, H.J., Betja, I. Joints with inclined screws. International council for research and innovation in building and construction. Working commission W18-Timber structures. Meeting thirty five. Japan, [5] Blass, H.J., Schmid M. Self-tapping screws as reinforcement perpendicular to the grain in timber connections. Proceedings PRO 22, International RILEM Symposium on Joints in Timber Structures, pp , [6] Blass, H.J., Schmid, M., Litze, H., Wagner, B. Nail plate reinforced joints with dowel-type fasteners. Proceedings of World Conference on Timber Engineering, Whistler, British Columbia, Canada, [7] Chen, C.J., Lee, T.L, and Jeng, D.S. Finite element modeling for the mechanical behavior of dowel-type timber joints. Computers & Structures. 81: , [8] Chen, C.J., Mechanical behavior of fiberglass reinforced timber joints. Ph.D Thesis N Swiss Federal Institute of Technology Lausanne EPFL, Switzerland, [9] Chen, C.J., Haller, P. Experimental Study on Fiberglass Reinforced Timber Joint. Pacific Timber Engineering Conference. Gold Coast, Australia, [10] EN Timber structures - Joints made with mechanical fasteners -General principles for the determination of strength and deformation characteristics. (ISO 6891; 1983), [11] Haller P., Wehsener J. and Birk T. Embedding characteristics of fibre reinforcement and densified timber joints. Paper CIB/W18/34-7-7, Proceedings of Meeting 34, Venice, Italy, [12] Haller, P., Chen, C.J. and Natterer, J. Experimental study of glassfibre reinforced and densified timber joints. Proceedings Timber Engineering Conference. New Orleans, USA, [13] Haller, P., Chen, C.J. Development of glass fibre reinforced timber joints proceedings. European COST C1 Workshop. Prague, Czech Republic , [14] Haller, P., Wehsener, J. Use of technical textiles and densified wood for timber joints. Proceedings RILEM Symposium on Timber Engineering. Stockholm, Sweden, [15] Jorissen, A. Double shear timber connections with dowel type fasteners. Delft University Press, Delft, [16] Rowlands, R.E., Van Dewehge, R.P., Laufenberg, T.L., Krueger, G.P. Fiber-reinforced wood composites. Wood and fiber science; 18:39-57, 1986.