Fibre Reinforced Polymer or sometimes called Fibre Reinforced Plastic is a composite material which capable to replace conventional steel bar for designing of structural elements. FRP is a combination of several polymers such as epoxy, polyester thermosetting plastic and venylester. Normally, fibre is usually made up from glass, carbon and aramid. Nowadays, usage of FRP as main material is very significant especially in aeronautic, military and construction. There are many researchers which carried out the research based on application of FRP in construction industry.
Figure 2.1: Composite Materials Usage in the Industrial Construction (Oprisan et al., 2010)
For example in Okinawa, Japan there is a pedestrian bridge with two continuous spans was reinforced using FRP (see figure 2.2). This bridge was constructed in 2000 with girder span lengths between 19.7 to 17.2m. The main factor why this bridge constructed using FRP is due to corrosive environment from sea salt.
Figure 2.2: The bridge pedestrian reinforeced using FRP in Okinawa, Japan
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Besides that, in Scandinavia, Sweden there is the first large application of FRP for strengthening of a chimney in 1997. In this chimney, almost 3000m of CFRP sheet system was wrapped around the column. After 10 years, there is no deterioration effects could be noticed on the strengthening.
Figure 2.3: CFRP strengthening of a chimney in Scandinavia, Sweden (Täljsten B. 2007)
2.2 Advantages of FRP
Fibre reinforced Polymer is the friendly material in construction material. This material is really useful and applicable for reinforced elements in structural members. The FRP have been proved as the best alternative material to change from using conventional steel bar in most structural elements such as beam, column, slab, retaining wall and pier in bridges construction.
Once of the advantages of FRP is high corrosion resistance. Corrosive problem in steel bar have effects the durability of reinforced structural members. In addition, to repairing this kind of problem, it will cause high maintenance cost. So, FRP can be used effectively as reinforced material in main structural elements. According to Benjamin M. Tang, (2003) the pre and post-tension concrete will have great workability when reinforced with FRP.
Secondly, the FRP can handle high loads even this material is low in self-weight. FRP is lower in weigh compared to steel. But, the load capacity of this material is higher based on ratio weight to density.
Besides that, the life period of structural members strengthened with FRP can be extending higher compared to steel bar. This is because, the corrosion of bar approximately zero compared to steel bar. Most of structure reinforced with conventional steel bar will has imperfect surface after some period because of corrosion effect. Consequently, the structure can't be longer used and rehabilitation process needs to be done immediately. Therefore, when using FRP as reinforced material in main structural members, it will reduce the probability of corrosion attack.
Cost for maintenance also can be reduced if the FRP be used in construction. Truly, when the structure is not facing with problem such as corrosion and crack, the maintenance and repairing works can be reduced indirectly. Hence, the owner of building can save their money for spending with maintenance of building.
2.2 Properties of Fibre Reinforced Polymer
Nowadays, most of type of FRP used for construction is Glass Fibre Reinforced Polymer (GFRP). But, other type that usually used is Class Fibre Reinforced Polymer (CFRP). The main mechanical properties to be considered in selecting of FRP are tensile stress, compressive stress, and modulus of elasticity. But, for this studies only focus more on tensile stress of FRP.
Tensile stress: The ability of a material to resist breaking under tensile stress is one of the most important and widely measured properties of materials used in structural applications. The force per unit area (MPa or psi) required to break a material in such a manner is the ultimate tensile strength or tensile strength at break. (A.S. Singha and Vijay Kumar Thakur, 2008)
According to Sakurada et al.,(2007), the stress-strain relations for the braided-FRP reinforcing bars in nominal diameter of 13 mm behaves linear elastic to failure and its ultimate tensile strength gains finally 1354 N/mm2, 2.6 times higher than that of the deformed steel bar with same diameter. The elongation rate at failure reaches 1.9 % whereas the mild steel in diameter of 13 mm extends at 27.4 %.
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While the tensile strength of CFRP laminates did not change significantly over 5 years of exposure, the in-plane shear strength showed a slight decrease. Bending strength of matrix resin decreased in early exposure stage. The result suggests the reduction of bonding properties between carbon fibers and resin. (Nishizaki et al., 1997)
2.3 Timber in Construction Industry
Timber strengthening with FRP is very significant material in order to minimize the usage of steel bar. Studies of strengthening timber beam with FRP is useful because of the properties of FRP can be alter and needs to be improve in term of strength and durability. Therefore, detail understanding of timber should happen in order to know the behavior of timber for becoming structural element.
Timber is an oldest material has been used for construction industry. This kind of material is a unique material compared to common structural materials such as concrete and steel. Unlike other material, the behavior of timber is influenced by the existence of its moisture contents. Other than that, the timber behavior will depends also by difference in strength when loads are applied either parallel or perpendicular to the grain direction. In fact, different species have different strength properties that need to be considered in selecting of timber. Hence, to enhanced usage of timber in construction industry, detail understanding of timber behavior is very important due to this material is more complex than those other materials.
2.4 Factors Affecting the Strength of Timber
Moisture Content: Once of the factor that affect the strength of timber is moisture content. Unlike other structural materials, the behavior of timber is influenced by the existence of its moisture contents.
Density: Density which is expressed as mass per volume is one of the main properties affecting strength of timber. Normally, the heaviest species have thick cell wall and small cavities. So, increasing of density will lead the increasing of strength of timber.
Slope of Grain: Angle of the grain direction in a cut section of timber is not parallel to the longitudinal axis. The effect of sloping grain have a significant influence on the bending resistance of a timber section.If the angle of sloping grain increases, the strength of the timber will decreases.
Temperature: The strength of timber is also affected by temperature, the general effect being a linear decrease in strength with increase in temperature. This effect is also very dependent on moisture content, dry timber suffering much less decrease in strength per oC rise in temperature than wet timber.
2.5 Tensile strength and compressive strength of timber
Tensile strength of timber will be higher if tensile forces parallel to the grain. While, the tensile strentgh of timber perpendicular to the grain is small. But, if there is defect such as knot on timber surface even forces applied parallel to the grain, the tensile strength of timber may be reduced.
Compressive strength of timber parallel to the grain is much higher compared that perpendicular to grain.
2.6 Timber Beam strengthening with FRP
Lately, there are many researches which focus on reinforcing timber with other material like FRP. T. Russell Gentry (2011) stated that tests on large-scale beams with longitudinal FRP reinforcement confirm that the strengthening technique can be used to increase the flexural strength and stiffness of glulams using densely finger-jointed low-grade wood to the level observed in high-quality glulams constructed of highgrade timber without longitudinal reinforcement. According to Francesco Micelli and Antonio Nanni (2001), different resin showed different performance durability of FRP rebars, especially for GFRP specimens, since glass fiber are more sensitive to external agents.
Besides that, Maurice Brunner and Marco Schnueriger (2005) showed that the use of prestressed carbon laminates will lead to a greater improvement of the load-bearing capacity of timber beams than when the laminates are bonded in a slack state. Other than that, the prestressing force was too small when the degrees of strengthening was rather slow. In a followup project, studies are being carried out to attach several layers of prestressed carbon laminates glued on top of each other.
In addition, Borri et al., (2001) stated that without necessitating the removal of the overhanging part of the structure, the use of CFRP as a strengthening technique can be applied. The technique used proved to be easy and fast to execute, even when on in situ parts. Other than that, Gentile et al., (2000) concluded the strength increase of 25 to 50 percent was obtained, depending on the reinforcement ratio and strength of the original timber.
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Thanasis C. Triantafillou (1997), shown that a little FRP reinforcement can increase shear stresses in timber structures. This reinforcement method is very efficient in term of cost and work as well as can be applied with minor influence on the esthetics of wood, especially when the FRP materials employed are made of glass fibers in a transparent matrix.
The ultimate tensile strain of the timber increased 64 percent for the reinforced beams. The FRP material arrests crack opening, confines local rupture, and bridges local defects in the adjacent timber, allowing the timber to support higher nominal stresses and strains before failing. (Christopher J. Gentile, 2000)
According to A. Yusof & A. L. Sale (2010), timber beam strengthened with GFRP rods had an increase in its ultimate load carrying capacity. The percentage of increase is between 20 to 30 percent. In addition, the strengthening of timber beams with GFRP also enhanced the stiffness of the beam with a percentage of increase between 24 to 60 percent.
James F. Brady and Annette M. Harte (2000) stated that the bonding of pre-tensioned FRP laminates in the tension zone of low-grade glulam beams can significantly increase their flexural strength and ductility. This indicates that prestressed beams have a higher load capacity but are less ductile than the non prestressed FRP reinforced glulam.
Tajnik et al.,(2007) showed if in practice after such reinforcement (with CFRP strip) become decisive shear resistance of section than is suitably to use the carbon strip in position of stirrups glued on web of timber beam similar like transverse steel reinforcement in steel reinforced concrete structures. Presented study show an improvement of 15% on initial and 24% on final bending capacity, further at the same time is achieved improvement of 11% on initial and 17.5% on final bending stiffness.
Yusof Ahmad (2010), proved the load carrying capacity of timber beams was increased as the percentage of GFRP used was increased. The ultimate load and the serviceability load were increased between 17.0 to 25.1% and 13.0 to 54.1%, respectively when the timber beams were strengthened using GFRP bar for area between 0.16 to 2481.27%. It is suggested that the percentage of GFRP between 0.6 to 0.7% should be considered in the design to satisfy both ultimate and service load.
Besides that, as reported by Ngu Wang Chung (2007), the timber beam strengthened with FRP increased between 7 to 25% of stiffness and bending strength will depend on the type of material and strengthening method. In addition, CPRP plate strengthened beam has higher bending strength compared to the GFRP bars strengthened beam. This is because, the modulus of elasticity of CFRP is greater than GFRP.
2.7 Finite Element Analysis (FEA) in LUSAS Software
Analysis of structural members is very important in designing stage. Appropriate analysis should be carried out to get the best dimension of any structural members. In structural analysis, the most popular method have been used is Finite Element Method (FEM). Finite Element Method is a solution based on partial differential equation and integral equation.
In addition, Finite Element Analysis (FEA) is used to analyzed specific problem by using computer program which to obtain approximate solution. Finite Element Analysis was proposed by R.Courant in 1943 to obtain approximate solution in vibration system. This analysis is very popular to solve structural analysis such as truss, beam and frame.
LUSAS software was developed based on Finite Element Analysis. LUSAS software can solve all types of linear and nonlinear stress, dynamics, composite material and thermal engineering analysis problem. For this study, LUSAS Software is very suitable to use due to the result analysis from this software almost approximate with manual solution. Moreover, this software can solve complicated and complex structures that ease the structural engineer in designing a building.
2.8 Gap of Studies
For this research, the analysis of timber beam strengthened using FRP is not much be studies in composite material field. Most of researches are focus more on concrete beam strengthened using FRP material. Furthermore, usage of timber beam as main structural members in construction is not much popular compared to concrete and steel.
BENDING BEHAVIOR OF TIMBER BEAMS STRENGTHENED USING FIBER REINFORCED POLYMER BARS AND PLATES
To determine the load carrying capacity, bending strength or modulus of rupture, stiffness and ductility of the timber beams strengthened with FRP,
thus the strength and stiffness modification factor can be developed.
Numerical Analysis of Strengthened Timber Beam With FRP
Ngu Wang Chung
To determine if strengthened beam with FRP would increase the stiffness and bending strength of timber
Timber Beams Strengthened with GFRP Bars: Development and Applications
Chris Gentile et. al.
To determine the behavior of sawn timber beams reinforced with GFRP bar
2.9 Theoretical of Background
There are several theories have been used for this study.
Deflection at mid span (Simply Supported) :
Where: P = Load
E= Modulus of Elasticity
I= Second Moment of Inertia
Maximum Bending Moment (Simply Supported):
Where: w = point load
L = length