AN INNOVATIVE FRP ANCHOR SYSTEM FOR THE SEISMIC RETROFIT OF REINFORCED CONCRETE SHEAR WALLS

Transcription

1 10NCEE Tenth U.S. National Conference on Earthquake Engineering Frontiers of Earthquake Engineering July 21-25, 2014 Anchorage, Alaska AN INNOVATIVE FRP ANCHOR SYSTEM FOR THE SEISMIC RETROFIT OF REINFORCED CONCRETE SHEAR WALLS J. Woods 1, C. Cruz-Noguez 2 and D. T. Lau 3 ABSTRACT A number of studies have shown that the use of externally bonded carbon fibre-reinforced polymer (CFRP) sheets in the seismic retrofit of reinforced-concrete (RC) shear walls has the potential to eliminate premature shear failure, increase the energy dissipation capacity, and enhance both the flexural strength and stiffness of walls in both repair and strengthening applications. However, it is also found that the efficiency of most FRP retrofitting schemes is limited by the debonding/slippage behaviour between the FRP material and the concrete substrate. Recently, several studies on the seismic retrofit of RC shear walls representative of both current and older design practices using externally-bonded CFRP tow sheets has been conducted at Carleton University. A crucial component of this CFRP retrofitting system is an innovative tube anchor system. The performance objective of this anchoring system is to ensure that the load carried by the vertical CFRP sheet(s) is effectively transferred to adjacent structural elements and premature failure of the FRP system due to FRP-concrete debonding is prevented. This paper presents the finite-element studies conducted to investigate the performance characteristics of the tube anchor system and to develop an optimized design procedure. A design example, design chart and preliminary methodology for the design and application of the tube anchor system in FRP-reinforced walls are also presented. 1 Graduate Student Researcher, Dept. of Civil Engineering, Carleton University, Ottawa, Canada, K1S5B6 2 Formerly Post-Doctoral Fellow, Carleton University; Present: Assistant Professor, Dept. of Civil and Environmental Engineering, University of Alberta, Edmonton, Canada, P6G2M7 3 Professor, Dept. of Civil and Environmental Engineering, Carleton University, Ottawa, Canada, K1S5B6 Woods J, Cruz-Noguez C, Lau DT. An Innovative FRP anchor system for the seismic retrofit of reinforced concrete shear walls. Proceedings of the 10 th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014.

2 10NCEE Tenth U.S. National Conference on Earthquake Engineering Frontiers of Earthquake Engineering July 21-25, 2014 Anchorage, Alaska An Innovative FRP Anchor System for the Seismic Retrofit of Reinforced Concrete Shear Walls J. Woods 1, C. Cruz-Noguez 2 and D.T. Lau 3 ABSTRACT A number of studies have shown that the use of externally bonded carbon fibre-reinforced polymer (CFRP) sheets in the seismic retrofit of reinforced-concrete (RC) shear walls has the potential to eliminate premature shear failure, increase the energy dissipation capacity, and enhance both the flexural strength and stiffness of walls in both repair and strengthening applications. However, it is also found that the efficiency of most FRP retrofitting schemes is limited by the debonding/slippage behaviour between the FRP material and the concrete substrate. Recently, several studies on the seismic retrofit of RC shear walls representative of both current and older design practices using externally-bonded CFRP tow sheets has been conducted at Carleton University. A crucial component of this CFRP retrofitting system is an innovative tube anchor system. The performance objective of this anchoring system is to ensure that the load carried by the vertical CFRP sheet(s) is effectively transferred to adjacent structural elements and premature failure of the FRP system due to FRP-concrete debonding is prevented. This paper presents the finite-element studies conducted to investigate the performance characteristics of the tube anchor system and to develop an optimized design procedure for the anchor system. A design example, design chart and preliminary methodology for the design and application of the tube anchor system in FRP-reinforced walls are also presented. Introduction The use of fibre-reinforced polymer (FRP) composites in the strengthening and retrofit of existing reinforced concrete (RC) elements has increased dramatically over the past two decades. The advantages of repair and strengthening schemes utilizing externally bonded FRP sheets include its ease of application, high strength-to-weight ratio and resistance to environmental degradation. When externally bonded FRP sheets are utilized in the strengthening or retrofit of reinforced concrete members (such as beams, columns, slabs and walls) the optimal mode of failure is controlled by crushing of the concrete and/or rupture of the FRP laminate after yielding of the steel reinforcement [1]. However, it is commonly recognized that in many cases, failure of the FRP strengthening system through debonding of the FRP laminate from the concrete substrate occurs prior to the FRP material reaching its ultimate tensile capacity, preventing the 1 Graduate Student Researcher, Dept. of Civil Engineering, Carleton University, Ottawa, Canada, K1S5B6 2 Formerly Post-Doctoral Fellow, Carleton University; Present: Assistant Professor, Dept. of Civil and Environmental Engineering, University of Alberta, Edmonton, Canada, P6G2M7 3 Professor, Dept. of Civil and Environmental Engineering, Carleton University, Ottawa, Canada, K1S5B6 Woods J, Cruz-Noguez C, Lau DT. An Innovative FRP anchor system for the seismic retrofit of reinforced concrete shear walls. Proceedings of the 10 th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014.

3 member from reaching its ultimate design strength [2]. In order to address this issue, various FRP anchorage systems have been developed in an attempt to eliminate or at least minimize FRP-concrete debonding prior to the FRP composite reaching its ultimate tensile strength. Experimental studies by authors [3-5] investigating the strengthening and repair of RC shear walls using FRP composites each concluded that special attention should be taken to providing adequate FRP anchorage in RC specimens strengthened or retrofitted with externally bonded FRP sheets, in order to prevent premature debonding failures. This paper presents the development of an innovative tube anchor system which eliminates undesirable premature debonding failures associated with many other FRP anchorage systems. Detailed finite element (FE) simulations are conducted on the tube anchor system and predicted behaviour is compared with measured data from experimental testing. It is concluded that the tube anchor system can successfully achieve the performance object of fully utilizing the tensile capacity of the FRP material. Preliminary design guidelines, a design chart and a design example for the implementation of the tube anchor system in the repair and strengthening of RC shear walls are also presented. Debonding Mechanisms in FRP-Strengthened Members The performance of FRP anchorage systems is a crucial component in the design of FRP retrofitting systems. By allowing the FRP laminate to reach its ultimate tensile capacity, a properly detailed anchor is capable of enhancing the efficiency of the FRP retrofitting scheme. However, premature failure of the FRP strengthening system characterized by the debonding or peeling of the FRP from the concrete substrate continues to be a commonly reported issue plaguing many past and ongoing investigations [1]. Several failure modes for FRP-strengthened members have been identified through experimental investigations and are summarized in detail by author [6]. The main failure modes in FRP-strengthened RC members include (1) concrete crushing, (2) FRP rupture, (3) shear failure, (4) concrete cover separation, (5) intermediate crack (IC) debonding, and (6) critical diagonal crack (CDC) debonding, and (7) plate end interfacial delamination (Fig. 1). Mode 7 (end interfacial debonding) is attributed to a lack of anchorage force within a region where the force cannot be fully developed within the FRP material leading to delamination of the FRP sheet from the concrete substrate. This is the most common failure mode observed in experimental investigations on FRP-strengthening systems [7]. By introducing an effective anchorage system into the retrofitting scheme, end interfacial delamination can be avoided, resulting in an increase in force sustained by the FRP sheet. This leads to more desirable failure modes in FRP reinforced members including rupture of the FRP laminate or flexural failure of the strengthened member. Figure 1. FRP debonding mechanisms (Adapted from [7]).

4 FRP Anchorage Systems To date, various anchorage systems have been developed with the goal of preventing premature debonding failure in RC members repaired or strengthened using externally bonded FRP sheets. These anchor systems have all been designed with the performance objective of allowing the FRP sheet to rupture without premature slippage/peeling from the concrete substrate. Anchor systems are capable of achieving this performance objective through one or more of the following design principles: 1) preventing premature debonding failure by resisting tensile forces developed within the FRP; 2) reducing the required development length to within appropriate lengths; or 3) transferring the full force from the FRP laminate to adjacent structural elements [1]. Detailed experimental investigations have demonstrated that steel mechanical anchorage systems generally provide higher anchorage strength than non-metallic anchor systems including both FRP and U-shaped anchor systems [6]. Along with providing higher anchor strength, their common availability in an array of sizes, materials and geometries, as well as their ability to be fabricated by non-specialized personnel make mechanical anchorage systems an attractive alternative to other FRP anchor systems. Mechanical Anchorage Devices One of the most commonly used mechanical anchor system is a steel angle anchor system, in which one flange of a steel angle is bonded to the vertical FRP sheet(s) while the other is bolted to adjacent structural elements using steel threaded anchor rods. This anchor system is designed to transfer stresses from the FRP sheet to the adjacent structural elements through the flange of the steel angle which is epoxy-bonded to the FRP (Fig. 2a). Observations by authors [3,8] have found that the steel angle anchor system performs poorly due to debonding of the FRP material from the concrete wall prior to the FRP material reaching its ultimate capacity. Failure of the steel angle anchor system occurs as a result of the eccentricity which exists between the tensile force in the FRP sheet and the tie-down reactions of the anchoring bolts. The eccentricity between these two forces causes a moment which ultimately leads to the rotation of the steel angle, also referred to as a prying action as illustrated in Fig. 2a. This prying action is detrimental to the anchors performance as the rotation of the flange results in debonding of the FRP sheet from the concrete substrate at the point of anchorage. This behaviour effectively reduces the load carrying capacity of the FRP sheet, rendering the reinforcing scheme less efficient resulting in a lower ultimate capacity in the retrofitted/strengthened RC member. (a) (b) Figure 2. (a) Prying action of the steel angle; (b) Balanced force distribution over tube section.

5 Motivated by the observed behaviour of the commonly used steel angle anchor system, this paper presents the development of an innovative anchor system consisting of a cylindrical hollow section (CHS) around which the FRP sheet is wrapped and epoxy bonded to adjacent structural elements. The design of the tube anchor is based on the pulley principle: as the FRP sheet is loaded in tension, the vertical force in the FRP sheet(s) is equated by the tension in the horizontal portion of the FRP sheet(s) which must be provided with the appropriate development length (Fig. 2b). The tube is bolted into the adjacent structural element at a 45 degree angle through the use of several properly spaced anchor rods. The anchor rods transfer the resultant component of the load from the FRP sheets into the desired adjacent structural element. By wrapping the CFRP sheet around the tube and placing the anchor bolts in the direction of the resultant load, the eccentricity between the force in the FRP sheets and anchor bolts which plagued the steel angle anchor system is eliminated. Finite Element Modelling In order to further study the response of the tube anchor system and ultimately aid in the development of design guidelines for the application of the innovative anchor system in the strengthening and retrofit of RC members, a rigorous finite element (FE) modelling procedure was developed using the FE computer package ABAQUS v6.10. Tube Anchor Modelling The initial geometry for the tube anchor system was selected according to a typical design load of one sheet of FRP laminate. In order to ensure the anchor remained elastic under the applied loading, the anchor was modelled using a 76.2mm (3.0") diameter cylindrical hollow section with a wall thickness of 12.7mm (0.5") measuring 1500mm in length. The anchor bolts were modelled as separate parts measuring 31.75mm in diameter, the spacing for which is shown in Fig. 3a. The interaction between the threaded anchor rods and steel pipe was modelled by surface-surface contact algorithm. Steel on steel tangential contact was modelled using a penalty friction model with the coefficient of friction of 0.2. The model was meshed using 4 node linear tetrahedral solid 3D stress elements for all parts. In regions where cylindrical contact surfaces were applied, such as adjacent to the anchor rods, a finer mesh was necessary for a smoother contact surface. Tube Anchor Loading As an FRP-strengthened RC member is loaded, the force in the FRP increases until the ultimate tensile strength is reached. It is assumed that the force in the FRP sheet follows a linear distribution as shown in Fig. 3b [9]. Depending on the implemented mechanical anchor system (tube, angle, plate etc...), this force is then transferred from the anchor system into adjacent structural elements through several anchor bolts. At regions in-between anchor bolts, the load from the FRP sheet must be resisted by the anchors structural member. The region over which the force from the FRP is distributed is the area of steel in contact with the FRP sheet (Fig. 3c). Therefore the resultant load from the FRP sheet(s) must be converted into an equivalent distributed pressure which acts over the base of the section. In the case of the tube anchor system, the maximum resultant load in the FRP sheet must be converted into an equivalent distributed pressure acting over the convex base of the tube. The maximum resultant load (P c,max )

6 acting on the anchor system, which is shown in Fig. 2b, can be determined based on the material properties of the FRP as well as the total number of sheets installed in the vertical direction P c, max = 2σ t u s n s (1) where σ u represents the ultimate tensile capacity of the FRP material; t s represent the thickness of one FRP sheet; n s represents the number of FRP sheets used in the design; The maximum resultant load can be converted into an equivalent uniform distributed pressure over the base of the tube by utilizing the following relationship Resultant Load π = 2 p 0 c 4 π cos x ro dθ (2) where p c represents the pressure over the curved surface, x is the distance along the radius of the tube, and r o is the outer radius of the tube. The solution to this integral can be converted into an expression for the maximum uniform pressure (p c,max ) acting over the area in which the FRP sheet is in contact with the tube anchor, which is shown in Fig. 3c p c, max σ ut = r s o n s (3) It is assumed that the maximum uniform pressure determined from equation (3) varies linearly from the end of the anchor to the theoretical location of the neutral axis. Through the use of these relationships, an accurate prediction of the load acting on the anchor system can be determined based on the material properties of the FRP sheet shown in Table 1. Based on a design load consisting of one sheet of FRP material, a maximum uniformly distributed pressure of MPa was applied in the FE model at the end of the tube decreasing linearly until the assumed location of the neutral axis, 1275mm from the anchors end (85% of the anchors length) [9]. Table 1. Material properties of the FRP composite. Tensile Modulus Tensile Strength Ultimate Elongation Density Weight (MPa) (MPa) (me) (g/cm 2 ) (g/m 2 ) 230, % (a) (b) (c) Figure 3. (a) Dimensions of the tube anchor system; (b) Force distribution over length; (c) Force distribution over cross section.

7 Results from the FE simulation of the tube anchor system are shown in Fig. 4. A maximum displacement of 0.351mm was observed over the longest unsupported length within the anchor model. This area lies between the first and second anchor bolts, a region which is subject to the largest uplift pressure. Maximum stresses within the tube were measured at roughly 40% of the yield strength of the material while the anchor bolts experienced maximum stresses of roughly 230MPa (58% of yield) in the two outermost anchor bolts. Small displacements of the anchor system are assumed to not be significant enough to cause debonding between the FRP and the concrete substrate and therefore would not significantly affect the load carrying capacity of the FRP sheet and allow the FRP laminate to approach its full rupture capacity. However, analytical results emphasize the need for the optimization of geometric and material properties of the tube anchor system for future modelling in order to maintain the performance objectives of the anchor system while ensuring maximum cost-efficiency. (a) Scale Factor: 150 (b) Figure 4. (a) von Mises stress distribution (MPa); (b) Displacement results (mm). Modelling of Steel Angle Anchor To provide evidence on the validity of the modeling procedure used for the tube anchor, the steel angle anchor discussed in Section 3 is modeled using the same approach. Experimental data are available on the performance of this anchor through an investigation by author [3]. As shown in Fig. 5, the displacement results from the steel angle model correlate well with the measured results from the experimental testing [3]. The analytical results confirm that the eccentricity between the force applied to the angle from the FRP and the reaction force within the anchor bolts results in the prying or rotation of the flange of the angle as shown in Fig. 5. This observed prying action in both experimental testing and analytical modelling led to the premature failure of the FRP strengthening scheme. Scale Factor: 150 Figure 5. Prying action of the steel angle anchors flange in experimental testing and analytical modelling (mm) [3].

8 Analysis on the Flexibility of the Tube Anchor System During the optimization of the anchor system it is assumed that the maximum deformation of the anchor systems is a key parameter in determining the effectiveness of the design. Large deformations of the tube anchor in between points of anchorage lead to a loss in load carrying capacity of the FRP due to premature failure prior to reaching its ultimate strength. In order to investigate how the FRP retrofitting system performs when coupled with the performance of the new tube anchor system at the base of a RC shear wall, a FE model of the shear wall developed in VecTor2 by author [9] is used to analyze the retrofitted shear wall with the FRP anchor system as shown in Fig. 6a. Within VecTor2, the concrete is modeled as an orthotropic material using 2D membrane elements under normal and shear stresses with smeared, rotating cracks [10]. The FRP materials are modeled as discretized truss elements bonded to the concrete substrate. Additional flexible link elements with varying stiffness are used to represent the behaviour of the anchor system at the base of the shear wall as shown in Fig. 6b. Discretized Truss Elements Concrete Elements Base Shear (kn) Wall Without Link Elements With Link Elements Flexible Link (a) (b) Elements (c) Top Displacement (mm) Figure 6. (a) Shear wall reinforcement detail; (b) Shear wall finite element model [10]; (c) Comparison of wall response with and without anchor. Results from the FE analysis indicate that small deformations in the anchor system (up to 0.5mm) do not have a significant effect on the ability of the FRP to reach its maximum load capacity in resisting the seismic lateral load, as shown in Fig. 6c. Therefore the deformation behaviour of the anchor system would not have an impact on the overall performance of the FRP retrofitting scheme so long as the displacement of the anchor system are limited to within appropriate values. Design Methodology for Applications of the Tube Anchor System Through both experimental and analytical observations, the tube anchor systems design has been shown to be a better alternative in terms of both performance and efficiency to other anchoring systems. However, the lack of a defined anchor design procedure could lead to the implementation of unsafe tube anchor designs. The following section outlines the design methodology with which a practicing engineering should follow when implementing the tube anchor system in the seismic retrofit of reinforced concrete shear walls.

9 Step 1: Calculate the total resultant load from the FRP which must be transferred to adjacent structural elements based on the material properties of the FRP and the number FRP sheets applied. Step 2: Determine the minimum number of anchor rods required in order to adequately transfer the load from the FRP sheet(s) into the concrete slab or foundation according to the manufacturer s recommendations. The number of anchor rods required shall be distributed evenly over the length of the anchor system. Step 3: Determine the geometry (diameter and wall thickness) of the tube required in order to ensure the loss in load carrying capacity of the FRP material due to the deformation of the anchor system is kept within allowable limits. Step 4: Optimize the tube anchor design by detailing additional anchoring rods in regions which are subject to higher load. The reduction in unsupported length between adjacent anchor rods allows for reduction in size of the tube anchor. Design Chart for Applications of the Tube Anchor System Based on the design methodology for the tube anchor system outlined above, the following design chart may be used as an aid in order to appropriately select the required tube dimensions. When selecting the tube size, key parameters such as diameter and wall thickness correlate with the amount of displacement the tube anchor will experience between anchor points. An increase in displacement results in relaxation of the FRP sheet between anchor rods, which translates into a decrease in strain at mid-span and higher strains at points of anchorage as shown in Fig. 7a. Higher strains at points of anchorage gives rise to higher stresses which will ultimately lead to premature rupture of the FRP material. The difference between the deformed and undeformed maximum stress in the FRP as a percentage represents the total loss in load carrying capacity of the FRP sheet due to deformation of the anchor system. In the design process, the size of the tube may be selected based on the percent loss in load carrying capacity the design engineer wishes to accept in exchange for detailing a smaller tube. The relative loss in load carrying capacity for variously sized hollow tubes (12.7mm wall thickness) subject to a design load of one sheet of FRP composite is plotted in Fig. 7b. (a) Loss in Capacity (%) Unsupported Length (mm) (b) 38.1mm (1.5") O.D. Tube 50.8mm (2") O.D. Tube 63.5mm (2.5") O.D. Tube 76.2mm (3.0") O.D. Tube Figure 7. (a) Stress distribution between anchor points; (b) Design chart for the variously sized tubes with 12.7mm (1/2") wall thickness. Application Example for the Tube Anchor System

10 A repair and strengthening scheme for RC shear walls using externally bonded FRP sheets has recently been investigated by author [4] which employed the aforementioned tube anchor system. The tests included a total of five 1.8x1.5x0.1 m cantilevered shear wall specimens subject to in-plane loading up to failure. The shear wall specimens were design according to the 1994 Canadian design provisions [11]. A typical wall specimen is shown in Fig. 6a. The test specimens were designed to exhibit a flexure-dominated behaviour [9]. Test specimens included one control wall (CW), two repaired walls (RW), and two strengthened walls (SW). Although focus of the experimental study was on the effectiveness of the FRP retrofitting and strengthening scheme, mechanical anchorage of vertical FRP sheets was of particular interest throughout the research program. The innovative tube anchor system was implemented in order to transfer the force from the FRP sheet to the foundation of the wall specimens. The anchor system was constructed using the same dimensions as those previously mentioned in section 4 and are shown in Fig. 8. Experimental results and observations show that the anchor system performs well in transferring the load between the FRP sheets and the foundation of the wall specimens. The FRP sheet is capable of reaching its ultimate tensile strength without premature failure of the FRP material due to prying of the anchorage system resulting in debonding of the FRP from the concrete substrate. Results of experimental testing are shown in Fig. 8, in which the rupture of the FRP material can be noted. Although the tube anchor system performed well during the experiment, the anchor was designed to experience little to no deformation in order to ensure the FRP material would reach its ultimate tensile capacity. These results correlate well with the aforementioned analytical model of the tube anchor system. Further research will focus on the implementation of a more efficient and cost effective anchor system. Fracture of FRP Figure 8. Performance of tube anchor system under the application of one sheet of FRP. Conclusions This paper presented both experimental and analytical results on two mechanical FRP anchor systems: a steel angle anchor system as well as a tube anchor system constructed from a cylindrical hollow section for applications in the seismic retrofit of a variety of RC members. The use of mechanical anchorage devices to provide adequate anchorage of FRP sheets in structural applications has been shown to be a key component in the repair or strengthening of RC members using FRP. An effective anchor system is capable of dramatically increasing the efficiency of the FRP retrofitting scheme by allowing the composite material to develop its ultimate tensile capacity without premature debonding from the concrete substrate. Experimental and analytical results demonstrate that the steel angle anchor system performs poorly due to an eccentricity which exists between the force within the FRP sheet and the reaction produced in the anchor bolts. However, the tube anchor system is shown to perform well in allowing the FRP

11 material to reach its ultimate capacity. Results from analytical models of both anchor systems correlate well with experimentally measured data. The FE models were able to accurately capture the prying action which plagued the steel angle anchor system as well as the limited deformation noted within experimental testing of the tube anchors system. The design methodology for the tube anchor systems is an accurate basis for continual research into the development of design equations and charts for the real-world application of this anchor system in the seismic retrofit and strengthening of RC shear walls using externally bonded FRP sheets. Acknowledgments Funding has been provided by the Canadian Seismic Research Network, Natural Sciences and Engineering Research Council Canada and Public Works and Government Services Canada. References 1. Grelle, S. V., & Sneed, L. H. (2011). An Evaluation of Anchorage Systems for Fibre-Reinforced Polymer (FRP) Laminates Bonded to Reinforced Concrete Elements. Proceeding of Structures Congress, Teng, J., Chen, J. F., Smith, S. T., & Lam, L. (2002). FRP-strengthened RC Structures. Journal of Engineering Structures, Lombard, J., Lau, D. T., Humar, J., Foo, S., & Cheung, M. (2000). Seismic Strengthening and Repair of Reinforced Concrete Shear Walls. Proceedings of 12th World Conference on Earthquake Engineering. 4. Hiotakis, S. (2004). Repair and Strengthening of Reinforced Concrete Shear Walls for Earthquake Resistance Using Externally Bonded Carbon Fibre Sheets and a Novel Anchor System. Master's Thesis, Carleton University, Department of Civil and Envirnomental Engineering. 5. El-Sokkary, H., Galal, K., Ghorbanirenani, I., Leger, P., & Tremblay, R. (2013). Shake Table Tests on FRP- Rehabilitated RC Shear Walls. Journal of Composites for Construction, Kalfat, R., Al-Mahaidi, R., & Smith, S. T. (2013). Anchorage Devices Used to Improve the Performance of Reinforced Concrete Beams Retrofitted with FRP Composites: State-of-the-Art Review. Journal for Composites for Construction 17:14, Pham, H., & Al-Mahaidi, R. (2004). Assessment of Available Prediction Models for the Strength of FRP Retrofitted RC Beams. Journal of Composite Structures Vol. 66, Kanakubo, T., Aridome, Y., Fujita, N., & Matsui, M. (2000). Development of Anchorage System for CFRP Sheet in Strengthening of Reinforced Concrete Structures. Proceeding of 12th World Conference on Earthquake Engineering, Cruz-Noguez, C. A., Lau, D. T., Sherwood, E. G., Lombard, J., & Hiotakis, S. (2012). Seismic Behaviour of RC shear walls with externally bonded FRP sheets- Part I: Experimental studies. Experimental paper, Carleton University, Department of Civil and Envirnomental Engineering, Ottawa. 10. Wong, P. S., and Vecchio, F. J. (2002). VecTor2 and FormWorks user's manual (Publication No ). Toronto, ON, Canada: University of Toronto, Department of Civil Engineering. 11. CSA A23.3 (1994). Design of Concrete Structures. Rexdale, Ontario, Canada: Canadian Standards Association.