Effect of FRP strengthening on the behavior of shear walls with opening

Transcription

1 CICE The 5th International Conference on FRP Composites in Civil Engineering September 27-29, 2010 Beijing, China Effect of FRP strengthening on the behavior of shear walls with opening M. Asfa (Asfa.mohaad@gmail.com) Graduate Student of Structural Eng., Yazd University, Yazd, Iran D. Mostofinejad Department of Civil Engineering, Isfahan University of Technology (IUT), Isfahan, Iran N. Abdoli Department of Civil Engineering, Yazd University, Yazd, Iran ABSTRACT: In recent decades, Fiber Reinforced Polymer (FRP) strengthening of concrete members is known as a unanimously acceptable method. An example of a proper application of this method is strengthening of shear walls with openings using FRP strips around them, especially at corners with high stress concentration, where FRP strips keep cracks closed and prevent brittle shear failure to the edges. The efficiency of FRP strengthening on shear walls and around the openings was examined in the current study using finite element (FE) software (ABAQUS). Available laboratory test results on concrete shear walls under lateral loads were used to calibrate the software and verify its application. Then, boundaries of the openings in the wall strengthened with FRP composites in different configurations and nonlinearly were analysed. The results showed considerable effectiveness of FRP strengthening on the overall behaviour of the walls with opening. 1 INTRODUCTION Shear walls are structures which provide resistance against lateral loads. In some cases, due to the architecture and installation needs, creating opening in the walls is inevitable. The openings divide solid wall into two or more separate walls that are connected to each other by special beams called coupled beams, depending on the strength and stiffness of the beams, the performance of separated walls changes to a solid one to some extent. Creating openings leads to severe loss of shear and flexural resistance. In recent decade FRP was used for shear, bending and in some cases twisting strengthening of beams and columns. Today, because of easy installation, high strength to weight ratio and resistance to corrosion, use of fibers in the RC walls and slabs due to the existing opening are also prevalent. These fibers can prevent diagonal cracks and brittle failure around the openings. In the field of coupled wall FRP strengthening, Meftah (2007a), using a numerical method, verified the effect of fiber reinforced polymer on the seismic behavior improvement of coupled walls. He applied flexural reinforcement on piers in bottom, middle and top the of wall's piers. It was concluded that the greatest effect in reducing the lateral displacement is to strengthen bottom of the wall and close to foundation. He also changed parameters such as FRP thickness and strengthened area of a damaged coupled wall, to evaluate the effect of FRP flexural retrofitting on frequency of first seven seismic modes on a 20 floors coupled shear wall. The results showed that increasing the two parameters leads to increasing the frequency of the wall (Meftah 2007b). Nagy (2007) also, studied the shear and flexural FRP strengthening effect on several damaged coupled walls, with different opening arrangements, under cyclic loading. They concluded that the load bearing capacity of all samples before damages is recovered. Antoniadis (2007) also retrofitted 11 models of coupled walls using FRP, after loading them up to failure. They evaluated the FRP effect on energy dissipation and lateral displacement of the wall. Dimitri (2009) investigated the FRP strengthening effect on the prefabricated panels with cut-out openings, before and after applying load. The results showed that, fiber reinforcements increased the load bearing capacity of both damaged and undamaged walls. In this paper, verification of software finite element modeling, using existing laboratory test has been investigated. And then, effect of proposed FRP retrofitting method on a shear wall with opening has been studied. By sticking FRP strips around the opening of coupled walls, an effective way to im- 1

2 prove overall behavior of the wall and reducing damages has been developed in this method. modulus of 230GPa and tensile strength of 3500MPa. 2 STUDY PROCESS For software calibration, two experimental models including a FRP strengthened beam (Harries 2007) and a coupled shear wall (Lu & Chen 2005) are verified. Then, FRP strengthening effect around openings of the coupled wall is investigated. The coupled wall is designed based on the high-rise building codes in China JGJ-391 (1991). It is a scaled model of five floors coupled wall and was tested under gravitational and lateral load. According to ACI-318 (2005), if the ratio of the coupled beam length to its height is less than 2, using diagonal reinforcement is required. The ratio in the studied wall is 1.6 and therefore, applying diagonal reinforcement in the coupled beams is necessary and the wall needs retrofitting. In this study, the wall behavior, in two cases, with the diagonal bars in coupled beam according to ACI-318 and excluding them is verified and then the latter is retrofitted using proposed methods and compared with the other walls in order to verify the efficiency of retrofitting methods, to improve the wall behavior. 5 FINITE ELEMENT PROCESS 5.1 Elements For this study analysis, ABAQUS software which can solve linear and nonlinear FE problems is used. For concrete, bar and FRP layer modeling, 8 node cubic elements (C3D8), 2 node rod element (T3D2) and 4 node Shell element (S4) are used, respectively. Figure 1. Parameters of the FRP strngthened beam (Harries 2007). 3 MODEL SPECIFICATION The characteristics of the beam and shear wall including dimensions with the type and arrangement of existing reinforcement are shown in Figure 1 and 2, respectively. Also reinforcement specification of coupled beam, according to the ACI-318, is illustrated in Figure MATERIALS STRENGTH 4.1 Beam The beam includes #4, #6 steel bars with the yield stress of 429MPa. The 28-day cylindrical compressive strength and Young's modulus of concrete is 23.3 and 22680MPa respectively. FRP strip thickness is 1.4 with elasticity modulus of 155GPa and tensile strength of 2800MPa. 4.2 Coupled wall As shown in Figure 2, the wall steel reinforcements include Φ4, Φ6 and Φ8 with the yield stress of 796.9, and 278.3MPa, respectively. The 28- day cylindrical compressive strength, tensile strength and Young's modulus of concrete were measured 35, 2.63 and 31100MPa respectively. Thickness of FRP layers is 0.16 with elasticity Figure 2. Parameters of the coupled wall (Lu & Chen 2005) Figure 3. Parameters of the coupled beam with diagonal bars according to the ACI Boundary conditions For simulation of restraint in software, the degree of freedoms are limited according to the type of restraints. Because of the syetry in the geometry 2

3 and loading condition, in order to reduce the analysis time and prevent out-of-plane displacement, a quarter of beam and half of the wall is modeled by applying syetrical boundary conditions. Wall loadings include first, a total 200kN vertical load on the wall piers and then applying the lateral load gradually up to wall failure through an upper beam attached to the wall top. As illustrated in Figure 1, beam loading is applied at mid span monotonically. 5.3 Materials specification Steel is introduced as an elastic-plastic material to the software and a completely linear and elastic material is defined for FRP. Concrete is defined using the Concrete Damage Plasticity option which exists in software library. This model is defined according to the Lubliner (1989) studies and was developed by Lee & Fenves (1998). In this model the development of yield surface is controlled by two strainhardening variables, one in tension and the other in pressure (Habbit et. al. 2008). Compressive stress-strain curve of concrete should be obtained from test result, but if it was not available, equation introduced by various researchers, can be used. In the current study, Hognestad equation in which the relationship of stress-strain is defined according to concrete compressive strength, strain related to maximum stress or, elastic modulus of the concrete (MostofiNejad 2009), is used. Effects associated with steel-concrete interface, such as bond-slip or dowel action, are approximately introduced to the concrete by applying strainhardening characteristic in tension to the concrete material. This characteristic simulates load transfer from concrete to the reinforcement through cracks. After concrete cracking under tensile stress, the stiffness of concrete does not reach to zero at once and it still transmits shear force, due to aggregate inter-locking and friction (Habbit et. al. 2008). In this study a complete bond between FRP and concrete is considered in the models and debonding is controlled according to the ACI-440.2R (2002), it limits the FRP strain to a specified extent for preventing FRP debonding. Figures 4 & 5 present experimental and analytical diagram of load-displacement of the beam and coupled wall respectively. The failure mode of the beam is FRP debonding and then flexural failure, and for the wall is flexural-shear mode. Obviously, the results are in good agreement with each other for both failure modes and load-displacements curve, expressing that the software modeling results are reliable. To evaluate the primary effect of diagonal bars in coupled beam on wall behavior, they were implemented in the coupled beam of the studied wall according to ACI-318 and nonlinearly analyzed. In Figure 6, the obtained results from software analysis of wall with diagonal bars and without them were compared. The results showed that diagonal bars in coupled beam increased ultimate load bearing capacity about 22 percent from the 145kN to 185kN and they also increased the ultimate failure strain up to 32 percent from 34 to 50. Generally, this type of bars improved the overall behavior of the shear wall. Figure 4. Comparison of load-displacement diagram of beam, from software FE test and experimental result. Figure 5. Comparison of lateral load-displacement diagram of wall, from software FE test and experimental result. 6 COMPARISON OF THE EXPERIMENTAL AND SOFTWARE MODELING RESULT Figure 6. Comparison of lateral load-displacement diagram, from software FE test of walls with and without diagonal bar. 7 FRP RETROFITTING OF WALL As depicted in Figure 7, three different methods for FRP retrofitting of wall are presented as follows: a) Connection of FRP sheets around opening corners 3

4 justified at the angle of 45 in 3 layers (Cw-45). b) Connection of FRP sheets parallel to the opening edges at the angles of zero and 90 in 2 layers (Cw- 90). c) A shape combination of two previous methods, in 1 and 2 layers (Cw-45,90). To compare the efficiency of retrofitting methods, in all cases the volume of FRP sheets are considered the same (324 cm on each side of the wall). 3 In Table 1, the FRP sheet dimensions are presented. Cw-45 Cw-45,90 Cw-90 Figure 7. FRP Retrofitting method, used in this study. Table 1. Dimensions of FRP Layers for wall strengthening. Type Horizontal strip 45 strip Vertical layers* a b c * This layer is extended 3150 from 275 above the foundation level to the 175 above the top opening of wall. 8 RESULTS Figure 8 presents load-displacement diagram from FE analysis of FRP reinforced walls, compared with unretrofitted walls in both cases, with diagonal bar (SWD) and without it (SW). As expected, the diagrams' slopes in the linear region are in excellent agreement, indicating that the strengthening method does not affect the shear walls stiffness in linear region and before the onset of considerable cracks. In non-linear region, however, the curves' slopes change based on their strengthening case. All of the studied wall models failed in a flexural-shear mode with no FRP debonding. Figure 8. Lateral Load-Displacement diagram of walls with different type of strengthening method. 9 CONCLUSION Among the FRP strengthened walls, the Cw-45 gained the most stiffness, while the highest ductility was belonged to Cw-45,90. In terms of efficiency of FRP strengthening methods, generally all the techniques increased the strength and ductility of the coupled wall. However, the most efficient method in compensation of lack of diagonal bar was Cw-45 which increased the ultimate load bearing capacity to 186kN, while this amount for Cw-45,90 and Cw- 90 were 185kN and 171kN, respectively. It can be concluded that using FRP strips at the angle of 45 in the corners of the opening can keeps cracks closed and thus increases the stiffness and ultimate load bearing capacity. On the other hand, vertical and horizontal FRP strips on the coupled beam and around the wall openings increase the ductility of coupled wall. Therefore, the combination of diagonal, vertical and horizontal FRP layers (Cw-45,90), leads to an efficient strengthening method that improves the overall wall behavior. 10 REFERENCES ACI Coittee Building Code Requirements for Structural Concrete (ACI ). American Concrete Institute, Farmington Hills. ACI Coittee 440.2R Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures. Antoniades, K.K., Thomas, N. & Kappos, A.J Evaluation of hysteric response and strength or repaired R/C walls strengthened with FRPs. Engineering Structures 29: Demeter, I., Nagy-György, T., Stoian, V., Dãescu, C. & Dan, D Precast RC wall panels with cut-out openingsretrofitted by CFRP composites. NOVI SAD. Harries, K.A., Reeve, B., & Zorn, A., Experimental evaluation factors affecting monotonic and fatigue behavior of FRP concrete bond in RC beams, ACI Structural Journal, 104-S62, Nov-Dec. Hibbitt, Karlsson & Sorensen Inc ABAQUS theory manual, user manual and example manual. Version 6.8, Providence, RI. Lee, J & Fenves, GL Plastic-damage model for cyclic loading of concrete structures. J Eng Mech 124(8). Lu, X. & Chen, Y Modeling of Coupled Shear Walls and Its Experimental Verification, Journal of Structural Engineering, 131(1). Lubliner, J., Oliver, J., Oller, S & On ate E. 1989, A plasticdamage model for concrete, Int J Solids Struct, 25(3): Meftah, S. A., Yeghnem, R., Tounsi, A. & Adda Bedia, E. A. 2007a. Seismic behavior of RC coupled shear walls repaired with CFRP laminates having variable fibers spacing. Construction and Building Materials Structures. 21. Meftah, S. A., Yeghnem, R., Tounsi, A. & Adda Bedia E. A. 2007b. Lateral stiffness and vibration characteristics of 4

5 damaged RC coupled shear walls strengthened with thin composite plates, Building and Environments. 42. Ministry of Construction. 1991, Code for design and construction of reinforced concrete high-rise buildings. JGJ Chinese Building Industry Press, Beijing (in Chinese). MostofiNejad, D Reinforced Concrete Structures. Vol th ed. Esfahan: Arkane Danesh. Nagy-Gyorgy, T., Stoian, V., Dan, D., Dãescu, C., Diaconu, D., Sas, G. & Mosoarca M Research Results on RC Walls and Dapped Beam Ends Strengthened with FRP Composites. FRPRCS-8, Patras, Greece. 5