Seismic Retrofitting of IBS Wall Connections with FRP Composites

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1 Fourth Asia-Pacific Conference on in Structures (APFIS 2013) December 2013, Melbourne, Australia 2013 International Institute for in Construction Seismic Retrofitting of IBS Wall Connections with Composites R.Vaghei, F. Hejazi 1, P. Khanzaei, H. Taheri, M. S. Jaafar, A.A. Abang Ali Housing Research Center, Department of Civil Engineering, Faculty of Engineering, University Putra Malaysia (UPM), Serdang, Selangor, Malaysia 1 Corresponding author (Tel: ; farzad@fhejazi.com ) ABSTRACT In the last two decade, Industrialized Building System (IBS) was promoted to enhance the importance of prefabrication technology rather than conventional method in construction. The components of IBS structure are floors, walls, columns, beams, and roofs which are assembled and erected on the site properly joined to form the final units. Among all the components, connection has the most influential role to join all elements together such as wall to wall, slab to wall and wall to foundation for transferring the earthquake lateral loads. In the present study, an efficient retrofit scheme for IBS wall connection has been developed in order to provide adequate strength and stiffness for wall connection to be resisting against the lateral seismic load. For this purpose, the inter-storey deformations of strengthened infill walls which are integrated to the boundary frame members is evaluated by numerical analysis. So, the analytical model of wall and connection is developed and retrofit design and analysis for actual deficient RC wall subjected to huge ground motion is conducted. From the analysis result, it was observed that application of the composite retrofit scheme reduced the damage induced to deficient wall by controlling story deformations. So in this case, it is possible to satisfy the collapse prevention performance state through an efficient and economical manner. KEYWORDS Industrialized Building System, Precast Concrete Connection,, FE Modelling INTRODUCTION There are different situations in which structures would require strengthening or rehabilitation due to the lack of stiffness, strength and durability. One of the most common situations where a structure needs strengthening during its lifetime is a seismic retrofit to satisfy current code necessities. However limited testing has been done on precast walls and their connections, many precast wall connection designs are mainly based on theory that does not adequately model the complex interaction between the concrete and connection material. These connections have been proved to be brittle (Hofheins et al. 2002), and because of their low strength, do not sufficiently absorb earthquake energy. Additionally, steel connectors are subjected to extreme levels of corrosion where this corrosion results in significantly decreased strength. Therefore, a reliable connection is needed for new and retrofit connection that will absorb earthquake energy and last the life of the structure. Recent developments in the composites industry have brought the use of fibre-reinforced plastics () into the construction market of civil structures. Because composites are light-weight and easy to install on site, they are considered to be the most favoured material in many strengthening applications. The overall cost of the whole strengthening job using materials can be as competitive as using conventional materials, in addition to being quick and easy to handle on site with minimum interruption to use of facility (Motavalli and Czaderski 2007). Moreover, materials have the potential to be viable alternatives to conventional steel joint connections because of their material properties that can give them a significant advantage over steel in terms of durability, and corrosion resistance (Bank 2006). In recent years, fibre reinforced polymer () composites have found increasingly wide applications in civil engineering, both in the retrofit of the existing structures and in new construction. composites consist of

2 fibres embedded polymeric resins and possess several advantages over steel, including their high strength-toweight ratio and excellent corrosion resistance. As a result, the use of composites as externally bonded reinforcement for the retrofit of structures has become very popular in recent years (Teng, Chen et al. 2002; Teng, Chen et al. 2003). Limited testing on precast wall connectors has been carried out. This is particularly true of fibre reinforced plastic () composites. Vertical shear key connections in precast walls were studied by (Chakrabarti, Nayak et al. 1988). The research developed equations to predict the ultimate shear load the joint could withstand. These equations depend on the strength of the infill concrete, amount of reinforcement in the joint, the shear area and effects due to shrinkage and creep. Chajes et al (1996) (Chajes, Finch Jr et al. 1996) investigated the bond strength between composites and concrete. Several parameters such as surface preparation, adhesive type and concrete strength are considered in this study. From their experimentation it was determined that, after a certain development length, no increase in failure load can be achieved. (Volnyy 1998) tested ten precast wall assemblies with C connections. Variations in lay-up, shear area and surface preparation were evaluated. The results indicated that the C connection is feasible. In particular, it was found that, the development length of the C depends upon the geometry and stiffness of the connection. A year after, an FE simulation procedure is introduced by(vecchio and Bucci 1999) for repairing/retrofitting concrete structures that consider engaging and disengaging elements in a structure, which models the sequence of implementation of the repair/retrofit scheme. The procedure was validated with a 2D numerical study of RC beams and slabs retrofitted by sheets and of RC shear walls retrofitted by concrete replacement. Furthermore, a perfect bonding between the retrofitting and the concrete was assumed in this study; therefore, debonding of the was not captured. (Aprile, Spacone et al. 2001) conducted numerical analysis of RC beams and slabs strengthened with externally bonded fibre-reinforced polymer () and steel plates. The FEs incorporated a strengthening component, which simulated bond slip. This study consisted of 1D nonlinear FE with fibre section models capable of predicting the strength-displacement response of retrofitted structures. Other numerical studies employed 3D FE models with perfectly bonded.(arduini, Di Tommaso et al. 1997), (Kachlakev, Miller et al. 2001), and(hii and Al-Mahaidi 2006) conducted studies of beams strengthened by subjected to monotonic loading. The models fell short in predicting the failure mode of the and the hysteretic response of the strengthened structure. Among the all research available in the literature just few studies have focused on retrofitted RC shear walls and even fewer on retrofitted RC shear walls subjected to cyclic loading. In addition, simulation of retrofitting details such as anchorage of externally bonded materials has not been extensively investigated, which is crucial to properly assess seismic behaviour. Therefore, the main aim of this paper is to provide guidance to design engineers and researchers in developing simple modelling procedures for various practical retrofitting strategies that improve the response of RC shear walls. Although this study focuses on static analysis, the modelling procedures are equally applicable to dynamic analysis. The retrofitting techniques investigated by considering all detail such as cross section and number of hooks, external bonding of sheets and IBS walls, and longitudinal bar in cast in-situ concrete as a connection filling. DESIGN AND DETAILING OF WALL CONNECTIONS In this study the influence of layers on the performance of IBS walls in terms of stress, maximum displacement and capacity are studied. Consequently different features are conducted which are mentioned as and also are shown in Figure1: (i) (ii) (iii) Case A: Connection (wall to wall connection without ) Case B: Connection (wall to wall connection with 1 layer of ) Case C: Connection (wall to wall connection with 3 layers of )

3 Left Co nn ect io Co nn ect io Left Connection FINITE ELEMENT MODELLING Figure 1: Three different types of connections The Panels including male and female of dimensions 1.2mx0.6m in XY plane, 0.125m thick in YZ plane and the gap between those s has 1.2mx0.15m in side view and 0.125m thick which both are supported on the ground. The elements types used in this study are C3D8R for concrete, T3D2 for reinforcement and S4R for modelling. The IBS walls are pinned at the bottom; all the translational degrees of freedoms are constrained. Loading above the IBS wall performed as a lateral displacement which is distributed at each node on the top surface of the left wall. Material Properties Material properties of concrete, steel and are given in Table1. Table 1: Material Properties and Mechanical Behaviour Material Concrete Steel Density (T/mm 3 ) 2.4E E E-009 Elasticity Modulus (MPa) Poisson s Ratio NUMERICAL ANALYSES OF IBS WALL CONNECTIONS WITH COMPOSITES The behaviour of IBS walls depends on the modes of connection deformation as well as the interface bonding/binding properties and also on how accurately it is accounted for the analysis. There can be different interface conditions between IBS walls and the connection. Attention is focused herein on determining the response of IBS connection under imposing loads with and without layers. Numerical analysis are performed using Abaqus, a 3D nonlinear FE program applicable to structure with 6DOF subjected to nonlinear static loading. The analysis has been carried out for the wall to wall connection subjected to lateral loading. The convergence criteria used for the analysis is displacement with the tolerance of To perform the pushover analysis, the values of displacement vs. time curve are applied on structure as a lateral displacement on topside of left IBS wall. Loading and boundary conditions used in this study are shown in Figure2.a. Figure2.b depicts the meshes which are generated and element size assumed 20mmx20mm for whole model. Interactions between concrete and reinforcements are assumed as embedded and the interaction of layers and concrete assumed as shell to solid coupling. The first step in the analysis of a moment resisting reinforced concrete frame is the development of a model of the actual structure. This process is schematically illustrated in previous section which shows how the IBS walls, connection and of the actual structure are modelled by the elements presented in the previous sections. To determine the response of the structure to imposing loads, the nonlinear geometry analysis should be performed which is applied step-by-step starting from the unloaded state. Once a particular step is completed the displacement and load increments are added to the corresponding values at the end of the previous step.

4 (a) (b) RESULTS AND DISCUSSION Figure 2: (a) Loading and Boundary Condition, (b) Mesh Generation Based on the pushover analysis on IBS wall which are subjected to lateral loading within 20 seconds until the displacement for the top node reach to 10mm, the capacity curve for maximum top displacement vs. shear base is derived in Figure3. It can be seen that, when the more layers of apply on the IBS walls, the more capacity of IBS connection will achieve. Furthermore, the absolute stresses for the wall to wall connection in IBS system are captured in Table 2. Three different cases are compared to each other in terms of stress values including S 11, S 22, S 33, S 12, S 13 and S 23. Case A, case B and case C are IBS connections without, with 1 layer of and 3 layers of respectively. Figure 3: Pushover Curve Comparison Table 2: Absolute Stresses in 6 Degrees of Freedom TYPE of Stress Reduction Stress Stresses B&A(%) C&A(%) S 11 (MPa) S 22 (MPa) S 33 (MPa) S 12 (MPa) S 13 (MPa) S 32 (MPa) Reduction Table 2 shows the reduction percentage of stress when was provided in 1 layer and 3 layers versus to the connection without. As the table shows, it can be clearly seen that the maximum values are belong to

5 contact surface of connection (S 12 and S 23 ) due to the friction of interface between contact surfaces and free surfaces has less value in comparison with contact surfaces (S 13 ). Moreover, the highest percentage of stress reduction when 1 layer of and 3 layers of provided in compare to common connection are belong to principal stress in x direction (11.72%) and shear stress in xz surface (20.7%) respectively. Figure 4: Stress Distribution on Layer of Figure 4 shows the mises stress distribution on the layers for case B and case C. Consequently, outcomes reveal that the more layers, the less stress value will generated in layers from 143.9MPa to 140.9MPa as well as in the connection and this phenomenon help to rise up the capacity of IBS walls due to more energy dissipation that can absorb by fibre polymers. Modelling of failure and fracture has become one of the controversial issues in structural mechanics. Among constitutive models for concrete, Concrete Damage Plasticity (CDP) has been chosen to identify the crack propagation. In order to account for the large inelastic deformations that can take place at the base of the IBS walls as well as in the contact zone between the IBS wall and connection. In this case Figure 5 shows the plastic strain occurred at the bottom of the IBS wall and along the interface simultaneously. Based on the contour results it can be mentioned that the crack will be occurred sequentially from the left concrete and then it moves toward the connection and finally reaches to right. Cracks are improved by inserting layers, furthermore the cracks move towards the control area. Figure 5: Absolute Plastic Strain on the IBS walls and Connections Distributions of displacement magnitude for different cases are presented in Figure 6. Figure 6: Distribution of Displacement Magnitude for Different Cases

6 It can be statistically proven that by applying more layers, the flexibility of IBS structure in terms of displacement almost remains constant. Figure 6 presents that the maximum displacement magnitude in Case A from 11.5mm is increased to 11.74mm and 11.88mm in case B and case C respectively. Based on the outcomes, it can be said, the horizontal layers doesn`t have significant effect on the flexibility. Conclusion In the present study, an attempt has been made to evaluate effect of implementing layer in precast wall to wall connections. The analysis result reveals that the stress capacity of wall to wall connection is increased by adding layers. So based on result, the maximum stress in model with three layers is less than model with two and one layer which is proved that more capacity is obtained. The point of initiation and evolution of fracture is estimated by using Concrete Damage Plasticity model which enables to proper defining of the failure mechanisms in concrete elements. The crack propagations in the IBS walls and connection indicated that cracks mostly occurred at the bottom of the IBS wall and along the interface simultaneously. Furthermore, the applying lateral inplane loads leads a few cracks on the bond between the IBS walls and their connection in model with two and three layer in comparison which shows adding layers as a retrofitting demand is resulted to cracks reduction. Based on the deformation results, it can assert that implementing layers shows low effectiveness on energy dissipation in walls connection. So, the role can be ignored in wall connection when the lateral inplane load is applied to wall. However, it predicted that the layer is more effective in the out-of-plane lateral loading especially when the load has eccentricity which causes the inplane torsion. ACKNOWLEDGMENTS This work received financial support from Housing Research Center of UPM and NAIM Company and was further supported by the Ministry of Higher Education of Malaysia under FRGS Research Projects No and No , these supports are gratefully acknowledged. References Aprile, A., E. Spacone, et al. (2001). Role of bond in RC beams strengthened with steel and plates. Journal of structural engineering 127(12): Arduini, M., A. Di Tommaso, et al. (1997). Brittle failure in plate and sheet bonded beams. ACI Structural Journal 94(4). Bank, L. C. (2006). Composites for construction: Structural design with materials., John Wiley & Sons. Chajes, M. J., W. W. Finch Jr, et al. (1996). Bond and force transfer of composite-material plates bonded to concrete. ACI Structural Journal 93(2). Chakrabarti, S., G. Nayak, et al. (1988). Shear Characteristics of Cast-in-Place Vertical Joints in Story-High Precast Wall Assembly. ACI Structural Journal 85(1). Hii, A. K. and R. Al-Mahaidi (2006). An experimental and numerical investigation on torsional strengthening of solid and box-section RC beams using C laminates. Composite structures 75(1): Hofheins, C. L., Reaveley, L. D. et al. (2002). Behaviour of welded plate connections in precast concrete s under simulated seismic loads. PCI journal 47(4): Kachlakev, D., T. Miller, et al. (2001). Finite element modelling of reinforced concrete structures strengthened with laminates. Final Report SPR 316. Motavalli, M. and C. Czaderski (2007). composites for retrofitting of existing civil structures in Europe: State-of-the-art review. International Conference of Composites & Polycon., American Composites Manufacturers Association. Tampa, FL, USA. Teng, J., J.-F. Chen, et al. (2002). : strengthened RC structures. : Strengthened RC Structures, pp ISBN Wiley-VCH, January Teng, J., J. Chen, et al. (2003). Behaviour and strength of -strengthened RC structures: a state-of-the-art review. Proceedings of the ICE-Structures and Buildings 156(1): Vecchio, F. and F. Bucci (1999). Analysis of repaired reinforced concrete structures. Journal of structural engineering 125(6): Volnyy, V. A. (1998). Precast shear wall assembly with carbon fibre composite connections. Department of Civil and Environmental Engineering, University of Utah.