Behaviour of Vertical Loop Bar Connection in Precast Wall Subjected To Shear Load

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Behaviour of Vertical Loop Bar Connection in Precast Wall Subjected To Shear Load
  Australian Journal of Basic and Applied Sciences , 8(1) January 2014, Pages: 370-380 AENSI Journals Australian Journal of Basic and Applied Sciences  Journal home page: Corresponding Author:  Nabila Rossley,   Department of Civil Engineering, Faculty of Engineering, Universiti Putra Malaysia, Selangor, Malaysia. Ph:06 0132646224. Behaviour of Vertical Loop Bar Connection in Precast Wall Subjected To Shear Load 1  Nabila Rossley, 1 Farah Nora Aznieta Abdul Aziz, 2 Heng Chiang Chew, 3  Nima Farzadnia   1 1Department of Civil Engineering, Faculty of Engineering, Universiti Putra Malaysia, Selangor, Malaysia. 2  Baxtium Construction Company, 63A, Jalan BPU2, Puchong Utama,47100 Puchong, Selangor, Malaysia. 3  Housing Research Centre, Faculty of Engineering, Universiti Putra Malaysia, Selangor, Malaysia. ARTICLE INFO ABSTRACT  Article history: Received 21 November 2013 Received in revised form 18 January 2014 Accepted 29 January 2014 Available online 25 February 2014  Keywords: Loop bar, shear loading, shear stress, wall to wall connection . Background: Connection design is one of the most important considerations for a successful construction of precast reinforced concrete structures in terms of the structural behaviour  . The main purpose of the structural connection is to transfer forces  between the precast concrete elements in order to obtain structural interaction once the system is loaded. Therefore, the structural connections should design properly as the same for the precast concrete elements. Objective: The main objectives of this experimental study are: a) to determine the maximum shear stress and b) to investigate failure mode of vertical connection using loop bars under shear load. Results:  The average maximum shear stress obtained from the test is 1.83 N/mm 2 , which exceeded the allowed maximum shear stress of 1.8 N/mm 2 . From the visual observation, most concrete crushing and spalling, were concentrated at the joint and the adjacent wall. Basically, the wall panel was failed under brittle failure mode, however the lab test proved that the loop connection was able to give crack lines at the interface before failure occurred. The critical stage was when inclined cracks formed near the loop bar connection as it almost reached its failure stage . Conclusion: In economic point of view, use of 6 mm diameter loop bar can be considered in order to achieve 1.8 N/mm 2  for prototype scale of the  precast panels that typically used for medium rise construction. To increase the ductility of the connection, the ratio of the transverse bar, overlapping length and concrete joint strength should be increased.  © 2014 AENSI Publisher All rights reserved . To Cite This Article:  Nabila Rossley, Farah Nora Aznieta Abdul Aziz, Heng Chiang Chew, Nima Farzadnia., Behaviour of Vertical Loop Bar Connection in Precast Wall Subjected To Shear Load.  Aust. J. Basic & Appl. Sci., 8(1): 370-380, 2014   INTRODUCTION In recent years, the construction trend has moved from the traditional to prefabricate manufacturing (Nor et al ., 2012; Lovell & Smith, 2010; Tam et al ., 2007). In Malaysia, according to the Survey on the Usage of Industrialised Building Systems (IBS) in Malaysian Construction Industry in 2003, the most popular IBS types and structure components were steel beams and columns as well as portal frames and cold-rolled roof trusses followed by precast concrete framing, panel and box systems. Nowadays, due to the high cost of structural steel sections, precast concrete construction has gained popularity among engineers and architects (Abd Rahman et al ., 2006). The precast concrete structural systems have shown advantages in concrete structure constructions such as,  providing easy standardisation, cost saving, efficient use of materials, speedier construction and improved quality control (Megally, S., F. Seible, M. Garg, 2002). However, the conventional wet cast in situ constructions, which relatively require more construction space at site, labour, longer pending time for concrete curing and hardening process and poorer quality control seem to be replaced with the precast concrete systems at a slow pace, but at a wide scale (Tiong et al ., 2011). There are three types of precast building systems which are skeletal structure, portal frame, and wall frame (Farsangi, 2010). Precast skeletal structure referred to as skeletal because it resembles a skeletal of rather small  but very strong components of columns, beams, floors, staircases and sometimes structural walls (Elliot, 2002). A second category of precast building is the portal frame which consisting of columns and roof rafters. This type of system is normally used in industrial buildings and warehouses. This research merely focussed on the last category of the precast structure, wall frame, which consists of walls and slabs. This system depends on load  bearing walls to resist vertical and horizontal loads. The precast wall frame structures provide an economical solution when compared to cast in situ walls and can increase the speed of the construction(Freedman 1999; Bhuskade & Mundhada, 2011). Precast wall frame structures may often be faster to build especially if the external walls are furnished with thermal insulation and decorative finishes at the factory (Elliot, 2002).  371 Nabila Rossley et al  , 2014   Australian Journal of Basic and Applied Sciences, 8(1) January 2014, Pages: 370-380 Connection design is one of the most important considerations for a successful construction of precast reinforced concrete structures in terms of the structural behaviour (Tiong et al ., 2011; Sadrnejad & Labibzadeh, 2006; Loo & Yao, 1995). The main purpose of the structural connection is to transfer forces between the precast concrete elements in order to obtain structural interaction once the system is loaded. By the ability to transfer forces, the connections should secure the intended structural behaviour of the superstructure and the precast subsystems that are integrated in it. Therefore the structural connections should be regarded as an essential part of the structural system and they should be designed properly as the same for the precast concrete elements. For the precast wall frame structures, the connections between load bearing wall panels are essential parts of the structural support system as the stability of the structure relies on their performance. According to Freedman (Freedman, 1999), it is desirable to design load bearing precast concrete structures with connections which allow lateral movement and rotation. Two methods can be used to tie precast wall panels with other panels namely: continuity reinforcement bar and protruding bar. Continuity reinforcement bars can be utilized when steel bars are placed across the joint by which the shear forces can be transferred between elements by dowel action. Example of continuous tie bars is the spliced steel reinforcing bars. However, this connection is susceptible to corrosion which could lead to deterioration of the strength of the structure if exposed to sodium chloride present in marine environments (J. Tibbetts et al ., 2009). Furthermore, in Malaysia, the splices steel available in the market is proprietary and this type of connection can be only purchased from foreign companies. Therefore, the cost of this connection is higher compared with protruding bar. Due to that, this type of connection is not widely used in Malaysia. Protruding tie bar is used to connect the walls by bolting, welding or loop connection filled with grout or concrete. In bolted connections the anchor precast concrete walls are connected directly to the adjacent panels (Ertas et al ., 2006). Bolted connections could transmit a combination of moment, shear force and axial force  between the panels(Mansur et al ., 2010). The main advantage of this connection is that this connection can be made immediately but it has restricted tolerance required for mating, in which oversize holes are not allowed  because of a significant slippage of the bolt (Yu et al ., 2011). Welded connections can be used to connect elements by heating the protruding bars. The behaviour of the welded connections is satisfactory, but the construction of these specimens requires accurate welding of the reinforcement (Ertas et al ., 2006). However, the bond of steel bars could be damaged by heat generated from welding and may result in cracking of the adjacent precast concrete, so the use of welded connections needs to  be minimised (Ochs, J. E., 1993; Stanton et al ., 1991). Naito et al . (2012) found out that welding through surface wetness has the potential to increase micro discontinuities and create visible cracking. Malaysia experiences wet and humid tropical climate throughout the years (Jamaludin & Jemain, 2007) and hence welding connection may need to be minimised. Other than that, the quality of the weld is totally dependent on the skill of the welder. As for the loop bar connections, loop bars that are protruding from the elements can be connected by lap splicing in the intermediate joint. The connection is activated as the joint is filled with grout or concrete. The splitting effect is large in the plane of the loop. To prevent premature brittle failure of the connection, transverse reinforcement should be placed through the overlapping part of the loop (CEB-FIP, 2008). In this research,  protruding reinforcement with loop bar was chosen because the connection is easy to fix on site due to the higher tolerances allowed. Other than that, it also provides protection for the loop bar from corrosion (Hao, 2004). It is well established that every precast concrete element has to transfer compressive forces such as self weight and live loads down to the foundations. Lateral loads change the compressive forces into tensile forces, or horizontal shear load. So far some studies targeted the behaviour of different wall to wall connections but they were merely concentrated on the behaviour of the connection subjected to lateral shear load due to earthquake induced loads (Arturo et al ., 1994; Perez et al ., 2004; Soudkl et al ., 1996). Shear force can also be transferred across joints where the structure has differential settlement or uneven load (Zheng & Burgoyne, 1994). Until today, no research has been conducted to investigate loop connections under shear loading. The overarching purpose of this study is: a) to determine the maximum shear stress and b) to investigate the  behaviour of vertical connections using loop bars in medium rise constructions under shear load. The main  purpose of determining the maximum shear stress is to make sure that shear loads could be effectively transferred between precast walls through in situ joints. Other than that, it is important to understand its  behaviour in terms of mode of failure since the connection plays a vital role in structures stability.  Experimental Work: Specimen Detailing: This experimental consists of two identical specimens denoted as WWC1 and WWC2 throughout the study. Each specimen consisted of two walls connected together using loop bars. The height, width and thickness of wall panels were 1200 mm, 600 mm and 125 mm, respectively. The dimensions correspond to a prototype scale of the precast panels typically used for medium rise construction. The diameter of loop bars was 8 mm. Bar with  372 Nabila Rossley et al  , 2014   Australian Journal of Basic and Applied Sciences, 8(1) January 2014, Pages: 370-380 a diameter of 10 mm were inserted into the loops and the gap between the walls was filled with concrete. A double layer steel reinforcement mesh (BRC-7) with dimension of 200 mm x 200 mm vertically and horizontally was used for both walls. The detailing for wall to wall connection is shown in Figure 1 and 2. Fig. 1: Top view of wall to wall connection. Fig. 2: Front view of wall to wall connection. Test Material: The specimen was ready-mixed concrete with grade of 30 N/mm 2 in both walls and concrete joint. High-tensile reinforcements were used to connect the walls side by side. Steel reinforcement mesh was used as a replacement of reinforcement bars in both precast walls to resist the flexural and shear induced on the wall. Loop bar of 10 mm and the transverse bar of 12 mm placed at the centre inside the loop were used. For each type of rebar, four tensile strength tests were done and average value of the test results was taken as shown in Table 1. Table 1: Properties of reinforcements. Type Diameter (mm) Cross Section (mm 2 ) Yield Strength (N/ mm 2 ) Tensile Strength (N/ mm 2 ) BRC 7 38.49 467.25 496 Loop bar 8 78.54 483.25 549.75 Transverse Bar 10 113.01 586.25 607 Sample Preparation: The construction process of the specimen was divided into two stages. Firstly, the precast concrete components were cast and secondly, the precast components were assembled with cast in situ concrete. Plywood was used as a formwork. The formwork was greased prior to concrete casting, to prevent concrete moisture loss and to ease up formwork removal. Subsequently, the BRC steel fabric was installed inside the formwork with spacer blocks of 20 mm thick to define the nominal concrete cover. The construction of the specimen was started by preparing the formwork, pouring the concrete and then followed by 28 days curing period. Test set-up: In order to determine the shear stress at the interface of the connection, the wall was rotated to a horizontal  position, and a shear loading was applied horizontally to the top panel. Top panel and bottom panel are denoted as wall 1 and wall 2 respectively. To prevent any unwanted movement of the bottom panel, wall 2 was restrained by a fixed support while wall 1 was set free by putting a roller support. The main reason the roller support was used is to allow the top wall to move in order to determine the displacement or slip of the connection in the direction of the applied load. The test setup is shown in Figure 3.  373 Nabila Rossley et al  , 2014   Australian Journal of Basic and Applied Sciences, 8(1) January 2014, Pages: 370-380 A load actuator was positioned horizontally in order to apply the shear load on the sample. The load actuator was bolted to a strong wall. To transfer the load accurately, the steel plate was placed in between the actuator and sample so that the load could be applied at the interface between the joint and wall. A total number of three Linear Variable Differential Transducers (LVDT) were placed on both side of the wall panels in order to record the displacement when the load was applied to the specimen. The location of LVDT is shown in Figure 4. LVDTs 1, 2 and 3 were used to determine the displacement of specimen once load was applied. In this test, the load was progressively increased up to the failure of the specimens when no further load could be sustained. At every load interval, the crack patterns, width and length of cracks were marked. Damage crack  pattern was highlighted during testing and captured using a camera. Fig. 3: Experimental set-up. Fig. 4: Location LVDT on the specimen.   A total number of three strain gauges were attached to the loop bars as shown in Figure 5. Strain gauges were installed to read the changes in strain of reinforcement due to alternate tension and compression stress during the experiment (Arturo et al . 1994). Fig. 5: Location of strain gauges on loop steel bars. Ten strain gauges were also placed on the concrete surface of sample WWC1 as shown in Figure 6. For further clarification of the connection behavior, five additional concrete strain gauges were placed on the concrete joint in sample WWC2 to measure the strain at the damage area as shown in Figure 7. This was  because the formation of the shear cracks was expected to occur near the loop joint due to high stress level in this area. All strain gauges were placed at 45 degree since the shear cracks were expected to occur in the specimens.  Experimental Result and Discussion: In this research, loop bar connection was used to connect adjacent walls. The specimens were subjected to a direct shear force as explained in details in previous section. The structural behaviour of this type of wall to wall connection is discussed in terms of shear stress, displacement, strain , and crack pattern. Wall Displacement: Figures 8 and 9 show load-lateral displacement curves of wall to wall loop connections as can be seen from figures, LVDT 1 has the highest displacement followed by LVDT 2 and 3. This stipulates the incurred rotation of the wall 1 as the load was applied. It is safe to state that when the load was applied on the wall 1, loop bars in  374 Nabila Rossley et al  , 2014   Australian Journal of Basic and Applied Sciences, 8(1) January 2014, Pages: 370-380 wall 1 transferred the load to the adjacent wall through the middle pin bar which pulled out the loop bars in wall 2 and let the specimen rotate. The roller support stationed at the top right corner of the setting constrained the vertical displacement, nonetheless, the wall had the freedom to rotate. Therefore, in order to acknowledge the rotation occurred, the shear displacement of the wall should be converted to shear deformation based on Equation 1 as according to (Gökgöz, 2008). Fig. 6: Location of concrete strain gauges (WWC1). Fig. 7: Location of concrete strain gauges (WWC2).   Shear deformation = (Eq. 1) The shear stress versus shear deformation of WWCs are shown in Figure 10 and 11. It Could be seen that two walls deformed as stress reached up to 0.64 N/mm 2 in WWC1 and 0.77 N/mm 2  inWWC2. As the loading increased, cracks propagation initiated and caused different displacement rate in the panels. In precast concrete frames rotation of less than 0.02 rad will be applied in the connection design process in order to control the shear failure (Kitayama et al ., 1991). In these specimens, the maximum rotation of WWC1 and WWC2 is 0.011 rad and 0.0129 rad which are below the maximum limit. Fig. 8: Shear Stress vs. Lateral Displacement of WWC1. Fig. 9: Shear stress vs. Lateral Displacement of WWC2.
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