Abstract
To study the antiseismic performance of the high-strength concrete composite shear wall with built-in steel plates, an experiment on a high-strength concrete composite shear wall with four built-in steel plates (SPRCW-1∼4) was set up. Based on the experimental result, the paper discusses the antiseismic performance, failure mode, and failure mechanism of the high-strength concrete composite shear wall with built-in steel plates under different steel ratios and different positions of steel plates. The experimental result has shown that the differences in steel plate position and steel ratios have certain effects on wall cracking. The use of high steel content and the placement of steel plates on both sides of the wall can limit wall cracking to some extent. When the bearing capacity of the steel plates located on both sides of the wall is larger than that in the middle of the wall, a high content of steel in the wall can effectively increase the bearing capacity of the test piece to some extent. Under a high axial compression ratio, the horizontal bar of the wall can substantially limit the vertical cracks in concrete arising from compression. Moreover, the built-in steel plates in the shear wall play a significant role in inhibiting the propaganda of the oblique cracks under the action of earthquakes. The research result has very good economy and operability and can provide a basis for promotion and application of the mid- and high-rise buildings in regions with high seismic intensity.
1. Introduction
When an earthquake occurs, shear walls are able to bear large seismic force and consume enough energy released during the earthquake and play a very important role in guaranteeing the safety of high-rise buildings. The shear walls of high-rise buildings in high seismic intensity regions have a large size, high rigidness, and high bearing capacity but poor ductility and energy dissipation capacity. In case of an intensive earthquake, they would lose their bearing capacity prematurely and the antiseismic performance of the walls would not be brought into full play. How to design a lateral force resistance structural system with excellent antiseismic performance and high economic applicability is the focus of the structural designer personnel’s attention.
Many scholars have conducted a large number of tests and studies on the steel plate concrete shear walls. Xu et al. [1] have conducted research on the antiseismic performance of the hexagonal steel tube concrete and proposed its simplified strength model. Mun et al. [2] studied the shearing properties of heavy-duty shear walls; Marsono and Hatami [3] analyzed and studied the octagonal single-reinforcement concrete shear walls. Hitaka and Matsui [4] have proposed a slotted shear wall structure system with both ends connected, i.e., shear walls only connected with the frame beam and found that such shear walls have excellent plasticity and hysteretic behavior. Wei et al. [5] have studied the antiseismic performance of the novel partially connected buckling constrained steel plate shear walls. Jin et al. [6, 7] have analyzed and studied the stability of inclined joint buckling constrained steel plate shear wall and found that the smallest concrete slab thickness is determined by the bolt interval. Guo and Yuan [8] have conducted experimental research on the steel-shear wall composite structures under cyclic loading. Chen et al. [9] investigated a composite system of reinforced concrete and shear walls. Shafaei et al. [10] have studied the framework structure in the composite shear wall and the steel-concrete interactions.
The research shows that the good premise of the mechanical performance of the composite shear wall with slots is that the plasticity of the wall panels can be brought into full play before the out-of-plane buckling, but the performance of the composite shear wall with slots will be affected by such factors as the form of the slots, the position of the slots, and the flexural-torsional buckling of the wall columns between the slots. Considering that the rigidness of the slotted steel plate concrete composite shear wall is adjustable and the failure mode can be induced, the paper attempts to propose that two strip-shaped steel plates are placed in the medium and tall concrete shear walls. Shear studs are welded to both sides of each steel plate. Different positions and sizes of the strip-shaped steel plates are used to substitute for different parameters of the slotted steel plates, thus decreasing the steel content and the degree of difficulty with building slotted steel plate composite shear walls and saving the construction cost. The results of the research on the law of structural rigidness, mechanical properties, and crack initiation of high-strength concrete composite shear walls under different positions of steel plates and different steel ratios.
2. Overview of Test
2.1. Test
2.1.1. Test Design
The dimension of the test piece of high-strength concrete mid-rise shear wall with built-in steel plates (SPRCW-1-SPRCW4) used in the test is shown in Figure 1. The horizontal load applied by the horizontal actuator is directly acting on the upper loading beam, and the lower pedestal is used to impose build-in restriction over the wall.
The numbers of the test pieces are arranged in sequence: SPRCW-1 (high steel ratio steel plates located in the middle), SPRCW-2 (low steel ratio steel plates located in the middle), SPRCW-3 (low steel ratio steel plates located on both sides), and SPRCW-4 (high steel ratio steel plates located on both sides). There are some differences in steel ratio and position among the four test pieces of high-strength concrete mid-rise shear wall with built-in steel plates, and their remaining test parameters are consistent. See the details of parameters of the 4 test pieces in Table 1.
See Figure 2 for the SPRCW-1 construction drawing. The differences among the 4 test pieces are shown below:(1)The steel ratio of SPRCW-1 is 1.6%. The outer edge of the steel plate is 247 mm from the axis of symmetry, and the inner edge of the steel plate is 25 mm from the axis of symmetry. For the edge members on both sides, cold-formed thin-walled sectional steel with a steel ratio of 1.43% is placed.(2)The steel ratio of SPRCW-2 is 1.0%. The outer edge of the steel plate is 270 mm from the axis of symmetry, and the inner edge of the steel plate is 50 mm from the axis of symmetry. For the edge member on both sides, cold-formed thin-wall sectional steel with a steel ratio of 1.43% is placed.(3)The steel ratio of SPRCW-3 is 1.0%. The outer edge of the steel plate is 195 mm from the axis of symmetry, and the inner edge of the steel plate is 125 mm from the axis of symmetry. For the edge member on both sides, cold-formed thin-walled sectional steel with a steel ratio of 1.43% is placed.(4)The steel ratio of SPRCW-4 is 1.6%. The outer edge of the steel plate is 170 mm from the axis of symmetry, and the inner edge of the steel plate is 104 mm from the axis of symmetry. For the edge member on both sides, cold-formed thin-walled sectional steel with a steel ratio of 1.43% is placed.
2.1.2. Mechanical Properties of Steel
The material characteristic test was conducted in the Laboratory of Material Mechanics, Chongqing University. 12 test pieces were fabricated with sampling and sizes in accordance with the national standard Tensile Test Methods for Metal Materials at Room Temperature (GB/T 228-2010) [11]. The main purpose of the material characteristic test was to determine the mechanical property parameters of the cold-formed thin-walled sectional steel, Q235A hot-rolled steel plates, and HPB300 reinforcement bars under uniaxial tension: elasticity modulus , yield stress , ultimate tensile strength , and yield strain . The uniaxial tension result is shown in Table 2.
2.1.3. Measured Strength of Concrete and Axial Pressure of the Test Pieces
The strength grade of the five test pieces was C60. The test blocks and test pieces were fabricated, subject to concrete placement, and cured simultaneously. The test block dimension was 150 mm × 150 mm × 150 mm. The test blocks were subject to cube compressive strength test at 28 d after curing and on the day of test, and the test results were averaged. See Table 3 for details.
2.2. Test Device and Loading System
2.2.1. Test Device
In accordance with Code of Seismic Testing Methods for Buildings (JGJ101-2015) [12], a horizontal low-cyclic repeated loading quasistatic test was conducted under the action of a fixed axial pressure. The devices for the low-cyclic repeated loading test comprised a vertical loading device and a horizontal loading device. The vertical load was controlled by the hydraulic jack through the ball bearing. The maximum bearing capacity of a single vertical ball bearing was 120 kN. During the test, it was manually controlled by the oil pump for guaranteeing the stability of vertical load. The horizontal loading devices mainly comprise a reaction wall, a horizontal actuator, and a horizontal connection device. The end of the horizontal actuator was hinged with the horizontal connection device. During the test, the horizontal load was mainly applied by the horizontal actuator. One end of the actuator was connected to the loading beam of the test piece, and the other end was fixed onto the reaction wall. The schematic diagram for the test equipment is shown by Figure 3.
2.2.2. Test Loading System
In the test, the quasistatic testing method was used for loading. During the test, the vertical load was kept unchanged based on the axial pressure ratio and the horizontal load changed continuously. The loading process is as follows:(1)In the preloading stage, an axial pressure of 150 kN was preloaded on the top of the test piece and then the pressure was unloaded to 0 kN for removing nonuniformity within the test piece. After the axial pressure was loaded to a preset value, a horizontal load of 20 kN was applied positively and negatively once for testing the operating state of the test equipment.(2)After the first stage was completed, a positive horizontal load was applied in an ascending order, 20 kN, 40 kN, 60 kN, 80 kN…… until the first crack occurred in the test piece. The cracking load was recorded. The horizontal load was decreased to 0 kN in three levels. The load was negatively applied to seek the negative cracking load.(3)After the second stage was completed, the displacement loading stage started. The initial position after the application of axial force served as the 0 point. Each loading was cycled twice based on the displacement control. One-cycle displacement was loaded in three sections, and two-cycle displacement was loaded at one time. The loading would terminate when the test piece completely lost its bearing capacity or underwent out-of-plane instability that caused difficulties with loading. See Figure 4 for the loading system in the test.
3. Analysis of the Failure Modes and Types of Test Pieces
The paper involves the following relevant rules:(1)Loading direction: positive loading, loading direction of the pushing force applied by the horizontal actuator, and negative loading, loading direction of the pulling force applied by the horizontal actuator.(2)Displacement direction: Δ represents the horizontal displacement of the central point of the crown of the shear wall. The displacement is zero when a target axial force is applied and no horizontal load is applied. It is positive in positive loading and negative in negative loading.(3)Direction of horizontal force: P represents the load applied by the horizontal actuator, positive in the case of pushing and negative in the case of pulling.(4)Axial pressure direction: positive when the axial pressure is downward and negative when the axial pressure is upward.(5)Strain of steel reinforcement bars and cold-formed sectional steel: positive when the measuring point of strain for the cold-formed sectional steel or reinforcement bars is being pulled and negative when being compressed.(6)Crack angle: an included angle between the crack and the horizontal line.(7)Front and back sides of wall: the front side of the wall is the side adjacent to the mould during concrete placement and the main wall surface for plotting crack diagrams and photographing and recording and observing various phenomena in the test. The back side of the wall is the wall surface away from the formwork during concrete placement. A dial gage and a strain gage output line were suspended from the back side as an auxiliary wall surface for observing tests and recording phenomena.
3.1. Test Phenomena of Test Piece SPRCW-1
3.1.1. Loading Stage of Load Control
A vertical axial pressure up to 826 kN was applied according to the loading system for the test. Then, the horizontal actuator started to apply positive horizontal force and entered the loading stage of load control. When the pushing force was increased to 134.983 kN, a thin and long horizontal crack, about 7 cm long, occurred at the bottom right part of the back side of the wall, 15.7 cm away from the pedestal. In the case of positive loading, when the horizontal pulling force was increased to 91.069 kN, a horizontal crack, about 15 cm long, occurred at the bottom left of the back side of the wall, about 12.3 cm from the pedestal. See Table 4 for the strain of cold-formed sectional steel and steel plates in this stage.
3.1.2. Displacement Control Loading Stage
(1)Δ = 3 mm, the first cycle: During the positive loading process, the original horizontal cracks in the tensile area of the wall started to extend. Also, new and tiny horizontal cracks occurred at the left edge within the middle part of the wall, about 8 cm long. With the “bang” from the wall, a new curved shear oblique crack, about 22 cm long and with a dip angle of 15°, developed at 45 cm from the pedestal. During the process of negative loading, the concrete still gave a low sound of fragmentation. Within the tensile area at 1/3 of the wall height, the concrete skin experienced three tiny horizontal cracks that were developing slowly, which was similar to the phenomenon in the process of positive loading. Δ = 3 mm, the first cycle crack is shown in Figure 5(a). The strains of the cold-formed steel and steel plates are shown in Table 5.(2)Δ = 3 mm, the second cycle: During the process of positive and negative loading, neither significant phenomena nor development of new cracks occurred. The horizontal force was and , respectively. The above result indicated that structural damage occurred in the wall but not serious. See Figure 5(b) for the cracks in the second cycle in the case of Δ = 3 mm.(3)Δ = 5 mm, the first cycle: During the process of positive loading, new thin and dense cracks about 10 cm long were developing on the left side of the tensile area of the wall. The wall interior gave a low sound of rupture. The original horizontal cracks were dipping downward at an angle of 30°. Meanwhile, several shear dipped cracks were developing around the longer dipped cracks, indicating that the failure mode of the wall shifted from bending failure to bending shear failure. With the increase in the horizontal force, the dipped cracks newly developed in the middle upper part on one side of the tensile area were blocked in the position of the steel plate, indicating that the steel plates play a certain role in controlling development of cracks. As the wall had a few horizontal bars and a weak middle area where steel plates were located, vertical cracks were developing in the two steel plates. Based on the width of the crack, no structural failure damage was caused to the wall. However, at this point, the cold-formed sectional steel and steel plates had yielded. The development of cracks in the middle and lower parts of the wall during the process of negative loading was symmetrical with the development of cracks during the process of positive loading. The original horizontal cracks of the wall started to extend towards the middle and lower parts of the wall, and the width of the cracks was increasing. Similarly, bending shear dipped cracks were developing. The new horizontal cracks occurring at the outer edge of the tensile area were more than that in the process of positive loading. The vertical cracks present in the middle of the two steel plates were extending downward. At this point, the vertical reinforcement bars of the embedded column in the tensile area had yielded, and the cold-formed sectional steel had also yielded. See Figure 6(a) for the cracks in this stage.(4)Δ = 5 mm, the second cycle: During the process of positive and negative loading, the dipped cracks continued to develop, and there were no significant new cracks. The original cracks were extending slightly, and the cracks on both sides were basically developing symmetrically. The horizontal loads corresponding to Δ = 5.0 mm of the second cycle were and . The extreme value of the horizontal bearing capacity loaded negatively in the second cycle decreased considerably than that in the first cycle, suggesting that the structural damage to the embedded column on the right is more serious. See Figure 6(b) for cracks in this stage.(5)Δ = 10 mm, the first cycle: During the process of positive loading, some fragmented concrete was detached at the left bottom of the tensile area, and some concrete was crushed in the compressed area. The dipped cracks in the upper part were widened and extended somewhat. The width of the dipped cracks increased significantly. Based on the propaganda of the cracks in the previous cycle, the dipped cracks started to extend and exhibited a trend of penetrating the wall but the cracks were still blocked in the position of the steel plate. The maximum value of the compressive strain of the embedded column on the compressive side was −2466 . The horizontal load corresponding to Δ = 10.0 mm was . During the process of negative loading, the horizontal cracks at the bottom of the wall in the tensile area were widening and developing significantly. Similarly, some fragmented concrete were detached, and the some concrete in the compressive area were crushed. A few dipped cracks in the upper part were developing and widening, with a width of about 1∼2 mm. A little concrete on the surface of the wall in the compressive area on the bottom right was detached. Several new vertical cracks were developed in the compressive area. The width of the original cracks increased. See Figure 7(a) for the cracks in this stage.(6)Δ = 10 mm, the second cycle: During the test, no significant increase in number of cracks occurred and only a small number of cracks developed. The corresponding horizontal loads were and . Compared with the first cycle Δ = 10.0 mm, the decrease in peak during the process of positive and negative loading with was not significant, suggesting that the test piece at this point has excellent energy dissipation capacity. See Figure 7(b) for the cracks in this stage.(7)Δ = 15 mm, the first cycle: During the process of positive loading, mass concrete in the compressive area on bottom right of the front side of the wall was crushed and detached. The section steel had yielded and been exposed. Several new dipped cracks occurred in the compressive area on bottom left of the front side of the wall. Some skin concrete was detached. With the increase in horizontal force, the reinforcement bars in the embedded column in the tensile area were stretched. At this point, the corresponding horizontal load was . The bearing capacity of the test piece decreased by the horizontal load was 83.13% of the extreme value of the pushing force. The bearing capacity of the test piece was lower than 85% of the ultimate bearing capacity of the structure. The test piece could be deemed ready to fail. During the process of negative loading, mass concrete at the bottom of the tensile area collapsed, and both the stirrups and horizontal reinforcement bars were exposed. During the process of unloading, the concrete on the right of the front side of the wall continued detaching, and the horizontal reinforcement bars in the embedded column were exposed. The corresponding horizontal load was . At this point, the horizontal bearing capacity of the test piece decreased by the horizontal load reached 88.35% of the extreme value of the pulling force. During the process of negative loading, the horizontal bearing capacity was insufficient to cause failure. See Figure 8(a) for the cracks in this stage.(8)Δ = 15 mm, the second cycle: During the process of positive loading, the cracks did not propagate. The concrete in the tensile area at the edge of the bottom left of the front surface of the wall was spalling. The vertical reinforcement bars and sectional steel in the wall were exposed. The vertical reinforcement bars bent outward and were significantly compressed. During the process of unloading, the range of concrete spalling on the bottom left of the front surface of the wall was increasing. In this stage, the horizontal load . During the process of negative loading, the concrete in the lower part of the tensile area on the bottom right of the front side of the wall was spalling. The horizontal load in this stage was . See Figure 8(b) for the cracks.(9)Δ = 20 mm, the first cycle: During the process of positive and negative loading, the spalling of concrete in the corner on both sides of the wall was continuing. With the increase in horizontal load, through horizontal cracks occurred at the bottom of the wall. Subsequently, vertical cracks occurred and developed, with concrete skin spalling. At this point, the horizontal bearing capacity decreased to and , respectively. See Figure 9 for the diagram of cracks in the first cycle Δ = 20.0 mm.
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3.2. Phenomena in the Test for the Test Piece SPRCW-2
3.2.1. Loading Stage of Load Control
A vertical axial pressure of 869 kN was applied after various preparation works and the microcirculation were completed. Then, the horizontal actuator started to apply positive horizontal force and entered the loading stage of load control. When the pushing force was increased to 109.375 kN, a thin and long horizontal crack about 10 cm long occurred at 1.7 cm from the pedestal on the bottom right of the back side of the wall. A negative load was applied to the test piece. When the horizontal pulling force was increased to 100.92 kN, a 5 cm long horizontal crack occurred at about 11.7 cm from the pedestal on the bottom left of the back side of the wall. See Table 6 for the data including cracking load.
3.2.2. Displacement Control Loading Stage
(1)Δ = 3 mm, the first cycle: During the process of positive loading, the original horizontal cracks at 25 cm from the pedestal in the tensile area of the wall started to dip at an angle of 15° and extend downward. Three bending shear dipped cracks, about 15 cm long and 0.01 cm wide, occurred at 40 cm, 53 cm, and 60 cm from the pedestal at the edge of the tensile area of the wall. With the development of cracks, the wall gave a low sound of “bang.” During the process of negative loading, two bent shear dipped cracks, about 30 cm long, occurred within the tensile area at 50 cm and 65 cm from the pedestal. The cracks were developing rapidly. Meanwhile, the original horizontal cracks also developed significantly. At this point, the vertical reinforcement bars and the cold-formed sectional steel had not yielded. See Figure 10(a) for the cracks.(2)Δ = 3 mm, the second cycle: During the process of positive loading, all of the original horizontal cracks extended about 3 cm. A new bending shear crack, about 25 cm long, occurred on the tensile side in the lower middle part of the wall at an angle of 30°. During the process of negative loading, a new bending shear oblique crack occurred in the middle of the wall. All of the original oblique cracks extended somewhat. The horizontal force in this stage was . See Figure 10(b) for cracks.(3)Δ = 5 mm, the first cycle: During the process of positive loading, the wall continuously gave a clear sound of “split.” All of the original cracks extended and developed to different extents. A standard oblique crack caused by shear failure developed in the middle of the tensile area on the left side of the wall. The oblique crack had a dip angle of about 40° and a length of 55 cm and stopped developing when extending to the middle of the wall. At the end of loading, an oblique crack, about 28 cm long, occurred in the vicinity of the pedestal in the middle of the wall. This indicates that the failure mode of the wall transformed from bending failure to significant bending shear failure. At this point, the vertical reinforcement bars and steel plates approximately yielded, and the cold-formed sectional steel had yielded. During the process of negative loading, a standard oblique crack caused by shear failure developed in the upper part of the tensile area on the right side of the wall. The oblique crack had a dip angle of about 40° and a length of about 70 cm and stopped developing after extending downward the middle of the wall and intersecting with the longitudinal oblique crack formed during the process of positive loading. At this point, the longitudinal reinforcement bars of the embedded column in the tensile area had exceeded the measuring range. See Figure 11(a) for the cracks in this stage.(4)Δ = 5 mm, the second cycle: During the test, new oblique cracks occurred. The oblique cracks on both sides in the vicinity of the pedestal in the lower middle part of the wall showed an increasingly significant sign of intersection and development. In the second cycle with Δ = 5.0 mm, the corresponding horizontal loads were and . The extreme value of the negatively loaded horizontal bearing capacity in the second cycle decreased substantially compared with that in the first cycle, indicating that the structural damage to the embedded column on the right side of the test piece is relatively serious. See Figure 11(b) for the cracks in this stage.(5)Δ = 10 mm, the first cycle: During the process of positive loading, many vertical cracks occurred in the tensile area, the horizontal cracks widened significantly, fragmented concrete on bottom right in the tensile area spalled, and some concrete was crushed in the compressive area. The oblique cracks in the lower part of the wall widened and extended somewhat. The development of cracks was sufficient. Based on the numerical value of the vertical reinforcement bars in the tensile area measured by the strain gauge, the longitudinal bars in the embedded column had slipped. Meanwhile, the sectional steel on both sides had also yielded and exceeded the measuring range. The steel plate had also yielded. This indicated that some concrete had failed, and the steel plates bore partial load. The corresponding horizontal load was . During the process of negative loading, the horizontal cracks at the bottom of the wall in the tensile area widened significantly; many vertical cracks occurred with fragmented concrete spalled; some concrete was crushed in the compressive area. The corresponding horizontal load was . See Figure 12(a) for the cracks in this stage.(6)Δ = 10 mm, the second cycle: During the process of positive and negative loading, a few oblique cracks developed and no significant new cracks occurred. After positive and negative cycles, mass concrete in the corner of the lower part of the right side of the wall spalled. The cracking on the left side was serious. The corresponding horizontal loads were and . Compared with the positive and negative loading in the first cycle with Δ = 10.0 mm, the positively applied bearing capacity decreased significantly, suggesting that the positive loading causes severe damage to the wall structure. During the process of negative loading, the wall had excellent energy dissipation capacity. See Figure 12(b) for the cracks in this stage.(7)Δ = 15 mm, the first cycle: During the process of positive loading, mass concrete in the compressive area on bottom right of the front side of the wall was crushed and detached. The section steel had yielded and been exposed. At the same time, the concrete skin spalled. With the increase in the horizontal force, the reinforcement bars in the embedded column were obviously bent, protruded, and extended. At this point, the corresponding horizontal load was . The horizontal bearing capacity decreased by the horizontal load had reached 47.18% of the extreme value of the pushing force. During the process of negative loading, mass concrete at the bottom of the tensile area collapsed, and both the stirrups and horizontal reinforcement bars were exposed. During the process of unloading, the concrete on the right of the front side of the wall continued detaching, and the horizontal reinforcement bars in the embedded column were exposed. The corresponding horizontal load was . The horizontal bearing capacity decreased by the horizontal load had reached 64.847% of the extreme value of the pulling force. There were certain brittleness characteristics. See Figure 13(a) for the cracks in the first cycle with Δ = 15.0 mm.
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3.3. Phenomena in the Test for the Test Piece SPRCW-3
3.3.1. Loading Stage of Load Control
A vertical axial pressure of 879 kN was applied to the test piece after various preparation works and the microcirculation were completed. Then, the horizontal actuator started to apply positive horizontal force and entered the loading stage of load control. When the pushing force was increased to 98.0743 kN, a thin and long horizontal crack about 8 cm long occurred at 2.5 cm from the pedestal on the bottom right of the back side of the wall. A negative load was applied to the test piece. When the horizontal pulling force was increased to 132.44 kN, a 8 cm long horizontal crack occurred at about 6 cm from the pedestal on the bottom left of the back side of the wall. See Table 5 for the cracking load and strain in this stage.
3.3.2. Loading Stage of Displacement Control
(1)Δ = 3 mm, the first cycle: During the process of positive loading, all of the original horizontal cracks extended about 10 cm. In addition, three horizontal cracks with a length of about 10 cm occurred at the edge of the tensile area in the upper middle part of the wall. With the progress of the test, bending shear oblique cracks with a length of about 30 cm at 45 cm from the pedestal in the tensile area of the wall started to dip at an angle of 15° and extend downward. With the development of cracks, the wall gave a low sound of “bang,” and the steel plates had a significant strain value, indicating that the steel plates had started to bear partial load together with the concrete when they were placed on both sides of the wall. During the process of negative loading, the wall surface still gave a low sound of “bang.” New horizontal cracks with a length of about 10 cm occurred within the tensile area at 50 cm and 30 cm from the pedestal. With the increase in horizontal force, a horizontal crack with a length of about 45 cm at 41 cm from the pedestal rapidly dipped and developed downward at an angle of 40°. An obvious bending shear oblique crack with a length of about 45 cm occurred at 61 cm from the original pedestal, parallel to the oblique crack at 41 cm from the pedestal and extended to the central line of the wall. At this point, the longitudinal and the cold-formed sectional steel did not yield. See Figure 14(a) for cracks.(2)Δ = 3 mm, the second cycle: During the process of positive loading, the original horizontal cracks and the oblique cracks extended 3–5 cm. All cracks occurred on the centre line of the wall. At this point, the horizontal force was . During the process of negative loading, the horizontal cracks in the first cycle extended somewhat and dipped downward. The original oblique cracks showed no sign of extension. The ultimate bearing capacity in this stage was . See Figure 14(b) for cracks.(3)Δ = 5 mm first cycle: During the positive loading, the original horizontal cracks in the upper middle part of the tensile area of the wall extended to different degrees. A thin and small oblique crack caused by shear failure occurred in the middle of the tensile area of the wall. The oblique crack had a dip angle of about 40° and a length of about 28 cm. As the wall had a large axial pressure ratio, the middle part of the wall had concentrated stress during the process of vertical loading, thus leading to large compressive strain. With the increase in the horizontal force, the small bending moment of the wall crown and the Poisson effect caused the concrete to suffer tensile strain. When such tensile strain exceeds the ultimate tensile strain of the concrete, the longitudinal reinforcement bars and the cold-formed sectional steel did not yield. This was because the horizontal bars in the wall were too few to block the cracking of wall. During the process of negative loading, a standard oblique crack caused by bending shear failure was developing in the upper middle part of the tensile area on the right side of the wall. The oblique crack had a dip angle of about 40° and a length of 60 cm and stopped developing when extending to the middle of the wall. All of the original bending shear cracks of the wall extended 8–10 cm. Also, new cracks caused by shear compression failure occurred at the top of the wall. The cracks caused by shear compression failure during Δ = 5.0 mm positive loading showed signs of oblique extension, stopped developing in the position of steel plate, and continued to develop towards the middle of the wall. Based on the failure mode, the two steel plates could block the through development of the oblique cracks. See Figure 15(a) for the cracks in this stage.(4)Δ = 5 mm, the second cycle: During the test, none of the cracks showed signs of extension, no new cracks developed, and the oblique cracks in the middle of the wall caused by failure exhibited a widening trend. In the Δ = 5.0 mm second cycle, the corresponding horizontal loads were and . The extreme value of the horizontal bearing capacity decreased significantly during positive loading in the second cycle compared with that in the first cycle, indicating that the structural damage to the embedded column during the positive loading is serious. See Figure 15(b) for the cracks in this stage.(5)Δ = 10 mm, the first cycle: During the process of positive loading, 3 obvious vertical cracks occurred in the tensile area of the wall, the horizontal cracks widened significantly, fragmented concrete on bottom left of the tensile area spalled, and some concrete in the compressive area was crushed. Many oblique cracks in the middle of the wall developed. The original oblique cracks started to widen, and the largest width was up to 1 mm. The new oblique cracks developed in the position of steel plate, indicating that the steel plate can no longer limit the development of the shear failure of the wall. The ultimate tensile strain of the steel plate with cracks well developed was 1541; the ultimate compressive strain was −667. At this point, the corresponding horizontal load was . During the process of negative loading, the horizontal cracks at the bottom of the tensile area of the wall developed significantly. Also, many vertical cracks occurred and fragmented concrete spalled. The oblique cracks in the upper part widened and the width was about 1∼2 mm. The horizontal bearing capacity in this stage was . The horizontal bearing capacity of the test piece had reached the extreme value of the pulling force. See Figure 16(a) for the cracks in the first cycle with Δ = 10.0 mm.(6)Δ = 10 mm, the second cycle: During the test, no significant new cracks occurred. After the positive and negative cycles were completed, the cracking of concrete in the corner of the lower part of the embedded column on the right side of the wall was more significant. With the increase in the horizontal force, the range of cracking in the corner on both sides of the wall was increasing gradually. The corresponding horizontal loads were and , respectively. During the process of negative loading, the wall had excellent energy dissipation capacity. See Figure 16(b) for the cracks in the second cycle with Δ = 10.0 mm.(7)Δ = 15 mm first cycle: During the process of positive loading, mass concrete in the compressive area on bottom right of the front side of the wall was crushed and detached. Shear oblique cracks still occurred in the middle of the wall. The shear oblique cracks at the top of the wall were extending upward. With the increase in horizontal force, the concrete in the corner of the compressive area was spalling continuously. The longitudinal reinforcement bars were exposed and bent. At the same time, the cracking of the through oblique cracks in the middle of the wall was serious. Concrete fragments were spalling. At this point, the corresponding horizontal load was . The horizontal bearing capacity of the test piece was 66.93% of the extreme value of the test pushing force. The test piece had failed before positive loading. During the process of negative loading, mass concrete at the bottom of the tensile area collapsed, shear oblique cracks occurred in the middle of the wall, and shear oblique cracks at the top of the wall were extending upward. The concrete in the corner of the compressive area was obviously crushed. The corresponding horizontal load was . The horizontal bearing capacity of the test piece decreased by the horizontal load had reached 75.784% of the extreme value of the pulling force. See Figure 17(a) for the cracks in this stage.(8)Δ = 15 mm, the second cycle: During the process of positive and negative loading, there were a few oblique cracks, indicating that the wall still has certain energy dissipation capacity. The range of failure of the concrete of the embedded columns on both sides of the test piece was also increasing continuously. The width of the through oblique cracks of the wall was up to 2.7 mm. The corresponding horizontal bearing capacity was and , respectively. During the negative loading, the strength of the wall was decreasing slowly. See Figure 17(b) for the cracks in this stage.(9)Δ = 18 mm cycle: During the process of positive and negative loading, no new cracks developed in the wall. The range of spalling of concrete in the corner on both sides of the wall was increasing. Spalling of mass concrete also occurred. At this point, the horizontal bars and the sectional steel within the embedded column were exposed. During the process of loading, the deformation of reinforcement bars and sectional steel was apparent. The corresponding horizontal loads were and . This indicated that the test piece failed but still has certain energy dissipation capacity. See Figure 18 for the cracks in this stage.
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3.4. Phenomena in the Test for the Test Piece SPRCW-4
3.4.1. Loading Stage of Load Control
A vertical axial pressure of 826 kN was applied to the test piece after various preparation work and the microcirculation were completed. Then, the horizontal actuator was started to apply positive horizontal force and entered the loading stage of load control. When the pushing force was increased to 98.5398 kN, a thin and long horizontal crack about 3 cm long occurred at 8 cm from the pedestal on the bottom right of the back side of the wall. A negative load was applied to the test piece. When the horizontal pulling force was increased to −130.47 kN, a 3 cm long horizontal crack occurred at about 10 cm from the pedestal on the bottom left of the back side of the wall. See Table 7 for the cracking load and strain in this stage.
3.4.2. Loading Stage of Displacement Control
(1)Δ = 3 mm, the first cycle: During the process of positive loading, the original horizontal cracks in the tensile area of the wall extended significantly and the length of cracking was about 10 cm. With the progress of loading, two horizontal cracks with a length of about 20 cm occurred in the tensile area of the wall, 30 cm and 35 cm from the pedestal on the left side of the wall. At the end of positive loading, a bending shear oblique crack with a length of about 50 cm occurred at 60 cm from the bottom of the tensile area on the left side of the wall. The cracking angle of the oblique crack was about 45°. The steel plates had a significant strain value, indicating that the steel plates had begun to bear partial load together with the concrete when placed on both sides of the wall. During the process of negative loading, new horizontal cracks with a length of about 10 cm occurred within the tensile areas at 41 cm and 32 cm from the pedestal. With the increase in horizontal force, a horizontal crack with a length of about 29 cm at 23 cm from the pedestal dipped rapidly at an angle of 30° and developed downward. See Figure 19(a) for the cracks in this stage.(2)Δ = 3 mm, the second cycle: During the test, the original bending shear oblique cracks extended 20 cm and 10 cm, respectively. The two oblique cracks had developed to the middle of the wall. At this point, the horizontal force was . During the process of negative loading, the horizontal cracks present in the first cycle extended 10 cm and showed a tendency of downward inclination. No other significant phenomena occurred. At this point, the horizontal forces were and Δ = 3.0 mm, respectively. See Figure 19(b) for the cracks in the second cycle with Δ = 3.0 mm.(3)Δ = 5 mm first cycle: During the process of positive loading, the shear oblique cracks not occurring in the first cycle with Δ = 3.0 mm continued developing towards the lower middle part. Meanwhile, 5 obvious shear oblique cracks were occurring in the upper middle part of the tensile area of the wall, extending to the middle line of the wall, and then stopped developing. Horizontal cracks caused by bending failure no longer occurred in the wall. At this point, the longitudinal reinforcement bars, steel plates, and cold-formed sectional steel did not yield. During the process of negative loading, 3 standard bending shear oblique cracks with a length of about 25 cm occurred in the lower part of the tensile area of the wall. At the end of loading, a standard oblique crack caused by bending shear failure developed in the middle of the tensile area on the right side of the wall. The oblique crack had a dip angle of about 40° and a length of about 45 cm and stopped developing after extending to the middle of the wall. The cracks developing the positive and negative cycles were asymmetric. The development degree of the cracks in the negative loading was one cycle slower than that in the positive loading. Based on the macroscopic observation of the test phenomena, the eccentricity of the test piece was subject to torsion. The loading head deformed laterally. The test proceeded after it was adjusted. See Figure 20(a) for the cracks in this stage.(4)Δ = 5 mm, the second cycle: During the test, all of the cracks extended somewhat, the cracks on both sides were converging into the middle of the wall, and the width of the cracks was increasing. During the process of positive loading, oblique cracks started to occur at the top of the wall. The oblique cracks stopped developing after extending to the edge of the interior of the steel plate due to the limitation of the steel plates. In the second cycle with Δ = 5.0 mm, the corresponding horizontal loads were and . See Figure 20(b) for the cracks in the second cycle.(5)Δ = 10 mm first cycle: During the process of positive loading, a few oblique cracks occurred in the tensile area of the wall, the crack length was small, and concrete fragments in the compressive area spalled. Many oblique cracks in the middle of the wall extended. The original oblique cracks started to widen, and the largest width was up to 1 mm. The sectional steel and longitudinal reinforcement bars on the compressive side yielded. Some longitudinal bars exceeded the measuring range. Meanwhile, the sectional steel and longitudinal reinforcement bars on the compressive side also yielded. Some longitudinal reinforcement bars exceeded the measuring range. The horizontal load corresponding to Δ = 10.0 mm was . During the process of negative loading, the horizontal cracks at the bottom of the wall in the tensile area widened. Many vertical cracks occurred. Fragmented concrete spalled. A few oblique cracks in the upper part developed and widened. The width was about 1∼2 mm. The cracks in the interior of the steel plate area in the middle of the wall were gradually developing like “8.” The horizontal load corresponding to Δ = −10.0 mm was . At this point, the horizontal bearing capacity had reached the extreme value of the pulling force. See Figure 21(a) for the cracks in the first cycle with Δ = 10.0 mm.(6)Δ = 10 mm the second cycle: During the test, no significant new cracks occurred. The cracking of concrete in the corner in the lower part of the embedded column on the right side of the wall was increasing obvious after the positive and negative cycles were completed. The horizontal bearing capacities were and , respectively. At this point, the wall had excellent energy dissipation capacity. See Figure 21(b) for the cracks in this stage.(7)Δ = 15 mm, the first cycle: During the process of positive cycle, the mass concrete in the compressive area in the right corner of the front side of the wall was crushed and started to spall. The vertical cracks in the tensile area on the left side increased. A few concrete fragments spalled. Shear oblique cracks still occurred in the middle of the wall. The shear oblique cracks at the top of the wall were extending upward. With the increase in horizontal force, new horizontal cracks and shear oblique cracks started to occur again at the edge of the tensile area of the wall. Meanwhile, the cracking of through oblique cracks in the middle of the wall was serious. The concrete skin spalled. At this point, the corresponding load was . The horizontal bearing capacity of the test piece had reached 89.11% of the extreme value of the pushing force. During the process of negative loading, mass concrete in the tensile area on the right side of the wall collapsed. Shear oblique cracks in the middle of the wall occurred and extended to the left side of the wall. The concrete in the corner of the compressive area was significantly crushed. The corresponding horizontal load was . The horizontal bearing capacity had reached 93.36% of the extreme value of the pulling force. See Figure 22(a) for the cracks in this stage.(8)Δ = 15 mm second cycle: During the process of positive and negative loading, a few cracks still developed, indicating that the wall still has certain energy dissipation capacity. The failure degree of concrete of the embedded column on the left side of the test piece increased. The through oblique cracks of the wall widened. The concrete skin spalled. The corresponding horizontal bearing capacities were and , respectively. During the process of negative loading, the strength of the wall decreased slowly. During the positive loading in the second cycle, the bearing capacity of the test piece decreased substantially, indicating that the structural damage during positive loading was significant. See Figure 22(b) for the cracks in this stage.(9)Δ = 20 mm cycle: During the process of positive and negative loading, the concrete in the corner on both sides of the wall continued spalling. With the increase in the horizontal load, the through oblique cracks of the wall widened rapidly. Subsequently, as the mass concrete spalled, the horizontal bearing capacities decreased to and , respectively. During the process of negative loading, the horizontal bearing capacity decreased to 75.70% of the extreme value of the pulling force. See Figure 23 for the cracks in this stage.
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4. Conclusion
The paper describes in detail the phenomena regarding the 4 test pieces during the test in different loading stages, compares and summarizes the failure modes of the 4 test pieces based on the diagram of cracks and failure of the 4 test pieces, and draws the following conclusions.(1)Based on the macroscopic presentation, the test pieces can be arranged by the mean value of cracking load: SPRCW-4 > SPRCW-3 > SPRCW- 2 > SPRCW-1. Based on the test result, the differences in position and steel ratio have certain effects on the cracking of wall. The use of a high steel ratio and the placement of steel plates on both sides of the wall can limit the cracking of wall to some extent.(2)The SPRCW-1 and SPRCW-4 and SPRCW-2 and SPRCW-3 are compared in test data and failure mode. The bearing capacity in the case of steel plates located on both sides of the wall was higher than that in the case of steel plates located in the middle of the wall.(3)SPRCW-1 and SPRCW-2 and SPRCW-3 and SPRCW-4 are compared in test data and failure mode. The wall can effectively increase the bearing capacity of the test piece to some extent when it has a high steel ratio.(4)Based on different phenomena in the tests for the test pieces, the failure modes of the 4 test pieces are all bending shear failure. In the failure of the SPRCW-2 test piece, certain brittle failure occurs and thus the shear failure is dominant. Bending failure is dominant in the SPRCW-1, SRHCW-3, and SRHCW-4 test pieces.(5)During the process of displacement loading in the test for SPRCW-1∼4, different vertical cracks and “8” shaped cracks occur in the middle of the two steel plates in the displacement cycle after Δ = 5 mm. In terms of the causes of cracks, under the control of a high axial pressure ratio, the wall of the test piece would experience substantial vertical strain. The Poisson effect causes the concrete to undergo large tensile strain. When the tensile strain is larger than the ultimate tensile strain of the concrete, the test piece cracks at the wall top. The built-in steel plate composite shear wall has a low ratio of horizontal bars and cannot effectively limit the cracking of concrete, thus leading to generation and development of vertical cracks. With the increase in displacement, the vertical cracks and the shear oblique cracks converge and develop downward the wall along the diagonal line. The steel plates have excellent shear capacity and limit the development of oblique cracks, thus causing the oblique cracks of the wall to extend in the steel plates and leading to “8” shaped oblique cracks.
In conclusion, under a high axial pressure ratio, the horizontal bars of the wall can significantly limit the vertical cracks in the concrete under compression. Moreover, the built-in steel plates in the shear wall play a significant role in inhibiting the development of oblique cracks under the seismic action. See the following references [13, 14] for possible solution in order to realize numerical models devoted to simulate the structural behavior of such kind of shear walls with steel plates.
Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This study was supported by the National Natural Science Foundation of China (grant no. 41372356). The authors gratefully acknowledge this support.