The subassembly test was performed at Chongqing University structural laboratory. The emphasis of the test was placed on the examination of the overall performance of the test subassemblies and revealing the extent of influences the test variables had on the behavior of the test subassemblies.
2.1. Description of Test Subassemblies
The test subassemblies were designed to possess two seismic fortification lines. The SCBs will first significantly yield and consume a great deal of inelastic energy. Then, the composite wall piers provide the further deformation and energy dissipation capacities. Three 1/3-scaled subassemblies, identified by CS-1 to CS-3, were designed and constructed according to the Chinese Code for Design of Composite Structures (JGJ 138-2016) [
23], Code for Seismic Design of Buildings (GB50011-2010) [
24] and AISC 341-2016 [
1]. As shown in
Figure 2a, the test subassembly consisted of a wall pier of one-and-a-half story and an SCB of a half-span length.
Figure 2b illustrates the boundary conditions of the actual subassembly corresponding to loading conditions depicted in
Figure 2a.
The overall dimensions are provided in
Figure 3. The clear length of the SCB was 400 mm, measured from the wall surface to beam end. Wall piers of all three subassemblies had identical overall dimensions, consisting of a 900 × 300 × 300 mm loading beam on top, a 2200 × 500 × 400 mm foundation at the bottom and a 1500 × 800 × 160 mm wall portion in-between.
In these three subassemblies, the SCB was welded to the VBE of the embedded SPSW. As shown in
Figure 4a, the SCB of CS-1 and CS-2 had a section of 200 × 80 × 6 × 10 mm, while that of CS-3 had a section of 200 × 80 × 6 × 8 mm. As shown in
Figure 4b, the VBE of the embedded SPSW of all subassemblies had the same total depth of 120mm. The flange thickness was 10mm for CS-1, CS-2 and 6mm for CS-3, respectively. The web thickness was 6mm for CS-1, CS-2 and 8mm for CS-3, respectively. The thickness of the steel plate of SPSW was 6mm for CS-1 and CS-2, while 8mm for CS-3. The overall dimensions and details of the embedded SPSW are shown in
Figure 5. The SPSW of CS-1 and CS-3 was fabricated and erected as whole with continuous steel plate and VBE, as depicted in
Figure 5a. The SPSW of CS-2, on the other hand, was fabricated by joining the upper and lower segments through a bolted faceplate joint at the web plate and a welded joint at the VBEs, as shown in
Figure 5b. In the lower SPSW segment, the top edge of steel plate was 165 mm beyond the edge of the VBEs; in the upper SPSW segment, the bottom edge of the VBE was 165 mm beyond that of the steel plate. Two faceplates with 5 mm thickness were placed on both sides of the steel plate at the joint and two rows of bolts were used to fasten the joints. The upper and lower VBEs were welded together. As shown in
Figure 6, six No.10 hot-rolled rebars were used as the longitudinal reinforcement at wall boundaries. No.8 hoops with a spacing of 100 mm were used as the transverse reinforcement, and No.6 rebars spaced by 100 mm were used as the horizontal and vertical distributed wall reinforcement. Shear studs with a diameter of 10 mm were welded to the steel plates with 100/120 mm spacing to strengthen bonding between steel and concrete.
In this experimental study, the shear-span ratio of the SPRC wall section,
Mw/
h0Vw, was chosen as a test variable, where
Mw is the most probable flexural capacity of the wall cross-section;
Vw is the shear force demand;
h0 is the effective depth, measured from the edge of the compressive zone to the center of the elements in tension of the wall cross-section. According to section 10.1.4 of JGJ 138-2016 [
23], the shear-span ratio should be between 1.5 and 2.2. Thus, 2.2 is selected for CS-1 and CS-2. Previous research also suggested that it should be greater than 2.0 to ensure ductile behavior and avoid brittle failure in shear [
25,
26]. Thus, 2.0 was chosen for CS-3.
The computation of the flexural and shear capacities of SPRC wall was based on JGJ 138-2016 [
23].
Figure 7 shows the calculating diagram for the flexural capacity of the SPRC wall section, which can be obtained by Equation (1):
where
α1 is the influence coefficient of the concrete compressive zone;
fc is the compressive strength of the concrete;
bw is the wall thickness;
x is the depth of the concrete compressive zone;
is the compressive strength of the longitudinal reinforcement;
is the section area of the reinforcement in the compressive zone;
is the distance from the center of the compressed reinforcement to the edge of the element in tension;
is the compressive strength of the steel used in the VBE;
is the section area of the VBE in the compressive zone;
is the distance from the center of the compressed VBE to the edge of the element in tension;
Msw,
Mpw are the moments applied on the vertical distributed bars and the steel plate. The shear capacity of the SPRC composite wall can be estimated by Equation (2):
where
λ is the shear-span ratio;
fct is the tensile strength of the concrete;
N is the axial load applied on the wall pier;
A is the shear wall section area, taking
Aw to equal to
A;
fyh is the tensile strength of the horizontal distributed bars;
Ash is the section area of the horizontal distributed bars;
s is the spacing of the horizontal distributed bars;
fa is the tensile strength of the steel used in the VBE;
Aa is the section area of the VBE in the element in tension;
fp is the tensile strength of the steel used in the steel plate;
Ap is the section area of the steel plate. The shear capacity of SCB,
Vp, was calculated as per AISC 341-2016 [
1]. Using the global limit state analyses on each test subassembly, the calculated lateral load capacities of the test subassemblies,
Vn, are obtained and listed in
Table 1.
To summarize, the three test subassemblies were designed to form two groups for comparison. The first group included test subassemblies CS-1 and CS-3, where the embedded SPSW was welded as a whole for erection while their shear-span ratios were 2.2 and 2.0, respectively. The second group consisted of CS-1 and CS-2, with the same shear-span ratios of 2.2, with different SPSW details. In CS-2, the SPSW was assembled using bolted faceplates to join the web plates of the upper and lower segments of the SPSW. The CS-1 versus CS-3 and CS-1 versus CS-2 comparison aimed to reveal the influences of the wall shear-span ratio and SPSW details on the behaviors of the test subassemblies, respectively. Whether the coupled mechanism can be realized was also an emphasis to examine out of the test results.
2.2. Loading Regime and Test Setup
Test subassemblies were subjected to constant axial load and lateral cyclic load at the top of the wall pier. A hydraulic jack with 1500 kN loading capacity was used to apply the constant axial load, representing an axial load ratio of 0.15 [
27]. To distribute the vertical load, a spreader beam was mounted between the vertical hydraulic jack and loading beam. A roller was placed between the reaction girder and jack so that the vertical jack could slide horizontally and remain vertical. The lateral cyclic load was applied at the top of the wall pier via a horizontal actuator of 2000 kN loading capacity with one end fixed on the reaction wall and the other on the loading beam, and the pushing of the actuator was designated as the positive direction and pulling the negative direction. The cantilevered SCB end was supported through a vertical hydraulic jack with a loading capacity of 500kN, which was connected using the hinge connection to allow the rotation of the joint. Rigid beams were used to compress the foundation to prevent overturning through post-tensioning to the strong floor. To prevent horizontal sliding, two jacks were placed against the front and rear surfaces of the foundation. Lateral supports were also provided at two-thirds of the subassembly height to prevent out-of-plane deformation. The depiction of the test setup is shown in
Figure 8.
Figure 9 depicts the boundary conditions corresponding to the test setup. It is noted that no bending moment is applied to the top of the wall pier, which is largely due to the difficulty in providing such external moment in the laboratory. However, according to the structural analysis on the coupled-wall system, it is reasonable to assume a very small bending moment at the midheight cross-section of the wall piers, which will not have noticeable influence on the structural responses of the test subassemblies.
After applying axial load, the lateral cyclic load was applied in two stages. The first stage was a preloading process, when two cycles of 50 kN load level were carried out to ensure the loading system was fully engaged with the test subassembly. The subsequent loading stage was displacement-controlled [
28]. The top lateral drift ratio
θ was chosen as the deformation index, defined as the ratio of the top lateral drift, Δ, to the total structural height measured from the centerline of the horizontal actuator to the top surface of the foundation. The wall pier was displaced two cycles to
θ equal to 0.25%, 0.5%, 0.75%, 1.0%, 1.5%, 2.0%, 3.0% and 4.0%. When the measured lateral capacity decreased to less than 85% of the peak lateral load, the test was terminated.