(3) Specimen SPE3

Specimen SPE3 was in the elastic stage with no observable behavior when the horizontal load was less than 180 kN. Numerous fine cracks appeared on the right side of the wall when the load was 200 kN. The cracks constantly expanded when the load was 280 kN. Then, the specimen yielded locally, and the load was applied by controlling the displacement.

The cracks at the control stage of displacement 1.0–3.0 δ*y* continued to expand and extend to the edge of the wall, thereby forming through cracks with a width of 2–3 mm along the diagonal direction (Figure 6a). The concrete partly fell off on both sides of the principal diagonal cracks when the displacement was 4.0 <sup>δ</sup>*y*, and the width of the cracks reached 4–5 mm (Figure 6b). Considerable concrete fell off at the intersection of the cracks along the diagonal when the displacement was 4.5 <sup>δ</sup>*y*. The local steel bars were exposed. Then, the flange at the end of the beam bulged upward. A large area of concrete fell off when the displacement was 5.5 <sup>δ</sup>*y*. Then, many holes appeared on the wall. Numerous steel bars were exposed and the wall was seriously damaged. Furthermore, the end-plate warped (Figure 6c). The middle part of the column bulged outward and the column base buckled (Figure 6d). Finally, the horizontal load decreased by more than 15%, and the specimen lost its carrying capacity.

 **6.** Local

failure of SPE3.

## 3.1.2. Prefabricated RACSWs

## (1) Specimen SPE4

Specimen SPE4 was in the elastic stage with no observable behavior when the horizontal load was less than 120 kN. An initial crack appeared at the lower right corner of the wall when the load was 140 kN. Multiple fine oblique cracks appeared in the wall center and extended when the load was 220 kN. The specimen yielded locally, then the load was applied by controlling the displacement.

In the displacement control stage, multiple short cracks with a width of approximately 1 mm appeared along the diagonal direction of the wall when the displacement was 1.5 <sup>δ</sup>*y*. The concrete at the lower right corner of the wall began to fall <sup>o</sup>ff, and the number of horizontal cracks gradually increased (Figure 7a). Cracks in the horizontal direction formed at the bottom of the wall when the displacement was 2.5 δ*y* (Figure 7b). The upper ear plate had a relative slip of approximately 10 mm with the T-shape connector (Figure 7c) accompanied by a friction sound among steel plates. Bending deformation occurred on the wall when the displacement was 4.5 <sup>δ</sup>*y*. Then, the corner concrete fell <sup>o</sup>ff, exposing the steel bars. The top flange of the steel beam exhibited a brittle fracture when the displacement was 5.0 δ*y* (Figure 7d), and the column base buckled locally. Finally, the horizontal load was reduced by more than 15%, and the specimen lost its carrying capacity.

**Figure 7.** Local failure of SPE4.

## (2) Specimen SPE5

Specimen SPE5 was in the elastic stage with no observable behavior when the horizontal load was less than 60 kN. Multiple fine oblique cracks appeared along the diagonal direction of the wall when the load was 80 kN. The number of fine cracks gradually increased and continuously expanded when the load was 120 kN. The specimen yielded locally, then the load was applied by controlling the displacement.

In the displacement control stage, intersecting cracks formed along the diagonal direction of the wall when the displacement was 2.5 <sup>δ</sup>*y*, and the principal cracks were connected (Figure 8a). The concrete at the lower right corner of the wall was crushed and fell off when the displacement was 3.5 <sup>δ</sup>*y*, exposing the steel bars. Multiple horizontal cracks with a width of 2–3 mm appeared at the bottom of the embedded T-shape connector and extended from right to left (Figure 8b). The lower left flange of the steel beam noticeably bulged upward when the displacement was 4.0 <sup>δ</sup>*y*. The upper ear plate had a relative slip of approximately 10 mm with the T-shape connector, accompanied by a friction sound. A large area of concrete fell off when the displacement was 6.0 <sup>δ</sup>*y*. Consequently, the steel bars were exposed, and cracks in the horizontal direction formed at the bottom of the wall (Figure 8c). The end plate warped (Figure 8d), and the column base buckled. Finally, the horizontal load decreased by more than 15%, and the specimen lost its carrying capacity.

**Figure 8.** Local failure of SPE5.

#### 3.1.3. Pure Steel Frame

The specimen SPE6 was in the elastic stage with no observable behavior when the horizontal load was less than 140 kN. The surface coating of the steel column webs fell o ff locally when the load was 160 kN. A slight bending occurred at the upper flanges of the two columns when the load was 180 kN, and the test entered the displacement control loading stage. The column bases yielded when the displacement was 3.0 <sup>δ</sup>y, and a slight out-of-plane instability occurred in the specimen. The flanges at the top portion of the left column and ends of the beam yielded when the displacement was 4.0 <sup>δ</sup>y. The structural capacity of the specimen constantly declined and was eventually lost [35].

Several key load points according to the test behaviors of specimens SPE1 to SPE6 are summarized in Table 3.


**Table 3.** Several key load points in the test process.

#### *3.2. Failure Modes*

From the behavior of the test specimens, the force process of the specimens can be divided into four stages: elastic, concrete cracking, yield, and damage stages. In the elastic stage, steel frames and infilled RACSWs combine to resist exterior loads. The initial sti ffness of the structure was high, and the load-displacement curves are linear with no observable behavior.

#### 3.2.1. Cast-in-Place RACSWs

The horizontal load applied to the specimens of infilled cast-in-place RACSWs was transferred to the walls by the shear studs. In the concrete cracking and yield stages, initial cracking occurred along the diagonal direction of the walls, and the sti ffness of the specimens decreased slightly after the cracking of the walls. The wall cracks gradually expanded and connected with the increase in horizontal load, finally forming three principal cracks with a width of 4–5 mm along the diagonal direction. The local concrete at the intersection of the principal diagonal cracks was crushed and fell <sup>o</sup>ff, and the energy was dissipated mainly by the coarse aggregate friction and bite of the cracked surface. A slight bulging deformation emerged at the column base and the end of the steel beam. Upon reaching the peak load, the specimens were in a damaged stage, and a large area of concrete fell

off on both sides of the principal cracks. The steel bars were exposed, and the local wall showed holes. The column base and beam end buckled. The bearing capacity and lateral stiffness of the specimens decreased sharply, and the failure modes are illustrated in Figure 9a.

**Figure 9.** Failure modes.
