*4.4. Strength Degradation*

The degradation coefficient (η) of the bearing capacity of the same displacement cycle is the ratio of the maximum loads of the last and first cycles. The degradation curves of the bearing capacity of the specimens are depicted in Figure 14. The values of η on the main stages are presented in Table 6. Figure 14 and Table 6 demonstrate the following:

(1) The steel frame is a typical flexible structure with good deformation capacity, and its strength degradation is unclear before the peak load. In SFIRACSWs, the walls act as the first seismic fortification lines that resist most of the horizontal load. The concrete at the diagonal compressive strips is gradually crushed and dropped, and the bearing capacity of the structure decreases sharply through the continuous expansion of the cracks in the wall.

(2) The degradation coefficients of the bearing capacity of SPE1, SPE2, and SPE3 are more than 0.97 when the horizontal drift angle is less than 0.01 rad. The degradation coefficients remain more than 0.80 when the horizontal drift angle is 0.02 rad, indicating that the specimens of infilled cast-in-place RACSWs also have a high safety reserve.

(3) The degradation coefficients of the bearing capacity of SPE4 and SPE5 are close when the horizontal drift angle is less than 0.02 rad. The degradation law is consistent, denoting that the damage degree of the specimens of infilled prefabricated RACSWs is the same under the same drift angle. Furthermore, the connecting stiffness of BCJs slightly influences the strength degradation of the specimens of infilled prefabricated RACSWs. The degradation coefficients of the bearing capacity of the specimens of infilled prefabricated RACSWs are more than 0.85.

**Figure 14.** Strength degradation curves.

**Table 6.** Values of η on main stages.


## *4.5. Ductility Analysis*

Displacement ductility factor is the ratio of damage displacement Δu to yield displacement <sup>Δ</sup>y, which is an important index for measuring the deformation capability of a structure. The inter-story drift angles and displacement ductility factors of the main stages are listed in Table 7, where Δcr, <sup>Δ</sup>y, Δmax, and Δu are the cracking, yield, peak, and damage displacements of the specimen, respectively; and θcr, <sup>θ</sup>y, θmax, and θu are the cracking, yield, peak, and damage drift angles, respectively. Table 7 shows the following:

(1) The displacement ductility factor of the pure steel frame is 3.47. The displacement ductility factors of the specimens are from 2.44 to 2.69 when the cast-in-place RACSWs are infilled. The infilled walls can increase the bearing capacity and initial stiffness of the structure while reducing the yield and damage displacement of the structure. Thus, the ductility of the specimens of infilled cast-in-place RACSWs is reduced.

(2) The displacement ductility coefficient in an SPE1 infilled ordinary concrete wall is 1.34 times that of SPE2, indicating that the wall made of recycled coarse aggregate has poor bonding performance and ductility.

(3) The inter-story drift angles are from 1/415 to 1/317 at the concrete cracking stage, from 1/116 to 1/114 at the yield stage, and from 1/66 to 1/64 at the peak point, thereby indicating that the specimens of infilled cast-in-place RACSWs have a good deformation capacity. The displacement ductility factor is approximately 10% higher in SPE3 than in SPE2, suggesting that the specimen of end-plate joints is simple to construct and has a good deformation capacity, and the ductility is better in the end-plate joints than in the welded–bolted joints.



*Appl. Sci.* **2019**, *9*, 4723

(4) The displacement ductility factors of the specimens are from 3.32 to 3.40 when the prefabricated RACSWs are infilled. The overall deformation is restrained by the walls, and the horizontal displacements are smaller in the main stages than those in the pure steel frame, although the bearing capacity and lateral sti ffness of the specimens of infilled prefabricated RACSWs are remarkably increased. Consequently, the displacement ductility factors are slightly lower in the specimens of infilled prefabricated RACSWs than those in the pure steel frame. The inter-story drift angle is from 1/619 to 1/337 at the concrete cracking stage, from 1/139 to 1/137 at the yield stage, and from 1/60 to 1/52 at the peak point.

#### *4.6. Energy Dissipation Capacity*

The energy dissipation capacity of the specimens is expressed by the relation curves between the hysteretic loop area and the horizontal drift angle, as shown in Figure 15. The values of energy dissipation in the main stages are presented in Table 8.

(1) The energy of the specimens of infilled cast-in-place RACSWs are dissipated mainly by the flexible deformation of the steel frames and the coarse aggregate friction and bite of the cracked surface of the RACSWs. The energy dissipation is 3.25 times higher in SPE2 infilled cast-in-place RACSWs than in the pure steel frame when θ = 0.005 rad, and 2.6 times higher in SPE2 than in the pure steel frame when θ = 0.02 rad.

(2) The energy dissipation of SPE2 infilled RACSWs is 41% that of SPE1 infilled ordinary concrete wall when θ = 0.005 rad, and 56% that of SPE1 when θ = 0.02 rad. The energy dissipation is approximately 13% higher in SPE3 than in SPE2 when θ = 0.005 rad, and approximately 28% higher in SPE3 than in SPE2 when θ = 0.02 rad, thereby indicating that the end-plate joints are fully deformed and characterized by excellent energy dissipation during loading.

(3) The energy of the specimens of infilled prefabricated RACSWs is dissipated mainly by the flexible deformation of the steel frames and the coarse aggregate friction and bite of the wall cracks and friction slip among the connectors. In the early stage of loading, the cracks on the wall of SPE5 occur early and the concrete cracking load is low; the energy dissipation is slightly higher in SPE5 than in SPE4. With the increase in displacement, the energy dissipation of the two specimens becomes the same. The energy dissipation capacity is approximately two times higher in the specimens of infilled prefabricated RACSWs than in the pure steel frame.

(4) Compared with the pure steel frame, infilled RACSWs can greatly improve the sti ffness and energy dissipation capacity of SFIRACSWs while reducing the ductility of the structure, thereby indicating that infilled RACSWs strongly influence the hysteretic behavior of SFIRACSWs.

**Figure 15.** Energy dissipation curves.


**Table 8.** Values of energy dissipation at main stages.
