## Numerical Simulation and Parametric Analysis
### Discussions  
#### 4.4.1 Effect of the Damping Layer and the Reinforcement Enhancement  
Figure 12 shows the compressive behavior of the SCS and SDC walls with different types of reinforcement enhancements. It can be observed that embedding rubber layers, steel sheets, and (or) sleeves in composite walls can significantly change their ultimate compressive resistance and ductility. Specifically, SDC-no enhan, the specimen without any reinforcement enhancements, shows the 9.6% decrease of ultimate compressive resistance, compared with the SCS wall. This is because: (i) the rubber layer fails to provide effective lateral confinement for the concrete core due to its low elasticity modulus and (ii) setting the damping interlayers causes the separation of concrete and steel plate as well as concrete and binding bars. The former one leads to the decrease in the confinement of steel plate on concrete directly, and the latter one results in the loss of embedment effect of binding bars on concrete and the decrease of the tensile resistance of the binding bars which can determine the magnitude of the confinement on concrete provided by the steel plate [3]. To ensure sufficient lateral confinement, the composite walls with steel sheets and (or) sleeves are conducted. It can be found in Figure 12b that compared with the SCS wall, the walls with steel sheets (specimen SDC-sheet), sleeves (specimen SDC-sleeves), and both of them (specimen SDC-REF) show 5.0%, 2.4%, and 0.2% reduction of compressive capacity, respectively. This is because the steel sheets and sleeves improve not only the steel content of the composite wall but also the lateral confinement on the concrete core. Figure 13 shows the lateral Von-Mises stress (S22) contours of the concrete core of the five composite walls under the 85% ultimate compressive resistance (post-peak). In Figure 13e, the lateral stress (S22) of the concrete core of SDC-no enhan is marginal due to the lack of lateral confinement in spite of stress concentration induced by holes. Figure 13c shows higher S22 of concrete elements near steel sheets of SDC-sheet because of the lateral confinement on concrete core provided directly by steel sheets. Figure 13d shows a uniform and higher S22 of concrete element near sleeves of SDC-sleeve. This is because larger lateral expansion occurred in the concrete near the sleeves under compression due to the high elastic modulus of steel, which leads to higher reaction force on the concrete provided by the rubber layer and the steel plates. Moreover, it can be seen in Figure 13b that the concrete core of the SDC wall-REF is subjected to the largest lateral constraint even more than that of the SCS wall due to the combined action of steel sheets and sleeves. In such a case, the SDC walls have an equivalent ultimate compressive resistance of SCS walls which is essential for being used in high-rise buildings or special structures. Figure 12: The compressive behavior of SCS wall and SDC walls with different enhancements. Figure 12b also shows that, compared with the SCS wall, a 0.3% decrease in the ductility of the SDC-no enhan while the SDC-steel sheet, SDC-sleeve, and SDC-REF show 14.7%, 6.3%, and 15.8% increases in the ductility, respectively. The decrease of ductility of SDC-no enhan is also because of the insufficient lateral restraint due to the separation of the concrete core and the steel plate caused by embedding the rubber layer. It is known that the longitudinal compression failure of concrete materials is caused by lateral tensile cracking. Compared with the concrete core of the SCS wall restrained by steel plates under two-axial compression, the concrete core of SDC-no enhan is subjected to lower confining stress. When the concrete core reaches the ultimate compressive load, the longitudinal crack expands rapidly and its low tensile deformation ability results in brittleness.