## 3 Test Results and Discussion
### Effects of Test Parameters on the Bending Resistance Capacity of the Strengthened Steel Pile Cap  
This section discusses the effects of test parameters such as the type of perfobond shear connectors, infilled concrete depths, and the number of L-shaped steel plates on the bending resistance capacity of the steel pile cap strengthened with the proposed perfobond connectors. Figure 9a,b show the load-displacement curves of the D05_L3 and D10_L3 specimens to investigate the effect of perfobond shear connector types on their peak strengths, respectively. Here, D05_L3 specimens refer to the specimens with concrete infilled depth equal to the half of steel cap diameter (D05) and three L-shaped plates (L3). Hereafter, this notation is used throughout the section for convenience. The load-displacement curve of the reference specimen SB_D10_NL is also plotted in the two figures for comparison. The peak strengths and peak strength ratios of D05_L3 and D10_L3 specimens are listed in Tables 4 and 5, respectively. From these results, it can be seen that the specimens strengthened with the open-hole type perfobond connectors have higher bending resistance than those with the closed-hole type connectors, and almost equal strength to that of the reference specimen. For example, the ratios of the peak strength of the open-hole type perfobond specimens PO_D05_L3 and PO_D10_L3 to that of the reference specimen are 93.3% and 95.3%, respectively. This is remarkable considering that the concrete infilled depths of these specimens are only in the range of 200 mm and 400 mm, while that of the reference specimen is 700 mm as indicated in Table 1. Furthermore, as can be seen from Figure 9, their initial stiffness is almost equal to that of the reference specimen, and they retain the level of ductility comparable to that of the reference specimen. In addition, Tables 4 and 5 show that their peak strengths are approximately 15% to 20% greater than those of the closed-hole type perfobond specimens. This seems to happen since the open-hole type perfobond connectors have a cross-sectional area larger than the open-hole type connectors as illustrated in Figure 3.  
In order to examine the effect of concrete infilled depths on the bending resistance capacity of the specimens, the load-displacement curves of PC_L3 and PO_L3 specimens are plotted in Figure 10a,b, respectively. The load-displacement curve of the reference specimen is also included in the plot. Tables 6 and 7 provide the peak strengths and peak strength ratios of these specimens. It can be noted from these results that the peak strength of the specimen is increased with increasing concrete infilled depth. For example, in the case of PC_D10_L3 specimen, its peak strength is increased by 9.2% due to the increase of concrete infilled depth from 200 to 400 mm, while that of PO_D10_L3 specimen only by 2.2%. Therefore, the increase of peak strength due to increasing concrete infilled depth is more pronounced with closed-hole type perfobond specimens. As already observed previously, the open-hole type perfobond specimens have an approximately 10%-20% higher peak strength than those with closed-hole type perfobond connectors, and almost the same initial stiffness as the reference specimen. Figure 11a,b show the load-displacement curves of PC_D05 and PC_D10 specimens, respectively, and the effects of the number of L-shaped plates on the bending resistance capacity of the specimens are investigated. The load-displacement of the reference specimen is also included in both figures. Tables 8 and 9 list the peak strengths and peak strength ratios of these specimens. It can be seen from Figure 11a and Table 8 that the specimen with three L-shaped plates (PC_D05_L3) has a slightly smaller peak strength than the one with no L-shaped plates (PC_D05_NL). In contrast, Figure 11b and Table 9 indicate that the specimen with six L-shaped plates (PC_D10_L6) has a slightly higher peak strength than the one with three L-shaped plates (PC_D10_L3).