## 1 Introduction  
In case the reinforced concrete footing does not have enough bearing capacity to support various types of loads transmitted from the upper structure, steel pipe piles can be used to provide additional bearing capacity to the foundation. In order to guarantee the safe load transfer from the upper structure to the supporting layer of rocks, it is highly important to integrate the steel piles into the concrete footing slab. This can generally be achieved by the use of steel reinforcing bars (rebars) embedded into the concrete footing. However, it is not easy to retain a sufficiently high level of bond strength between the reinforcing bars and surrounding concrete due to their limited rib size and small cross-sectional area unless a large number of rebars are used at the pile cap connection. This may complicate the processes required with the use of rebars, such as their distribution into concrete footing and the usage of shear keys, and become a critical issue when designing a pile cap connection subjected to seismic loads as discussed in [1,2]. In order to address this issue, the perfobond shear connector illustrated in Figure 1 was proposed in [3]. This type of perfobond shear connector can guarantee an excellent composite behavior among the structural components at the steel pile cap. This device is basically a steel plate with holes and embedded into concrete to ensure the perfect integration of steel members and surrounding concrete. As external loads are applied, the dowel action of concrete inside the hole is activated and enables the load from the upper structure to be safely transferred to the steel pipe piles. This perfobond shear connector was originally developed to ensure the composite behavior of steel beams and slab concrete by [4]. Since then, it has been extensively utilized in the construction industry and developed into various forms by many researchers [5; 6; 7; 8; 9]. The proposed perfobond shear connector can be installed at the pile cap connection by following the four steps illustrated in Figure 2. In the first step, the unnecessary portion of the steel pile cap is removed via gas cutting with the help of a guide supporter as shown in Figure 2a. Then, bolting holes are punched at the upper locations of the steel pile cap using a punching machine in Figure 2b. As a third step, a bottom circular form is installed inside the steel pile, and the perfobond shear connectors are connected into the steel pile using high-tension bolts as illustrated in Figure 2c. In the last step, steel rebars are distributed in the concrete footing, and penetration rebars are put into the holes of the perfobond connectors as shown in Figure 2d. After all of these steps are completed, concrete can be poured into the footing. It can be noted from this that the construction procedure of the proposed perfobond connector is much simpler and less time consuming than that of the strengthening method using steel rebars. Figure 1: Perfobond shear connector for steel pile cap strengthening. (**a**) Perfobond shear connector with closed holes; (**b**) Perfobond shear connector with open holes; (**c**) Connection between the steel pile and perfobond shear connectors. Figure 2: Perfobond shear connector for steel pile cap strengthening. (**a**) Cutting of steel pile cap; (**b**) Hole punching on the steel pile cap; (**c**) Installation of perfobond shear connectors by bolting; (**d**) Distribution of steel reinforcing bars at concrete foundation. Continuing the research in [3], the structural performance of the steel pile cap strengthened with the proposed perfobond connectors under lateral loading were experimentally investigated. A bending test was conducted on seven specimens by considering the type of perfobond shear connectors, infilled concrete depth, and the number of L-shaped steel plates as test parameters. The failure characteristics of the test specimens were examined, and their load-displacement curves were analyzed.