## 1 Introduction  
Steel and concrete composite structures have various constructional, mechanical, and economic advantages that enable them to be applied to wide range of civil engineering applications. The most benefits of composite structures can be achieved through the efficient balanced design between steel and concrete members. A shear connector is the key element in composite structures, which should guarantee the composite action and transfer the shear force between two different material bodies. Due to the various types of composite structures, many different types of shear connectors such as a stud [1], channel [2], plate perfobond [3], Y-type perfobond rib [4], and composite dowel [5] have been developed to maximize the efficiency of composite action. The evaluation of shear resistance of a shear connector is essential in the composite structure design. Many studies have developed the shear resistance formula considering various design variables. Many shear resistance formulas for a stud shear connector have been proposed considering various conditions since [PERSON] et al. [6] first suggested it. Recently, many studies have been developed considering various design variables, such as shank diameter, stud length, material properties, loading directions [7, 8], and stud spacing [9]. The plate perfobond shear connector was developed first in Germany to overcome the fatigue problems of stud. [PERSON] and [PERSON] [10] suggested a shear resistance formula for a plate perfobond shear connector considering strengths of steel and concrete as well as the rib shape. [PERSON] et al. [11], [PERSON] et al. [12], [PERSON] al. [13], and [PERSON] et al. [14] have considered the distance between two adjacent plate perfobond shear connectors, geometries of the perfobond rib, and material properties. [PERSON] et al. [15] compared the previously proposed shear resistance formulas and suggested a new formula. [PERSON] et al. [16] presented a shear resistance formula for composite dowels with PZ and CL shapes, and verifications for a beam-type section. [PERSON] et al. [4] introduced the Y-type perfobond rib shear connector as a new type of perfobond shear connector. The Y-type perfobond rib shear connector is one of the improved perfobond types that provides better structural performances and workability in the rebar assembly. Based on the push-out tests as well as numerical investigations, a shear resistance formula of the Y-type perfobond rib shear connector has been suggested and improved with additional experimental works by changing the basic design variables, such as rib size, transverse rebar size, concrete strength, etc. The previous shear resistance formula has been designed to represent the individual contributions, such as the end bearing resistance, transverse rebar resistance, dowel hole resistance, Y-shape rib resistance, etc. The basic design variables, such as concrete compressive strength, Y-rib size, the number of Y-ribs and rebars, the dowel hole size for transverse rebars, the number and size of concrete blocks between adjacent Y-ribs, etc., have been included [4,17,18,19]. Even though the previous formula provides high accuracy to predict the shear resistances, it is a little complicated and a limitation to extend the application range. This study is planned to extend the application range of the shear resistance formula for the Y-type perfobond rib shear connector and to improve the accuracy of the shear formula. Additional experimental specimens are designed based on the previous experiments. The new shear resistance formula is developed to represent the contributions of the main design variables, such as number of ribs and rebars, concrete strength, rib size and material strength of steel plate, rebar size and material strength of rebar, etc., rather than the contributions of individual resistance actions. The additional push-out test specimens are designed to supplement previous experimental works.