## 2 Review of Previous Studies of Steel Composite Bridge Shear Connectors
Despite limited testing, it was reported that reductions in the strength of shear connectors with bedding layers were prevented by installing shear keys. [PERSON] et al. (2020) reviewed the static strength and fatigue design formulas for shear connector designs in various countries and used two-face symmetric shear tests and analysis to reevaluate the single face asymmetric shear tests by [PERSON] and [PERSON] (2017) that are the basis for the fatigue design curve presented in AASHTO LRFD. In total, 150 data were collected from 12 fatigue test programs around the world, and comparative analysis was performed. The analysis results confirmed that most of the experimental results exceeded those of [PERSON] and [PERSON] (2017). From this finding, it was determined that the AASHTO LRFD fatigue design produces conservative results because a relatively low fatigue load bearing capacity was measured due to imbalances during the one-sided shear tests at that time. In addition to experimental research, various studies have been conducted using analytical methods. [PERSON] et al. (2018) compared the experimental results of other researchers through 3D FE nonlinear analysis of steel-concrete composite girders. A concrete damaged plasticity (CDP) model was loaded in the ABAQUS program to simulate the cracking and fatigue modes of concrete in a stable manner. [PERSON] (2019) compared analytical reviews regarding the use of the ABAQUS smeared crack model and CDP model on a concrete cube 3D model with a size of 150 mm. [PERSON] et al. (2019) assessed the ultimate strength behaviors of steel-concrete composite structures through 16 push-out experiments and bending tests on six beams. In addition, they verified the experimental results through FE analysis. In contrast, relatively few studies have been performed on bolted shear connectors in comparison to welded shear connectors, and national design codes do not contain rules on bolted shear connectors. Bolted shear connectors are presented as alternatives that enable quick installation and removal of precast slabs, as shown in Figures 2 and 3 (2020). Figure 2: Prefabrication with use of casted bolted shear connectors (2020): (**a**) assembling method; (**b**) connection after assembling; (**c**) assembled composite beam. Figure 3 shows the shapes of the main bolted shear connectors that have been studied previously. The connecting action is affected by the presence of embedded nuts in the concrete slab, number of embedded nuts (single or double embedded nuts), and resultant preloading of bolts. Friction grip bolts transfer shear force through friction between the steel girder flange and the concrete slab caused by the bolt load. Initially, at less than a certain level of shear force, resistance occurs via frictional force; then, slipping occurs and the shear connector ultimately resists shear. Because the initial slip resistance increases in proportion to the bolt load, the slab receives high compressive stress in two directions. Therefore, there are many cases in which helix-shaped reinforcers are used around the bolt holes to reinforce the concrete, which receives high local stress [21]. [PERSON] et al. [25] performed static push-out tests and composite beam tests using friction grip bolts with a diameter of 16 mm to evaluate the effects of various concrete strengths. In these tests, a bolt load of approximately 90 kN was used, and a friction coefficient of approximately 0.45 was reported in the case of precast slabs. [PERSON] [26] performed single bolt shear tests on stud bolts without embedded nuts. The bolt diameter and height and the concrete strength were used as the experimental parameters. It was concluded that the shear strength was low, at 80% of the shear strength of welded studs, and the shear stiffness due to the slip around the hole perforation was only 15%. [PERSON] [27] used friction grip shear connectors that employ high-tensile bolts to reinforce a conventional non-composite bridge.