1. Introduction
Steel–concrete composite structural systems with shear connectors have excellent structural performance and economic feasibility, and have been employed in various fields for decades. In particular, beam–slab composite systems have been widely used in building and bridge structures, and are beneficial for withstanding extreme conditions such as fire and earthquake. Kodur et al. [
1,
2] studied the behavior of composite steel girders during a fire through experiments and analyses, while Kim et al. [
3], Bursi et al. [
4], and Nakashima et al. [
5,
6] experimentally analyzed composite structures under cyclic loading. The shear resistance secured by the shear connectors is determined using the design shear force. In addition, shear stiffness determines the degree of shear connection, and ductility prevents brittle failure of the shear connectors. The behaviors of composite beams with respect to shear connectors have been investigated by numerous researchers. Kim and Jeong [
7] conducted an experimental study to verify the ultimate behavior of a composite deck system with respect to steel sheets and perfobond rib shear connectors. They performed beam and push-out tests of the shear connectors and composite beams, and verified their load-carrying capacity. Qureshi et al. [
8] developed a three-dimensional nonlinear numerical model for a composite beam using profiled sheeting and stud shear connectors, and used the model to obtain the shear strength, relative slip, and failure modes. Vasdravellis and Uy [
9] performed an experimental and numerical study on the shear capacity and moment–shear interaction of composite beams. They aimed to assess the contribution of the concrete slab to the ultimate shear capacity of a composite section and develop a moment–shear interaction law for composite beams subjected to combined positive bending and shear. They found that a higher degree of shear connection increases the shear capacity of a composite beam, and the level of increase is higher in beams with a larger slab-depth slenderness ratio (depth of slab/depth of beam). In addition, the contribution of the slab is a linear function of the slab-depth slenderness ratio of the composite section. Shariati et al. [
10] conducted push-out tests of the channel and angle shear connectors in high-strength concrete to compare their shear strengths. The channel shear connectors were found to have higher ductility than the angle shear connectors, and higher performance due to the higher height of the connectors. Lasheen et al. [
11] compared the behavior of lightweight and normal weight concretes in eight composite beams with channel shear connectors. The findings showed that the lightweight concrete only slightly affected the load capacity in comparison to the normal weight concrete.
Studies on composite structures were first conducted in the 1920s. Caughey [
12] stressed the need for shear connectors that can resist horizontal shear force. The stud shear connector, which is commonly utilized in steel–concrete composite systems, has been studied for many years. In 1956, Viest [
13] performed a static load test by using a stud connector to propose an equation for shear strength, and modified this equation in the 1960s [
14]. Subsequently, the shear strength of stud shear connectors was studied by considering various variables, such as the cross-section, height, and tensile strength of the stud, as well as the elastic modulus and compressive strength of the concrete [
15,
16,
17]. Large stud shear connectors greater than 22 mm in diameter have also been studied [
18,
19,
20]. At a German design company, Leonhardt and Zellner [
21] developed a new type of a shear connector—that is, the perfobond rib shear connector—to solve the fatigue problem of stud shear connectors. Oguejiofor and Hosain [
22,
23,
24] compared the behaviors of the perfobond rib shear and stud connectors by analyzing the differences in their failure modes in push-out and beam tests. They then proposed an equation for evaluating the strength of the perfobond rib shear connector by considering the tensile strength of concrete, amount of transverse rebar, and location of holes. Valente and Cruz [
25] conducted an experimental analysis to compare shear behaviors, such as shear strength and ductility, of various connector types and conducted push-out tests for three types of shear connectors: stud, perfobond, and T-connector. Vianna et al. [
26,
27,
28] conducted a push-out test and numerical analysis on the T-type shear connector in a composite beam girder. The results displayed that the performance of the T-perfobond connector and shear resistance were affected by the thickness of the concrete slab. Lorenc et al. [
29,
30] performed an experimental study and a numerical analysis on composite dowels with puzzle-like shapes. Papastergiou et al. [
31] proposed a new type of shear connector using friction and bond effects, and identified its behavior through experimental analysis. The Y-type perfobond shear connector developed based on various types of shear connectors was observed to have outstanding shear resistance and ductility [
32], and exhibited good structural performance under the cyclic design load of bridges [
33]. To predict the shear strength of Y-type perfobond shear connectors, Kim et al. [
34,
35,
36] conducted push-out tests, beam tests, and numerical analysis and proposed shear resistance formulas by considering design variables.
In building structures, the shear force exerted on the composite frame by design loads is smaller than that on composite bridges. The existing Y-type perfobond rib shear connectors [
32,
33,
34,
35,
36] are designed for the girder slabs of composite bridges. Therefore, the rib and transverse rebars of the conventional Y-type perfobond rib shear connectors are extremely large for the composite frames of building structures. This conventional connector has a rib height and width 100 and 80 mm respectively, and the spacing of the dowel hole is 120 mm. However, the slabs of building structures have a smaller thickness than those of composite bridges, and the spacings of reinforcements for building structures are narrow. In addition, the diameter of transverse rebars used in composite buildings is smaller than that of composite bridges. Thus, the dimension of the shear connector should be modified to the stubby Y-type perfobond rib shear connector. To use Y-type perfobond rib shear connectors in composite frame structures, various design factors, such as the compressive strength of concrete, the height of the slab, and the diameter of the transverse rebar, must be considered. To this end, the current study proposed the stubby Y-type perfobond rib shear connectors for composite frames, and experimentally examined their shear strength and ductility through push-out tests. All the dimensions of the specimens were determined considering the concrete slab, and then the shear resistance, ductility, and fracture modes were confirmed at the shear connection area compared with the shear resistance equation.
3. Shear Strength and Ductility of Composite Structures using Stubby Y-Type Perfobond Rib Shear Connectors
The test objective was to analyze the change in the shear force according to the diameter of the transverse rebar for which the dimensions of the stubby Y-type perfobond rib shear connectors were fixed. This is because the stubby Y-type perfobond rib shear connector can be applied to various sizes of a transverse rebar used in composite building structures. To compare the shear strength and ductility based on push-out tests, the shear strength (P
u), characteristic resistance (P
rk), initial relative slip (δ
90), characteristic slip capacity (δ
uk), and slip capacity (δ
u) were defined, as shown in
Figure 4 [
32]. Eurocode-4 [
37] defines a shear connector as ductile if δ
uk > 6 mm. In addition, Kim et al. [
32] suggested using the ratio of the slip capacity and initial relative slip (δ
u/δ
90) to estimate the ductility in the inelastic behavior region of a shear connector by considering initial stiffness. Moreover, Kim et al. [
35] proposed Equation (1) to predict the shear strength of a Y-type perfobond rib shear connector.
Table 5 compares the tested and predicted shear strengths of SY-D13-M and SY-D16-M.
where
represents the shear resistance (kN),
is the diameter of the dowel hole (mm),
is the individual rib height (mm),
is the rib thickness (mm),
is the compressive strength of the concrete (MPa),
is the number of transverse rebars,
is the cross-sectional area of the transverse rebar (mm
2),
is the yield strength of the transverse rebar (MPa),
is the number of dowel holes,
is the number of dowel areas formed between the ribs bent in a Y-shape, and
is the net distance between the ribs bent in the same direction (mm).
Figure 5 and
Table 5 present the push-out test results. In the cases of SY-D13-M1/M2/M3, the shear strengths obtained were 925.2, 904.4, and 898.7 kN, respectively, and the average shear strength was 897.3 kN. The ductilities calculated according to Eurocode-4 [
37] 6.90 mm and the result obtained by the evaluation formula (δ
u/δ
90) suggested by Kim et al. [
32] was 4.82. In the cases of SY-D16-M1/M2/M3, the shear strengths obtained were 904.1, 907.7, and 939.7 kN, respectively, with an average of 912.17 kN. Moreover, the ductilities calculated according to Eurocode-4 [
37] were 10.01 and the result of the evaluation formula (δ
u/δ
90) suggested by Kim et al. [
32] was 6.21.
The difference between the shear strengths of SY-D13-M and SY-D16-M was 12.8 kN, with SY-D16-M exhibiting 1.4% higher shear strength. Based on these results, the effect of the change in shear strength due to the rebar sizes of D13 and D16 is not much. However, the load reduction is greater for SY-D13-M than for SY-D16-M, both of which satisfied the ductility standard for shear connectors defined by Eurocode-4 [
37]. The δ
uk of SY-D13-M was 6.90 mm, which slightly exceeds the ductility standard suggested by Eurocode-4 [
37], while that of SY-D16-M was 10.01 mm, which significantly exceeds the same standard. When evaluating ductility based on the initial stiffness, δ
u, δ
90, and δ
u/δ
90 of SY-D13-M were 7.67 mm, 1.59 mm, and 4.82, respectively, while those of SY-D16-M were 11.12 mm, 1.79 mm, and 6.21, respectively. The difference between the δ
90 values of SY-D13-M and SY-D16-M was 0.02 mm (11% for δ
90 of SY-D16-M), and the difference between their δ
u values was 36.45 mm (31% for δ
u of SY-D16-M). That is, larger-diameter transverse rebars show more ductility after yield strength than the initial shear behavior. Based on both ductility evaluation methods, shear connectors with large-diameter rebars are preferable in terms of ductility.
The variables applied to Equation (1) in [
32] to evaluate the shear resistance are listed in
Table 6. As a result, the shear strengths of SY-D13-M and SY-D16-M predicted using Equation (1) in [
32] were 803.5 and 1082.6 kN, and the experimental results were 894.6 and 907.4 kN, respectively. In the case of SY-D13-M, the average shear strength estimated in the push-out tests was 1.1 times the shear strength estimated using the equation. Moreover, the average shear strength of SY-D16-M in the push-out tests was 0.84 times the shear strength estimated using the equation. In other words, the measured shear strength of SY-D13-M was greater than the predicted shear strength, while that of SY-D16-M was lower than the predicted shear strength. As the difference between the measured and predicted strengths was approximately 13%, the shear strength equation for Y-type perfobond rib shear connectors can also be applied to stubby Y-type perfobond rib shear connectors. However, the influence of the transverse rebar was found to be overestimated.