## 4 Steel-Concrete Composite Joint Test Setup  
### Description of Composite Joints  
A steel-concrete composite continuous truss bridge (3 \(\times\) 60 m) was built with the intention of bearing a China Railway standard live loading [48]. This bridge of 12.2 m width consisted of an upper concrete slab, steel truss webs, and a bottom concrete slab, as shown in Figure 5. The maximum axial force (8200 kN) of the composite bridge appeared at E9. Hence, E9 was selected as a typical composite joint to be tested. Considering the limits of the reaction floor and testing devices, a geometric similarity ratio of 1:2 between the specimen and prototype joint was adopted in this test. In addition, the load similarity ratio was 1:4. Therefore, the design load of the composite joint with a scale of 1:2 was  
\begin{table}
\begin{tabular}{c c c c c c c} \hline
**Specimen** & **N\({}_{\textbf{y}}\) (kN)** & **N\({}_{\textbf{u}}\) (kN)** & **N\({}_{\textbf{u}}\)/N\({}_{\textbf{y}}\)** & **S\({}_{\textbf{y}}\) (mm)** & **S\({}_{\textbf{u}}\) (mm)** & **S\({}_{\textbf{u}}\)/S\({}_{\textbf{y}}\)** \\ \hline PA1 & 750 & 1150 & 1.53 & 0.5 & 12.13 & 24.26 \\ PA2 & 750 & 1100 & 1.47 & 0.54 & 9.06 & 16.78 \\ PA3 & 750 & 1150 & 1.53 & 0.47 & 10.05 & 21.38 \\ Average & 750 & 1133.33 & 1.51 & 0.5 & 10.41 & 20.81 \\ PB1 & 700 & 900 & 1.29 & 0.45 & 1.24 & 2.76 \\ PB2 & 700 & 1000 & 1.43 & 0.58 & 3.4 & 5.86 \\ PB3 & 650 & 900 & 1.38 & 0.4 & 1.5 & 3.75 \\ Average & 683.33 & 933.33 & 1.37 & 0.48 & 2.05 & 4.12 \\ PC1 & - & 540 & - & - & 1.53 & - \\ PC2 & - & 540 & - & - & 1.31 & - \\ Average & - & 540 & - & - & 1.42 & - \\ \hline \end{tabular} N\({}_{\textbf{y}}\): the yield load; N\({}_{\textbf{u}}\): the ultimate load; S\({}_{\textbf{y}}\) and S\({}_{\textbf{u}}\): the slip corresponding to yield and ultimate load, respectively.  
\end{table}
Table 2: Major experimental characteristics of pull-out specimens.  
Figure 4: Load–slip curves of specimens. (**a**) PA specimens; (**b**) PB specimens; (**c**) PC specimens; (**d**) the comparison of PA2, PB2, and PC2.  
2050 kN. These composite joint specimens were meticulously designed based on the prototype design, which was proposed by the China Railway First Survey and Design Institute Group.  
To understand the mechanical behavior of composite joints with different embedded depths, three composite joints with a scale of 1:2 were tested. In contrast to the welded joints in the previous study, high-strength bolted connections were installed between the gusset plate and steel web member in all composite joint specimens (denoted CJA, CJB1, and CJB2). The embedded depths of CJA and CJB were 200 mm and 130 mm, respectively, as shown in Figure 6. The composite joint consisted of the concrete chord (3500 \(\times\) 600 \(\times\) 800 mm), gusset plates (1640 \(\times\) 1240 \(\times\) 24 mm), and steel web members (350 \(\times\) 375 \(\times\) 1180.4 \(\times\) 16 mm). The gusset plates were perforated with 36 openings of 40 mm diameter. In addition, perforated rebars of 12 mm in diameter traversed the openings to transfer force between the concrete chord and steel gusset plate. The mechanical properties of the materials are listed in Table 1.