## 6 Conclusions  
The mechanical behavior of a steel-concrete connection in a composite truss bridge was experimentally investigated. Pull-out tests of eight specimens were carried out to assess the bearing capacity of PBL shear connectors at different embedded depths. The connection performance in the composite truss bridge was evaluated through a model test on three composite joint specimens. Based on the experimental results, the following conclusions can be drawn:  
1. The experimental results of the pull-out and composite joint test indicated that the pull-out specimens (except PC) and the steel-concrete composite joints showed excellent resistance to an external load. The ultimate load capacities of the PA and PB specimens were about 1133 kN and 933 kN, respectively. The ultimate bearing capacities of all composite joints in this study were higher than 6000 kN and over 2.93 times the design load. Hence, these results show that PBL shear connectors exhibit excellent connection performance in resisting and transferring shear force.
2. The load-slip relations and the experimental phenomena of pull-out specimens showed that the embedded depth and the existence of perforated rebars played an important role in the ultimate bearing capacity, deformability, and even the failure mode of PBL shear connectors. Resulting from increased embedded depth, the ultimate load and the corresponding slip of PA specimens were on average 21% and 408% bigger, respectively, in comparison with those of PB specimens. Furthermore, compared with PB specimens, PA specimens underwent ductile failure rather than brittle failure. Similarly, the average ultimate bearing capacity and the corresponding slip of PA specimens were increased by 110% and 633%, respectively, in comparison with those of PC specimens, because of the existence of perforated rebars in the PA specimens.
3. Increased embedded depth can enhance the stiffness of composite joints in a composite truss bridge. For instance, the joint stiffness increased from 37.61 \(\times\) 10\({}^{4}\) kN/m in CJB1 to 43.38 \(\times\) 10\({}^{4}\) kN/m in CJA because the embedded depth was increased from 130 mm to 200 mm.
4. The experimental data of composite joints revealed that there was an interesting distribution of strain on the exposed gusset plate and the concrete chord. The strain decreased from G8 to G1 in the same row and from G13 to G23. Similarly, the diminution of strain occurred in the same column from G9 to G12 as well as from G14 to G17. Consequently, the area enclosed by G8, G9, G14, and G13 was the key area of the exposed gusset plate. In addition, the compressive strain in the first column of gauging points was bigger than that on the concrete chord, and the strain on the concrete chord increased from top to bottom in the vertical direction.  
Writing--original draft manuscript, Y.T.; Conceptualization, B.Z.; Investigation, T.Y., B.H. ([PERSON]) and X.W.; Visualization, Y.T.; Writing--review and editing, Y.T., W.Y. and B.H. ([PERSON]).  
The research described in this paper was supported by the National Natural Science Foundation of China (No. 51178397), the Major Applied Basic Research Frontier Projects of Sichuan Province (No. 2017Y0003), the Railway Scientific Research and Development Project of the Ministry of Railways of China (No. 2010G019-E), and the Scientific Research and Development Project of the China Railway First Survey and Design Institute Group Co. Ltd. (No. 16-18).  
The authors declare no conflict of interest.