## 5 Conclusions  
This paper proposes an optimal layout method of shear connectors for a steel-concrete composite beam with interface slip that is based on a one-dimensional composite beam element model. The proposed composite beam element model consists of two beam components, which are considered to be a Euler-Bernoulli beam, and the corresponding element stiffness matrix is derived by adopting the total potential energy method. After the efficiency of the developed element is validated by comparisons with experimental data and closed-form solutions, we arrive at an optimal layout method for the uniform distribution of shear connectors based on the developed element. The main conclusions drawn from this study can be summarized as follows:  
(1) The proposed composite beam element model is a unified formulation and is applicable to any complex load conditions, particularly cases with an arbitrary stiffness and arbitrary distribution of shear connectors. The results obtained using the developed program are in good agreement with experimental data in the elastic stage, and are also in good agreement with those of closed-form solutions.  
(2) Compared to the uniform distribution method, the proposed connector layout method can effectively decrease the interface slip and deflection of composite beams and increase the composite effect and stiffness without increasing the number of shear connectors.  
(3) Based on the developed composite beam element model, the proposed connector layout method can be applied under arbitrary load conditions, and the type of distribution does not need to be determined afresh for different load cases.  
(4) The 3-stepped rectangle distribution method is an optimal layout for the shear connectors of composite beams, since the distribution type of the shear connectors can not only decrease the interface slip and deflection, but will also clearly never result in the locations at which maximum interface slip occurs moving from the beam ends toward the mid-span.  
However, the developed composite element model was verified only by using the experimental data and closed-form solutions provided in the paper, and the proposed connector layout method is applied to a simply supported beam. In the future, further numerical simulations and experimental research need to be conducted to verify the developed element model and to confirm the efficiency of the proposed connector layout method.  
**Author Contributions:** [PERSON] and [PERSON] contributed to the idea and funding support for this paper, [PERSON] contributed to the methodology, [PERSON] and [PERSON] contributed to the writing and revision of this paper.  
**Funding:** The work is supported by the National Natural Science Foundation of China (No. 51678156), the National 13 th Five-Year Research Program of China (Grant No. 2016 YFC0700706) and Traffic Science and Technology Project of Fujian Communication Department (No. 201536), Educational and Scientific Project of Education Department of Fujian Province, China (No. JT180348).  
**Acknowledgments:** The authors would like to thank the anonymous reviewers for their constructive comments and suggestions.  
**Conflicts of Interest:** The authors declare no conflict of interest.