# Seismic Performance of LRS-FRP-Concrete-Steel Tubular Double Coupling Beam
## 5 Conclusions  
In this paper, a PET-FRP sheet and steel tube were applied to form FRP-concrete-steel double-skin tubular (DST) composite coupling beams. The seismic performance was tested on three types of double coupling beams (DCBs) (namely RC-DCB, LRS-DCB, and DST-DCB), through an experimental program. The following conclusions were drawn:  
1. The ductility and energy dissipation performance of the double coupling beams either wrapped with a PET-FRP sheet or consisting of an FRP-concrete-steel DST composite system is improved significantly compared to the traditional RC-DCB. The ductility coefficient and accumulate energy dissipation of the DST-DCB members based on the rupture of the PET-FRP sheet are similar to that of the LRS-DCB, which improved above 170% and 2300%, respectively, compared to the RC-DCB.
2. The external wrapped PET-FRP sheet does not affect the initial stiffness and peak strength of the RC-DCB. However, under the same conditions, the inner steel tube will improve the initial stiffness, yielding strength, and peak strength.
3. The DST double coupling beams still have considerable deformability after 85% of peak strength has been distributed since the external PET-FRP sheet was able to provide an effective constraint effect on the core concrete and the inner steel tube allowing it to bear excellent shear deformation. The improved strength of the DST-CB can now serve as energy storage for the double coupling beam.  
What should be mentioned here is that, due to the limited number of test specimens, some parameters were not considered. For example, the influence of steel pipe diameter-thickness ratio on the seismic performance of the composite structure, which could be analyzed by using the finite element method. However, the numerical method in reference [33] indicated that the finite element model should use a reliable three-dimensional constitutive model for the concrete, which needs to fully consider the unique characteristics of PET-FRP confined concrete. Furthermore, based on the test and finite element analysis,  
Figure 15: Analysis of energy dissipation: (**a**) cumulative energy dissipation; (**b**) equivalent viscous damping coefficients.  
the equations for predicting the bearing capacity of the LRS-FRP-concrete-steel DST double coupling beams could also be further researched.  
Conceptualization, Y.Z.; methodology, L.S.; validation, B.H. and C.C.; formal analysis, Z.Z., B.H. and C.C.; investigation, Z.Z., Y.L. and Y.Z.; resources, Y.Z.; writing--original draft preparation, Z.Z. and Y.L.; writing--review and editing, Y.Z. and L.S.; visualization, Z.Z. and Y.L.; funding acquisition, L.S. and Y.Z.; project administration, Y.Z. and L.S. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation of China (Grants No. 51778371, 51878414 and 51978412), the National Key Research and Development Project of China (2018 YFE0125000), the Shenzhen City's basic research project (JCYJ2019080154805456), and the Guangdong Provincial Key Laboratory of Durability for Marine Civil Engineering (SZU, 2020B1212060074).  
Not applicable.  
The data presented in this study are available on request from the first author and corresponding authors.  
The authors declare no conflict of interest.