# Seismic Performance of LRS-FRP-Concrete-Steel Tubular Double Coupling Beam
## 1 Introduction  
Shear wall structures and frame-shear wall structures are widely used in high-rise buildings. As an important structural member of these structures, the coupling beam is usually designed as the energy-dissipating member that produces a plastic hinge after yielding, making it the first line of defense for anti-seismic energy dissipation. Under the action of seismic load, the coupling beam will yield and fail, and act as energy dissipation, to avoid the premature failure of the wall limb. Due to the characteristics of the small span-height ratio and large stiffness of the connected shear wall, the coupling beam is prone to brittle shear failure. To improve the mechanical behavior of the coupling beam, scholars have developed many strategies since the 1970s. The research plans generally focus on improving the following three aspects: new reinforcement forms [1; 2], steel-concrete composite members [3], and using high-performance concrete [4]. However, these strategies are not ideal for improving the ductility or energy consumption of the coupling beam [4], and some of them even limit the thickness of the beam. With the development and application of fiber-reinforced polymer (FRP) in civil engineering, [PERSON] et al. developed a new type of hybrid FRP-concrete-steel double-skin tubular (DST) composite member in 2004 [5; 6], which consists of an external FRP tube, internal steel tube, and reinforced concrete (RC) in the sandwich. Different components in the system work well together. The outer FRP tube has a good restraint effect on thesandwich concrete, which improves the strength, plasticity, and toughness of concrete. At the same time, the confined concrete can also prevent the buckling of the internal steel tube. This type of composite member presents excellent anti-seismic performance, ductility, and corrosion resistance. In 2006, [PERSON] et al. [7] researched the flexural mechanical behavior of hybrid glass FRP (GFRP)-concrete-steel beams. They found this DST-GFRP composite beam to have excellent flexural capability and ductility, especially when the inner steel tube is reasonably offset toward the tensile zone. However, the bonding behavior between concrete and its steel tube interface needs to be improved. The test results indicate that interface slippage would result in obvious load fluctuations--even sudden drops, which is a threat to structural safety. The research carried out by [PERSON] and [PERSON] [8; 9] shows that the bond behavior could be improved significantly by using mechanical connectors. Meanwhile, the novel composite system presented in this article also presents good ductility under cyclic loading, and the ductility performance of the composite beam would not be significantly weakened by the inner steel tube filled with concrete. Moreover, the compressive behaviors of FRP-concrete steel double-skin tubular columns under both monotonic and cyclic loads have also been investigated by some scholars [10; 11; 12; 13; 14; 15; 16; 17; 18]. Except for the good ductility and anti-seismic performance, [13; 14] also indicated that the ductility could be improved by filling concrete at both ends of the inner steel tube, while [19; 20] recommended using a stiffener-reinforced steel tube. For the compressive behavior of large rupture strain FRP (LRS-FRP), the rupture strain was usually larger than 5% (as in the case of a polyethylene terephthalate, PET) wrapped concrete-steel column, some researchers [21; 22; 23] have also reported that the corresponding composite system has a good bearing capacity and ductility, with an ultimate compressive strain in some cases reaching up to 15%. Notably, [PERSON] et al. 's [24] test results show that the failure mode of the composite column tends to be a brittle failure when using ultra-high-performance fiber-reinforced concrete instead of traditional concrete. As just mentioned, the application of an FRP-concrete-steel DST composite member is mainly concentrated in the RC column and beam.