## 1 Introduction  
Concrete Filled Tube (CFT) structures are widely used in engineering due to their excellent structural and constructional performance [1; 2]; this is especially the case for typical forms of Concrete Filled Steel Tube (CFST) structures [3; 4; 5; 6; 7; 8; 9; 10]. Attention also has been paid to Concrete Filled Fiber Reinforced Polymer (FRP) Tubes, in view of their anti-corrosion properties, which can be better adapted to corrosive environments than CFST [11; 12; 13; 14; 15; 16]. However, compared with concrete filled FRP Tubes (CFFT), CFSTs are preferred when involving transverse impacts, because steel has much better toughness than FRP when subjected to direct impacts. Throughout the whole life cycle, structures, including the CFTs, may be subjected to transverse impacts from accidental or deliberate events. Accurate vulnerability assessments are crucial in the design process to ensure the safety of CFT composite structures when faced with unexpected loads [17]. At present, structural damage detection [18; 19; 20] and health monitoring methods [21; 22; 23; 24] have been developed to monitor the damage status and well-being of a structure in real time. Simultaneously, experimental and numerical simulation studies have also been conducted to predict the behavior of structure under unexpected loads, such as impacts, explosions, and other high dynamic loads [25; 26; 27; 28; 29].  
In recent years, experimental and theoretical studies on the behavior of CFSTs under transverse impact have been broadly conducted. Axial impact experiments on twenty-one circular CFST columns were carried out by [PERSON] et al. [30]. [PERSON] [31] and [PERSON] et al. [32] experimentally investigated the performance of square CFST members subjected to lateral impacts at the beam mid-span. [PERSON] et al. [33; 34; 35] experimentally investigated the impact performance of CFST members and FRP-concrete-steel double skin tubular members. The failure modes and the time history of the impact forces for those members under lateral impact were obtained. In addition, an investigation of the behavior of concrete filled double steel tubular members under lateral impact using ABAQUS 2016 was presented. Subsequently, the experimental and theoretical studies on the behavior of high-strength concrete-filled steel tubes under transverse impact loading were conducted by [PERSON] et al. [36]. [PERSON] et al. [37; 38] experimentally and theoretically developed a study on the performance and dynamic behavior of carbon FRP strengthened CFST members subjected to lateral impact.  
Apart from the aforementioned work [30; 31; 32; 33; 34; 35; 36; 37; 38], some other studies [39; 40] have been reported on CFST members subjected to transverse impact. The majority of these studies demonstrate that CFST members with different configurations have excellent transverse impact resistance. However, there is a lack of investigation into the performance of CFST members after exposure to transverse impact loads. [PERSON] et al. [41; 42] conducted research on residual axial bearing capacity, but the sample was only aimed at hollow steel tubes. Therefore, it is of great importance to investigate the residual bearing capacity of CFCSTCs after transverse impact.  
This paper presents an experimental and theoretical investigation into the residual compressive bearing capacity of CFCSTCs after transverse impact. In this research, the effects of drop-hammer transverse impact height (\(H\)), and ratio of outer diameter to thickness (\(D\)/t) of outer steel tube on the residual compressive bearing capacity of CFCSTCs after transverse impact load, are experimentally investigated. Moreover, the failure mode, load-displacement curves, load-strain curves, and residual axial bearing capacity of those samples are extensively investigated based on the experimental results. Furthermore, a finite element analysis (FEA) model is developed to predict the residual compressive bearing capacity of CFCSTCs after transverse impact load with different \(D\)/t ratios.