## 1 Introduction  
Over the past few decades, the prevalence of concrete-filled steel tube (CFST) arches has steadily increased in the construction industry and bridge engineering, especially in large-span roofs and bridges. The application of CFST arches owes to their strong bearing capacity, large span potential, beautiful appearance, and convenient construction [1,2]. Also, as a composite structure, the CFST arches can enhance structural performance by fully utilizing the compressive and tensile strengths of steel and concrete, respectively, which greatly promotes the usage efficiency of materials [3]. In China, it has been roughly estimated that more than 400 CFST arch bridges have been built, and the largest span is over 500 m [4,5]. Among these various structure forms of CFST arches, single tube CFST arches make up a large proportion in small- and medium-span bridge constructions. Compared to arches with other types of cross-sections, such as dumbbell-shaped sections and truss sections, single tube CFST arches have simpler structural forms and mechanical behaviors. However, to date, the extensive use of single tube CFST arches has not led to deeper studies on their working behaviors. Theoretical and experimental research about arches has simply involved a calculating method, bearing capacity under in-plane loads or spatial loads [6; 7], and dynamic performance [8; 9], inclusive of considering the effects of initial stress [10] and initial imperfection, geometric and material nonlinearity, et al. [11; 12; 13]. [PERSON] [14] conducted a model experiment of two CFST rib arches under in-plane loads and found that they have outstanding ductility and bearing properties. [PERSON] et al. [15] used the finite element program to study the ultimate load-bearing capacity of long-span CFST arch bridges with fly-bird-type and the results indicated that the effect of both geometric and material nonlinearity could not be neglected. Using a CFST arch bridge with a span of 80 m, for example, [PERSON] et al. [16] studied its stability using the general finite element program. [PERSON] et al. [17] proposed a co-rotational formulation in the setting of finite elements for three-dimensional nonlinear analysis of the ultimate bearing capacity of a circular (CFST) arch, considering both geometric and material nonlinearities. [PERSON] et al. [18] tested six fixed CFST parabolic arches to study the effects of the rise-span ratio and the non-uniform moment. [PERSON] et al. [19] analyzed the out-plane elastic stability of typical single tube CFST arch bridges using the finite element method and provided a calculation method. From the literature above, the lack of sufficient large-scale experiments on single tube CFST arches to investigate their complex in-plane or out-plane working behaviors and failure mechanisms can be seen. In existing experimental analysis, the measured strain data has always not been fully utilized, leading to the neglect of extensive unseen information of structural working behavior characteristics, which could have been extracted from experimental strain data using the appropriate methods. Using the finite element (FE) method, the mechanics performance of structures will have greater clarities. Inevitably, the finite element model is often simplified or assumed by researchers with experience in structural design. The problem arises that the calculation results of structural responses using these simplified models always have deviation from the actual test results in varying degrees. Besides, all the experimental or simulative studies mainly investigated the effect of configurational/material parameters on the arch's responses from the displacement-load or strain-load relations. According to the existing empirical and statistical approximation, various assumptions and preconditions have been used to derive formulas for calculating the bearing capacity of specific arches in theoretical studies.