## 1 Introduction  
Concrete-filled steel tubes (CFT) exhibit a satisfactory load capacity, sufficient ductility, and adequate energy absorption capability. Additionally, the steel tube functions as a mold for pouring the concrete, leading to cost savings in construction. With the tube serving as both longitudinal and lateral reinforcement for the concrete core, no additional reinforcement is required. The enhanced structural characteristics of CFT members are primarily attributed to the combined effect of the steel hollow section and the core concrete. This composite action results in the core concrete being subjected to triaxial stress, while also preventing the steel hollow section wall from buckling inward [1]. [PERSON] [2] proposed analytical models for predicting the strength of circular CFT beam-columns, assuming that confinement enhances both the strength of the concrete core and its ductility. Furthermore, [PERSON] and [PERSON] [3], and [PERSON] and [PERSON] [4] have proposed analytical models for predicting the strength of rectangular CFT beam-columns where these models assume that confinement only enhances the ductility of the concrete core, not the strength. [PERSON] et al. [5] proposed design formulas to predict the axial compressive load capacity and ultimate moment of CFT columns with circular or square cross-sections. [PERSON] and [PERSON] [6] suggested that the hoop stress on steel tubes for CFT columns is about 20% of the tensile yield stress of steel. [PERSON] et al. [7] developed a model for circular tubes that considers the effect of geometry and material properties on strength enhancement and post-peak behavior. [PERSON] et al. [8] proposed appropriate stress-strain relationships for concrete confined by steel tubes of various shapes, adopting [PERSON] et al. 's [7] model for circular columns. [PERSON] et al. [9] conducted model tests to determine a stress-strainrelationship for confined concrete and proposed a method to analyze the ultimate behavior of concrete-filled box columns, considering the local buckling of component plates and initial imperfections. However, there is limited research on the pure flexural behavior of concrete-filled hollow structural section beams. [PERSON] [10] proposed that the flexural strength derived solely from the capacity of steel tubes was approximately 50% lower than that of the tested specimens. Subsequently, [PERSON] et al. [11], [PERSON] and [PERSON] [12], and [PERSON] and [PERSON] [13] conducted experimental investigations on numerous tests, revealing that the beam specimens exhibited adequate behavior and failed in a highly ductile manner. This research offers a thorough and detailed examination aimed at forecasting the complete behavioral range of both rectangular and circular concrete-filled tube (CFT) members when subjected to a combination of axial forces and bending moments. The study is systematically organized into two distinct yet complementary sections, each significantly enhancing the overall comprehension and practical application of CFT structural components. The initial section focuses on the creation and application of various advanced finite element models. These models are intended to replicate a diverse array of loading conditions, including both concentric and eccentric axial loads on columns, as well as beams experiencing pure flexural loading. The main goal of this section is to establish a comprehensive and reliable response database that encapsulates the intricate behavior of CFT members under different loading scenarios.