Performance And Design Method Of Square Concrete-encased CFST Members Under Combined Compression And Bending

Concrete-encased concrete-filled steel tube(CFST) has attracted the interests of structural engineers due to its good structural performance. It is necessary to provide the design methods based on the study on the performance of concrete-encased CFST. This paper investigated the performance and design method of two square concrete-encased CFST members under combined compression and bending, i.e. one is the circular CFST in the middle encased by square RC component(concrete-encased CFST member in the paper), another is a reinforced concrete(RC) box component and four CFSTs in the corners(concrete-encased CFST box member in the paper). The main study and achievements are summarized as follows:1. A finite element analysis(FEA) model of concrete-encased CFST member was developed, and the constitutive relations of different material and the interaction between concrete and steel tube were considered. The tested results were verified by the FEA model. Full range analysis on concrete-encased CFST member under axial compression, combined compression and bending and pure bending was presented by the FEA model. The failure mode, material stress distribution, interaction between steel tube and concrete, load transfer mechanism and load distribution between CFST and RC components were investigated. The performance of concrete-encased CFST member and the RC and CFST members was compared.2. Experiments on concrete-encased CFST box member under axial compression, combined compression and bending, and pure bending were carried out. The investigated parameters were stirrup ratio(axial compression), load eccentricity and height-to-width ratio(combined compression and bending) and shear span-to-depth ratio(pure bending). The failure mode, load transfer mechanism and the parametric influence were investigated.3. The FEA model of concrete-encased CFST box member was carried out, which combined the FEA model of concrete-encased CFST member and the characteristic of box section, i.e. the division of concrete according to different confinements. The tested results were verified by the FEA model. Full range analysis on concrete-encased CFST box member under axial compression, combined compression and bending and pure bending was presented by the FEA model. The failure mode, material stress distribution, contact between steel tube and concrete and load distribution between CFST and RC components were investigated. The performance of concrete-encased CFST box member and the RC and CFST members was compared.4. The influence of investigated parameters, including parameters of RC and CFST components and the geometric dimension ratio of two components(D/B), on the performance of the composite member was summarized. The design methods and suggestions of square concrete-encased CFST members under combined compression and bending were established. The fiber models of square concrete-encased CFST members were developed by the subroutines of different matieals and the combination of different sectional components. The applicability of the fiber model in the analysis of the seismic performance of the whole concrete-encased CFST structure was verified.