Experimental And Numerical Investigations On Concrete-filled Steel Tubular Stub Columns Under Axial Compression

Due to the interaction between the concrete and the steel section, concrete-filled steel tubular (CFST) columns have a series of excellent structuralperformance. To enable better understanding of the compressive behavior andmechanism of CFST stub columns under axial compression, and assess theconfinement effect between the concrete and the steel section, experimental andnumerical studies were concducted in this thesis:This thesis presents an experimental study to investigate the compressivebehavior of circular CFST stub columns when subjected to pure axial loading.An experimental programme comprising a total of6test specimens, with threediameter-to-thickness (D/t) ratios (48.7,30and22) and two concrete grades(C30, C50) has been performed. The effects of diameter-to-thickness (D/t) ratios,concrete strength on ductility and ultimate capacity were studied.Based on the test data of CFST stub columns under axial compression, thebehaviour of empty steel tube under combined axial compression and internalpressure was investigated experimentally, the purpose was to simulate the stateof biaxial stress in steel tube and gain an in-depth understanding of themechanism of CFST under axial compression.The measured ultimate axial capacities were compared to theircorresponding theoretical values predicted by six different international codesand standards: the American Institute of Steel Construction (AISC), theAmerican Concrete Institute (ACI318), the Australian Standard (AS),Architectural Institute of Japan(AIJ)，British Standard(BS5400)and Eurocode4. Result comparisons also included some suggested equations found in theliterature.Due to the passive confinement provided by the steel jacket for the concretecore, the behavior of the concrete in a CFST column is always very challengingto be accurately modelled.Although considerable efforts have been made in thepast to develop finite element (FE) models for CFST columns,these models maynot be suitable to be used in some cases. The simulation was based on theconcrete damaged plasticity material model, where new models were introducedfor a few material parameters used in the concrete model and a stress–strainmodel which is dependent on the confining pressure was developed for confined concrete. The compressive behaviour, which includes ultimate capacity,load-end displacement relationship and failure modes, were obtained from thenumerical models and compared against the experimental results, and goodagreements were obtained. This indicated that the proposed model could be usedto predict the compressive characteristics of CFST stub columns.