Creep Behavior Of Steel-Tube-Confined Concrete-Filled Steel Tube Stubs Under Sustained Axial Compression

Steel-tube-confined concrete-filled steel tube(TCFST)column is a new type of structural element.The lateral restraint of the outer steel tube improves the plasticity and ductility of the core concrete,thereby enhancing the compressive strength.TCFST columns also have the advantages of significant economic effect;good corrosion resistance and good fire resistance,so they have a wide range of application scenarios.When used as structural columns,due to the creep characteristic of the core concrete,the TCFST columns may undergo creep deformation and stress redistribution under longterm load,which may cause potential danger to the structural safety.However,the research on the creep of TCFST columns still in initial stage,which is the focus of this paper.The main work is presented as follows:Long-term tests were carried out on 4 groups of 8 TCFST columns under axial compression and the loading time is over 600 days.Based on the long-term deformationtime curve measured by the experiment,the influence of the core concrete strength grade and loading age on the creep of TCFST columns is compared and analyzed.Four plain concrete creep prediction models are compared and analyzed through finite element macro model.The CEB-FIP model is suitable for TCFST columns and is selected for indepth theoretical research.Parametric studies of the creep of TCFST columns were carried out.Considered parameters included core concrete strength grade,loading age,steel ratio and ratio of inner and outer steel tubes.The results show that the total steel ratio and the ratio of inner and outer steel tubes within the critical value have a greater impact on the creep coefficient of TCFST.Proposed that the improved CEB-FIP model is suitable for the mortal in TCFST columns.Studied the creep mechanism of TCFST columns on the meso scale and analyzed the stress redistribution between the inner steel tube and the core concrete and between the aggregate and the mortar.The results show that under the combined action of the two sets of stress redistributions,the vertical stress of the mortar gradually decreases and the stress unevenness increases.